Dr. Rob Dillon, Coordinator





Monday, May 16, 2022

Freshwater Gastropods of the Tennessee/Cumberland

Today we are pleased to announce the expansion of our FWGTN coverage from its East Tennessee origins though the entirety of the Tennessee and Cumberland River drainage basins, increasing our sampling area from approximately 22,000 square miles to over 58,000.  We document 54 species of freshwater gastropods with 16 additional subspecies in this malacologically rich region, offering ecological and systematic notes for each, as well as detailed distribution maps, a dichotomous key and a photo gallery.  This expanded web resource, coauthored by R.T. Dillon, M. Kohl and R. Winters, is available here:

The Tennessee/Cumberland

The previous version of our FWGTN website, brought online in 2011 by Dillon & Kohl, covered only the Tennessee River drainage system from SW Virginia and western North Carolina through East Tennessee to skim the top of North Georgia and stop at the Alabama border.  Our 2011 database included 1,674 records from approximately 767 discrete sites, documenting 39 species and 2 subspecies.  The expanded database we release today includes 4,003 records from approximately 1,700 discrete sites, ranging though North Alabama and Middle Tennessee to clip the corner of NE Mississippi, plus a big slice across southern Kentucky as well.

Click for larger

Among many interesting findings, we report here that three pleurocerid species previously thought restricted to East Tennessee range significantly further west: Pleurocera simplex (with its subspecies ebenum), Pleurocera troostiana (with subspecies perstriata, edgariana, and lyonii) and Pleurocera clavaeformis (subspecies unciale).  We have also discovered that Pleurocera semicarinata, previously unknown further south than Kentucky, ranges through Cumberland drainages well into Tennessee.  The distributions of several hydrobioid species are also clarified and expanded – more about this in coming months.

Our complete FWGNA database, covering the drainages of The Ohio as well as Atlantic drainages from Georgia to the New York line, now contains 22,044 records documenting 107 species of freshwater gastropods, with 21 subspecies.  We have updated our overall website with a new continent-scale biogeographic analysis, dividing records into North Atlantic, South Atlantic, Ohio, and Tennessee/Cumberland subsets.  Our analysis suggests that natural selection has been more important in the evolution of freshwater pulmonates than gene flow restriction, but that gene flow restriction has been more important in the evolution of freshwater prosobranchs than natural selection.

We also announce today the publication of an updated “Synthesis v3.1,” ordering our 107 species by their incidence in our continental database and assigning fresh FWGNA incidence ranks to all.

So, visit the FWGNA web resource again, for the first time!

Tuesday, April 5, 2022

The ham, the cheese, and Lithasia jayana

Editor’s Note – This is the fifth and final installment of my series on the population genetics and systematic relationships of the Duck River Lithasia.  If you are interested in the science, you probably ought to review my posts of 7Dec21, 4Jan22, 3Mar22, and 28Mar22 before proceeding.  Or you could simply enjoy this essay for what it is, a true tale from the wild west days of American malacology.

“Well, Ahlstedt, is this jayana or is it not?”  That was the punchline of a story told me by my major advisor, Dr. George Davis, shortly after I arrived at the Philadelphia Academy of Natural Sciences in the summer of 1977.  Why was that question so weighty?  And why have I remembered the saga from which it springs over 40 years now, to pass it along to future generations of malacologists?

Previously, on the FWGNA blog!  In December and January, we obsessed at great length over Lithasia geniculata, with its three subspecies, extending down the 275 mile length of the Duck River of Middle Tennessee, bearing smooth, oblong shells in the headwaters, developing robust, bumpy shoulders in the lower reaches. In his seminal (1940) monograph [1], Calvin Goodrich also identified a second species of Lithasia in the Duck, bearing a more acute apex and angled (sometimes even tuberculate) shell, extending only from about river mile 186 to the mouth.  These he identified as Lithasia duttoniana.

This month’s episode begins in Washington, walking the hallowed halls of Congress.  In 1969, just before the dawn of the environmental movement, federal funding was secured for the construction of two new dams on the Duck River: the smallish Normandy Dam at RM 248, and a larger and more expensive Columbia Dam, downstream around RM 145.  The Normandy Dam was completed in 1976.

Normandy Dam (TVA)

But the National Environmental Policy Act went into effect January 1, 1970, the Clean Water Act in 1972, the and the Endangered Species Act in 1973.  The TVA found itself required to file an environmental impact statement for the Columbia Dam, even as construction proceeded apace.  And Dr. George M. Davis, fresh out of the Army and sitting in the Pilsbry Chair of Malacology at the ANSP, was awarded a contract to survey the Duck for potentially-endangered pleurocerid snails.

George Davis’ pleurocerid taxonomy was idiosyncratic.  He began by noticing that in almost all aspects of their biology, including body size, life habit, and shell morphology, pleurocerid populations of the Haldeman's 1840 genus Lithasia are not strikingly different from Lea's monotypic genus Io of 1831. Davis therefore synonymized Lithasia under Io and recognized five taxa in the Duck River: Io geniculata geniculata, Io geniculata pinguis, Io salebrosa, Io armigera duttoniana and Io armigera jayana.  The identities of Davis’ Io geniculata geniculata, Io geniculata pinguis, and Io armigera duttoniana will by now be obvious to my readership.  “Io salebrosa” was Davis’ name for what Goodrich would have called Lithasia geniculata fuliginosa.  What was “Io armigera jayana?”

The idea that a pleurocerid population matching Isaac Lea’s nomen “jayana” in the Duck River was especially controversial when Davis filed his report in 1974 [2].   Isaac Lea [3] published a brief, Latinate description of “Melaniajayana in July [4] of 1841: “Hab. Cany Fork, DeKalb Co., Tenn. – Dr. Troost [5].”  In his more complete English description of 1846, Lea emphasized “It very closely resembles the M. armigera (Say), in most of its characters, but may at once be distinguished by the double row of tubercles, the armigera never possessing distinctly more than one row [6].”

Lea’s selection of Mr. Say’s Melania armigera as a point of comparison is especially significant.  Described by Thomas Say in 1821 from “The Ohio River” [7], the range of populations today identified as Lithasia armigera extends through much of the lower Cumberland and Tennessee Rivers as well [1], including the Tennessee at the mouth of the Duck.  And the resemblance between Mr. Lea’s jayana and Mr. Say’s armigera is indeed close, as shown in the montage of Tryon’s [8] figures below.

Two L. armigera left, two L. jayana right [8]
Goodrich recognized neither armigera nor jayana in the Duck River.  Goodrich identified all the Lithasia bearing acutely-spired shells with tubercles or spines inhabiting the Duck River as L. duttoniana, a species Lea described in brief Latinate form about five months prior [4] to his description of M. jayana: “Hab. Waters of Tennessee, Dr. Troost.  Duck River, Maury Co. Tenn., Mr. Dutton [9].”  Lea never compared his M. duttoniana to any other species of what we would identify today as a Lithasia: armigera, jayana or anything else.

The only population of L. jayana that Goodrich recognized in his 1940 monograph was the Caney Fork type population, and that population seems to have been extincted by the closing of the Center Hill Lake dam and associated development in 1948.  So if Davis’ Duck River record of “Io armigera jayana” was to be believed, the last remaining population of a genuinely endangered species would be smack in the middle of the Duck River where the Columbia Dam was even at that time being constructed.

The scene now shifts to the sleepy little East Tennessee company town of Norris, were in 1974 the TVA hired a promising young biologist named Steven A. Ahlstedt to work in its Department of Forestry, Fisheries, and Wildlife Development [10].  Steve is an excellent scientist, with superb field skills and a great eye for freshwater mollusks.  He had a reprint of Tryon’s monograph on his shelf, and all of Goodrich’s papers in the top drawer of his filing cabinet, but very little of Goodrich’s work is illustrated, and to match Goodrich with Tryon is a bitch.  And this was six years before the EPA published The Gospel According To Jack Burch [11].  So the TVA sent Steve to the University of Michigan Museum of Zoology to learn freshwater malacology hands-on.

And the stage is now set.  In the fall of 1975 the telephone rang on George Davis’ desk at the Academy of Natural Sciences in Philadelphia.  It was the receptionist downstairs.  She reported that a team of TVA biologists had presented themselves at the front door and were requesting admission to the Malacology Department to examine his Duck River pleurocerid collections.  They had no appointment.

Two L. duttoniana [8]

The members of the TVA team were John Bates, Billy Isom, Steve Ahlstedt and Donelly Hill.  Bates was, at that time, a professor at Eastern Michigan University and Research Associate at the UMMZ [12].  Isom worked for the TVA at their Muscle Shoals office [13].  Hill was a supervisor, with a fisheries background.

Like my Momma used to say, “You could have called.”  George Davis was still livid about the surprise nature of this audit when he told me the story two years later.  The TVA team asserted that they had a right to examine the Duck River collections, since the agency they represented had paid for them.  Davis countered that he would need reimbursement for the time required by his curatorial staff to pull the lots.  The TVA team went away threatening to get a court order but returned the next day with a purchase order instead.

Davis would not admit either Bates or Isom into the collection.  They were notorious guns-for-hire in the wild west era of American freshwater malacology, and Davis did not trust them.  Neither did Steve Ahlstedt, for that matter.  So, in the end, Donelly Hill sent the most junior member of the team, alone, up the elevator into the ANSP malacology collection to face the Wrath of George.  Steve tells me that he felt like he was “caught between the ham and the cheese.”

At high noon the fateful shot rang out over Benjamin Franklin Parkway, “Well, Ahlstedt, is this jayana or is it not?”  And Steve’s reply was … (dramatic pause)… affirmative.

The lower reaches of the Duck River, from about RM 60 to the mouth, are indeed inhabited by a population of Lithasia bearing shells indistinguishable from type collections made of L. jayana in the Caney Fork.  And since Isaac Lea and his contemporaries defined gastropod species by shell morphology alone, and since the Duck shells in George Davis’ right hand matched the Caney Fork types, they were, by definition, Lithasia jayana.

So, Davis’ pleurocerid identifications stood.  And in 1984, after a ten-year period of comment, revision and study, his taxa were included in a gigantic laundry-list of candidate invertebrates offered for review by the USFWS as Lithasia jayana, L. duttoniana, L. geniculata, L. pinguis, and L. salebrosa [15]. Nothing ever came of that proposal, however, and Lithasia jayana receded back into sincere obscurity, as opposed to the mere obscurity it had enjoyed in the 1970s.

The abandoned Columbia Dam, 1986 [14]

But it did not matter.  By that point, Steve Ahlstedt and his colleagues had documented populations of genuinely-endangered unionids in the Duck – species that had already been entered into the Federal list, as early as 1977.  Construction was halted on the 90% complete Columbia Dam in 1983, never to resume.  According to Steve, “They left their coffee cups on their desks.” The $83M project was demolished in 1999, and the 12,800 acres it was planned to inundate turned over to the state wildlife resources agency.

But although the name of Lithasia jayana disappeared from the public eye, it remained printed in the pages of dusty tomes, and scrawled on the labels of moldering museum collections.   And in 2003 our colleagues Russ Minton & Chuck Lydeard pulled it back from sincere obscurity into mere obscurity once again in the paper we reviewed early last month [16].  Russ and Chuck reported no mtDNA sequence divergence between the robust, doubly-spined populations that Davis identified as jayana and more lightly shelled populations, tuberculate at best, that Davis (and everybody else prior to 2003) identified as Lithasia duttoniana.

Nor is there any allozyme difference between jayana and duttoniana.  Late last month I posted an unusually technical report on this blog, which I did not advertise at the time, so most of you probably missed it.  If you’re serious about the science, you might want to open this link in a new window [28Mar22].  Or you could just scroll down directly below the present post, skim that essay and scroll back.  Or you could just believe the paragraph that follows.

When Paul Johnson sent me all those Lithasia samples from the Duck River back in the summer of 2002, there were three bags from Site E and three bags from Site F – one labelled L. geniculata geniculata (which we talked about in December, January, and early March), one labelled L. duttoniana (which we talked about in January, early March, and late March) and one labelled L. jayana.  Using gene frequencies at three allozyme-encoding loci, I was indeed able to distinguish L. geniculata from L. duttoniana.  But there is no genetic distinction whatsoever between L. duttoniana and L. jayana.  They seem to be simple shell forms of the same biological species.

So now the time has come to sum up, over all five essays in this extended series.  The Duck River is inhabited by two biological species of large-bodied pleurocerid snails referable to the genus Lithasia.  One of those species bears smooth, high-spired, shells in the headwaters, becoming more robust, oblong and bumpy downstream and out into the main Tennessee River.  The modern era of American malacology was born in 1934, when Calvin Goodrich realized that populations bearing all those smooth-to-bumpy shells are conspecific, lowering the specific nomina assigned to two upstream forms, pinguis and fuliginosa, to subspecific status under the downstream nomen Lithasia geniculata.

Gastropod magafauna of Caney Fork, 500 A.D. [17]

Living together with Lithasia geniculata in the lower reaches of the Duck River is a second species of Lithasia, genetically quite similar to L. geniculata, but morphologically distinct and reproductively isolated.  These snails also demonstrate a cline in shell morphology, from a single row of light tubercules upstream, becoming more robust and heavily (even doubly) spined downstream, out into the main Tennessee River.  Then by analogy with L. geniculata, let us lower the specific nomina assigned to the two upstream forms, duttoniana and jayana, to subspecific status under the downstream nomen Lithasia armigera.

This hypothesis is supported by the genetic data, both by the duttoniana/jayana allozyme results offered late last month [28Mar22], and by the L. armigera CO1 sequences published by Minton & Lydeard, reviewed early last month [3Mar22].  It was originally suggested on the basis of shell morphology by Calvin Goodrich himself, way back in 1921 [18], although by 1940, he had apparently changed his mind [1].  It was also advocated by Davis in 1974 [2].

So this morning I have uploaded three fresh pages to the FWGNA website: Lithasia armigera duttoniana, L. armigera jayana, and L.armigera armigera.  And I remind my readership that the FWGNA defines the word “subspecies” to mean “populations of the same species in different geographic locations, with one or more distinguishing traits.”  This is the definition of the term as developed by the architects of the Modern Synthesis, and means precisely what it says, neither more nor less [19].

There need be no additively-genetic basis for the “distinguishing traits.”  Indeed, in the genus Pleurocera, a very closely analogous trend from gracile shells in the headwaters to robust shells in the big rivers has been convincingly attributed to cryptic phenotypic plasticity [20]. The heritable component of the shell morphological variance among populations identifiable as jayana, duttoniana, and armigera (s.s.) remains an open question.


Notes

[1] Goodrich, C. 1940. The Pleuroceridae of the Ohio River drainage system. Occasional Papers of the Museum of Zoology, University of Michigan 417:1 - 21.

[2] Davis, G.M. 1974.  Report on the rare and endangered status of a selected number of freshwater Gastropoda from southeastern U.S.A. U.S. Fish & Wildlife Service. Washington, DC. 51 p.

[3] For more about “The Nestor of American Naturalists” see:

  • Isaac Lea Drives Me Nuts [5Nov19]

[4] Lea, I (1841) Proceedings of the American Philosophical Society 2 (19): 83.  There has historically been much controversy regarding the exact publication dates for Lea’s descriptions.  The date on the cover of PAPS 2(19) is “July – October 1841,” a four month window.  Lea’s description of jayana appears in the “Stated Meeting of July 16,” prefaced by a statement that the paper itself was “read on the 18th of June last.”  That’s as far down the rabbit hole as I care to go.

[5] For a brief biography of Gerard Troost, see:

  • On the Trail of Professor Troost [6Dec19]

[6] Lea, I (1846) Continuation of Mr. Lea’s paper on fresh water and land shells.  Transactions of the American Philosophical Society 9(1) 1 – 31.

[7] Say, T. (1821) Journal of the Academy of Natural Sciences of Philadelphia (First Series) 2: 178.

[8] Tryon, G. W. (1873)  Land and Freshwater shells of North America Part IV, Strepomatidae.  Smithsonian Miscellaneous Collections 253: 1 - 435.

[9] Lea, I. (1841) Proceedings of the American Philosophical Society Volume 2(16) 15.

[10] I met Steve in 1975, the summer after my sophomore year at Virginia Tech, when I was blessed to be offered an hourly job with the TVA in Norris.  Malacological posterity will owe him a debt of gratitude for sharing his recollections of the events recorded here.

[11] This is a difficult work to cite.  J. B. Burch's North American Freshwater Snails was published in three different ways.  It was initially commissioned as an identification manual by the US EPA and published by the agency in 1982.  It was also serially published in the journal Walkerana (1980, 1982, 1988) and finally as stand-alone volume in 1989 (Malacological Publications, Hamburg, MI).

[12] John Morton Bates (b. 1932) began his career at the ANSP 1956 – 1966, then accepted a tenured position at Eastern Michigan University, with a research associate appointment at the UMMZ.  He left Michigan to found “Ecological Consultants” of Shawsville, VA, with Sally Dennis.  I’m not sure what became of him after that.

[13] Billy G. Isom (b. 1932) was an ecologist for the Tennessee Stream Pollution Control Board 1957 – 1963 before joining the TVA as a supervisor in the Limnology Section at Muscle Shoals, AL.  I think he is still alive and living in Killen, AL at the age of 89.

[14] Tennessee Valley Authority (1999) Use of Lands Acquired for the Columbia Dam Component of the Duck River Project. Final Environmental Impact Statement.

[15] Federal Register FR-1984-05-22. Endangered and threatened wildlife and plants; Review of invertebrate wildlife for listing as endangered or threatened species.  49(100) 21664 – 21675.

[16] Minton, R. L. and C. Lydeard. 2003. Phylogeny, taxonomy, genetics, and global heritage ranks of an imperiled, freshwater snail genus Lithasia (Pleuroceridae). Molecular Ecology 12:75-87.  For my review, see:

  • The third-most amazing research results ever published for the genetics of a freshwater gastropod population. And the fourth-most amazing, too. [3Mar22]

[17] This necklace was discovered by Cousin Bob Winters in a rock shelter on the north shore of Center Hill Reservoir, DeKalb County, TN.

[18] Goodrich, C. (1921)  Something about Angitrema.  The Nautilus 35: 58 – 59.

[19] If the clean, clear definition of the word “subspecies” confuses you, read:

  • What is a subspecies? [4Feb14]
  • What subspecies are not [5Mar14]

[20] Cryptic phenotypic plasticity is defined as “intrapopulation morphological variation so extreme as to prompt an (erroneous) hypothesis of speciation.”  For examples, see:

  • Goodrichian Taxon Shift [20Feb07]
  • Goodbye Goniobasis, Farewell Elimia [23Mar11]
  • Pleurocera acuta is Pleurocera canaliculata [3June13]
  • Elimia livescens and Lithasia obovata are Pleurocera semicarinata [11July14]

Monday, March 28, 2022

No reproductive isolation between Lithasia populations of the duttoniana and jayana forms in the Duck River, Tennessee

Editor’s Note.  We have always tried to avoid excessively technical posts on the FWGNA blog.  I typically publish formal research results elsewhere first and subsequently refer to those results here in a more casual tone.  Two of the three essays I have posted in recent months about the Lithasia of the Duck River [7Dec21] and [4Jan22] were, for example, preceded by brief, technical notes published in Ellipsaria (the newsletter of the Freshwater Mollusk Conservation Society) in 2020.

The post that follows, however, is technical.  Its publication was suppressed by FMCS Newsletter editor Dr. John Jenkinson because one of our mutual colleagues on the FMCS Board [1] called to Dr. Jenkinson’s attention that Ellipsaria content is being indexed by Google Scholar, and hence that the hypotheses I have proposed below [2] might fall upon na├»ve and uncritical eyes.  Why is the research that follows so dangerous?  You be the judge!

The taxonomic history of the pleurocerid genus Lithasia (Haldeman 1840) is a long and complicated one.  Goodrich (1940) recognized 16 taxa of Lithasia with smooth shells, 3 with tuberculate shells, and 5 taxa bearing shells with spines or acute protuberances.  This last category comprised Lithasia duttoniana, described by Lea (1841) from the Duck River, L. jayana, also described by Lea (1841) from Caney Fork (of the Cumberland), and three subspecies of L. armigera, described by Say (1821) from the Ohio River.

Goodrich considered both Lithasia jayana, a heavily-shelled species bearing two rows of spines, and Lithasia duttoniana, a more lightly-shelled species typically bearing (at most) a single row of small protuberances, endemic to the rivers from which they were described.  Lithasia armigera, as understood by Goodrich, ranged from the lower Ohio and Wabash Rivers through most of the Cumberland River and much of the Tennessee River as well.

In an unpublished report to the U.S. Fish and Wildlife Service, Davis (1974) suggested that Lithasia be subsumed under the genus Io of Isaac Lea (1831).  Davis then went on to recognize three smooth-shelled taxa in the Duck River, which do not concern us here, and two spiny taxa, which he identified as Io armigera duttoniana and Io armigera jayana.  After a period of comment and revision, Davis’ spiny taxa were offered for review under the Endangered Species Act in the Federal Register (May 22, 1984) as Lithasia duttoniana and Lithasia jayana.

Minton & Lydeard (2003) surveyed mitochondrial CO1 sequence variation in Lithasia populations from the Duck River and many other river systems of the American southeast. The 4 unique sequences they obtained from 19 Duck River snails (1 jayana, 4 duttoniana, and 14 of smooth-shelled taxa, in 9 Genebank submissions) did not resolve into consistent clades.  Thus Minton & Lydeard synonymized all Duck River populations, bearing both smooth and spiny shells, under a single smooth-shelled nomen, L. geniculata (Haldeman 1840).

Dillon (2020 b, c) has recently reported, however, a survey of allozyme variation in Duck River Lithasia confirming Goodrich’s (1940) hypothesis that populations of the spiny duttoniana form are reproductively isolated from the more smooth-shelled form, which Dillon followed Goodrich in identifying as Lithasia geniculata.

The large and diverse samples of Lithasia analyzed by Dillon (2020 b, c) were collected incidentally during a survey of the Duck River mussel fauna conducted by Ahlstedt et al. (2017).  In addition to populations bearing shells of the (smooth-shelled) geniculata form and the lighter, single-spined duttoniana form, the collections made by Ahlstedt and colleagues at two of their most downstream sites also contained Lithasia bearing shells of the heavy, doubly-spined jayana form.  Here I compare those doubly-spined jayana samples to sympatric samples of the lightly-shelled duttoniana form using gene frequencies three allozyme-encoding loci.

Fig. 1. The Duck River, showing sample sites.

My methodology for the resolution of allozyme polymorphism by horizontal starch gel electrophoresis has been previously detailed (Dillon 1982, 1985, 1992).  For the present study, variation interpretable as the product of codominant, Mendelian alleles was resolved at the mannose phosphate isomerase (Mpi) locus using buffers TrisCit6 and TEB8, at the octopine dehydrogenase (Odh) locus using buffers TisCit6 and Poulik, and hexanol dehydrogenase (hexdh) using buffers TEB8 and Poulik.

Sample sites and example shells are shown in Figure 1 above.  (The shell length of dutF is 24.6 mm; the other shells are to scale.)  At their most downstream site, site F, Ahlstedt and colleagues collected 35 Lithasia of the duttoniana form (dutF) and 30 of the jayana form (jayF).  This site, the Watered Hollow Boat launch at Duck River Mile 26.0 (35.9322, -87.7475), was the point at which Minton & Lydeard (2003) collected their sample of L. jayana for mtDNA sequencing.  Upstream at Wright Bend Site E (TNC110, DRM 38.7, 35.8267, -87.6657), Ahlstedt collected 44 Lithasia of the duttoniana form (dutE) and 40 of the jayana form (jayE).

Lithasia bearing shells of the jayana morphology become increasingly rare further upstream and are not effectively collectable above Duck River mile 60.  But populations of the more lightly-shelled duttoniana type extend as far upstream as DRM 186.  Gene frequencies in duttoniana population dutD, collected from the Fountain Creek confluence at DRM 145.5 (TNC 94, 35.5695, -86.9682), are included here for comparison.

Table 1 below shows that no significant allele frequency differences were apparent between samples bearing shells of the duttoniana and jayana forms at either site where they co-occurred.  This was true for the Odh locus (chi-square = 0.688, 2 df at site E, chi-square = 1.62, 3 df at site F), the Mpi locus (Fisher’s p = 0.334 at site F) and the Hexdh locus (Fisher’s p = 0.513 at site E, p = 0.832 at site F).  Judging by Nei (1978) genetic distance, sample jayE was more genetically similar to sample dutE (D = 0.050), and sample jayF was more similar to dutF (D = 0.080) than jayE was to jayF (D = 0.161) or dutE to dutF (D = 0.164).

Gene frequency differences were very significant longitudinally, however, at two of the three loci examined.  Combining the 44 + 40 = 84 samples from site E (DRM 38.7) and comparing to the 35 + 30 = 65 samples from site F (DRM 26.0), chi-square = 26.2 (3 df, p < 0.00001) at the Odh locus and chi-square = 9.91 (1 df, p = 0.002) at the Hexdh locus.  The dutD sample collected upstream at DRM 145.5 also differed significantly at the Odh locus from the combined site E sample (chi-square = 7.84, 2df, p = 0.02).

Tab 1. Gene frequencies at three loci in five samples of Lithasia.

These results reflect no evidence of reproductive isolation between Lithasia bearing the duttoniana shell morphology and those bearing the jayana shell morphology.  The genetic evidence is strong, however, for isolation by distance among the spiny Lithasia populations down this length of river, similar in magnitude to that documented by Whelan et al. (2019) in Alabama Leptoxis, and Dillon (2020a) in North Carolina Pleurocera.

The similarity between these results and those previously published by Dillon (2020b) for the smooth-shelled Lithasia of the Duck River is striking.  Dillon confirmed the hypothesis of Goodrich (1934) that the shells borne by Duck River Lithasia geniculata also become more robust when sampled in a downstream direction, adding bumpy shoulders to the point that 19th-century authorities recognized two additional species, L. fuliginosa and L. pinguis.  Here an identical phenomenon is documented in the spiny Lithasia, populations identified by Goodrich as Lithasia duttoniana developing such robust and heavy shell spines downstream that some authorities have recognized a second species, L. jayana.

Given such levels of shell variability, neither nominal Lithasia duttoniana (Lea 1841) nor Lithasia jayana (Lea 1841) can be distinguished at the specific level from the much more broadly-distributed Lithasia armigera (Say 1821).  The suggestion of Davis (1974) that both of Lea’s 1841 nomina be lowered to subspecific status under Say’s L. armigera would seem to have substantial merit.


References

Ahlstedt, S. A., J. R. Powell, R. S. Butler, M. T. Fagg, D. W. Hubbs, S. F. Novak, S. R. Palmer and P. D. Johnson. 2017. Historical and current examination of freshwater mussels (Bivalvia: Margaritiferidae: Unionidae) in the Duck River basin Tennessee, USA. Malacological Review 45:1-163.

Davis, G.M. 1974.  Report on the rare and endangered status of a selected number of freshwater Gastropoda from southeastern U.S.A. U.S. Fish & Wildlife Service. Washington, DC. 51 p.

Dillon, R. T., Jr. 1982. The correlates of divergence in isolated populations of the freshwater snail, Goniobasis proxima (Say). Ph.D. Dissertation, The University of Pennsylvania.

Dillon, R. T., Jr. 1985. Correspondence between the buffer systems suitable for electrophoretic resolution of bivalve and gastropod isozymes. Comparative Biochemistry and Physiology 82B: 643-645. [pdf]

Dillon, R. T., Jr. 1992. Electrophoresis IV, nuts and bolts. World Aquaculture 23(2):48-51.

Dillon, R. T., Jr. 2020a. Fine scale genetic variation in a population of freshwater snails. Ellipsaria 22(1): 24-25. [pdf]

Dillon, R. T., Jr. 2020b. Population genetic survey of Lithasia geniculata in the Duck River, Tennessee. Ellipsaria 22(2):19 – 21. [pdf]

Dillon, R. T., Jr. 2020c. Reproductive isolation between Lithasia populations of the geniculata and duttoniana forms in the Duck River, Tennessee. Ellipsaria 22(3): 6 – 8. [pdf]

Goodrich, C. 1934. Studies of the gastropod family Pleuroceridae - I. Occasional Papers of the Museum of Zoology, University of Michigan 286:1-17.

Goodrich, C. 1940. The Pleuroceridae of the Ohio River drainage system. Occasional Papers of the Museum of Zoology, University of Michigan 417:1 - 21.

Minton, R. L. and C. Lydeard. 2003. Phylogeny, taxonomy, genetics, and global heritage ranks of an imperiled, freshwater snail genus Lithasia (Pleuroceridae). Molecular Ecology 12:75-87.

Nei, M. 1978 Estimation of average heterozygosity and genetic distance from a small number of individuals. Genetics 89:583–590.

Whelan, N. V, M. P. Galaska, B. N. Sipley, J. M. Weber, P. D. Johnson, K. M. Halanych and B. S. Helms. 2019. Riverscape genetic variation, migration patterns, and morphological variation of the threatened Round Rocksnail, Leptoxis ampla. Molecular Ecology 28:1593-1610.


Notes

[1] Here is the relevant passage from the minutes of the FMCS Board, November 19, 2020:

“Nathan Whelan noticed recently that some Contributed Articles in Ellipsaria are now findable on Google Scholar. This was unexpected for our informal, non-peer-reviewed newsletter.  Nathan also recognized that some recent articles in the newsletter include the analysis of data and/or what could be viewed as proposed taxonomic revisions. In a series of emails, monitored and, occasionally, participated in by the Executive Committee, Nathan and Ellipsaria Editor John Jenkinson agreed that articles including data analysis and/or taxonomic revisions should be peer-reviewed and, therefore, are outside of the intended scope and purpose of our newsletter.”

[2] Science is the construction of testable hypotheses about the natural world.  It is not the handing down of fact.  It appeals to no authority, nor does it merit any.  It is independent of context or culture.  Whether published in a slick international journal or a humble newsletter is irrelevant.  The quality of a work of science is dependent only upon the extent to which the hypothesis proposed matches the natural world, upon rigorous test.  And it stuns me – literally stuns me – to see how few scientists actually understand any of this.

Thursday, March 3, 2022

The third-most amazing research results ever published for the genetics of a freshwater gastropod population [1]. And the fourth-most amazing, too.

Editor's Note:  This is the third installment of what will turn out to be a five-part series on the Lithasia of the Duck River in Middle Tennessee.  It will help to familiarize yourself with my posts of [7Dec21] and [4Jan22] before continuing.

In 2003, Russ Minton and Chuck Lydeard published a CO1 gene tree for the North American pleurocerid genus Lithasia in Molecular Ecology [2].  Nestled among the branches of that tree was the third-most amazing research result in the history of freshwater gastropod population genetics.  What Minton and Lydeard found was… nothing.

The Minton & Lydeard study was very good by the standards of its day.  Our colleagues did a thorough job collecting U1S2NMT3 individuals [7] from 30 Lithasia populations representing 11 nominal species and subspecies.  We first touched on the M&L results back in [4Sept19], focusing on a single outside branch, labeled “L. geniculata pinguis.”  And here is a quote from that 2019 essay: “To completely unpack the message being telegraphed to us by the enigmatic arboreal specimen (of Minton and Lydeard) would require at least 6 – 8 blog posts of standard length.”  So, what follows is another installment [8].

Minton & Lydeard included in their analysis 19 Lithasia individuals from the main Duck River, identified as follows: 6 geniculata pinguis, 7 geniculata fuliginosa (from three sites), 1 geniculata geniculata, 4 duttoniana (from two sites), and 1 jayana.  And on the basis of mtCO1 sequence, they were unable to distinguish among any of those 19 individuals.  I was agog in 2003 and remain agog 20 years later.

Detail from Minton & Lydeard [2] fig 4, modified

To contextualize.  With no difficulty whatsoever, even at very small sample sizes, workers have easily been able to document 23% CO1 sequence divergence within Pleurocera simplex populations, 21% within Pleurocera catenaria, 19% within Pleurocera proxima, 15% within Leptoxis carinata, and 12% within L. ampla [9].  Then In 2003, in the pages of an international journal, Russ Minton and Chuck Lydeard stunned the world by reporting no more than a couple lousy nucleotides of difference in 19 individual Lithasia sampled down the 200-mile length of the Duck River, bearing five different Latinate nomina.

Well, maybe the M&L result is not terribly surprising for the 14 individual Duck River Lithasia geniculata they included in their survey, of the three subspecies.  In December [7Dec21], we documented evidence of gene flow among subpopulations representing all three of those nomina, attenuated by distance but not much else [11].  The M&L pinguis sample seems to have been sampled from below the falls.

Nor am I terribly shocked by the absence of sequence divergence among the 4 duttoniana sampled by M&L.  In January [4Jan22] we documented similar levels of isolation-by-distance between Duck River subpopulations bearing the DUT shell morphology that we had previously seen in the GEN form [12].

Nor am I even terribly surprised by the absence of sequence divergence between the 4 duttoniana and the singleton snail that M&L identified as “Lithasia jayana.”  This is the first time the specific nomen “jayana” has appeared on the FWGNA blog.  It will not be the last.  We will have much more to say about Lithasia jayana in posts upcoming.  But for now, please accept that there is no significant genetic difference between snails that have been identified as L. jayana on the basis of shell morphology and sympatric Lithasia populations that Goodrich, Minton, and everybody else has always called L. duttoniana.

Rather, the third-most amazing research result ever registered in the annals of freshwater gastropod population genetics is the absence of any detectable sequence divergence between the 14 individual geniculata (all subspecies) and the 5 individual duttoniana + jayana.  Populations historically identified by those two sets of nomina really are morphologically distinct throughout their entire 100+ miles of sympatry in the Duck River, and everywhere else through their combined ranges across the Ohio, Cumberland, and Tennessee.

Can you tell us apart? [13]

Isaac Lea and George Tryon and Calvin Goodrich and all modern workers even unto the present day have all drawn a clear and unambiguous distinction between Lithasia populations bearing a robust, oblong, bumpy shell morphology and those bearing a more acutely-spired shell morphology, with angular whorls often tuberculate or even slightly-spiny.  In our January essay we abbreviated that former morphology “GEN” and that latter morphology “DUT.”  Snails bearing shells of the DUT morphology range only up to around Duck River mile 186 (as opposed to 275 for GEN) and seem more common in the shallows, rather than on rocks in the middle.  No prior worker has ever questioned the distinction between those two groups of taxa.

Setting 200 years of field observation aside, however.  On the basis of their sequence data, Minton and Lydeard synonymized all the Duck River Lithasia taxa: pinguis, fuliginosa, duttoniana, and jayana, under Haldeman’s (1840) geniculata.  Russ Minton then went on, in papers published in 2008 and again in 2018, to perform detailed morphometric analyses on the entire five-taxon GEN/DUT mishmash combined [14].  Bless his heart.

Well, the GEN and DUT populations do differ genetically, but not by much.  When Johnson, Ahlstedt, and their colleagues sent me those Lithasia samples from Fountain Creek (site D), Wright Bend (site E) and Watered Hollow (site F) back in 2002, they divided them (quite naturally and conventionally) into oblong-bumpy subsamples they identified as Lithasia geniculata, and acute-angular subsamples they identified as Lithasia duttoniana.  And in my January essay [4Jan22] I used the allozyme results I obtained from Wright Bend (site E) as an example of stable character phase disequilibrium between GEN and DUT.  Results were the same at Fountain Creek and Watered Hollow.  Hit this link for a pdf of my technical results [Ellipsaria 22(3)].

Lithasia bearing the GEN shell morphology and Lithasia bearing the DUT morphology sympatric in the Duck River do not constitute a single randomly-breeding population.  There is some sort of reproductive isolation between them [15].  They are distinct biological species.

How many species can you see? Click to zoom [16].

But my goodness, the allozyme divergence between GEN and DUT is tiny!  The gene frequencies I published in Table 1 of my paper in Ellipsaria 22(3) were certified in 2020 as the fourth-most amazing research results in the history of freshwater gastropod population genetics, at 88.7 international amazingness units [1].

Again, some context would seem to be in order.  I ran allozyme gels on scores of pleurocerid populations during the 35 years I had access to a biochemical laboratory, 1980 – 2016.  Typically, I would do an initial screening across 15 – 20 allozyme loci (17 in the case of the Duck River Lithasia), and then focus on the polymorphic loci for a detailed analysis.  And very rarely did I ever find a pair of distinct biological species sharing alleles any more than at a couple loci, out of 15 or 20 [17].

Even among populations within pleurocerid species, fixed allozyme differences are not uncommon [18].  Across the 25 populations of P. proxima I surveyed for my 1984 dissertation, for example, it was possible to find conspecific populations sharing no alleles at five loci.  And even within individual pleurocerid populations, sampled from single creeks or rivers, significant allozyme differences among subpopulations are not uncommon, as we witnessed in the P. proxima of Naked Creek in October [12Oct21], and in the L. geniculata of the Duck in December [7Dec21].

So seen in that context, to find no difference between a pair of reproductively-isolated pleurocerid species at 14 of 17 allozyme loci, and merely-statistical differences at the other three, shocked me back in 2002.  And I’m obviously still not over it, any more than I am over the CO1 sequence results published by Minton and Lydeard in 2003.  I was aghast at the time and remain aghast to this day.

Let this be a lesson to any of you high school seniors out there, looking for science fair projects.  There are a lot of online purveyors of simple kits advertising “DNA barcoding” services, promising to identify any sort of unknown bug or slug you might pluck into a tube and mail to Canada.  That’s fun, and I’m sure you’ll learn more from the experience than lying around your bedroom, watching Tik-Tok videos.  But please understand that no serious scientist would ever publish a paper in the peer-reviewed literature relying on “DNA barcoding.”

Do I have time to touch on one additional feature of the 2003 M&L gene tree before you run out of patience with me this month?  Notice this.  Not only is there essentially zero divergence among their 19 Duck River samples of two reproductively-isolated species, we really don’t see much sequence divergence anywhere in the entire top half of the Minton & Lydeard Lithasia tree.

Detail from Minton & Lydeard [2] fig 4, modified.

If you back down one limb below the big Duck River cluster at the top, you’ll see a couple samples labeled, “geniculata fuliginosa” from 23 miles back up a tributary of the lower Duck River called the Buffalo.  M&L did uncover 2.0% sequence divergence between their Duck River N = 19 and their Buffalo River N = 2, upon which basis Russ described a new species, “Lithasia bubala” in 2013 [19].  The allozyme data I reported on [7Dec21] did not support that [11].

Then if you back down two limbs from the M&L Duck River cluster, you find a set of five sequences identified as Lithasia armigera.  These represent 14 individuals collected from five far-flung rivers: the Harpeth River and the Stones River (both tributaries of the Cumberland), the main Tennessee River way down in Alabama, the Wabash River (in Illinois) and the main Ohio River on the IL/KY border.  All 14 of these snails, from five populations, were genetically indistinguishable.  And all differed by just 3.8% from the Duck River group.

And if you back down three limbs from the M&L Duck River cluster, you’ll find a set of three sequences (representing 8 individuals) labelled “geniculata fuliginosa,” two from the Red River (a tributary of the Cumberland about 80 miles north of the Duck) and one sequence from Garrison Fork, an upstream tributary of the Duck River itself.  The sequence divergence between that set of N = 8 and the set of N = 19 from the main Duck was 4.3%.

Let me say that again.  There is less sequence divergence between L. duttoniana of the Duck River and L. armigera of the Wabash River almost 200 miles away, than between L. geniculata fuliginosa of the Duck River and L. geniculata fuliginosa of Garrison Fork, 25 miles upstream.  What in the world does that mean?  Stay tuned!


Notes

[1] I apologize for the overly-dramatic title.  For the record, the CO1 sequence homogeneity in the Duck River Lithasia as reported by Minton & Lydeard in 2003 [2] scored 91.5 international amazingness units.  The Bianchi et al. (1994) report of hybridization between P. virginica and P. semicarinata livescens [3] holds first place in the freshwater gastropod population genetics division at 93.2 international amazingness units, with Nathan Whelan’s [4] discovery of a wildebeest sequence in the population of bison he sampled at Shades Creek in second place at 91.9 iau.

For context, in the freshwater gastropod transmission genetics division, Yoichi Yusa’s discovery of multigenic sex determination in Pomacea [5] scored a whopping 98.7 iau in 2007, pushing  Boycott’s (1923) paper on maternal inheritance of chirality in Lymnaea [6] to second all time, at 98.4 iau.

[2] Minton, R. L. and C. Lydeard. 2003. Phylogeny, taxonomy, genetics, and global heritage ranks of an imperiled, freshwater snail genus Lithasia (Pleuroceridae). Molecular Ecology 12:75-87.

[3] Bianchi, T. S., G. M. Davis, and D. Strayer 1994.  An apparent hybrid zone between freshwater gastropod species Elimia livescens and E. virginica (Gastropoda: Pleuroceridae).  Am. Malac. Bull. 11: 73 - 78.

[4] Whelan, N.V. & E. E. Strong (2016)  Morphology, molecules and taxonomy: extreme incongruence in pleurocerids (Gastropoda, Cerithiodea, Pleuroceridae). Zoologica Scripta 45: 62 – 87.  I reviewed Nathan’s findings in a series of posts back in 2016, see note [10] below.

[5] Yusa, Y. 2007. Nuclear sex-determining genes cause large sex-ratio variation in the apple snail Pomacea canaliculata. Genetics 175: 179-184.  For more, see:

  • Ampullariids star at Asilomar [11Aug05]

[6] Boycott, A.E. and C. Diver (1923) On the inheritance of sinistrality in Limnaea peregra.  Proceedings of the Royal Society of London, Series B, Biological Sciences 95: 207 – 213.

[7] Usually 1, Sometimes 2, Never More Than 3.  This has always been the rule-of-thumb in sampling for gene trees.  See:

  • The Lymnaeidae 2012: Stagnalis yardstick [4June12]

[8] Actually, looking back on this post from the bottom, I am afraid I have written an essay of twice what ought to be my standard length.  And this is two installments.  Sorry.

[9] I coined the term “mitochondrial superheterogeneity” on this blog in 2016 to describe double-digit intrapopulation sequence divergence [10].  Here are several prominent examples from the pleurocerids:

  • Dillon, R. T., and R. C. Frankis. (2004)  High levels of DNA sequence divergence in isolated populations of the freshwater snail, Goniobasis.  American Malacological Bulletin 19: 69 - 77.  [PDF]
  • Lee, T., J. J. Kim, H. C. Hong, J. B. Burch, and D. O’Foighil (2006)  Crossing the Continental divide: the Columbia drainages species Juga hemphilli is a cryptic member of the eastern North American genus Elimia.  J. Moll. Stud. 72: 314-317. 
  • Dillon, R T. and J. D. Robinson (2009)  The snails the dinosaurs saw: Are the pleurocerid populations of the Older Appalachians a relict of the Paleozoic Era?  Journal of the North American Benthological Society 28: 1 - 11. [PDF]
  • Dillon, R. T. Jr, and J. D. Robinson (2016)  The hazards of DNA barcoding, as illustrated by the pleurocerid gastropods of East Tennessee.  Ellipsaria 18: 22-24. [PDF]
  • Whelan, N.V. & E. E. Strong (2016)  Morphology, molecules and taxonomy: extreme incongruence in pleurocerids (Gastropoda, Cerithiodea, Pleuroceridae). Zoologica Scripta 45: 62 – 87.

[10] For more about the origin and significance of the phenomenon, see:

  • Mitochondrial superheterogeneity: What we know [15Mar16]
  • Mitochondrial superheterogeneity: What it means [6Apr16]
  • Mitochondrial superheterogeneity and speciation [3May16]

[11] Dillon, R. T. (2020) Population genetic survey of Lithasia geniculata in the Duck River, Tennessee.  Ellipsaria 22(2): 19 - 21. [PDF]

[12] Dillon, R. T. (2020) Reproductive isolation between Lithasia populations of the geniculata and duttoniana forms in the Duck River, Tennessee.  Ellipsaria 22(3): 6 - 8.  [PDF]

[13] From upper left: GEN, GEN, DUT, DUT, GEN, GEN.

[14] Papers in which Russ Minton lumped L. geniculata and L. duttoniana:

  • Minton, R. L., A. P. Norwood & D. M. Hayes (2008) Quantifying phenotypic gradients in freshwater snails: a case study in Lithasia (Gastropoda: Pleuroceridae)  Hydrobiologia 605: 173-182.
  • Minton, R. L., K.C. Hart, R. Fiorillo, & C. Brown (2018) Correlates of snail shell variation along a unidirectional freshwater gradient in Lithasia geniculata (Haldeman 1840) (Caenogastropoda: Pleuroceridae) from the Duck River, Tennessee, USA.  Folia Malacologia 26(2): 95 – 102.

[15] But I’ll bet dollars to donuts that they hybridize.  I think hybridization is widespread in the North American family Pleuroceridae.  See the paper by Bianchi et al from footnote [3] above.

[16] Five pleurocerid species are visible grazing across this rock in the Duck River at the Watered Hollow Boat Launch (RM 26): Pleurocera canaliculata canaliculata, Pleurocera laqueata laqueata, Leptoxis praerosa praerosa, Lithasia geniculata geniculata, and Lithasia armigera jayana.  Notice that no juveniles are apparent whatsoever.  All massively-shelled adults!  I could write an entire essay on that phenomenon alone.

[17] A selection of papers showing typical levels of allozyme divergence between pleurocerid species:

  • Dillon, R.T. and G.M. Davis (1980) The Goniobasis of southern Virginia and northwestern North Carolina: Genetic and shell morphometric relationships. Malacologia 20: 83-98. [PDF]
  • Dillon, R. T., and S. A. Ahlstedt (1997) Verification of the specific status of the endangered Anthony's River Snail, Athearnia anthonyi, using allozyme electrophoresis. The Nautilus 110: 97 - 101. [PDF]
  • Dillon, R. T. and A. J. Reed (2002)  A survey of genetic variation at allozyme loci among Goniobasis populations inhabiting Atlantic drainages of the Carolinas.  Malacologia 44: 23-31. [PDF]

[18] A selection of papers showing typical levels of allozyme divergence among populations within species:

  • Dillon, R.T. (1984) Geographic distance, environmental difference, and divergence between isolated populations. Systematic Zoology 33:69-82.  [PDF]
  • Dillon, R.T. (1988) Evolution from transplants between genetically distinct populations of freshwater snails. Genetica 76: 111-119. [PDF]
  • Dillon, R.T., and C. Lydeard (1998) Divergence among Mobile Basin populations of the pleurocerid snail genus, Leptoxis, estimated by allozyme electrophoresis.  Malacologia. 39: 111-119. [PDF]
  • Dillon, R. T. and J. D. Robinson (2011)  The opposite of speciation: Population genetics of  Pleurocera (Gastropoda: Pleuroceridae) in central Georgia.  American Malacological Bulletin  29: 159-168.  [PDF]

[19] Minton, R. L. 2013. A new species of Lithasia (Gastropoda: Pleuroceridae) from the Buffalo River, Tennessee, USA. The Nautilus 127:119-124.

Friday, February 4, 2022

Character phase disequilibrium in the Gyraulus of Europe

I get a lot of requests for freshwater gastropod samples for sequencing studies.  And I confess, I can sometimes be a jerk about it.  In reply, I always ask, “What hypothesis do you plan to test?”  Note that answers of the form, “I want to work out the phylogeny of gastropod family X” or “I want to figure out how many species there are in the gastropod genus Y” do not qualify as hypotheses [1].

So back in October of 2020 I was pleased to receive an email from Jeff Nekola of Masaryk University in Brno, the hometown of Gregor Mendel, posing a very well-considered hypothesis to test with sequence data from specimens that I might provide him.  Jeff introduced me to his colleague Michal Horsak, and the two of them together pitched me a project to test whether their European Gyraulus laevis (Adler 1838) might be conspecific with our North American Gyraulus parvus (Say 1817).  Let’s back up for a bit of context.

G. laevis [4]
All I know about the planorbid genus Gyraulus in the Old World, I learned from Prof. Claus Meier-Brook, late of the University of Tubingen [3].  The symphony of malacology he composed in 1983, entitled “Taxonomic Studies on Gyraulus [4]” was a masterwork of rigor, scholarship, technical brilliance and timeless beauty, both scientific and otherwise [5].  Meier-Brook examined 492 Gyraulus sampled from 94 Eurasian populations, thoroughly cataloguing variation in scores of anatomical and morphological characters both common and obscure, carefully documenting ecophenotypic variance as well as variance due to method of preservation.  Ultimately, he recognized eight species in Europe (excluding Macedonia [6]) and seven in Asia (one shared) for a total of 14 species of Gyraulus in the Palearctic.

So G. laevis and G. parvus were both among the eight European species recognized as valid by Prof. Meier-Brook, the former ranging widely across Europe but “rather stenotopic and rare.”  The latter he considered native in Iceland, but introduced into Germany by the early 1970s, and rapidly spreading through Europe as he wrote his monograph.

The two species are quite difficult to distinguish in the field, and even as early as 1952 European workers were suggesting that laevis might be a junior synonym of parvus [7].  Meier-Brook disagreed.  And here is the couplet by which he suggested that we distinguish them, transcribed verbatim:

6A. Penultimate whorl distinctly elevated, distal portion of spermoviduct slender, not wider than widest portion of sperm duct; distal half of vas deferens much wider (2:1 on an average) than proximal half (introduced from N America)… G. parvus.

6B. Penultimate whorl not or not distinctly elevated, distal portion of spermoviduct wider than widest portion of sperm duct and vas deferens, distal half of vas deferens not conspicuously widened (1:1) … G. laevis.

The six samples of G. parvus upon which Meier-Brook based his diagnosis, however, were collected from Ann Arbor, two places in Canada, two places in Iceland, and Germany – none from Thomas Say’s type locality in the Delaware River near Philadelphia.  So the hypothesis advocated by my colleagues in Brno back in October of 2020 was an interesting one, and it did, indeed, seem likely to me that sequence data might cast some light on upon it.

And my loyal readership may remember an expedition I undertook up the Delaware River in 2012, hunting the type locality of Thomas Say’s Lymnaea catascopium [8].  I had picked up incidental samples of G. parvus at two sites on that field trip, which I agreed to post to Brno at my next opportunity.

G. parvus [4]

Jeff and Michal also asked if I might be able to spare a couple samples of Gyraulus circumstriatus.  My antennae raised slightly at this request – it was hard to see how sequence data from G. circumstriatus might bear on the hypothesis before us.  And all I had on my shelves was a couple odd samples from Montana and NW Pennsylvania, nowhere near the Connecticut type locality.  But OK – in for a penny, in for a pound [9].

So in late January of 2021 I was pleased to receive an email from Michal reporting success in sequencing the samples that I had sent, and that “the preliminary phylogenetical analysis clearly says that these are the same species and there is also no difference from all European populations of G. parvus/laevis.”  And in mid-April a first draft manuscript arrived, courtesy of Ms. Erika Lorencova, the lead author on the project.

I found some things to like about the research as the manuscript unfolded before my eyes last spring.  Lorencova, Nekola, Horsak, and five additional coauthors [10] had sequenced four genes (COI, CYTB, ITS1, ITS2) from 65 individual snails representing seven nominal species of Gyraulus, plus a Planorbis outgroup – just one single individual per population, in almost all cases, alas.  They did not sequence all four genes for all 65 individuals, alas again.  Their data set included 34 individuals identified as G. parvus (27 Czech, 2 Slovak, 1 Croat, 1 Aust, 1 Serb, 2 USA) and 14 individuals identified as G. laevis (12 Czech, 2 Croat), none of the latter collected from anywhere near Alder’s type locality in England, which pissed me off royally.  Their data set did, however, include one individual collected from Scotland, one from Northern Ireland, and two from Wales, all identified as “parvus/laevis,” none of which helped one little bit.

Erika and our mutual colleagues opted to analyze their (mitochondrial) COI + CytB dataset separately from their (nuclear) ITS1 + ITS2 dataset.  They generated gene trees using four different methods for each data set: neighbor-joining, maximum parsimony, maximum likelihood, and Baysian inference, for a total of eight separate analyses.  All of these “yielded very similar tree topologies.”  If not, I suppose our friends in Brno could have picked the one they liked best.

In any case, the Baysian tree based on CO1 + CytB is shown on the left in their Figure 2 modified below, and the Baysian tree based on ITS1 + ITS2 is shown on the right.  Both trees vividly demonstrate character phase disequilibrium of a high and aggravated nature.  If at any point during the next seven paragraphs you fail to understand anything I am saying to you, please go back to my essay of [4Jan22] and read forward.
Fig 2 of Lorencova et al [16] modified

Let us first focus on the COI+CytB mtDNA tree at left.  Erika, Jeff, Michal and colleagues divided the 42 individuals depicted on the giant branch at the bottom of their tree into three “races,” which they distinguished using Roman numerals I, II, and III.  The mean uncorrected p-difference between Race I and Race II was 4.4%, between Race I and Race III it was 5.1%, and between Race II and Race III it was 5.5% [11].

Right-click on the figure above, open it in a new window, and count with me.  In Race I we see 6 parvus (red) and 12 laevis (blue), setting aside the 1 parvus/laevis (no count).  In Race II we see the two circumstriatus [12].  And in Race III we see 15 parvus, 2 laevis, and 4 parvus/laevis (no count).

In Table 1 below I have combined all 35 of the (classified) Gyraulus and categorized them simultaneously by nominal species and mtDNA race. The contingency chi-square is a very significant 10.98 (1 df).  This is character phase disequilibrium.  These 35 snails are not drawn from a single randomly-breeding population.

Tab 1. Gyraulus classified by species and mtDNA

Shifting our attention to the nDNA tree at right, we find the percent sequence divergence much less, but the character phase disequilibrium even more significant.  Lorencova and colleagues did not divide the gigantic branch at the bottom of their nDNA tree into “races” as they did for their mtDNA tree, but they easily could have done so.  There seem to be just two ITS1+ITS2 races, rather than the three we saw for mtDNA, and they differ by only a couple nucleotides.  But the match between the top half of the big branch of the nDNA tree is very close to the top half (“Race I”) of the big branch of the mtDNA tree.  And ditto for the two bottom halves.  Note that the same two individuals labelled “laevis” in the lower (Race III) clade on the mtDNA tree at left are exactly the same two individuals labelled “laevis” in lower clade by their nDNA at right, for example.

And indeed, laevis is again very much over-represented in the upper clade in the nDNA tree, which we might as well go ahead and call Race I, and parvus very much over-represented in the lower clade, which we might as well call Race III.  For the nDNA, Table 2 below shows that the contingency chi-square is 12.89 (1 df), even more significant than the mtDNA results.  What gives?

The 42 individual Gyraulus hanging on the big, low branches of the two gene trees shown above were not drawn from a single randomly-breeding population.  In large and complexly-structured environments, such as Erika’s six-country sample area most certainly is, the first thought of an evolutionary biologist would be that any such non-random mating might arise from barriers to dispersal or isolation by distance.  So let’s re-run our analysis focusing just on the 29 individual Gyraulus collected from the Czech Republic.  The mtDNA breakdown is Race I parvus = 5, Race I laevis = 11, Race III parvus = 11, Race III laevis = 2, chi-square (1 df) = 8.26, still quite significant.

Tab 2. Gyraulus classified by species and nDNA

I suppose it is possible that there might be some sort of geographic structure in the 29 Czech samples below the level of the country in which they were sampled, but I do not have the patience to plot 29 sets of lat/long coordinates and look for it.  I will simply note that there is another very plausible explanation for the character phase disequilibrium apparent in the branches of the gene trees developed by Erika, Jeff, Michal and our mutual colleagues, beyond some sort of fine geographic structure that I cannot see.  Perhaps the two sets of populations they have identified as G. parvus and G. laevis are in fact good, biological species, genetically and morphologically similar but reproductively isolated, with a smattering of misidentifications [13] or a hybridization event here and there.  In any case, the results of Lorencova et al. (2021) do not support their hypothesis that G. parvus and G. laevis are conspecific.

All through the second half of the month of April I carried on a detailed and earnest email correspondence with my colleagues in Brno, desperately trying to get them to see the reasoning I have outlined in the seven paragraphs above.  I explained character phase disequilibrium every way I know how to explain it, backing all the way up to Mendel and coming forward, drawing diagrams with circles and arrows and a paragraph on the back of each one explaining what each one was [14].  I calculated probabilities to the fourth decimal place and highlighted them in bold.  And I simply could not make Jeff, Erika, or Michal understand character phase disequilibrium, or its origins in non-random mating, or the likelihood that the disequilibrium evident in the branches of their trees might reflect reproductive isolation, or indeed (in retrospect) the connection between reproductive isolation and speciation.

Alas, we seemed to be speaking different languages.  Rather than the term, “species,” my colleagues preferred the term,  “species-level clade.”  And they concluded that G. parvus and G. laevis were “part of the same species-level clade” because “although reasonably well-supported races exist in the mtDNA tree their base pair variability is three times smaller than that seen between the other analyzed Gyraulus species” [15].

Nor apparently could the editor of the international journal to which my colleagues submitted their paper see the character phase disequilibrium in their Figure 2 as modified above, nor understand its significance, nor apparently could the reviewers he chose.  Apparently, there is widespread blindness in evolutionary biology today to simple principles of transmission genetics that I have considered fundamental for my entire professional career.  The paper by Lorencova and colleagues was published in July [16].  Going beyond their proposed synonymization of G. laevis under G. parvus, our colleagues in Brno went so far as to suggest that “North American G. circumstriatus might simply represent another race within G. parvus.”  Good grief.

Well, science is a self-correcting process.  Someday some bright young student will realize that the answer to the parvus/laevis question does not lie in sampling a single individual from 30 populations, but in sampling 30 individuals from a single population.  He will find a lake or a pond inhabited by Gyraulus bearing both the Race 1 and Race 3 haplotypes [17].  He will collect and sort 30 adults by the elevation of their penultimate shell whorl, sequence those genes, and look for disequilibrium between mtDNA race, nDNA race, and shell morphology.  And here is his hypothesis: big excesses of Race 1/flat whorl and Race 3/elevated whorl.  Will our bright young student find any Race 1/elevated or Race 3/ flat recombinants?  Will he find any ITS heterozygotes?  Good questions.

I will conclude with a caveat.  I am not aware of any research directly bearing on mode of reproduction in Gyraulus, but my biological intuition suggests to me that populations of trashy, weedy planorbids such as G. parvus most certainly is might demonstrate significant frequencies of self-fertilization.  Asexual reproduction voids the biological species concept and necessitates a retreat to the firm rock of morphology, not a blundering-forward into the slough of DNA [18]

So if our bright young student of the future confirms preferential selfing in Gyraulus, and googles up this essay and reads it, I would ask him to go back to my essay of [4Jan22], read to note [3] and stop.  The first nine paragraphs of my January essay will have turned out to be important for Gyraulus, but the remainder not.  And then I would suggest that he come forward to the present blog post, re-read from the top of my essay to Claus Meier-Brook’s couplet distinguishing G. parvus and G. laevis morphologically, and then stop again.  Because none of the rest of this month’s post will have turned out to be relevant, either.  To the extent that Gyraulus populations self-fertilize, Professor Meier-Brook’s brilliant 1983 monograph stands as the final word thus far.


Notes

[1] If, however, the opening lines of the present essay happen to fall upon the eyes of a bright young student with access to a molecular lab and an interest in real science [2], I stand ready to help in any way I can.  I even have a list of genuine hypotheses about the evolution of freshwater gastropods ripe for testing with sequence data, for students in the market for thesis topics.  A long list.

[2] Science is the construction of testable hypotheses about the natural world.

[3] Claus Meier-Brook (1934 - 2018) has occupied a niche in my malacological pantheon since the dawn of my professional career.  I never met him, although we enjoyed a cordial correspondence in the 1980s.  He was best known for his work on pisiidid clams, but also made important contributions to our understanding of the medically-important planorbids.  He was unable to resist the Willi Hennig fad, alas, but the cladist gobbledygook is easy to subtract from his otherwise inspiring oeuvre [5].  For a biography and bibliography, see:

  • Jungbluth, J. H. (2006) Claus Meier-Brook 70-Jahre (28  April  1934).   Mitteilungen  der Deutschen Malakozoologische  Gesellschaft 75:  39-47.

[4] Meier-Brook, C. (1983)  Taxonomic studies on Gyraulus (Gastropoda: Planorbidae).  Malacologia 24: 1 – 113.

[5] And I feel like Salieri.  Rifling the pages of his papers, hearing the beauty of the science in my mind’s ear – a rusty squeezebox, high above it a single oboe – my heart breaking.  But jeeze, crap!  Hit the fast-forward button through Movement VI, Cladiste.

[6] Meier-Brook recognized six Gyraulus species endemic to Macedonian lakes.

[7] Jaeckel, S. (1952) Zur Oekologie der Mollusken-fauna in der westlichen Ostsee. Sctiriften des Naturwissenscfiaftlictien Vereins fur Sctileswig-Holstein. 26: 18-50.

[8] It is traditional to assume that Thomas Say’s type localities for both G. parvus and L. catascopium were in the Delaware River at his home town of Philadelphia.  But 200 years of commercial development at the Philadelphia waterfront have rendered that environment unrecognizable today.  For an account of my explorations upstream, see:

  • The Type Locality of Lymnaea catascopium [14July15]

[9] Or more accurately, “In for 200 mg of snail flesh, might as well go all in for a half gram.”

[10] Michal offered me a coauthorship, which was very considerate of him, and I did appreciate the honor, but ultimately declined.

[11] MtDNA sequence divergences in the 4 – 5% range are quite consistent with the values typically reported among valid, biological species of freshwater pulmonate snails.  But that is not the direction I want to go in the present essay.

[12] The two G. circumstriatus sequences, sampled as they were from Montana and British Columbia, geographically remote from all 40 of the samples of parvus and laevis available for comparison, have no bearing on the specific status of circumstriatus whatsoever.

[13] Meier-Brook considered G. laevis and G. parvus sibling species, which means that they cannot be distinguished morphologically, and then spent many pages developing evidence to the contrary, ironically.

[14] By now I imagine that the origin of last month’s blog post will have become obvious to my perceptive readership.  My essay of 4Jan22 was an adaptation of the extensive email correspondence I carried on with Jeff Nekola and Michal Horsak in April of 2021.  And the reason for all those examples involving Mendel’s peas was that I was trying to communicate with geneticists in Brno.  And vainly searching for a language that we might have in common.  See:

  • What is character phase disequilibrium? [4Jan22]

[15] Darn it, I know I said in footnote [11] that this is not a direction I want to go with the present essay.  But it’s eating at me.  Although the “base pair variability” observed among the laevis, parvus, and circumstriatus clusters may indeed be “three times smaller than the values seen between the other analyzed Gyraulus species,” the values of 4 – 5% mtDNA sequence divergence among those groups reported by Erika, Jeff, Michal and colleagues are NOT small when compared to interspecific values typical for biological species of freshwater pulmonates worldwide.  See, for example:

  • The Lymnaeidae 2012: Stagnalis yardstick [4June12]

[16] Lorencova, E., L. Beran, M. Novakova, V. Horsakova, B. Rowson, J. Hlavac, J. Nekola, and M. Horsak (2021).  Invasion at the population level: a story of the freshwater snails Gyraulus parvus and G. laevis.  Hydrobiologia 848: 4661 – 4671.

[17] A clean answer to the question that has sent nine scientists, including yours truly, scampering around the world to not answer may be waiting 60 km south of Brno.  For according to the supplementary materials available with their paper, Erika, Michal, Jeff & Company did collect snails they identified as both parvus and laevis together in a fishpond on the northern edge of the Czech town of Hlohovec.

[18] I vividly illustrated the folly of attempting to classify asexually-reproducing populations by DNA sequence data in two recent series of blog posts: one on the (parthenogenic) prosobranch Campeloma and a second on the (self-fertilizing) pulmonate Galba.  See:

  • Fun with Campeloma! [7May21]
  • What Lymnaea (Galba) schirazensis is not, might be, and most certainly is [3Aug21]