Dr. Rob Dillon, Coordinator





Wednesday, September 7, 2022

Just 125 species of Pyrgulopsis in the American West

Editor’s Note.  If you are looking for something citable, the gray-literature report upon which the following essay is based can be downloaded as FWGNA Circular #6 from footnote [1] below.

In July [6July22] we reviewed the career of the USNM-Smithsonian’s Dr. Robert Hershler, for over 30 years the undisputed authority on the hydrobioid freshwater gastropods of North America.  Running our finger through the checklist that my buddy Bob published with Hsiu-Ping Liu in 2017 [2], we counted “126 species of Pyrgulopsis inhabiting the waters of the Western United States, 107 of which Bob Hershler was the author.”

Journey back with me now to a bleak Tuesday morning in May of 2020.  The ivory towers of Academia were locked and bolted, the store shelves stripped bare of toilet paper [3], the streets silent save for the rattle of wooden handcarts, and rhythmic appeals to bring out the dead.  And I opened my email inbox and found a message from Hsiu-Ping.  She asked me if I would be “interested in working on a project to determine the species status of Pyrgulopsis vinyardi and P. gibba.”  And here was my reply:

“Well, OK, maybe.  It would take me a while to get up to speed on the Western hydrobioids, and the maximum speed I could ever achieve would look like standing still, next to Bob Hershler.  But Bob’s not taking any more laps around the track, and I am.”

Thus encouraged, sort-of, Hsiu-Ping proceeded to lay out the situation.  Bob had described Pyrgulopsis gibba from the northwestern Great Basin Desert of California, Nevada, and Oregon in a relatively small paper published in 1995 [4] and followed with a description of P. vinyardi endemic to "two springs in the Squaw Valley drainage" of north-central Nevada in his big monograph of 1998 [5].  The two bore shells strikingly different in the relative size of their body whorls, and penises notably different in their morphologies as well.

Pyrgulopsis vinyardi [5] and P. gibba [4]

Regarding penial morphology, see the figure below.  I have marked the part of the penis that actually does the job, really just a simple filament, with the red letter “P.”  Everything else in Bob’s dorsal-and-ventral figures labelled P. vinyardi (boxed), and in his three dorsal-and-ventral figures labelled P. gibba, is the ridiculously enlarged and elaborate penial lobe characteristic of Pyrgulopsis. If you’re curious to see a whole mount of the actual organ itself, look back at my July post [6July22].

The distribution of glandular regions on the surface of this spatulate or blade-shaped lobe has for many years been considered diagnostic of hydrobiid species.  Bob has marked dorsal glands as “Dg,” ventral glands as “Vg” and terminal glands as “Tg.”

Bob wrote in his description of P. gibba: “This species is unique among members of the genus (as of 1995) in having penial ornament of terminal gland, Dg3, and ventral gland.”  He went on to observe, however, “Dg3 often present, either as a small papule (sometimes double) or large raised unit.”  Note the modifier, “often.”  Bob figured one P. gibba penis that had no Dg3 at all.  See the Dg3 regions encircled in red.  And in 1998 he noted that the development of the ventral gland often varies as well.

Penial morphology, modified from Hershler [6]
In his description of P. vinyardi three years later, Bob wrote, “Penial ornament a small terminal gland, large Dg1, small Dg2, small Dg3, additional dorsal gland on lobe, and large ventral gland.”  So, upon dissection, P. vinyardi males are expected to demonstrate the entire smorgasbord of glands, including everything seen on P. gibba plus Dg1 and (usually) Dg2.  He noted later, however, that Dg2 was “rarely absent.”

The obvious analogy is to “lock-and-key” reproductive isolation such as has been widely documented across the Phylum Arthropoda, except that the gastropod lock is a bag, and the gastropod key is a sock.

I'm not buyin' it!
I’m skeptical.  In the case of Physa, with which I do have a great deal of experience, even very different penial morphologies do not preclude mating [7].  There is (indeed) some prezygotic reproductive isolation between biological species of Physa, but it is behavioral (or possibly chemical) on the part of the snail mounted as female.  My personal observations do not suggest that mechanical barriers play a significant role in interspecific copulation in gastropods.

And besides.  Anybody who has walked through the weeds by a pond on a warm summer night, or hell, anybody who has a friend with a male dachshund, you all know.  Males will do it with anything.  Coke bottles.  It just does not matter.

So, both Bob’s 1995 description of P. gibba and his 1998 description of P. vinyardi were published absent any genetic data, before he met Hsiu-Ping.  Between 2003 and 2008 Bob and Hsiu-Ping did, however, publish mitochondrial CO1 gene sequences for three individual P. gibba and one P. vinyardi [9].  And it materialized that gibba and vinyardi are very similar genetically, mtDNA percent sequence divergence ranging just 0.5% to 1.1%.

All of which now brings us back up to the dark days of May 2020, and my email exchange with Hsiu-Ping.  Hsiu-Ping explained to me that she had recently agreed to provide molecular identifications for a set of 21 Pyrgulopsis samples [10] collected from northern Nevada by Ms. Diana Eck of the environmental consulting firm, Stantec.  And that she had sequenced the CO1 gene from approximately 4 – 6 individuals from each population, for a total sample size of N = 88.  The Baysian tree below shows the N = 29 unique CO1 haplotypes Hsiu-Ping discovered, with unidentified population number (“unk”), setting aside duplicates.  Also shown are the three control P. gibba sequences from GenBank, and the one control P. vinyardi.

CO1 sequence diversity in N. Nevada Pyrgulopsis

So, the second branch of the tree does indeed divide the control vinyardi from the control gibba, as one might expect.  But look at the samples from unidentified population #22, collected 23 miles NE of Lovelock, Nevada.  Sequences 22-A and 22-B cluster with vinyardi, while sequence 22-C clusters with gibba!  Could this be evidence that Spring #22 is inhabited by both P. gibba and P. vinyardi?  And that the two populations demonstrate reproductive isolation in sympatry?  My knowledge of the vast and weighty literature is far from encyclopedic, but I cannot recall any case of sympatric Pyrgulopsis species ever previously documented in the American West.

Hsiu-Ping and I resolved to test population #22 for character phase disequilibrium [11].  And so it came to pass that in June of 2020 a fresh sample of Pyrgulopsis collected from population #22 arrived on my doorstep, courtesy of Ms. Eck.  My half of the study was to dissect these snails and characterize their penial morphology as either matching P. vinyardi or matching P. gibba.  Then sending the residual tissues to Hsiu-Ping, she would characterize the individuals as either matching vinyardi or matching gibba by their CO1 gene sequence.  A significant relationship between penial morphology and CO1 sequence would suggest reproductive isolation within the sample, confirming the specific distinction between vinyardi and gibba.

Pyrgulopsis from Site 22

And so, I went to work with tiny forceps and even tinier dissecting needles, cracking and dissecting 30 adults [12], identifying 15 females and 15 males.  I ignored dg3, which is an unreliable character.  Then five males demonstrated both dg1 and dg2, matching P. vinyardi.  Three males did not demonstrate either dg1 or dg2, matching P. gibba.  And seven males demonstrated either dg1 or dg2, intermediate between gibba and vinyardi.  And in July of 2020 I forwarded 15 little tubes onward to Hsiu-Ping, 5 marked G for gibba, 3 marked V for vinyardi, and 7 marked I for intermediate.

The Baysian tree below shows Hsiu-Ping’s results for 13 of the 15 snails I dissected (setting aside 2 duplicate sequences), plus all five of the sequences she obtained from Ms. Eck’s original sample, plus the four control sequences from GenBank.  Hsiu-Ping’s analysis did resolve two sort-of distinct clusters of CO1 sequence within population #22, but those two clusters did not correspond to penial morphology, nor indeed, did they correspond especially well to the four CO1 sequences previously deposited in GenBank, three from nominal gibba and one from nominal vinyardi.  There is no pattern in the distribution of samples labelled G, V, and I.

Hence there is no evidence of character-phase disequilibrium between penial morphology and CO1 sequence in Pyrgulopsis population #22.  Hence there is no evidence of reproductive isolation between P. gibba and P. vinyardi.  The gray-literature report we filed with Ms. Eck on 22Oct21, available for download as FWGNA Circular #6 from footnote [1] below, concluded “that P. gibba and P. vinyardi should be synonymized into one species.”  My buddy Bob’s (1995) gibba would have priority over his (1998) vinyardi.

From Liu & Dillon [1]

OK, I know that’s a lot of technical detail for a silly, frivolous blog post.  So come back up to the surface with me and let’s take a big, fresh breath of air together.  Look at those two little shells I figured at the top of this essay, and then look at those four sets of penis diagrams four column inches below.  Both of the shells figured above, and all of those penises, were borne by a single biological species of Pyrgulopsis.

I should conclude this essay, however, emphasizing once again that science is the construction of testable hypotheses about the natural world.  Science is not right, it is testable.  And over the course of a distinguished career spanning almost 40 years, my buddy Bob rigorously constructed 126 testable hypotheses about the Pyrgulopsis fauna of the great American West.  One day, I feel sure, somebody will come behind him and test the 125 that remain. I cannot imagine when, or by whom.  Not it.


Notes

[1] Liu, H-P, and R. T. Dillon, Jr. (2021) Resolving the species status of Surprise Valley Pyrg (Pyrgulopsis gibba) and Vineyard Pyrg (Pyrgulopsis vinyardi).  Report to Stantec Environmental Consulting. FWGNA Circular 6: 1 – 5. [pdf]

[2] Hershler, R. & H-P. Liu (2017) Annotated Checklist of Freshwater Truncatelloidean Gastropods of the Western United States, with an Illustrated Key to the Genera.  US Bureau of Land Management Technical Note 449: 1 – 142.

[3] We always used pine cones when I was growing up, too poor for corn cobs.

[4] Hershler R. 1995. New freshwater snails of the Genus Pyrgulopsis (Rissooidea: Hydrobiidae) from California. The Veliger 38(4): 343-373.

[5] Hershler R. 1998. A systematic review of the hydrobiid snails (Gastropoda: Rissooidea) of the Great Basin, western United States. Part I. Genus Pyrgulopsis. The Veliger 41: 1-132.

[6] The boxed figure of the P. vinyardi penis was scanned from figure 39 of Hershler [5], showing dorsal aspect on the left and ventral aspect on the right.  The remainder of the figure, showing penial morphology for three different P. gibba males, was scanned from figure 12 of Hershler [4].  Again, dorsal aspect on the left, ventral on the right.  Abbreviations Dg = dorsal gland, Vg = ventral gland, Tg = terminal gland, P = penial filament.  The Dg3 region is encircled.

[7] The literature on prezygotic reproductive isolation in Physa is extensive.  Here’s a good entry:

  • Dillon, R.T., A.R. Wethington, and C. Lydeard (2011) The evolution of reproductive isolation in a simultaneous hermaphrodite, the freshwater snail Physa.  BMC Evolutionary Biology 11: 144. [html] [pdf]

[8] Penial morphology of Pyrgulopsis sadai, from Hershler [5], figure 39.

[9] The three CO1 sequences for P. gibba and the one sequence for P. vinyardi were published in four different papers:

  • Hershler, R., Frest, T.J., Liu, H.-P., Johannes, E.J. 2003a. Rissooidean snails from the Pit River basin, California. Veliger 46:275-304.
  • Hershler, R. and Liu, H.P. (2004) A molecular phylogeny of aquatic gastropods provides a new perspective on biogeographic history of the Snake River Region. Mol. Phylogenet. Evol. 32 (3), 927-937.
  • Hershler, R., Liu, H.-P. 2008. Ancient vicariance and recent dispersal of springsnails (Hydrobiidae: Pyrgulopsis) in the Death Valley system, California-Nevada. In: Reheis, M.C., Hershler, R., Miller, D.M., eds. Late Cenozoic drainage history of the southwestern Great Basin and lower Colorado River region: geologic and biotic perspectives. Geological Society of America Special Paper 439:91-101.
  • Hershler, R., Liu, H.-P. and Gustafson, D.L. (2008) A second species of Pyrgulopsis (Hydrobiidae) from the Missouri River basin, with molecular evidence supporting faunal origin through Pliocene stream capture across the northern continental divide. J. Molluscan Stud. 74 (4), 403-413.

[10] Ms. Eck actually sent 22 populations for analysis, but population #11 turned out to be a lymnaeid.  So just 21 populations of Pyrgulopsis.

[11] In January of 2022 I defined character phase disequilibrium as “any violation of independent assortment between one or more morphological characters and one or more characters of demonstrably genetic origin.”  Although CPD can arise from any violation of the assumption of random mating, the most likely explanation in Spring #22 would be reproductive isolation.  See:

  • What is character phase disequilibrium? [4Jan22]
  • Character phase disequilibrium in the Gyraulus of Europe [4Feb22]

[12] Actually I messed up a few, so no count.

Tuesday, August 9, 2022

Startled by Fontigens, sort-of, I suppose

It takes a lot of forbearance to follow the FWGNA Blog.  I am aware of this.  I break every rule of social media – exploring arcane topics at great depth, extending the attention span of my readership over many months.  Sometimes years.  It is not uncommon for me, your petulant host, to expect you to remember some petty anecdote or obscure factoid that may have flickered through my brain years ago, and if you do not remember it, scold you and assign it for homework.  And then forget it, myself.

So, welcome to snail science – a gradual unfolding, a cautious move forward, a startling, and an extended period of dormancy.

The stimulus for our gradual unfolding this month is the publication late last year of a paper by Liu, Schroeder, Berry, and Dillon on sequence divergence in phreatic hydrobioid snails of the genus Fontigens [1].  You might be forgiven if your memory of my 2006 essay on the Springsnails of the Blue Ridge is a bit fuzzy.  But I do trust that you remember our three-part series on Lori Schroeder’s tiny snails back in 2017, yes?  And surely, our July 2019 report of the discovery of a single putative Fontigens cryptica in a spring at the Bernheim Forest in central Kentucky is fresh on your mind.  And you must have read my Bob Hershler tribute last month, right?  Don’t tell me that you didn’t.

So all told, that’s six essays on Fontigens, written over a span of 16 years, with which I expect you to be familiar, before going forward into the blog post below.  What, no?  Tut-tut!  See note [4] below for your homework assignment.

Moving forward now, cautiously.  Down toward the bottom of my essay of [3July19] I wrote, “together Hsiu-Ping (Liu) and I worked up a small proposal to the Bernheim Board for a study on the evolution of Fontigens across the eastern USA.”  I am here pleased to report that our proposal was funded just two weeks after that essay was posted, and that we subsequently sampled 13 additional populations of Fontigens beyond the singleton putative F. cryptica that kicked off our research effort, and that the results are startling, on a malacological scale, sort-of, I suppose.

Topotypic F. nickliniana (Font14)

Hsiu-Ping and I settled on a study of sequence divergence in the mitochondrial CO1 gene quite early in our study.  But as I have emphasized many times, indeed as many as the 18 posts currently indexed at the margin of this blog under “Gene trees,” molecular phylogenies are dependent variables, not independent variables.  By themselves, they are at best weak, null models of population relationship.  Only the scientist who brings with him a hypothesis regarding the evolution of a group of organisms can understand a gene tree that might be derived from that evolutionary process.

Thank heaven, then, that the 1990 monograph by Hershler, Holsinger, and Hubricht on the North American Fontigens has been sitting at the front of the thick Hershler file in my reprint cabinet for thirty years [5].  Last month [6July22] I sang the praises of that wonderful work – complete, scholarly, detailed, and unsullied by any taint of molecular phylogenetics.  Entirely upon the basis of morphological data, demonstrating a remarkable appreciation for both intrapopulation and interpopulation variation, my buddy Bob recognized nine species of Fontigens in North America, plus F. cryptica of “uncertain status.”  I cannot imagine any finer standard against which to calibrate a gene tree such as the one Hsiu-Ping and I proposed to construct.

So, the late summer of 2019 found me cruising along the Blue Ridge of Old Virginia, very much as I described in my essay of [26July06].  And as my idyllic journey unfolded, I paused to collect three of the many populations of Fontigens orolibas Bob Hershler catalogued in his monograph.  These included Leslie Hubricht’s (1957) type locality at Hawksbill Spring (Font17), beside the Appalachian Trail at northbound mile 930.5, a prettier habitat it is difficult to imagine.  And I sampled a second F. orolibas population (Font18) just 80 km south on the Blue Ridge Parkway at the pioneer springhouse figured in my 2006 post.  And our good friends Wil Orndorff and Tom Malabad of the VaDCR contributed a third population of F. orolibas from Hugh Young Cave (Font19), 250 km further SW, to slide the scale. 

And ditto for Fontigens nickliniana.  Although Isaac Lea’s (1838) type locality for “Paludina” nickliniana (Hot Springs, Virginia) is obscure, I felt as though a sample from Blowing Springs (about 9 km N of Hot Springs) could reasonably substitute (Font14).  Then I added a second population of F. nickliniana listed by Bob Hershler 140 km NE at Lantz Mills, VA, (Font15), and a third population about 250 km SW at Fleenor Spring, VA, (Font16).  To these samples Hsiu-Ping was gratified to contribute an individual F. nickliniana she found in her freezer from Indiana (Font3).  And you might remember, but are excused if you do not, that as of 2019, GenBank already held exactly one CO1 sequence for any Fontigens whatsoever, that of Fontigens nickliniana, an individual collected in Michigan [6].

Blowing Springs, VA [7]

I was also able to sample the type locality of Fontigens morrisoni on my 2019 field trip through the green rolling hills of Old Virginia (Font13).  Hsiu-Ping had samples in her freezer from the type localities of F. tartarea (Font1) and F. bottimeri (Font6), and our colleague Bob Weck sent us samples from the type locality of F. antroecetes in Illinois (Font22).  And our good friends Wil and Tom contributed a second population of F. bottimeri sampled from Ogden’s Cave about 100 km W of the type locality (Font11).  Comparison to the genetic diversity demonstrated by these 13 carefully-chosen populations (plus the singleton sequence from GenBank) should, we felt, allow us to place the CO1 sequence divergence demonstrated by our single putative F. cryptica individual from Kentucky (Font12) into its proper evolutionary context.

A neighbor-joining gene tree based on our CO1 results is shown below [8].  And the most striking result of our analysis was, without question, the tremendous mtDNA sequence divergence we discovered among conspecific populations.  Sequence variation within the Blowing Springs type population of F. nickliniana (Font14) was negligible, for example, and ditto within Font16 from Fleenor Spring, and ditto within Font15 from Lantz Mills – just a few nucleotides.  But in striking contrast, the mean between-population sequence divergence was 10.8% between Font14 and Font16, 10.4% between Font15 and Font16, and 11.0% between Font14 and Font15.

Similarly negligible values of sequence variation within populations, contrasted with strikingly-high sequence variation between conspecific populations, prevailed across our entire control sample of six Fontigens species.  Mean interpopulation sequence divergence reached as high as 14.3% between the F. orolibas inhabiting the Hawksbill Spring type locality (Font17) and Hugh Young Cave (Font19).

This situation was so reminiscent of the population genetics characteristic of pleurocerid snails [9] that I suggested to Hsiu-Ping we might want to look for mitochondrial superheterogeneity within our three F. nickliniana populations.  We upped our samples sizes to 30 for Font14, 10 for Font15 and 10 for Font16, but didn’t find any.

The levels of interpopulation mtDNA sequence divergence we discovered within species did not, however, swamp out divergence between the species.  The mean CO1 divergence among species ranged from a high of 21.0% (between F. nickliniana and F. morrisoni of Virginia) to a low of 8.7% (between Virginia F. bottimeri and Illinois F. antroecetes).

NJ tree from CO1 sequence

Our second-most striking result was the nesting of the two F. nickliniana sequences we analyzed from the Midwest, Font3 from Indiana and JX970609 from Michigan, within our Font15 cluster from Lantz Mills, Virginia.  I admit that I do not have as much experience pioneering through forests of gene trees as most of my colleagues.  But this is the clearest example of a “shared primitive” mtDNA sequence I have ever seen.  Cladists made up the term “symplesiomorphy” to describe this phenomenon and used it for tails on fish.

And finally.  The only unknown sample hanging on our entire tree was Lori Schroeder’s tiny snail, Font12, the singleton individual that she and Andrew Berry discovered under that rock at the springhead in the Bernheim Forest back in 2019.  The minimum mean CO1 sequence divergence between that little snail and any of the control Fontigens species in our study was 14.4%, with F. bottimeri of Virginia.  Thus, our hypothesis that Lori’s snail was sampled from a distinct species is confirmed.  And since the shell and the animal both match Leslie Hubricht’s (1963) description of Fontigens cryptica [10], it is F. cryptica.

The confirmation that a population of Fontigens cryptica inhabits the subterranean waters of central Kentucky is not surprising, I don’t suppose, especially after five years of breathless advertisement in the columns of this blog.  But the high levels of interpopulation mtDNA sequence divergence we documented across the genus Fontigens on the way to that unsurprising conclusion, my goodness!

Twenty years of published research on hydrobioid evolution in the American West led us to expect much less than 1% divergence between conspecific populations.  Bob and Hsiu-Ping haven’t typically documented much more than a couple percent CO1 sequence difference among their scores of newly-described species.  But in Virginia, even a very small sample of three F. orolibas populations returned interpopulation sequence divergences as high as 14.3%!  Has the evolution of hydrobioid populations in isolated springs of the American West been so very different from the hydrobioids inhabiting isolated springs of the American East?  Startling, indeed.


Notes

[1] Liu, H-P., L. Schroeder, A. Berry, and R.T. Dillon, Jr. (2021) High levels of mitochondrial DNA sequence divergence among isolated populations of Fontigens (Truncatelloidea: Emmericiidae) [2] in eastern USA. Journal of Molluscan Studies 87.  [PDF] [html]

[2] No, the genus Fontigens has not properly been assigned to the family Emmericiidae since 2013 [3].  The parenthetical material was added to our title by some clueless naïf in the editorial office of the journal.

[3] Wilke T., Haase M., Hershler R., Liu H-P., Misof B., Ponder W. (2013)  Pushing short DNA fragments to the limit: Phylogenetic relationships of “hydrobioid” gastropods (Caenogastropoda: Rissooidea).  Molecular Phylogenetics and Evolution 66: 715 – 736.  For a review, see:

  • The Classification of the Hydrobioids [18Aug16]

[4] Here is your homework assignment.  Yes, this will be on the test.  All of it:

  • Springsnails of the Blue Ridge [26July06]
  • Lori Schroeder’s tiny snails [17July17]
  • The most cryptic freshwater gastropod in the world [6Aug17]
  • Not finding Fontigens cryptica [6Sept17]
  • Finding Fontigens cryptica [3July19]
  • My buddy Bob [6July22]

[5] Hershler, R., J.R. Holsinger & L. Hubricht (1990) A revision of the North American freshwater snail genus Fontigens (Prosobranchia: Hydrobiidae). Smithsonian Contributions to Zoology 509: 1-49.

[6] In my essay of [18Aug16] I mentioned that “a Fontigens nickliniana sample from Michigan” was the only fontigentine included in the gigantic worldwide CO1+16S+18S gene tree of Wilke and colleagues [3].

[7]  Air exactly 58 degrees F blows out of pipes set in the mountainside above the spring, 365 days per year.  I don’t know why.

[8] Lori showed this gene tree in a talk she gave at the OVUM meeting in 2019.  A couple notes might be helpful for those of you comparing the 2019 tree to the one we published as Figure 2 in our paper [1].  We changed our population codes slightly between 2019 and 2021, so that Font14 became “nic14” and Font17 became “oro17” and so forth.  And the figure in our published paper was a Baysian tree, rather than simple neighbor-joining.  And we added a couple additional outgroups from GenBank.

[9] The phenomenon of double-digit mtDNA intrapopulation sequence variation is so common in pleurocerid populations that I have coined the term "mitochondrial superheterogeneity" to describe it.  For a review, see:

  • Mitochondrial superheterogeneity: What we know [15Mar16

[10] Hubricht, L. (1963)  New species of Hydrobiidae.  Nautilus 76: 138 - 140.

Wednesday, July 6, 2022

My buddy, Bob

Although we studied at different institutions, Bob Hershler was my best friend in graduate school.  He was enrolled at Johns Hopkins way down in Baltimore, and I at the University of Pennsylvania in Philadelphia.  But we shared George Davis as our major advisor, so for several years we both spent most of our hours many of our days in the Malacology Department at the Academy of Natural Sciences.

In many ways, Bob was closer to George than I was, ultimately completing his dissertation on a morphological study of an endemic radiation of hydrobioids in northern Mexico [1], really rather similar to George’s masterpiece in southeast Asia [2].  Meanwhile, I was in the lab running allozyme gels on the opposite of an endemic radiation in the American pleurocerids [3].

Rob & Bob in Philadelphia, 1977

Bob often slept on the carpet under the big table in the malacology library.  And there was a night watchman, stationed at the desk at a side door downstairs, who patrolled regular rounds inside the building.   The watchman knew Bob was sleeping under there, which he most certainly was not supposed to be doing, but nevertheless turned a blind eye.

One evening Bob was running a set of exploratory electrophoresis gels, and after he got his samples on, decided to go out for supper.  And he walked by the watchman at the side door and out onto 19th Street and was gone for perhaps an hour or so.  And when he returned to the 19th Street door, the watchman was on his rounds.  And the door was locked.

And Bob panicked.  Fearing that his gels would cook and might ultimately (I suppose?) start a fire, he pulled the fire alarm on the side of the building.  I will leave to your imagination what response might follow a fire alarm at a 150-year-old building crammed with rotten alcohol and animal skins in downtown Philadelphia in the middle of the night.

Bob was awarded his Ph.D. in 1983, the same year that I was awarded mine, and by the blessing of divine providence, was able to win a curatorial position at the U.S. National Museum a couple years later.  Such plums are few and far between for scientists with our very specialized (and quite possibly obsolete) backgrounds.  My self-effacing buddy Bob confided to me shortly thereafter, “It was a weak field” [4].

But at the USNM Smithsonian, Bob’s career flourished.  His research focused almost entirely on the hydrobioids, of which he became the unchallenged North American authority, publishing over 100 peer-reviewed papers, many monographic in their size and scope.  His malacology was neoclassical, featuring lovely, detailed drawings of the dissected animals themselves, in the style of a Henry Pilsbry or our shared mentor George.  But he often whistled modern notes.  At his best, Bob was able to recognize intrapopulation and interpopulation variation along with the most evolutionary of his contemporaries.  His unification of Pyrgulopsis robusta, for example, will have a lasting impact [6].

The 1990 monograph of the phreatic genus Fontigens he published with John Holsinger and Leslie Hubricht [7] was a thing of beauty.  It opens with a marvelous and detailed examination of morphological variation in the most common species, Fontigens nickliniana, depicting shells of all shapes and penial complexes of in all their elaborate forms and twisted wonder.  His section subtitled “material examined” lists hundreds of distinct springs and spring runs in ten different states where populations of F. nickliniana can be found, with locality data of sufficient quality so that any subsequent worker can go and look at the things himself, if he has any questions.  That’s the best part of the monograph.

F. nickliniana penis variation [8]

Then moving forward from a rock-solid understanding of morphological variation in Fontigens nickliniana, Bob proceeded by rigorous methodology to distinguish nine additional species in the genus.  Four of these are more or less sympatric with nickliniana in the Valley-and-Ridge Province of Virginia, most strikingly F. orolibas, which even co-occurs mixed with F. nickliniana, lending credence to the hypothesis of reproductive isolation.  Again, Bob offers lovely figures of shell, radula, and reproductive anatomy for each, with excellent locality data.  And he iced the cake with a dichotomous key to cleanly distinguish among the ten total.  This work is easily on par with that of a Hubendick or a Meier-Brook [9].  Bob’s 1990 Fontigens monograph is as good as classical malacology ever got or can get.

Almost as good, equally important, and even larger was his 1994 monograph on the North American Pyrgulopsis [10].  I keep a copy by my desk and refer to it often.  In this 115-page tour-de-force Bob recognized 11 Pyrgulopsis species from Eastern North America as well as the 54 that (by that early date) had been described from the American West.  Of the 54, Hershler had himself described 21.  We will have much more to say about both the Eastern subset (now referred to Marstonia) and the western subset (still Pyrgulopsis) in coming months.

I think it is fair to say, by a margin of approximately 54 to 11, that Bob’s greatest love was his first love – the hydrobioid fauna of the American West.  Running my finger through the “Annotated Checklist of Freshwater Truncatelloidean Gastropods of the Western United States” he published with Hsiu-Ping Liu in 2017 [11], I count 43 papers with Hershler as the senior author, including such major contributions as the (63 pg) review of the Arizona hydrobiids he published in 1988, his (140 pg) Cochliopine monograph of 1992, his second (132 pg) Pyrgulopsis monograph of 1998, his (41 pg) Fluminicola monograph of 1996, and his (53 pg) Tryonia monograph of 2001 [12].

From his hypotheses regarding evolutionary relationships among western hydrobioid populations he developed a secondary interest in biogeography, publishing reconstructions of ancient drainage systems [13].  He had little interest in population biology, ecology, or the broader malacofauna beyond the hydrobioids, however.  When I approached him about collaborating on the FWGNA project back in 1998, he replied, “I don’t do checklists” [14].

It should not surprise you, after having read the anecdote with which this essay opened, that Bob did not run any sort of genetic laboratory.  But in 1999 he developed a tremendously productive relationship with someone who did, Dr. Hsiu-Ping Liu, and for 20 years they made beautiful malacological music together.  Of the 43 papers on the western hydrobioids I counted above, 30 were published after 1998, and of those 30, Hsiu-Ping is also listed as an author on 26.  In addition, the Literature Cited section of the Hershler & Liu catalog also includes 10 papers with Hsiu-Ping as lead author, Bob’s name following.

Pyrgulopsis penis from Hershler [15]
Alas, the rising tide of mtDNA sequence data that began to slosh across Bob’s desk in the 2000s pushed his species concept in a more typological direction, as became common in the generation that followed us.  How many species did he ultimately describe?  I do not know.  Bob and Hsiu-Ping’s 2017 catalog listed 126 species of Pyrgulopsis inhabiting the waters of the Western United States, 107 of which (I count) Bob Hershler was the author.  To put that figure in perspective, the total number of valid, biological species of freshwater gastropods inhabiting all U.S. Atlantic drainages, plus all the drainages of The Ohio, including the Tennessee/Cumberland, summing all families, including all pulmonate species and all pleurocerid species, as well as all the hydrobioids, across all or part of 17 eastern U.S. states, amounts to exactly 107.

For many years, I took it for granted that Bob would be behind his desk at the USNM, replying to my occasional email requests for consultation, and to the emails (many greater in their number) I forwarded to him from others.  And then, one day, he was gone.

Bob retired in 2018, the same year I myself would have retired from the College of Charleston, had all gone according to best-laid plans [16].  I tried emailing, and I tried calling, and nothing.  Finally, I called the front office at the USNM Department of Invertebrate Zoology where I learned the news.  He hadn’t left any forwarding info.

So now, perhaps you, my readership, can dimly see why I led this blog post with such a potentially-embarrassing anecdote about such a shy colleague, who is famous in the small world we share for many things, but not for his sense of humor.  Where are you, Bob?  Do you want to set me straight on any of the slander I have broadcast above?  I have a surprise for you, Old Buddy.


Notes

[1] Hershler, R. (1985) Systematic revision of the Hydrobiidae (Gastropoda: Rissoacea) of the Cuatro Cienegas Basin, Coahuila, Mexico.  Malacologia 26: 31 – 123.

[2] Davis, G. M. (1979)  The origin and evolution of the Gastropod family Pomatiopsidae, with emphasis on the Mekong River Triculinae.  Monograph of the Academy of Natural Sciences of Philadelphia 20: 1 – 120.

[3] Dillon, R.T., Jr. (1984) Geographic distance, environmental difference, and divergence between isolated populations. Systematic Zoology 33:69-82.  [pdf]

[4] I didn’t apply for the 1986 Smithsonian opening.  I had accepted an assistant professorship at The College of Charleston in 1983, which was a mighty quick turnaround, but that wouldn’t have stopped me.  Curatorships at the USNM are very, very sweet plums [5].  What did stop me from applying for the position at the Smithsonian that Bob Hershler ultimately won in 1986 might be grist for a future post on the FWGNA blog.

[5] I happened to notice on a website called federalpay.org that at his retirement, Bob was earning a base salary of $161,900.  That is way more than twice what I ever made, in 33 years of labor at a mid-sized college of regional reputation, grading thousands of lab reports written by entitled 19-year-old sorority girls.

[6] Hershler, R. & H-P. Liu (2004) Taxonomic reappraisal of species assigned to the North American freshwater gastropod subgenus Natricola (Rissooidea: Hydrobidae). The Veliger 47: 66-81.  Hershler, R. & H-P. Liu (2000) A molecular phylogeny of aquatic gastropods provides a new perspective on biogeographic history of the Snake River region. Molec. Phyl. Evol. 32: 927-937.  For a review of the tempest these two papers stirred up in the teacup of freshwater gastropod conservation, see:

  • Idaho springsnail showdown [28Apr05]
  • Idaho springsnail panel report [23Dec05]
  • When pigs fly in Idaho [30Jan06]

[7] Hershler, R., J.R. Holsinger & L. Hubricht (1990) A revision of the North American freshwater snail genus Fontigens (Prosobranchia: Hydrobiidae). Smithsonian Contributions to Zoology 509: 1-49.  For an appreciation, see:

  • Springsnails of The Blue Ridge [26July06]

[8] This is a scan of Figure 7 from Hershler, Holsinger & Hubricht [7].  It shows camera lucida outline drawings of penes from ten different populations of F. nickliniana, collected mostly from Virginia.  Pl = proximal penial lobe, Dl = distal penial lobe, Pf = Penial filament.

[9] For appreciations of the work of those two icons of neoclassical malacology, see:

  • The classification of the Lymnaeidae [28Dec06]
  • Character phase disequilibrium in the Gyraulus of Europe [4Feb22]

[10]  Hershler, R. (1994)  A review of the North American freshwater snail genus Pyrgulopsis (Hydrobiidae).  Smithsonian Contributions to Zoology 554: 1-115.  I mentioned idly  “leafing through” this important work back in 2016, for example:

  • Marstonia letsoni, quite literally obscure [5Feb16]

[11] Hershler & Liu (2017) Annotated Checklist of Freshwater Truncatelloidean Gastropods of the Western United States, with an Illustrated Key to the Genera.  US Bureau of Land Management Technical Note 449: 1 – 142.

[12] Hershler, R. 1998. A systematic review of the hydrobiid snails (Gastropoda: Rissooidea) of the Great Basin, western United States. Part I. Genus Pyrgulopsis. Veliger 41:1-132.  Hershler, R. 2001. Systematics of the North and Central American aquatic snail genus Tryonia (Rissooidea: Hydrobiidae). Smithsonian Contributions to Zoology 612:1-53.  Hershler, R., Frest, T.J. 1996. A review of the North American freshwater snail genus Fluminicola (Hydrobiidae). Smithsonian Contributions to Zoology 583:1-41.  Hershler, R., Landye, J.J. 1988. Arizona Hydrobiidae. Smithsonian Contributions to Zoology 459:1-63.  Hershler, R., Thompson, F.G. 1992. A review of the aquatic gastropod subfamily Cochliopinae (Prosobranchia: Hydrobiidae). Malacological Review Supplement 5:1-140.

[13] Hershler, R., D.B. Madsen, and D.R. Currey (eds) Great Basin Aquatic Systems History. Smithsonian Contributions to the Earth Sciences 33: 1 – 405 (2002).

[14] Ironic in light of the title of his 2017 publication, cited at note [11] above.

[15] This is a detail from Figure 2 of Hershler [10].  It depicts three views of a whole mount penis dissected from Pyrgulopsis californiensis.  Tg = terminal gland, Pg = penial gland, Vd = ventral gland, Dg = dorsal glands. 

[16]  I was banned from campus and forced into retirement for a Woodrow Wilson quote four weeks into the spring semester of 2016.  A press release issued by the Provost’s Office at the College of Charleston on the afternoon of February 21 stated, in part, “We have endured that sanctimonious asshole for 33 years, 5 months, 21 days, 13 hours and 15 minutes, and cannot stand him for one second more.”  For a review, see:

  • Inside Higher Education [8Aug16]

I used my settlement from the lawsuit to set up the FWGNA as a sole proprietorship.

Thursday, June 9, 2022

Cytoplasmic Male Sterility in Physa!

Three cheers for our good friend Dr. Patrice David of the CNRS-CEFE, who together with a team from the Universities of Montpellier and Lyon, has opened a new window into the mysteries of sex allocation in pulmonate snails, casting fresh light on mitochondrial evolution across the entire animal kingdom in the process.  Our colleagues from France have discovered the first case of cytoplasmic male sterility ever documented outside the vascular plants [1].  And they have done so in the most underappreciated experimental model of our time, the freshwater gastropod Physa acuta.

Male-fertile & sterile [3]
Genetic phenomena now understood to result from cytoplasmic male sterility were reported in plants as early as 1884 [2]. Many wild populations of the thistle Cirsium oleraceum, for example, are composed of mixtures of (normal) monoecious plants producing monoecious offspring and (male-sterile) female plants producing only female offspring, although always pollinated from monoecious plants. In a series of papers published between 1916 and 1937, the German geneticist Carl Correns demonstrated that the “female C. oleraceum plants contained in their cytoplasms some properties which inhibited the development of male floral parts.”  And in 1939, the obscure Argentinian geneticist E. Gini reported controlled crosses conclusively demonstrating cytoplasmic male sterility in certain varieties of maize, which might, however, be modified by a nuclear gene or genes segregating in the male parent.

Thistles are ordinary dicots, in the sense that their flowers bear both stigma and anther, both pollen and ova.  Maize is a monocot, and among the just 5% of all angiosperms with separate male and female inflorescences; the pollen being produced by the tassel on top, the ovum in the cob below.  What unites maize and thistles is that at least sometimes both demonstrate a mating system called “gynodioecy,”  where both hermaphroditic and female individuals co-exist in a single population.

Between 1940 and 1970, cytoplasmic male sterility was discovered in hundreds of species of (at least occasionally) gynodioecious plants, including many important as crops.  Nuclear genes that moderated the effects of CMS were discovered as well.  In the hands of plant breeders, CMS became a powerful tool to promote hybridization in normally self-pollinating varieties of maize, wheat, rice, carrots, onions, and beets.  And as the beet fields blossomed forth, so too did funding for research into CMS.

By the mid-1980s it was clear that CMS was caused by mitochondrial gene alteration.  By the mid-1990s sterility had been pegged to a variety of genes variably-inserted into otherwise normal plant mitochondria, encoding proteins that interfered with pollen formation [3].  And by the mid-2000s, the sequence data were rolling in.  It materialized that the plant mitochondrial genome is strikingly different from animal mtDNA in its evolutionary dynamics: variably sized, highly repetitive, and characterized by frequent gain, loss, duplication and rearrangement of genes [4].

Now, back to France.  You have heard me sing the praises of Dr. Philippe Jarne, his (1993-96) student Dr. Patrice David, and a bouquet of Montpellier friends and colleagues in many previous essays posted on this blog.  Most recently we featured their collaboration with a host of researchers from across the Americas to work out the evolution of the lymnaeid subgenus Galba, the crappy little amphibious snails serving as intermediate hosts for the sheep liver fluke Fasciola worldwide [5].  Philippe and Patrice also have research interests in Physa, primarily as a model for the study of sex allocation, extending all the way back to 1995.  See note [6] below for a selection of my favorite Physa papers published by Philippe, Patrice, and their colleagues over the years.
Graphical abstract from David et al. [1]
That said.  A phenomenon as startling as cytoplasmic male sterility in little brown trash snails is not something one can design an experiment to discover.  Patrice tells me that the breakthrough was initially made by Dr. Emilien Luquet of the University of Lyon, who has adopted Physa acuta as a model for the study of phenotypic plasticity.  Routinely sequencing the CO1 gene from a sample of Physa collected in a backwater of the Rhone River east of Lyon, Dr. Luquet obtained a couple sequences that were, to use the precise, technical terminology especially developed in Lyon to describe the phenomenon, “weird” [7].

Dr. Luquet, together with a team of four colleagues from the University of Lyon, (Plénet, Lefébure, Konecny and Deglétagne) then developed and characterized 34 isofemale lines of Physa from the Rhone River backwaters, 29 with normal mitochondrial genotypes and 5 weird.  The weird lines bore mitochondria with a median CO1 sequence divergence more than 20% different from the other members of their population, or indeed, any of the hundreds of Physa sequences in GenBank.  Sequences of the mitochondrial 16S gene returned the same remarkable result.

At this point Dr. Luquet sent samples to our mutual friend Patrice, saying “I apparently have Physa that came from outer space, look at them and tell me if this is really Physa acuta.”  Patrice, aware that here in North America we have several species of Physa that are indeed rather cryptic, began a series of experimental crosses with an albino line of P. acuta developed in the CNRS-CEFE laboratory.

When mounted by albino controls mating in the male role, the weird lines of Physa outcrossed readily, and actually showed some evidence of fecundity improved over the normal lines from Lyon.  But it materialized that the weird lines were male-sterile.  They showed much-reduced behavioral tendency to mount partners in the male role, their seminal vesicles apparently shrunken, containing very little sperm.  They could not self-fertilize.  This is the first demonstration of cytoplasmic male sterility in the animal kingdom

… but Amy Wethington and I had a shot at it 20 years ago.

Amy and I started working with Physa when she was an undergraduate student at the College of Charleston in the late 1980s.  And our first samples came from a pond in the state park just around the corner from my house, Charles Towne Landing.  That population of Physa became a standard for 20 papers published over the 20 years that followed [8].  We called it population C.

Amy at Charles Towne Landing

Almost immediately, Amy and I became interested in the evolutionary relationships between our population C, which we initially identified as “Physa heterostropha pomilia,” and all the other populations of trash Physa worldwide, identified with a wide variety of other names.   And my readership with admirably long memory may recall that it was from Philippe Jarne back in 2000 that Amy and I received the batch of French Physa acuta we used in our experiments demonstrating no reproductive isolation between population C and five other populations of Physa that had been known by three different names: Physa acuta, Physa heterostropha, and Physa integra [9].

So given that all six of these populations of Physa from four states and two European countries were conspecific, Amy and I were next curious to estimate the amount of genetic divergence among them.  A CofC undergraduate named Matt Rhett and I ran the allozyme gels [10], and Amy (now in the PhD program at the University of Alabama) sequenced two mitochondrial genes (CO1 and 16S) for approximately 20 snails from each of those six populations: C = Charleston, P = Philadelphia, D = Douglas Lake, Michigan, N = New Harmony, Indiana, F = France, I = Ireland.  The gigantic 16s+CO1 neighbor-joining gene tree that resulted is shown below.

Let’s look at that tree from the bottom up.  At the bottom you see two other good biological species of Physa: three individual Physa gyrina, and three individual Physa pomilia, which we were calling Physa hendersoni at that time [11].  The gigantic middle branch of the tree shows all six of our Physa acuta populations all mixed up: C, P, D, N, F, and I, with a little bit of geographic structure but not much.  Now what the heck is that branch sticking way out at the top?

That top branch, labelled C10, C13, C15 and C18, shows four snails from Charles Town Landing bearing mitochondria with sequences approximately 30% different from all other Physa acuta from four states and two European countries.  Amy called those sequences “whacky.”

So in 2004 Amy drafted a manuscript by Wethington, Rhett, and Dillon to report all our allozyme data and all our sequence data across all six Physa populations.  Meanwhile, I recruited another undergraduate student, Nick DiNitto, and together Nick and I went back to the pond at Charles Town Landing, collected 26 adult Physa, and started isofemale lines.  Once the original mothers had laid eggs, our plan was to enlist the aid of my colleague Bob Frankis to sequence their CO1 genes, hoping to found pure whacky [12] cultures for breeding experiments.

From FWGNA Circular #5 [pdf]

But alas.  Working with Bob, Nick was able to sequence just six of the original mothers before the semester came to a close and the project foundered.  None of those six bore whacky mitotypes [12].  Nick’s poster for the 2004 meeting of the American Malacological Society in Sanibel, Florida, reporting just N=6 unremarkable CO1 sequences, is available for download from note [13] below.

Meanwhile Amy was having no success whatsoever interesting journal editors in publishing the Wethington, Rhett & Dillon manuscript.  In the eyes of reviewers, the whacky result depicted at the top of our Figure 1 invalidated the entire paper.  This must be some sort of lab accident!  Or cryptic speciation?  Either Amy’s technique was hopelessly sloppy, or our premise was hopelessly wrong.  Amy ultimately gave up trying to publish the separate paper, cut the four whacky sequences, and folded the rest of the data into her 2004 dissertation [14].  Her amazing discovery was forgotten.

Until today.  Today I am making the original 2004 manuscript of A. R. Wethington, J. M. Rhett and R. T. Dillon, entitled “Allozyme, 16S, and CO1 sequence divergence among populations of the cosmopolitan freshwater snail, Physa acuta” available for download as FWGNA Circular #5, here: 

-  Wethington, Rhett & Dillon (2022)  -

And perhaps I should have written “almost forgotten” two paragraphs above.  For in footnote (3) of my [15Mar16] essay on mitochondrial superheterogeneity, I made passing reference to “a striking case of mtSH (both COI and 16S) in a Charleston population of Physa acuta.”  Which leads me to offer two final hypotheses, ask two final questions, and make an appeal.

First hypothesis.  It seems quite likely to me that mtSH = CMS in pulmonate snails broadly.  The first report of the phenomenon I subsequently dubbed “mitochondrial superheterogeneity” came in the 1996 paper of Thomaz and colleagues, working with the land snail Cepaea nemoralis [15].  Cases of mtSH have been reported at least three additional times in the land snails that I know of, and one other time in a freshwater pulmonate, the limpet Laevapex fuscus.  See footnotes (2) and (3) of my [15Mar16] essay.

Second hypothesis. It seems possible that CMS -> DUI in bivalves.  Doubly-uniparental inheritance of mitochondria was first reported in the marine mussel Mytilus in the early-1990s and has subsequently been documented in a wide variety of bivalves, including the unionid mussels of American freshwaters.  As the name implies, researchers have found that mitochondria can be passed by sperm as well as by egg in these bivalve groups.  And most intriguingly, the genomes of the male mitochondria and the female mitochondria are always found to be strikingly divergent.  Sophie Breton of the University of Quebec and a team of bivalve researchers including our friend Randy Hoeh have just very recently published a nice paper making this argument in compelling fashion [16].

First question.  Could CMS -> MtSH in pleurocerids and other prosobranch snails?  That is a stretch I am not intellectually limber enough to make.  Sexes are separate in the pleurocerids, as in almost all other prosobranch groups, mechanism of sex determination unknown.  If you wade out into the creek, fish up a nice sample of adult pleurocerids and crack them open, you will almost always find sex ratios significantly biased toward the female [17].  This is a secondary sex ratio, of course.  I don’t think anybody knows the primary sex ratio for any prosobranch population.

I would love to hypothesize that the mitochondrial superheterogeneity so often demonstrated by freshwater prosobranch populations – ten cases listed in footnotes (4), (5), (6), and (7) in my essay of [15Mar16] alone – might be related to such sex ratio biases.  But Whelan & Strong [18] did not find any relationship between sex and mt haplotype in their superheterogeneous populations of Leptoxis from Alabama, and I don’t know where else to look for evidence.

And second question.  What is the origin of CMS in the Physa population of the Rhone River?  Patrice and colleagues ultimately sequenced the weird mitochondrial genome in its entirety, documenting extreme divergence everywhere they looked, summing to an eye-popping 44.4% median deviation from the normal mitotype.  Yet no divergence in the nuclear genome was apparent.  Sequencing the (nuclear) 28S gene returned no significant difference across their entire sample of 34 lines, nor were any differences in microsatellite gene frequencies apparent.  The Mendelian genetics confirm that the Rhone River at Lyon is inhabited by a single randomly interbreeding population of Physa acuta.

Which brings us to my appeal.  To all of our colleagues who persist in defining species with mtDNA gene trees, please stop.  You’re embarrassing us.

Patrice went on to offer several compelling lines of evidence suggesting that the weird mitotype results from an acceleration of mutation rate, perhaps due to some defect in the mitochondrial DNA repair mechanism, rather than selection on mitochondrial functions more broadly.  He further observed that high mutation rates might both produce male-sterility genes, and help them persist over evolutionary time, counter to the evolution of nuclear genes restoring male fertility.  There are all sorts of selfish-DNA-type directions one could go from here.

And so, to conclude.  None of the thoughts, questions, hypotheses and speculations outlined above could have flickered through my brain before the marvelous paper by Patrice David and his colleagues appeared last month.  Job well done, all of you!  La reussite scientifique de la France est intimement lie au bonheur de l’Amerique [19].


Notes

[1] David, Patrice, Cyril Degletagne, Nathanaëlle Saclier, Aurel Jennan, Philippe Jarne, Sandrine Plénet, Lara Konecny, Clémentine François, Laurent Guéguen, Noéline Garcia, Tristan Lefébure, Emilien Luquet (2022) Extreme mitochondrial DNA divergence underlies genetic conflict over sex determination.  Current Biology 32: 2325-2333.  https://doi.org/10.1016/j.cub.2022.04.014.

[2] The historical review in this paragraph is extracted from:

  • Edwardson, J.R. (1956) Cytoplasmic male-sterility.  Botanical Review 22: 696-738.
  • Edwardson, J.R. (1970) Cytoplasmic male-sterility.  Botanical Review 36: 341 – 420.

[3] Schnable, P.S., and R.P. Wise (1998)  The molecular basis of cytoplasmic male sterility and fertility restoration.  Trends in Plant Science 3: 175 – 180.

[4] Galtier, N. (2011) The intriguing evolutionary dynamics of plant mitochondrial DNA. BMC Biol. 9, 61. http://www.biomedcentral.com/1741-7007/9/64

[5] Alda, Pilar, M. Lounnas, A.Vázquez, R. Ayaqui, M. Calvopiña, M. Celi-Erazo, R.T. Dillon Jr., L. González Ramírez,  E. Loker, J. Muzzio-Aroca, A. Nárvaez, O. Noya, A. Pereira, L. Robles, R. Rodríguez-Hidalgo, N. Uribe, P. David, P. Jarne, J-P. Pointier, & S. Hurtrez-Boussès (2021) Systematics and geographical distribution of Galba species, a group of cryptic and world-wide freshwater snails.  Molecular Phylogenetics and Evolution 157: 107035. [pdf] [html].  For a review, see:

  • The American Galba and the French Connection [7June21]
  • The American Galba: Sex, Wrecks, and Multiplex [22June21]
  • Exactly 3ish American Galba [6July21]
  • What Lymnaea (Galba) schirazensis is not, might be, and most certainly is [3Aug21]

[6] Here are a few of my favorite Physa papers from Philippe and colleagues:

  • Jarne, P., M-A Perdieu, A-F Pernot, B. Delay, and P. David (2000)  The influence of self-fertilization and grouping on fitness attributes in the freshwater snail Physa acuta: population and individual inbreeding depression. J. Evol. Biol. 13:645-655.
  • Bousset, L., P-Y. Henry, P. Sourrouille, & P. Jarne (2004)  Population biology of the invasive freshwater snail Physa acuta approached through genetic markers, ecological characterization and demography. Molec. Ecol., 13: 2023-2036.
  • Tsitrone A, Jarne P, David P: Delayed selfing and resource reallocations in relation to mate availability in the freshwater snail Physa acuta. Amer Natur 2003, 162:474-488.
  • Henry PY, Bousset L, Sourrouille P, Jarne P: Partial selfing, ecological disturbance and reproductive assurance in an invasive freshwater snail. Heredity 2005, 95:428-436
  • Noel, E., Chemtob, Y., Janicke, T., Sarda, V., Pelissie, B., Jarne, P., and David, P. (2016). Reduced mate availability leads to evolution of self-fertilization and purging of inbreeding depression in a hermaphrodite. Evolution 70, 625–640.

[7] Quoting Patrice, “Initially we called them W for weird; we later opted for D as in Divergent which sounds more serious and doesn't induce confusion with W chromosomes.”

[8] I have reviewed the many years of fruitful collaboration that Amy and I enjoyed in quite a few essays previously posted on this blog.  Here are my favorites:

  • To identify a Physa, 1989 [3Oct18]
  • Albinism and sex allocation in Physa [5Nov18]
  • To identify a Physa, 2000 [6Dec18]
  • TRUE CONFESSIONS: I described a new species [7Apr10]
  • What is a Species Tree? [12July11]

[9] Dillon, R. T., A. R. Wethington, J. M. Rhett and T. P. Smith (2002) Populations of the European freshwater pulmonate Physa acuta are not reproductively isolated from American Physa heterostropha or Physa integra.  Invertebrate Biology 121: 226-234.  [pdf]

[10]  I know that an allozyme survey sounds hopelessly outdated.  A postcard from 1982.  But it was allozyme markers that allowed us to do all our breeding studies.  We could not have verified outcrosses without baseline allozyme surveys such as my students and I labored over for years.  There are a lot of research groups who wish they had that hopelessly-outdated 1982 technology today.

[11] Dillon, R. T., J. D. Robinson, and A. R. Wethington (2007)  Empirical estimates of reproductive isolation among the freshwater pulmonates Physa acuta, P. pomilia, and P. hendersoni.  Malacologia 49: 283 - 292.  [pdf]

[12] Actually, Nick preferred to call the weird sequences “zany” rather than “whacky.”  So that’s what we called them on his poster…

[13] DeNitto, N. W., R. C. Frankis and R. T. Dillon (2004) Extensive mitochondrial CO1 sequence diversity in a population of the freshwater snail, Physa: Admixture or cryptic speciation?  American Malacological Society, Sanibel Island, FL. [Poster, pdf]

[14] Amy’s dissertation was ultimately published as: Wethington, A.R., & C. Lydeard (2007) A molecular phylogeny of Physidae (Gastropoda: Basommatophora) based on mitochondrial DNA sequences. Journal of Molluscan Studies 73: 241 - 257. [pdf]  For a review, see:

  • The classification of the Physidae [12Oct07]

[15] Thomaz, D., Guiller, A. & Clarke, B. (1996) Extreme divergence of mitochondrial DNA within species of pulmonate land snails. Proc. of the Royal Society of London B: Biological Sciences, 263, 363–368.

[16] Breton, Sophie, Donald T. Stewart, Julie Brémaud, Justin C. Havird, Chase H. Smith, and Walter R. Hoeh (2022)  Did doubly uniparental inheritance (DUI) of mtDNA originate as a cytoplasmic male sterility (CMS) system?  Bioessays 44: 2100283. https://doi.org/10.1002/bies.202100283

[17] Cipiaris, S., W. F. Henley and J.R. Voshell (2012)  Population sex ratios of pleurocerid snails (Leptoxis spp.): Variability and relationships with environmental contaminants and conditions.   Amer. Malac. Bull. 30: 287 - 298.

[18] Whelan, N.V. & E. E. Strong (2016)  Morphology, molecules and taxonomy: extreme incongruence in pleurocerids (Gastropoda, Cerithiodea, Pleuroceridae). Zoologica Scripta 45: 62 – 87.  For a review, see:

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

[19] The scientific success of France is closely tied to the good fortune of America.  I offer this benediction as an echo of the Marquis de Lafayette quote with which I ended my essay of [7June21].

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.

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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!