My buddy, Bob

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2023b)  My Buddy, Bob.  Pp 203 – 210 in The Freshwater Gastropods of North America Volume 6, Yankees at The Gap, and Other Essays.  FWGNA Project, Charleston, SC.

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.

Cytoplasmic Male Sterility in Physa!

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2023c)  Cytoplasmic male sterility in Physa!  Pp 195 – 206 in The Freshwater Gastropods of North America Volume 7, Collected in Turn One, and Other Essays.  FWGNA Project, Charleston, SC.

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].

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!

The ham, the cheese, and Lithasia jayana

Editor’s Notes – 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. 

This essay was ultimately published as: Dillon, R.T., Jr. (2023b)  The ham, the cheese, and Lithasia jayana.  Pp 183 – 192 in The Freshwater Gastropods of North America Volume 6, Yankees at The Gap, and Other Essays.  FWGNA Project, Charleston, SC 

“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, as a whispered exchange in 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 its 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 its brief, Latinate description 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 18^th 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]

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

Editor’s Notes - 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 naïve and uncritical eyes.  This essay was ultimately published as: Dillon, R.T., Jr. (2023b)  No reproductive isolation between Lithasia populations of the duttoniana and jayana forms in the Duck River, Tennessee.  Pp 175 – 182 in The Freshwater Gastropods of North America Volume 6, Yankees at The Gap, and Other Essays.  FWGNA Project, Charleston, SC 

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.