Friday, February 1, 2008
Last month we reviewed the experiments of Urabe (1998, 2000) vividly demonstrating that shell morphology in the East Asian pleurocerid genus Semisulcospira has a substantial non-genetic component. Since the taxonomy of this important group is based almost entirely on its shell, such results suggest that the true number of biological species in the genus Semisulcospira may be much different from the number of nomina conventionally applied.
This month we’ll update that message with a second chapter from Taehwan Lee, Dairmaid O’Foighil, and two colleagues from Pai Chai University in Daejon, South Korea (1). What might the application of modern molecular techniques add to our understanding of evolution in this enigmatic genus from The East?
The range of Semisulcospira libertina extends from Japan onto the Korean Peninsula, where it shares rivers and streams with ten other nominal Semisulcospira species, as judged by their shell morphology. Lee and his colleagues sampled eight of those species from nine sites – libertina (four populations), coreana (two populations), extensa (two populations), and single populations of forticosta, gottschei, multicincta, nodiperda, and tegulata. They sequenced two genes, the mitochondrial 16S and the nuclear 28S ribosomal genes, in most cases just one individual per population but in some cases up to ten or even twenty per population. They built phylogenetic trees using three different methods (maximum parsimony, maximum likelihood, and Baysian) to mollify all constituencies among their reviewers (2), with a couple other genera of Korean pleurocerids added as outgroups (3).
It should come as no surprise, to all of us familiar with Urabe’s work, that the phylogenetic trees obtained by Taehwan, Dairmaid and their colleagues showed a mish-mash of nominal species bearing almost no relationship to the traditional taxonomy based on shell. Only Semisulcospira extensa appeared monophyletic. Populations of the other seven species displayed a genetic structure we have seen much of recently (4) – heterogeneous mixtures with numerically predominant modal clades and the scattered highly-divergent genotypes that (in my post of 7/07) I have called "sports."
Not only were the nominal species polyphyletic, the populations and even the individual snails were polyphyletic. I am ashamed to confess that I was entertained by the author’s discovery of a single S. libertina individual heterozygous at the 28S locus. I should have been compassionate – analysis of this individual snail was apparently a real pain in the can. Taehwan and colleagues had to clone the PCR products amplified from this individual in plasmid vectors and sequence a bunch of recombinant bacterial colonies. The two alleles turned out to differ by 23 nucleotides, leaving the single individual S. libertina that bore them splayed across opposite ends of their 28S maximum likelihood tree. I think I may read phylogenetic systematic research for the same reason I watch NASCAR - the wrecks (5).
Taehwan, Dairmaid and their colleagues reviewed the same list of explanations for this phenomenon that they previously examined in their paper on Laevapex with Andrea Walther in the lead (6) – paralogous mt markers, cryptic species, introgression, and (their choice) retention of ancestral polymorphism through incomplete lineage sorting (7). They concluded with the recommendation that coreana, forticosta, gottschei, multicincta, nodiperda, and tegulata all be synonymized under the single, phenotypically variable biological species Semisulcospira libertina, broadly distributed across East Asia.
In summary, the messages borne to us this month by Taehwan, Dairmaid and their colleagues from The East are that “biologists interested in freshwater cerithiodean molecular phylogenetics approach these taxa as potential morphospecies complexes,” and that in the future “meaningful phylogenetic study of these organisms may well require the use of both mitochondrial and nuclear markers together with population level samples of all nominal taxa within regional drainages.” Message received, guys! And in other news … Ronald Reagan elected President (8).
1) Lee, T., H. C. Hong, J. J. Kim and D. O’Foighil (2007) Phylogenetic incongruence involving nuclear and mitochondrial markers in Korean populations of the freshwater snail genus Semisulcospira (Cerithioidea: Pleuroceridae). Molec. Phylog. Evol. 43: 386-397.
2) The tiny fringe group who prefers neighbor-joining techniques is not likely to read MP&E, much less referee their manuscripts.
3) It will be recalled from last month’s essay that Semisulcospira is ovoviviparous. As outgroups, Lee et al. selected four species from two strictly oviparous genera of Korean pleurocerids - Hua and Koreoleptoxis.
4) The parallel between these findings and our own (2004) results is striking, although perhaps not of the same magnitude. Dairmaid tells me that the sequence divergence ranged up to 8.9% within his Semisulcospira populations, while we reported up to 14.5% intrapopulation 16S sequence divergence in American Goniobasis. See Dillon & Frankis, Amer. Malac. Bull 19:69-77.
5) With Joe Gibbs running Toyotas in 2008, it may not be too much longer until Sprint Cup fans receive our own “message from The East.”
6) Walther, A., T. Lee, J. B. Burch, and D. Ó Foighil. 2006. E Pluribus Unum: A phylogenetic and phylogeographic reassessment of Laevapex (Pulmonata: Ancylidae), a North American genus of freshwater limpets. Molecular Phylogenetics and Evolution, 40: 501-516.
7) That’s close, but I don’t think exactly right. The likelihood that an ancestral polymorphism will be preserved in a pair of sorted lineages decreases with internodal time, which may be great in populations as old as these. I think such polymorphisms may not the product of incomplete lineage sorting, but rather have evolved endogenously in large, ancient, and isolated populations. And I think independently-evolved genes may converge on the same adaptive peak, or may be collected outside the population cluster in which they evolved by long branch attraction.
8) Dillon, R. T. and G. M. Davis (1980) The Goniobasis of southern Virginia and northwestern North Carolina: Genetic and shell morphometric relationships. Malacologia 20: 83-98.