To the FWGNA group:
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 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 (the
obvious 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).
Keep in touch,
Rob
Notes
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 are not the product of 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.
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