Great Unanswered Questions Continued - Deep Molluscan Phylogenetics

The January 2013 issue of American Malacological Bulletin included eight papers from 11 presentations from the James H. Lee symposium, “Great Unanswered Questions in Malacology,” which was held at the 77^th Annual American Malacological Society meeting in Pittsburgh, Pennsylvania, July 23-27 2011.  The organizers, Timothy Pearce and Charles Sturm, introduced each paper (Pearce and Sturm, 2013).  I highlighted one paper previously in my April blog posting, but one that I would like to highlight further as the topic for this blog posting is entitled “Recent advances and unanswered questions in deep molluscan phylogenetics” by Kevin M. Kocot (Kocot, 2013).  Kocot provides a terrific brief review of the leading hypotheses of molluscan phylogeny that have been proposed based on morphological and sequence data such as nuclear small subunit (SSU or 18S) and large subunit (LSU or 28S) ribosomal gene sequences.  Many of these hypotheses have been debated about over many years with each hypothesis having a leading advocate or group of advocates supporting them.  Regrettably, molecular sequence data, which often provides useful data when morphology conflicts offered little information to resolve any of the conflicts and often resulted in bizarre findings such as the lack of monophyly of the Bivalvia and Gastropoda or a paraphyletic Mollusca.  Recently, with the development of phylogenomics, large amounts of nuclear protein-coding gene data derived from genomes and transcriptome data instead of PCR to amplify targeted gene fragments has been generated and found useful in examining the relationships of animals.  In 2011, two papers were published applying phylogenomics to the test of examining deep molluscan relationships (Kocot et al. 2011, Smith et al., 2011) and one examined PCR-amplified regions of seven genes in a target-gene approach (Vinther et al. 2011).  

A consensus tree based on the findings of the three studies was provided as follows:  (((Gastropoda, Bivalvia, Scaphopoda)(Cephalopoda, Monoplacophora))(Polyplacophora,(Neomeniomorpha, Chaetodermomorpha))).  Unlike some previous hypotheses, it is evident that the Aplacophora is monophyletic and sister to Polyplacophora rather than being a paraphyletic grade that was basal and plesiomorphic.  This finding alters our notion of character states for an hypothetical ancestral mollusk.  Also, there is no support for the recognition of the Cyrtosoma (Gastropoda + Cephalopoda), which alters our notion from a comparative framework for those interested in neurobiology of Cephalopods, which may actually be sister to Monoplacophora (although Monoplacophora was only examined in one of the three studies – Smith et al., 2011).

The fact that there was general agreement among the three studies is comforting and leads one to think perhaps we are making progress towards understanding deep phylogenetic relationships of the Mollusca, but many more molecular studies need to be done and sample sizes increased to determine whether the consensus tree will stand the test of time.  Also, in addition to molecular sequence data, as Kocot concludes “more traditional morphological and developmental studies will undoubtedly continue to improve understanding of molluscan evolution while simultaneously raising new questions about this fascinating group of animals.”   

Literature Cited

Kocot, K. M.  2013.  Recent advances and unanswered questions in deep molluscan phylogenetics.  American Malacological Bulletin 31(1):195-208.

Kocot, K. M., J. T. Cannon, C. Todt, M. R. Citarella, A. B. Kohn, A. Meyer, S. R. Santos, C. Schander, L. L. Moroz, B. Lieb, and K. M. Halanych.  2011.  Phylogenomics reveals deep molluscan relationships.  Nature 477:452-456.

Pearce, T. A. and C. F. Sturm.  2013.  Introduction to the James H. Lee symposiu, “Great Unanswered Questions in Malacology,” 77^th annual meeting of the American Malacological Society.  American Malacological Bulletin 31:105-107.

 Smith, S. A., N. G. Wilson, F. E. Goetz, C. Feehery, S. C. S. Andrade, G. W. Rouse, G. Giribet, and C. W. Dunn.  2011.  Resolving the evolutionary relationships of molluscs with phylogenomic tools.  Nature 480:364-367.

Vinther, J. , E. A. Sperling, D. E. G. Briggs, and K. J. Peterson.  2011.  A molecular palaeobiological hypothsis of the origin of aplacophoran molluscs and their derivation from chiton-like ancestors.  Proc. Of the Royal Society B: Biol. Sciences 279:1259-1268.