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 77th
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,” 77th 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.
