Molecular investigation of the evolutionary origins of hydrothermal vent gastropods




McArthur, Andrew Grant

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Hydrothermal vent communities exhibit great taxonomic novelty with 88% of species, 51% of genera, and 21% of families new to science. Given the severe physiological barriers to invasion presented by hydrothermalism and the energetic independence of the community due to in situ primary production by chemoautotrophic bacteria, it has been previously proposed that hydrothermal vents may have acted as a refugia for groups of metazoan animals that originated during the Cambrian, Paleozoic, or Mesozoic. The alternate explanation is of rapid change of recent immigrants from the adjacent deep-sea and false taxonomic inflation. Six major groups of hydrothermal vent endemic gastropods exhibit high taxonomic novelty and a lack of known fossils. Discovery of these hydrothermal vent endemic groups has resulted in dramatic changes in how we view the evolution and phylogeny of the Gastropoda, particularly in regards to the novel anatomy of the Neomphalina (Neomphalidae + Peltospiridae). Recent cladistic examinations of gastropod phylogeny using anatomical and morphological characters disagree on the placement and monophyly of the Neomphalina or find few characters supporting their position in the overall gastropod phylogeny. In this dissertation, a molecular systematic investigation of gastropod phylogeny was performed to examine the antiquity of the vent endemic Neomphalina. Twenty-three new D1 domain and thirty new D6 domain DNA sequences of the 28S ribosomal RNA gene were obtained from fresh-frozen and formalin-ethanol preserved specimens. These were combined with previously published molluscan 28S ribosomal RNA sequences for a total of 159 sequences. Gastropod phylogeny was examined using both parsimony and distance-based analyses. The 28S ribosomal RNA gene exhibited saturation of substitutions beyond 15% divergence between sequences, estimated using Kimura’s two-parameter model. Alone, either domain exhibited poor resolution of gastropod phylogeny but together (32 genera only) monophyly of the Neritimorpha, Neomphalina, Vetigastropoda, Patellogastropoda, Caenogastropoda (including Viviparus, Ampullaria, and Campanile), and Heterobranchia (Euthyneura plus Valvata) was supported by bootstrap values. Relationships among these groups could not be resolved due to saturation of substitutions. Evidence of elevated evolutionary rates in the Patellogastropoda conformed to previous studies and confounded analyses. Regardless, the hydrothermal vent Neomphalina exhibited divergence values and phylogenetic novelty equivalent to the other early-Paleozoic radiations, supporting its consideration as a vent refugial phylogenetic relic. 28S ribosomal RNA sequences cannot resolve Cambrian or early Paleozoic radiations of the Gastropoda and use of diverse specimens limits reliability of sub-ordinal relationships due to long-branch attraction. Sequences of 28S ribosomal RNA are best used to examine within-order gastropod relationships due to saturation of substitutions at higher levels and among-order evolutionary rate variation.



Gastropoda, Molecular biology