Molecular phylogenetics of cupped oysters based on partial 28S rRNA gene sequences

Partial sequences of 28S-like rDNA were amplified using PCR and sequenced for eight species of oyster and one species of mussel. Phylogenetic relationships among seven species of Crassostreinid oyster were inferred from aligned sequences by parsimony and maximum-likelihood methods. Of the 315 sites that varied, 90 were phylogenetically informative in parsimony analysis. Inference by maximum parsimony (MP) is consistent with maximum-likelihood (ML) analysis for the major lineages, yielding a tree with the topology (Mytilus edulis (Ostrea edulis ((Crassostrea rivularis (C. belcheri, C. gigas))(C. virginica, C. rhizophorae, Saccostrea cuccullata, S. commercialis)))). MP and ML analyses resolved the systematic relationships of the Saccostrea and Atlantic Crassostrea differently such that a polytomy linking these four taxa is preferred with the data available. Molecular data support a later divergence of the tropical Pacific Saccostrea from a common ancestor of the Atlantic Crassostrea species. Molecular data from domains D1, D2, and partial D3 of the 28S rDNA supply sufficient phylogenetic information to determine systematic relationships among the extant oyster taxa, from the major species groups to the family level, thus providing valuable characters that are able to supplement the paucity of morphological characters so far recognized.