Phylogenetic analysis of southern hemisphere flat oysters based on partial mitochondrial 16S rDNA gene sequences

Oysters are among the most familiar, best studied, and morphologically variable of all marine invertebrate taxa. However, our knowledge of oyster phylogeny and systematics is rudimentary, especially for the subfamily Ostreinae (flat oysters). It is unclear, for instance, whether the predominant flat oysters occurring between latitudes 35 and 50 degrees S constitute a single circumglobal species, or multiple, phylogenetically distinct, regional taxa. We have performed the first DNA molecular phylogenetic analysis of ostreinid taxa to distinguish among competing phylogenetic and systematic hypotheses for Southern Hemisphere Ostreinae. An approximately 450-nucleotide fragment of the mitochondrial large ribosomal subunit (16S) was sequenced for 41 individual oysters, representing 14 taxa of brooding oysters: 5 Southern Hemisphere Ostreinae, 5 Northern Hemisphere Ostreinae, and 4 outgroup species of the subfamily Lophinae. Phylogenetic analyses of the resulting data set yielded consensus tree topologies that are comprehensively incongruent with prevailing morphologically based interpretations of systematic relationships among the Ostreinae. Three ostreinid mitochondrial clades were evident, each containing representatives of Southern Hemisphere regional ostreinid taxa, some of which robustly cocluster with Northern Hemisphere taxa. These three clades represent the first well-supported phylogenetic framework for this ecologically prominent and commercially important oyster subfamily.