NEOGASTROPOD PHYLOGENY - A MOLECULAR PERSPECTIVE

The origin and evolution of the gastropod order Neogastropoda was inve stigated using an iterative, two gene (18S rDNA and cytochrome c oxida se I) approach to phylogeny reconstruction. Partial sequences spanning approximately 450 base pairs near the 5' end of the 18S rDNA gene con firmed the monophyly of Apogastropoda and its two subclades, the Caeno gastropoda (including Neogastropoda and Architaenioglossa) and the Het erobranchia, but were incapable of resolving relationships among neoga stropod families, or between Neogastropoda and higher Caenogastropoda. The monophyly of Heterobranchia is additionally supported by the pres ence within this group of a large insert of variable length in the 18S rDNA gene in the region corresponding to the E-10-1 helix of the RNA molecule. Cytochrome c oxidase I sequences were able to resolve fully the relationships among representatives of ten families of Neogastropo da. Maximum parsimony, maximum likelihood and neighbor-joining analyse s of these data all revealed that Buccinoidea and Muricoidea [sensu Th iele, 1929] each represent a clade, while the families assigned by Thi ele and some subsequent authors to the superfamily Volutoidea comprise a grade. Although the two toxoglossan taxa included in our study emer ged as a grade rather than a clade, denser taxonomic sampling of this group will be undertaken to investigate further the paraphyly of Conoi dea. Based on percent transversions at third codon positions of the CO I gene, differences among neogastropod families as well as those betw een the neogastropod families and Cerithium are comparable to genetic differences between orders of mammals, but are only slightly greater t han differences between genera of penaeid shrimp.