DETAILS OF GASTROPOD PHYLOGENY INFERRED FROM 18S RIBOSOMAL-RNA SEQUENCES

Some generally accepted viewpoints on the phylogenetic relationships w ithin the molluscan class Gastropoda are reassessed by comparing compl ete 18S rRNA sequences. Phylogenetic analyses were performed using the neighbor-joining and maximum parsimony methods. The previously sugges ted basal position of Archaeogastropoda, including Neritimorpha and Ve tigastropoda, in the gastropod clade is confirmed. The present study a lso provides new molecular evidence for the monophyly of both Caenogas tropoda and Euthyneura (Pulnonata and Opisthobranchia), making Prosobr anchia paraphyletic. The relationships within Caenogastropoda and Euth yneura data turn out to be very unstable on the basis of the present 1 8S rRNA sequences. The present 18S rRNA data question, but are insuffi cient to decide on, muricacean (Neogastropoda), neotaenioglossan, pulm onate, or stylommatophoran monophyly. The analyses also focus on two s ystellommatophoran families, namely, Veronicellidae and Onchidiidae. I t is suggested that Systellommatophora are not a monophyletic unit but , due to the lack of stability in the euthyneuran clade, their affinit y to either Opisthobranchia or Pulmonata could not be determined. (C) 1998 Academic Press.