PRELIMINARY RIBOSOMAL-RNA PHYLOGENY OF GASTROPOD AND UNIONOIDEAN BIVALVE MOLLUSKS

Sequences of about 150 nucleotides in the D6 region of the large (28S) ribosomal molecule were obtained from 20 unionoidean bivalves and 13 gastropods, including 9 truncatellids, 1 muricid, 1 cancellariid, 1 me longenid, and 1 pleurotomariid. These were analyzed along with sequenc es from Emberton et al. (1990) for 8 pulmonates, a helicinid and a pom atiopsid. Rates of divergence varied by a factor of two, with unionoid ean and the helicinid sequences differing by 15-20% relative to mouse, pulmonates by 22-24%, neogastropods by 24-27%, and rissooideans by 27 -32%. Length variation among sequences occurred mainly in the D6 loop, and complementary mutations were seen in the D6 stem. Cladistic analy sis found 24 equally parsimonious trees; the strict consensus supports monophyly of Unionoidea, Rissooidea, Pulmonata and Stylommatophora. M onophyly of Neogastropoda is not contradicted. Some groupings are anom alous when compared to morphology-based phylogenies. Helicinidae group s with Unionoidea, Pleurotomariidae with Neogastropoda, and Geomelania (Truncatellidae) with Pomatiopsidae. In each case, addition of taxa t hat intersect long branches (e.g. Chitonidae, Patellogastropoda) might show that characters interpreted as synapomorphic are plesiomorphic o r convergent. The observed grouping of Muricidae and Cancellariidae is well-supported, indicating that cancellariids are a highly derived gr oup within the Stenoglossa. Pleurotomariidae are more closely related to the other gastropods in the analysis than are Helicinidae, supporti ng Haszprunar's (1988) anatomy-based conclusion. To date, sequence stu dies of mollusks have not overturned phylogenies based on morphology, but rather have helped in choosing among competing morphology-based hy potheses. Like morphological data, sequence data are subject to proble ms of convergence, unequal rates of evolution, and choice of taxa. Cla ssifications must be based on all available data to maximize the poten tial for detecting convergences and correctly resolving phylogenetic r elationships.