Phylogenetic relationships of the lower Caenogastropoda (Mollusca, Gastropoda, Architaenioglossa, Campaniloidea, Cerithioidea) as determined by partial 18s rDNA sequences

Phylogenetic analyses of partial sequences spanning approximately 450 nucleotides near the 5'end of the 18s rDNA strongly support the monophyly of Apogastropoda and its constituent clades, Caenogastropoda and Heterobranchia. Representatives of the architaenioglossan groups Cyclophoroidea, Ampullariidae and Viviparidae invariably emerge within Caenogastropoda in all analyses. While the Cyclophoroidea and Ampullariidae are monophyletic, the varying position of Viviparidae in all outcomes contradicts its hypothesized sister group relationship with Ampullariidae, and thus the monophyly of Ampullarioidea. Because of the position of Viviparidae, Architaenioglossa does not emerge as a clade in any of our analyses. Campanile consistently emerges between Cyclophoroidea and Cerithioidea, or in a clade with Cyclophoroidea and Ampullariidae, a position not predicted by previous morphological studies. Maximum parsimony analyses of sequence data show Caenogastropoda to comprise a series of sequentially diverging higher taxa. However, maximum likelihood analyses as well as maximum parsimony analyses using only trans-versions divide Caenogastropoda into two clades, one containing the architaenioglossan taxa, Campaniloidea and Cerithioidea, the other containing the higher caenogastropod taxa included in Eucaenogastropoda (Haszprunar, 1988) = Hypsogastropoda (Ponder & Lindberg 1997)l. Denser taxon sampling revealed insertions to be present in the 18s rDNA gene of several caenogastropod taxa. Earlier reports (Harasewych et al. 1997b) of reduced sequence divergence levels in Caenogastropoda are shown to be restricted to Hypsogastropoda. Based on a broader taxonomic sampling, divergence levels within Caenogastropoda are comparable to those found within Heterobranchia.