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 monop hyly of Apogastropoda and its constituent clades, Caenogastropoda and Heterobranchia. Representatives of the architaenioglossan groups Cyclo phoroidea, Ampullariidae and Viviparidae invariably emerge within Caen ogastropoda in all analyses. While the Cyclophoroidea and Ampullariida e are monophyletic, the varying position of Viviparidae in all outcome s contradicts its hypothesized sister group relationship with Ampullar iidae, and thus the monophyly of Ampullarioidea. Because of the positi on of Viviparidae, Architaenioglossa does not emerge as a clade in any of our analyses. Campanile consistently emerges between Cyclophoroide a and Cerithioidea, or in a clade with Cyclophoroidea and Ampullariida e, 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 likel ihood analyses as well as maximum parsimony analyses using only transv ersions divide Caenogastropoda into two clades, one containing the arc hitaenioglossan taxa, Campaniloidea and Cerithioidea, the other contai ning the higher caenogastropod taxa included in Eucaenogastropoda (Has zprunar, 1988) [= Hypsogastropoda (Ponder & Lindberg 1997)]. Denser ta xon sampling revealed insertions to be present in the 18S rDNA gene of several caenogastropod taxa. Earlier reports (Harasewych et nl. 1997b ) of reduced sequence divergence levels in Caenogastropoda are shown t o be restricted to Hypsogastropoda. Based on a broader taxonomic sampl ing, divergence levels within Caenogastropoda are comparable to those found within Heterobranchia. (C) 1998 The Norwegian Academy of Science and Letters.