A molecular phylogeny of Heterodonta (Bivalvia) based on small ribosomal subunit RNA sequences

Within Heterodonta, phylogenesis has so far been studied almost exclusively on the basis of morphological data. Results have often been discordant, an d an exhaustive molecular approach has not yet been attempted. The present study was undertaken to clarify the phylogenetic relationships obtaining am ong Heterodonta families through the analysis of ISS rRNA gene. To do this, the whole sequence of this gene was analyzed in 29 species of eight superf amilies of the order of Veneroida (Arcticoidea, Cardioidea, Galeommatoidea, Mactroidea, Solenoidea, Tellinoidea, Tridacnoidea, and Veneroidea) and in two superfamilies of Myoida (Pholaloidea and Myoidea). The study was extend ed by constructing phylogenetic trees using partial sequences. This strateg y made it possible to include 11 additional species by introducing three fu rther superfamilies: Chamoidea, Corbiculoidea, and Hiatellinoidea. At varia nce with the conclusions reached on the basis of morphological features, th e molecular data clearly show that the Myoida species included in this stud y belong to Veneroida, thus undermining the legitimacy of the division of H eterodonta into two orders, and that considerable differences in the phylog enetic relationships obtain among superfamilies. (C) 2001 Academic Press.