Study of molluscan phylogeny using complete small ribosomal subunit RNA sequences

Mainly due to the large phenotypical diversity in the phylum Mollusca, phylogenetic relationships of the different molluscan classes could not be traced using morphological and embryological data. There is also considerable disagreement about several aspects of animal phylogeny such as the position of the Mollusca. Molecular data were used to assess metazoan phylogeny but taxa and nucleotide sampling was too limited.; We used complete 18S rRNA sequences to assess several aspects of molluscan phylogeny viz. (1) The position of the Mollusca; (2) molluscan mono- or polyphyly; (3) relationships of the molluscan classes; 4) relationships within the class Gastropoda.; Complete 18S rRNA sequences of 44 Metazoa, among which 29 Mollusca, were determined and analysed together with published 18S rRNA sequences. Hypotheses were derived on the position of the Mollusca and more generally, on the relationships of the different metazoan phyla. The monophyly of the Mollusca was questioned, but needs further evaluation due to instability of the branching pattern in the clade to which Mollusca belong. The 18S rRNA sequences contain insufficient information to determine the relationships between the molluscan classes. New information on the relationships within the class Gastropoda was obtained.