Testing the utility of partial COI sequences for phylogenetic estimates of gastropod relationships.

Recent studies on phylogenetic relationships within the molluscan class Gastropoda have involved morphological (Kay et al., 1998), ultrastructural (Healy, 1996), and molecular (e.g., Lydeard et al., 2002; McArthur and Koop, 1999) approaches. These investigations have provided new insights into gastropod affinities and classification and have enabled a vigorous testing of taxonomic schemes for the group. The most generally accepted system of classification now partitions the Gastropoda into five subclasses (Tudge, 2000), two of which, the Heterobranchia and the Caenogastropoda, are extremely diverse. The other three subclasses (Patellogastropoda, Neritopsina, and Vetigastropoda) are much less speciose, but are thought to represent the basal lineages of the class. Over the past decade molecular approaches have proven their value not only in resolving phylogenetic issues, but also in providing a sense of the time scales of evolutionary divergence. Because of their slow rates of evolution, nuclear rRNA genes have been widely used in studies that attempt to resolve relationships among groups that have a long history of evolutionary divergence. In contrast, the more rapidly evolving mitochondrial (mt) genes have generally been employed to infer relationships among groups with a more recent ancestry. However, it has become apparent that the latter gene regions can also provide insights concerning deeper divergences, as shown by a study that employed 16S rDNA sequences to examine the affinities of major gastropod lineages (Thollesson, 1999).