A molecular phylogeny of the bivalve mollusks.

A phylogenetic reconstruction based on 506 nucleotides near the 5' end of the 18S subunit of ribosomal DNA (rDNA) in 2 gastropod, 3 chiton and 28 bivalve mollusks supported the monophyly and sister group relationship of the subclasses Heterodonta and Palaeoheterodonta but could not confidently establish either the monophyly or the phylogenetic relationships of the morphologically well defined subclasses Pteriomorphia, Protobranchia, and Anomalodesmata. When both gastropods and chitons were included in the analysis, one or the other invariably emerged within Bivalvia. Some evidence indicates that this apparent polyphyly may be the consequence of unequal rates of evolution and of rapid changes in the protobranch and anomalodesmatan lineages. The taxa usually included in Pteriomorpha emerge as a grade rather than a clade, although in a sequence that differs from morphologically based phylogenies.

Phylogeny and relationships of pleurotomariid gastropods (Mollusca: Gastropoda): an assessment based on partial 18S rDNA and cytochrome c oxidase I sequences.

The phylogenetic position of the ancient family Pleurotomariidae within the Molluscan class Gastropoda, as well as the relationships of its Recent genera and species, were assessed using an iterative, two-gene (18S rDNA and cytochrome c oxidase I) approach to phylogeny reconstruction. In order to orient the Pleurotomariidae within Gastropoda, partial 18S rDNA sequences were determined for 7 pleurotomariid and 22 other gastropods that span the major groups within the class as well as for one cephalopod and two polyplacophorans, which serve as outgroups. Cladistic analyses of a sequence of approximately 450 base pairs (bp) near the 5' end of the 18S rDNA support the monophyly of the following higher gastropod taxa: Patellogastropoda, Vetigastropoda, Neritopsina, Apogastropoda, and its subclades Caenogastropoda and Heterobranchia. The 18S rDNA sequences and 579 bp of cytochrome c oxidase I (COI) analyzed separately and together, indicate that Pleurotomariidae are included within Vetigastropoda but comprise a clade that is the sister group to the other families referred to this order. Monophyly of the Pleurotomariidae is also supported by the unique presence of seven separate inserts (ranging in length from 1 to 68 bp) within the V2 variable region of the 18S RNA. Relationships of the genera and species within Pleurotomariidae are fully resolved using "total molecular evidence" consisting of partial sequences of 18S rDNA and COI and including data on length variation within the inserts.