Phylogenomics reveals deep molluscan relationships

The evolutionary relationships among the eight major lineages of Mollusca remain unresolved despite their diversity and importance. Previous investigations of molluscan phylogeny, based primarily on nuclear ribosomal gene sequences or morphological data have been unsuccessful at elucidating these relationships. Limited genomic resources spanning the diversity of molluscs have prevented the use of a phylogenomic approach in resolving molluscan relationships. Here we use analyzed sequencing data from the tradiational expressed sequence tag (EST) and to the high throughput expression reads as well as genomic data, from all major lineages except Monoplacophora, in phylogenomic analyses and recover a well-supported topology. Our results strongly support the Aculifera hypothesis placing Polyplacophora (chitons) as the sister taxon of a monophyletic Aplacophora (worm-like molluscs). Additionally, within Conchifera, a sister-taxon relationship between Gastropoda and Bivalvia is supported. This grouping has received little consideration and thus we propose the node-based name Pleistomollusca. Our results also suggest advanced cephalization may have arisen more than once in Mollusca.