Comparative genomics reveals evolutionary drivers of sessile life and left-right shell asymmetry in bivalves

Abstract

Bivalves are species-rich mollusks with prominent protective roles in coastal ecosystems. Across these ancient lineages, colony-founding larvae anchor themselves either by byssus production or by cemented attachment. The latter mode of sessile life is strongly molded by left-right shell asymmetry during larval development of Ostreoida oysters such as Crassostrea hongkongensis. Here, we sequenced the genome of C. hongkongensis in high resolution and compared it to reference bivalve genomes to unveil genomic determinants driving cemented attachment and shell asymmetry. Importantly, loss of the homeobox gene antennapedia (Antp) and broad expansion of lineage-specific extracellular gene families are implicated in a shift from byssal to cemented attachment in bivalves. Evidence from comparative transcriptomics shows that the left-right asymmetrical C. hongkongensis plausibly diverged from the symmetrical Pinctada fucata in expression profiles marked by elevated activities of orthologous transcription factors and lineage-specific shell-related gene families including tyrosinases, which may cooperatively govern asymmetrical shell formation in Ostreoida oysters.