Convergent evolution of barnacles and molluscs sheds lights in origin and diversification of calcareous shell and sessile lifestyle

The calcareous shell and sessile lifestyle are the representative
phenotypes of many molluscs, which happen to be present in barnacles, a
group of unique crustaceans. The origin of these phenotypes is unclear,
but it may be embodied in the convergent genetics of such distant groups
(interphylum). Herein, we perform comprehensive comparative genomics
analysis in barnacles and molluscs, and reveal a genome-wide strong
convergent molecular evolution between them, including coexpansion of
biomineralisation and organic matrix genes for shell formation, and
origination of lineage-specific orphan genes for settlement. Notably, the
expanded biomineralisation gene encoding alkaline phosphatase evolves a
novel, highly conserved motif that may trigger the origin of barnacle
shell formation. Unlike molluscs, barnacles adopt novel organic matrices
and cement proteins for shell formation and settlement, respectively, and
their calcareous shells have potentially originated from the cuticle
system of crustaceans. Therefore, our study corroborates the idea that
selection pressures driving convergent evolution may strongly act in
organisms inhabiting similar environments regardless of phylogenetic
distance. The convergence signatures shed light on the origin of the shell
and sessile lifestyle of barnacles and molluscs. In addition, notable
nonconvergence signatures are also present and may contribute to
morphological and functional specificities.