Abstract
Molluscs are one of the most diverse groups of animals and exhibit a rich and diverse variety of form and lifestyle. Most molluscs live with a free-moving lifestyle, while some molluscs are sessile. The adaptation to the two distinct lifestyles required complex changes, from molecules to organs, and physiology to morphology. In this study, we conducted a genome-wide scan for positive selection by comparing the available genomes of two sessile molluscs with four free-moving molluscs. A total of 40 genes were identified undergoing positive selection in the sessile molluscs by the branch-site model. Functional characterization showed that they were mainly enriched in two pathways, Oxidative phosphorylation (OXPHOS) and TGF-beta signaling pathway. The unexpected positive selection on OXPHOS genes in sessile molluscs suggests that the adaptation of OXPHOS involves many factors beyond enhancing ATP production. A modified OXPHOS regulatory system may allow sessile molluscs to better cope with biotic and abiotic stresses. Moreover, positively selected genes in TGF-beta signaling pathway probably have played a key role in the patterning of body plans and growth in metazoans. We speculate that these genes are associated with the body structure and organic adaptation to a sedentary lifestyle in sessile bivalve molluscs.
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