Digging into bivalve miRNAomes: between conservation and innovation

Bivalves are a diverse mollusc group of economic and ecological importance. An evident resilience to pollution, parasites and extreme environments makes some bivalve species important models for studying adaptation and immunity. Despite substantial progress in sequencing projects of bivalves, information on non-coding genes and gene-regulatory aspects is still lacking. Here, we review the current repertoire of bivalve microRNAs (miRNAs), important regulators of gene expression in Metazoa. We exploited available sncRNA data for Pinctada martensii, Crassostrea gigas, Corbicula fluminea, Tegillarca granosa and Ruditapes philippinarum and we produced new sncRNA data for two additional bivalves, the Mediterranean mussel Mytilus galloprovincialis and the blood clam Scapharca broughtonii. We found substantial heterogeneity and incorrect annotations of miRNAs; hence, we reannotated conserved miRNA families using recently established criteria for bona fide microRNA annotation. Total 106 miRNA families could not be found in the previously published bivalve datasets whereas 89 and 87 miRNA complements were identified in the two additional species. The overall results provide a homogeneous and evolutionary consistent picture of miRNAs in bivalves and enable future comparative studies. The identification of two bivalve-specific miRNA families sheds further light into the complexity of transcription and its regulation in bivalve mollusks.