Evidence of widespread presence/absence gene variability in the highly heterozygous genome of Mytilus galloprovincialis. Mussel's genomics

The Mediterranean mussel is a widespread commercially, socially and ecologically important edible marine bivalve. Although pathogens and environmental alterations cause recurrent massive mortalities in other cultured species, mussels are extremely resilient to such conditions. Unfortunately, currently available genomics resources for this species are too limited to support molecular studies and improve farming practices. Here we report a high quality 1.28 Gb genome sequence, obtained through a tailored assembly strategy designed to overcome the inherent complexity of the mussel genome. We show that mussels display one of the highest levels of heterozygosity among metazoans and reveal that widespread genomic structural variations and gene presence/absence variability (PAV) are the main factors underlying the high levels of intraspecific genetic diversity observed. Moreover, we show that the ~15,000 mussel dispensable genes are enriched in functions related to immune defense and survival. This unique genomic trait might explain the impressive capabilities of adaptation and invasiveness of this species.