Comparative genomics and signatures of selection in North Atlantic eels

Comparative genomic approaches can identify putative private and shared signatures of selection. We performed a comparative genomic study of North Atlantic eels, European eel (Anguilla Anguilla) and American eel (A. rostrata). The two sister species are nearly undistinguishable at the phenotypic level and despite a wide non overlapping continental distribution, they spawn in partial sympatry in the Sargasso Sea. Taking advantage of the newly assembled and annotated genome, we used genome wide RAD sequencing data of 359 individuals retrieved from Sequence Nucleotide Archive and state-of-the-art statistic tests to identify putative genomic signatures of selection in North Atlantic eels. First, using the F-ST and XP-EHH methods, we detected apparent islands of divergence on a total of 7 chromosomes, particularly on chromosomes 6 and 10. Gene ontology analyses suggested candidate genes mainly related to energy production, development and regulation, which could reflect strong selection on traits related to eel migration and larval duration time. Gene effect prediction using SNPeff showed a high number of SNPs in noncoding regions, pointing to a possible regulatory role. Second, using the iHS method we detected shared regions under selection on a total of 11 chromosomes. Several hypotheses might account for the detection of shared islands of selection in North Atlantic eels, including parallel evolution due to adaptation to similar environments and introgression. Future comparative genomic studies will be needed to further clarify the causes and consequences of introgression, including the directionality of these introgression events.