Genome divergence between European anchovy ecotypes fuelled by structural variants originating from trans-equatorial admixture.

The formation of ecotypes is driven by evolutionary mechanisms that reduce gene flow through complex interactions among ecological, historical and genomic factors. In the European anchovy (Engraulis encrasicolus), marine and coastal ecotypes have been identified in the northeast Atlantic and the Mediterranean Sea, yet the genomic basis of their divergence remains unclear. Here, we present the first genome-scale analysis of this species complex, integrating whole-genome sequencing (WGS) and RAD-seq data from populations spanning its distribution range. In addition to the known marine and coastal ecotypes, we identify a previously undetected lineage extending from southern Morocco, through the Canary Islands, to South Africa. This southern Atlantic lineage exhibits a gradient of admixture with northern populations near the Atlantic-Mediterranean transition zone. Genomic differentiation landscapes reveal large regions of high linkage disequilibrium, probably corresponding to 13 structural variants (SVs) segregating within or between lineages. Notably, three of the six SVs contributing to the gene flow barrier between northern ecotypes originated in the southern lineage, supporting a partially shared evolutionary history between the coastal ecotype and the southern lineage. This study highlights how SVs that arose in geographically isolated lineages can act as key genetic elements in ecotype formation, reinforcing reproductive isolation through distinct evolutionary pathways.