Abstract
In the marine realm, unidirectional ocean currents often lead to high migration rates of marine organisms and, therefore, inhibit the formation of their latitudinal genetic structure. In contrast, cryptic latitudinal structures associated with local adaptation may frequently exist in widespread species generally exposed to a strong environmental heterogeneity. However, our understanding of the evolvability of locally adapted populations in open marine environments still needs to be completed. The coastal area along the Sea of Japan, where the Tsushima Warm Current flows from south to north in the Japanese Archipelago, provides a good model system for exploring this question. This study explored evidence for latitudinal local adaptation along with the prevailing ocean current in the ice goby Leucopsarion petersii at the phenotypic and genomic levels. Common garden experiments clearly showed genetically based clinal variation in growth rate, strongly suggesting local adaptation through conutergradient selection of this fitness-related trait. Analyses based on reduced-representation sequencing revealed a slight signal of genetic differentiation between the southern and northern populations, although continuous historical gene flow between them was supported by demographic modelling. Also, whole-genome resequencing showed their independent demographic history during the last glacial period. Thus, these results suggest that gene flow along with the prevailing ocean current is somewhat limited, and the populations are not completely panmictic. Furthermore, the selection scan based on low-coverage genome-wide sequencing detected putative genomic signatures of latitudinal adaptation of growth-related genes. Thus, our integrative study provided a novel example of marine local adaptation under a large ocean current.