Phenotypic and genomic signatures of latitudinal local adaptation along with prevailing ocean current in a coastal goby

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 counter gradient 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 modeling. Also, whole-genome
resequencing showed their past 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.