Evolution of gigantism and size-based female mate choice in ice goby (Leucopsarion petersii) populations in a semi-enclosed sea basin

Geographical isolation associated with Pleistocene glacial cycles has often caused allopatric genetic divergence, even in the marine realm. Therefore, an historically enclosed sea basin could harbour high endemism of both phenotypes and species. Divergent allopatric lineages have appeared inside and outside of a unique sea basin, the Sea of Japan, providing a valuable opportunity to explore marine phenotypic evolution and speciation in relation to Pleistocene glacial isolation. We examined divergence in body size-related and behavioural traits, which may be important drivers of reproductive isolation, between the allopatric Japan Sea (JS) and Pacific Ocean (PO) lineages of the ice goby Leucopsarion petersii. Our common garden experiments revealed that the JS lineage had rapid growth rates, and consequently evolved towards a larger size. Divergence in female mate choice patterns was also detected by within- and between-lineage mate choice experiments. The JS females demonstrated preferences for larger and same-lineage males, whereas the PO females did not. However, our genetic analysis of a contact zone population showed that such phenotypic divergence did not appear to contribute to reproductive isolation. Thus, the present study clarified the evolution of endemic phenotypes in this unique sea basin, together with the position of the diverging pair on the speciation continuum.