Some freshwater organisms have wide geographic ranges despite their habitats are discontinuous. Medaka fishes is a typical example of such organisms, and many species have wide ranges. Some medaka species are known to be euryhaline, and it is considered to be a reason for their wide distributions; they could disperse from one freshwater system to another through brackish areas. However, it is unknown whether or not the salinity tolerance is really a determinant of the distributions in medaka fishes. In this study, we compare the salinity tolerance between a species inhabiting multiple river systems and having a wide geographic range (Oryzias celebensis) and a species that inhabits only a single river system (O. dopingdopingensis), both of which are riverine and endemic to Sulawesi Island. Laboratory experiments revealed that the widely distributed species has high salinity tolerance as in other congeners, while the narrowly distributed species were less tolerant. Interestingly, the narrowly distributed species appears to have almost completely lost salinity tolerance, even though they are expected to have the same opportunity to be exposed to seawater as other riverine species. In the presentation, we will show a result of quantitative trait locus mapping for loci responsible for the stenohalinity and discuss the reason of the loss of salinity tolerance.
Keywords: Euryhaline, QTL mapping, riverine species, species range, stenohaline.