In many organisms, environmental temperature is a key determinant of the reproductive season, although physiological thermal responses related to reproduction often vary among taxa. Comparative analyses of reproductive seasonality within and between species can provide insights into the physiological constraints and local adaptation of reproductive traits in wild populations. This study examined wild populations of medaka (Oryzias latipes species complex). Although the photo-thermal thresholds for gonadal maturation in this species have been studied extensively in laboratory settings, geographical patterns of seasonality in gonad maturation, fecundity, and recruitment have not been thoroughly investigated in wild populations. We examined seasonal changes in gonad weight and the proportions of juveniles in wild populations of medaka at different latitudes. Our findings showed that seasonal changes in gonad weight were more pronounced in a high-latitude population in Aomori compared to a low-latitude population in Okinawa, irrespective of sex. Annual average proportions of juveniles tended to be higher in the Okinawa population compared to the Aomori and Chiba populations, suggesting a shorter recruitment period in high-latitude populations. A comparison of body size and egg number indicated that fecundity was relatively higher in high-latitude populations. We discuss whether gonad maturation at high latitudes can be explained by seasonal changes in temperature. Interpopulation differences in the gonadosomatic index and fecundity support the adaptive evolution of increased reproductive investment in high-latitude environments with shorter reproductive seasons. Additionally, trade-offs between survival and reproduction may influence ecological longevity and annual recruitment dynamics in these populations.

In fishes, maternal size typically influences the number of offspring. Although this size-fecundity relationship often varies among species and is considered to be a consequence of the coevolution of life-history traits, the genetic basis of such size-fecundity relationships remains unclear. We explored the genetic basis underlying this size-fecundity relationship in two small medaka species, Oryzias latipes and O. sakaizumii. Our findings showed that O. sakaizumii has a higher fecundity than O. latipes, and quantitative trait locus analysis using interspecific F2 hybrids showed that chromosome 23 is linked to the size-fecundity relationship. In particular, the genes igf1 and lep-b in this region are known to be associated with life-history traits. Because O. sakaizumii is distributed at higher latitudes and has a shorter spawning season than O. latipes in the wild, we propose that adaptation to high latitudes is responsible for the relatively high fecundity observed in O. sakaizumii. We also discuss the potential ecological ramifications by the evolution of increased fecundity in this species.

Single nucleotide variations (SNVs) in the nuclear genome have been used widely to analyze phylogenetic and population genomic structure. Cost-effective genotyping can be achieved by sequencing PCR amplicons using short 3-10 base sequences as primers to arbitrarily amplify thousands of sites in the genome using only a few primers. While previous methods have produced an insufficient number of SNVs to perform population genomic analyses, we designed a new primer set to improve the sequencing efficiency. To demonstrate the effectiveness of our method, we examined the population genetic structure of the small freshwater fish, medaka (Oryzias latipes). Specifically, we attempted to reconstruct the genetic admixture of the orange mutant strain, Himedaka. Although the strain is widely kept as an ornamental fish and for experimental purposes, the genetic background of the nuclear genome of commercial stock is less clear. We obtained 2987 informative SNVs with no missing genotype calls for 67 individuals from 15 wild populations and three artificial strains using the HiSeq X platform. The estimated phylogenic and population genetic structures of the wild populations were consistent with previous studies, corroborating the accuracy of our genotyping method. Admixture analysis focusing on Himedaka showed that at least two wild populations contributed SNVs to the nuclear genome of this mutant strain. Population genomics analyses based on nuclear SNVs data are indispensable to identify admixture events, including natural hybridization and anthropogenic introductions. The method developed in this study will be useful for future population genomics studies on medaka and on other organisms.