Seasonal adaptation is essential for insects to expand their habitat. Some insects that overwinter in the temperate zone to the subarctic zone induce a physiological state called reproductive diapause to adapt to the winter season. The insects that induce reproductive diapause for overwintering suppress the development of eggs or sperm in their reproductive organs. These insects are likely to be sensing the changes in temperature and day length as signals to determine the timing of diapause induction. Short-day conditions could allow insects to predict seasonal changes and induce diapause at optimal timing, depending on the latitude. In general, temperature affects photoperiodic responses. Thus, it is likely to interact with day length to determine the timing of diapause.
Drosophila triauraria, as well as some other species of Drosophila, has been observed to induce reproductive diapause for overwintering in the field. A previous study reported that the isofemale strain of D. triauraria from a high-latitude locality (42.0°N, 140.4°E) in northern Japan induced reproductive diapause in a short-day condition. However, two strains from southern Japanese islands (27.2°N, 128.3°E and 26.2°N, 126.5°E) did not. These results have suggested that D. triauraria is adapted to the regional climate. However, comparing the diapause conditions of only those strains does not tell us how this species has genetically adapted to winter in a wide range of latitudes. Therefore, it is necessary to examine the diapause conditions and to perform genomic analyses using strains from various localities of the Japanese archipelago.
In this study, we first investigated the diapause phenotype in the females of 21 strains originating from various localities under long- and short-day photoperiod at 12°C, which is just above the critical temperature for growth. The strains originated from mid-latitude localities (36.1°N, 140.1°E to 31.5°N, 131.3°E) induced reproductive diapause regardless of the day length. In contrast, the strains from southern islands did not induce reproductive diapause regardless of the day length. Interestingly, the strains from high-latitude localities (42.6°N, 141.6°E to 42.0°N, 140.4°E) went into reproductive mode under the long-day photoperiod, while they induced diapause under the short-day condition. These results indicate that the response of this species to low temperature and day length differs significantly among regional populations. The strains from northern Japan may be adapted to take an opportunity to reproduce even at a low temperature if the day length is long. Thus, the critical photoperiod to induce reproductive diapause at a given temperature may vary among the regional populations.
Next, we compared the genomes of all strains to investigate the genomic regions involved in geographical variation in reproductive diapause. The analyses revealed that the degree of divergence between regional populations is small even though the genetic variation for the diapause trait is differentiated. We also found that some genomic regions are highly differentiated between the strains from low-latitude island populations that did not induce diapause and those from the rest that induced diapause at 12°C.