Read/Search this Article
Temperate species of the Drosophila melanogaster species group enter reproductive diapause for overwintering in response to short daylength. During the prediapause period, they accumulate triacylglycerols (TAGs) as energy resources for winter. Under laboratory conditions, the capacity for storing TAGs differs in different species, and appears to be closely correlated with diapause and cold-hardiness; cool-temperate species, such as those of the auraria species complex which enter a deep diapause and are highly cold-hardy accumulate a larger amount of TAGs than warm-temperate species, such as D. rufa and D. lutescens which enter a weak diapause and are less cold-hardy. On the other hand, a subtropical species, D. takahashii, which has no diapause in nature and is not cold-hardy, is unable to store as much TAGs as the temperate species. These species were tested winter survival and TAGs content under outdoor conditions in Sapporo (a cool-temperate region) , northern Japan. In the strains of cool-temperate species from northern Japan, individuals which eclosed in mid autumn accumulated TAGs up to 163 μg/ mg body weight and 50 to 70% of them survived until spring, while those which eclosed later in autumn accumulated less TAGs and had a lower ability to overwinter. The TAG content was lower in the warm-temperate species and the subtropical strain of D. triauraria, and dropped to very low levels by mid winter, and these species and strain were unable to survive until spring. These observations suggest that TAG level plays an important role in overwintering of the Drosophila species. In addition, differential scanning calorimetry analysis revealed that the transition temperatures of TAGs were lower in diapausing adults than in reproducing ones, and also lower in species or strains adapted to cooler climates than those adapted to warmer climates. These phenomena were correlated to the fatty acid compositions of the TAGs. Furthermore, in the temperate species of the montium species subgroup (D. subauraria, D. biauraria, D. triauraria and D. rufa) , the amount of saturated TAGs was smaller than the value expected on the assumption that fatty acids are randomly distributed in the TAGs, suggesting the non-random distribution of unsaturated fatty acids among TAGs. This may facilitate the lowering of the transition temperature of TAGs, and hence may be related to the ability of Drosophila to cope with temperate climates.