イチモンジセセリ Parnara guttata guttata の卵サイズおよびその表現型可塑性の変異に関する研究

In this study, we examined the adaptive significance of egg size plasticity and the genetic variations of egg size and its reaction norms to day-length and temperature in the skipper butterfly Parnara guttata guttata (Lepidoptera : Hesperiidae), which shows a large phenotypic variation in egg size among generations. First, larvae were reared under the typical conditions of temperature and photoperiod experienced during the immature stages in the first, second, and third (overwintering) generations (LD 16 : 8 at 25℃, LD 14 : 10 at 25℃, LD 14 : 10 at 20℃). Females reared under LD14 : 10 at 20℃ produced more and smaller eggs, on the other hand, those reared under LD14 : 10 at 25℃ produced larger but fewer eggs. The trend was consistent with the difference of egg size among generations, though females laid relatively larger eggs in this study than those observed in the field. Fecundity was highest under LD 14 : 10 at 20℃ followed by LD 16 : 8 at 25℃ and LD 14 : 10 at 25℃. Survival rates of first instar larvae derived from females reared under the three photoperiod/temperature treatments were measured on young soft rice leaves, or tough, old rice leaves. Survival rates of hatchlings reared on soft and tough leaves did not differ when females were reared under LD16 : 8 and LD14 : 10 at 25℃. However, hatchling survival rate was significantly higher on soft than on tough leaves when females were reared under LD14 : 10 at 20℃. These results suggest that the plasticities of egg size in response to day-length and temperature are adaptive, and temperature and day-length are important factors as environmental cues to make the optimal reproductive allocation between size and number of eggs in each generation of P. g. guttata. Second, life history traits of P. g. guttata hatched from large-sized and small-sized eggs were compared to determine the influence of egg size differences on fitness under a fixed condition. Eggs were grouped into two sizes, larger than 0.19mm^3 (L group) and smaller than 0.16mm^3 (S group). Larvae from these groups were reared individually under LD16 : 8 at 25℃. The survival rate in immature stages was significantly higher in the L group than in the S group. The developmental period during immature stages of the L group tended to be shorter than that of the S group. These findings suggest that egg size in P. g. guttata is an important element in the adaptation not only between generations but also within a generation. Third, genetic and environmental sources of egg size were examined. Phenotypic and genetic correlations and heritabilities were estimated for these traits under different day-length and temperature conditions. Egg size had relatively high heritabilities. Negative phenotypic and genetic correlations between egg size and fecundity were estimated in treatments correspond to the natural conditions during larval development of the first and second generations. Next, genotype-environment interactions were estimated by comparing reaction norms to day-length or temperature of these traits among families. Mixed-model ANOVAs showed significant genotype-environment interactions in egg size and forewing length of both sexes for day-length and temperature. These results suggest that large genetic variations for egg size and its reaction norms to day-length or temperature have been maintained in a population of P. g. guttata. We discuss the possibility of maintaining genetic variation for egg size and its reaction norms in relation to antagonistic pleiotropy, environmental heterogeneity and other factors.