Genetic and Environmental Contributions to Geographic Variation in the Ovipositor Length of a Cricket

Abstract

Females of many insects posses an ovipositor, which in crickets is usually used to place eggs deep into the soil. Ovipositor length in female striped ground crickets (Allonemobius socius) increased at higher latitudes and altitudes in North America, despite an opposing trend in overall body size. Crickets inhabiting regions with short growing seasons and long, cold winters invariably had longer ovipositors than crickets inhabiting warmer regions. The only exception to this observation was for first generation crickets in bivoltine populations where there was no pattern of geographic variation in ovipositor length. However, second generation females from these same populations always had longer ovipositors than their first generation ancestors,and followed a pattern of geographic variation similar to that found for univoltine crickets. Overall, these results support Masaki's (1986) model, which predicts that longer ovipositors will evolve in northern cricket populations as an evolutionary response to the selective advantages of overwintering eggs being oviposited deep in the soil in cold habitats. In order to determine the mechanisms underlying patterns of phenotypic variation in the wild, and to test the hypothesis that interpopulation variation reflects genetic differentiation while intergeneration variation within a population reflects environmentally induced phenotypic plasticity, crickets from eight populations were brought to the laboratory and reared through three generations in a common garden. It was found that laboratory—reared populations varied significantly in ovipositor length, with the pattern of variation mimicking that found for field—collected crickets. This finding supports the hypothesis that geographic variation in the wild has a significant genetic basis. The influence of environment on ovipositor length was examined in 20 full—sib families from a bivoltine population. Crickets reared in "autumn—like" conditions (30°C, 15:9 h, L:D), such as might be experienced by second generation bivoltine crickets in the wild, were generally larger and had significantly longer ovipositors than siblings reared under "spring—like" conditions (27°C 11:13 h, L:D). This pattern corresponds to that observed for bivoltine populations in the wild and suggests the importance of environmental cues for phenotypic plasticity, or polyphenism in ovipositor length in bivoltine populations of this cricket.