Genetic and phenotypic sources of life history variation along a cline in voltinism in the cricket Allonemobius socius

Clinal variation in life histories can be genetically based, resulting from selection imposed by different environments, or it may be due to the differential expression of phenotypically plastic traits. We examined the cline in voltinism in the egg-diapausing cricket Allonemobius socius, with populations spanning the switch from a univoltine to a bivoltine phenology. A common garden experiment was employed, using environments that mimicked photoperiod and temperature conditions found in the field. There were only small differences in development time among populations, and the difference in phenology observed in the field is likely due to clinal variation in the length of the growing season. We found large genetically-based differences in the reaction norm for egg diapause that were further magnified by environmental cues. The synergism of genetic and environmental effects was an example of cogradient selection. In the zone of transition between phenologies, voltinism appeared to be a conditional strategy, rather than a genetic polymorphism. First-generation females from this area can lay either direct-developing or diapause eggs depending on the likelihood that a second generation will have sufficient time to develop. For this species, the cline in voltinism is the result of a combination of environmental effects on development, and genetic and environmental influences on egg diapause propensity.