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 exam ined the dine in voltinism in the egg-diapausing cricket Allonemobius socius, with populations spanning the switch from a univoltine to a bi voltine phenology. A common garden experiment was employed, using envi ronments that mimicked photoperiod and temperature conditions found in the field. There were only small differences in development time amon g populations, and the difference in phenology observed in the field i s 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 sy nergism of genetic and environmental effects was an example of cogradi ent selection. In the zone of transition between phenologies, voltinis m appeared to be a conditional strategy, rather than a genetic polymor phism. First-generation females from this area can lay either direct-d eveloping or diapause eggs depending on the likelihood that a second g eneration will have sufficient time to develop. For this species, the dine in voltinism is the result of a combination of environmental effe cts on development, and genetic and environmental influences on egg di apause propensity.