Genetic and Maternal Influences on Life History Plasticity in Response to Photoperiod by Milkweed Bugs (Oncopeltus fasciatus)

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Abstract
Using two geographically adjacent, temperate-zone populations of the milkweed bug Oncopeltus fasciatus, we employed a split-family experimental design to estimate genotype-environment interaction (g-e) and across-environment genetic correlations for two female life history characters, age at first reproduction and fecundity over the first 5 days of reproduction. For each of a number of families, some sisters were reared in a short-day environment (LD 11:13, 23.degree. C) and others in a long-day environment (LD 14:10, 23.degree. C); the environments were chosen to simulate the temporal environmental heterogeneity experienced by lineages of temperate-zone bugs. Bugs normally exhibit a plastic response to such heterogeneity by delaying the onset of reproduction in short days relative to that in long days, the delay providing time for migration to occur. In one population, there is significant variation caused by g-e for the age at first reproduction, indicating the existence of genetic variance for plasticity in this character. This result suggests that there is potential for the evolution of phenotypic plasticity in the timing of reproduction. In each population, however, there is a positive genetic correlation between the timing of reproduction in the long- and short-day environments, implying that the evolution of life history in one environment can influence evolution in the alternative environment. In particular, the potential rate of evolution toward a joint optimum of early reproduction in long days and delayed reproduction in short days will be less than if the across-environment correlation were zero or negative. During the attainment of equilibrium at the joint optimum, selection can prevent optimization of the age at first reproduction and maintain genetic variation for this character within environments. However, a maternal effect that prevents long delays in the onset of reproduction from occurring in two successive generations of bugs provides a means for lessening the influence of selection in the alternative environments. This maternal effect also ensures that a spring generation of bugs does not exhibit an inappropriate diapause. For fecundity, one population has a positive across-environment genetic correlation, indicating that performance generalizes across environments for this fitness component. In the other population, primarily because of the performance of one sibship, a negative across-environment correlation implies that performance is specialized. The populations also differ in life history. The basis for such microgeographic differentiation, the first recorded for migratory temperate-zone populations of this species, may be sampling, differential migration, or adapation. More work is necessary to distinguish among these hypotheses. Consistent with previous studies, we found high levels of genetic variation for life history characters within populations and, in general, a lack of genetic correlation between the age at first reproduction and fecundity.