Seasonally varying diet quality and the quantitative genetics of development time and body size in birch feeding insects

Genetic variance-covariance structures (G), describing genetic constraints on microevolutionary changes of populations, have a central role in the cur rent theories of life-history evolution. However, the evolution of Gs in na tural environments has been poorly documented. Resource quality and quantit y for many animals and plants vary seasonally, which may shape genetic arch itectures of their life histories. In the mountain birch-insect herbivore c ommunity, leaf quality of birch for insect herbivores declines profoundly d uring both leaf growth and senescence, but remains stable during midsummer. Using six sawfly species specialized on the mountain birch foliage. we tes ted the ways in which the seasonal variation in foliage quality of birch is related to the genetic architectures of larval development time and body s ize. In the species consuming mature birch leaves of stable quality, that i s. without diet-imposed time constraints for development time, long develop ment led to high body mass. This was revealed by the strongly positive phen otypic and genetic correlations between the traits. In the species consumin g growing or senescing leaves, on the other hand, the rapidly deteriorating leaf quality prevented the larvae front gaining high body mass after long development. In these species, the phenotypic and genetic correlations betw een development time and final mass were negative or zero. In the early-sum mer species with strong selection for rapid development, genetic variation in development time was low. These results show that the intuitively obviou s positive genetic relationship between development time and final body mas s is a probable outcome only when the constraints for long development are relaxed. Our study provides the first example of a modification in guild-wi de patterns in the genetic architectures brought about by seasonal variatio n in resource quality.