Evolutionary trade-offs in a small mammal : a quantitative genetics approach

Abstract

Limited resources force individuals to trade-off between life-history traits. A vast diversity of life-history strategies, which optimally combine life history traits, can be found. Knowledge of the genetic basis of this phenotypic variation is key to understanding evolutionary processes. I approached life-history evolution by estimating quantitative genetic parameters for a set of life-history traits in the bank vole (Myodes glareolus). The work is based on a laboratory-kept colony subjected to two-way selection for litter size (High- (H) and Low- (L) lines). Costs of the created reproductive strategies were observed in natural conditions. Selection increased the difference in litter size between lines, even though the response was asymmetric. The difference persisted in outdoor enclosures. A life-history trade-off between offspring number and offspring size was explained by environmental correlations as associated genetic correlations were even positive. Lines did not diverge in offspring size. In the enclosures maternal body mass had greater effect on pup survival in the L-line. Litter and maternal environment explained most of the phenotypic variation in pup body mass during the nursing period, while direct genetic variation emerged later, implicating selection on early size in the bank vole. Male and female metabolic rate did not seem to share genetic background; the first lacking additive genetic variation and the latter being bound to fecundity by genetic correlation close to one. Testosterone, a principle determinant of male reproductive success in the bank vole, was contrasted with humoral immune system in the both sexes. In general, with the exception of metabolism, the estimated genetic parameters did not set absolute constraints for life-history evolution. However, dimensionality of the additive genetic co(variance) matrix constrains evolution to act along certain trait combinations. In the future, research should strive to find genetic mechanisms, which, through the endocrine system, produces phenotypic co(variation) in different life-history traits.