A review and perspective on context-dependent genetic effects of extra-pair mating in birds
Résumé
The evolutionary origin and the maintenance of extra-pair mating in birds has been a major field of study in the last decades, but no consensus has been reached on the adaptive significance of this behaviour for female birds. The genetic benefit hypothesis proposes that extra-pair sires provide alleles of superior quality and/or better compatibility compared to the social mate, resulting in offspring of higher reproductive value. One frequently adopted approach to test this idea compares the performance of maternal half-siblings in broods with multiple paternity. However, results from such comparisons are inconsistent. Here I discuss the concept that the magnitude of genetic fitness benefits from extra-pair mating depends on the environmental context. To date, context-dependent genetic effects in maternal half-sibling comparisons have been demonstrated for only five passerine bird species. In none of the studies were the crucial environmental conditions experimentally manipulated, and the potentially confounding effects of differential maternal investment in relation to paternity were also largely not accounted for. A number of high-quality data sets on fitness consequences of extra-pair mating behaviour are available that could be (re-) analysed for context-dependence given that relevant gradients of the environment have been recorded and their use is well justified a priori. Such relevant variation may include, for example, the time of breeding in temperate regions, hatching order, but also offspring sex. Primarily, however, experimental approaches are required that systematically and gradually vary fitness-relevant environmental gradients, such as food availability or parasite abundance, and analyse the resulting differential fitness effects while controlling for differential investment. The context dependency of the genetic effects of extra-pair mating behaviour may offer an opportunity for reconciling conflicting results from different extra-pair paternity studies within and across species. More generally, it could allow a better understanding of under which environmental conditions will selection act to maintain a female mating bias towards extra-pair males with potentially far-reaching implications for the ecology and evolution of mating preferences and the maintenance of genetic variation in (sexually) selected traits.
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