Social environment and indirect genetic effects on life history and behavioural traits in mammals

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Finding the causes for variation among individuals in behaviours and other traits has been, even before Darwin, one of the central quests in evolutionary biology. Not only will an answer to this question help us understand why individuals differ, but can also identify environmental factors that will affect the expression of traits ranging from behavioural, developmental to complex diseases phenotypes. With knowledge about the underlying genetics, one may be able to predict to some degree how organisms develop and respond to changing environmental conditions. It has been known for almost a century that individuals can differ due to differences in their genes (direct genetic effects due to genotype) or the environment they experience. While the significance of interactions between genotype and environmental conditions has been recognized and investigated, much less appreciated is the fact that the environment is only partly physical (e.g. temperature, habitat etc.). Indeed, most animals, including humans, are exposed to a social environment of interacting individuals. During the most influential period in early development these comprise siblings and parents for most mammals. A very important insight is that effects of the social environment can have a genetic basis in that genes expressed in individuals with whom you interact affect the expression of your traits or characters. The rationale behind these so-called indirect genetic effects is easy to see in the case of mothers but it equally applies to siblings. Understanding how much variation in phenotypes is due to genes expressed in interacting individuals is pivotal to most traits characteristic of socially living animals and is the topic to be addressed by the proposed research. Yet, only with information about the underlying mechanisms do we truly gain a comprehensive picture of how indirect genetic effects work. For example, if we wish to explain variation among individuals in e.g. growth or a behavioural trait, a result of an indirect genetic analysis might show that a significant proportion of this measure is dependent on the genes expressed in siblings, which affects their competitive behaviour towards siblings and, as a consequence, the phenotype in the focal individual. This project aims to unravel the mechanisms by which indirect genetic effects work using an analysis that not only takes data from the current project but also from previously conducted studies using the same model system. This systems-genetics approach is only possible with such a large genetic reference panel as the BXD mouse system, for which genetic variation is defined and which has been extensively used in other studies. From an evolutionary perspective, indirect genetic effects are important because the social environment can respond to selection and evolve (since there is a genetic basis to it). Further, interactions between direct and indirect genetic effects might facilitate or constrain trait evolution. Models about how trait expression changes due to selection may thus yield quite different predictions of evolutionary dynamics and outcomes depending on whether indirect genetic effects are considered or not. Clearly, as recent theoretical research shows, we have good reasons to assume that we need to consider indirect genetic effects to advance our understanding of the causes of variation in complex phenotypes.