Chimerism and altruism

Chimerism spans the tree of life, from mammals and corals to plants and fungi. In such organisms, individuals contain within them cells and genomes from another once distinct member of the population. This chimeric genetic composition may subsequently alter patterns of relatedness not only between those individuals, but also within them. Consequently, we may expect unique patterns of social behaviour in such species. To explore the social evolutionary consequences of chimerism, here we develop a kin-selection model of a structured population. First, we show how somatic and germline chimerism influence patterns of relatedness and play an important role in modulating social behaviour. Specifically, we find that increased heterogeneity of the soma relative to the germline boosts the opportunity for altruism between individuals. We then explore how differences in chimerism levels within the body may generate within-organism differences in the valuation of social partners and thus foment internal conflicts between tissues and organs. Finally, we show how differences in the development of male and female germlines in chimeras provides a novel source of relatedness asymmetry between maternal-origin and paternal-origin genes. Overall, we find that chimerism introduces additional opportunities for internal conflicts over the development of behavioural phenotypes, most of which have been unexplored by empiricists.