Hybridization enables the fixation of selfish queen genotypes in eusocial colonies

The success of a eusocial colony depends on two main castes: queens that reproduce and sterile workers that help them. This division of labour is vulnerable to selfish genetic elements that enforce the development of their carriers into queens. Several factors, e.g. intra-colonial relatedness, can influence the spread of such selfish elements. Here we investigate a common yet understudied ecological setting: where hybrid larvae can develop into workers. Using mathematical modelling, we show that the coevolution of hybridization with caste determination readily triggers an evolutionary arms race between larvae that increasingly develop into queens, and queens that increasingly hybridize to produce workers. Even where hybridization reduces fitness, this race can lead to the loss of developmental plasticity and genetically hard-wired caste determination. Overall, our results help understand the repeated evolution towards complex reproductive systems (e.g. social hybridogenesis) and the special forms of parasitism (e.g. inquilinism) observed in many eusocial insects.