Towards a universal understanding of sex ratio in termites

Termites are eusocial cockroaches whose altruist caste is constituted of males and females. While sex ratio theory predicts a balanced investment between sexes in diploid organisms, extreme deviations are observed in termites, both in altruists and alate reproductives. Here, we expand the theoretical framework for the prediction of alate population sex ratio by considering partitioned sexual and parthenogenetic reproduction, and female/male relatedness asymmetries arising from their sex-linked chromosome complexes. We consider the viewpoint of either the primary reproductives or the altruists while accounting for the effect of caste developmental systems on the sex ratio. We compile all data on alate sex ratios available to date (97 species), and found the direction of the sex ratio bias to be consistent within major taxonomic groups. We test our models, along with models of intrasexual competition, on an exploratory set of 13 species with available demographic data. Our analyses indicate that the factors explaining bias in alate sex ratio are variable and include sexual dimorphism, sex-asymmetric inbreeding, imperfect use of sexual and parthenogenetic reproduction, sex-linked genomic inheritance, intrasexual competition and caste developmental constraints. Our study provides an integrative framework for sex ratio and conflicts in termites, and closes in on a universal theory.