Unmatedness promotes the evolution of helping more in diplodiploids than in haplodiploids

The predominance of haplodiploidy (where males develop from unfertilized haploid eggs and females from fertilized diploid eggs) among eusocial species has inspired a body of research that focuses on the possible role of relatedness asymmetries in the evolution of helping and eusociality. Previous theory has shown that in order for relatedness asymmetries to favor the evolution of helping, there needs to be variation in sex ratios among nests in the population (i.e., split sex ratios). In haplodiploid species, unmated females can produce a brood of all males, and this is considered the most likely mechanism for split sex ratios at the origin of helping. In contrast, in diploidiploids unmatedness means total reproductive failure. We compare the effect of unmatedness on selection for male and female helping in haplodiploids and diplodiploids. We show that in haplodiploids, unmatedness promotes helping in females but not in males within the empirical range. In diplodiploids, unmatedness promotes helping by both sexes, and the effect is stronger than in haplodiploids, all else being equal. Our study highlights the need to consider interactions between ecological and genetic factors in the evolution of helping and eusociality.