Egg size-mediated sex allocation and mating-regulated reproductive investment in a haplodiploid thrips species

The partitioning of resources between male and female offspring is a fundamental aspect of the reproductive biology of animals. Sex allocation can be adjusted by controlling offspring sex ratio or by sex-specific resource allocation towards individual eggs. The latter is well-known in birds and reptiles, but less studied in other organisms, especially haplodiploids with females that can adjust offspring sex via fertilization control. Here, we investigated whether egg size can affect sex allocation in a haplodiploid thrips, Pezothrips kellyanus. First, we established that larger eggs are more likely to be fertilized and become females than smaller eggs, which remain unfertilized and become males. This size-dependent fertilization may be regulated by anatomical constraints that allow sperm access only to larger eggs. Furthermore, across both sexes, larger eggs developed into offspring with higher fitness, suggesting that egg size is a reliable proxy for resource allocation in this species. During the early phase of reproduction, mating increased investment by females towards egg size thereby facilitating fertilization success and increased female offspring survival. However, this came with a reduced lifetime fecundity, reflecting that mating manipulates reproductive strategies in favour of increased daughter production. This may be in conflict with the optimal offspring sex ratio of females which is theoretically less female-biased than for males. We detected both egg size-mediated sex allocation and a mating effect on resource allocation under both laboratory and field conditions, suggesting that they are maintained by natural selection. We provide evidence for a finely tuned egg size-dependent fertilization mechanism that mediates sex allocation strategies and is moderated by mating. Furthermore, our findings establish a new perspective about potential effects of sexual conflicts on egg size and sex allocation. A free Plain Language Summary can be found within the Supporting Information of this article.