Egg size and reproductive allocation in eusocial thrips

Reproductive allocation, in terms of fecundity and egg size, has been given little consideration in eusocial societies. To begin to address this, absolute and body size-adjusted egg volumes were compared, along with fecundity, between the foundress and her subfertile soldier offspring in the eusocial, gall-inducing thrips, Kladothrips hamiltoni, Kladothrips waterhousei, and Kladothrips habrus, and a congeneric, Kladothrips morrisi, with fully fecund soldiers. Soldiers produced significantly larger eggs than the foundress in all species except K. morrisi, where egg volumes did not differ. After accounting for body size, soldiers produced significantly smaller eggs than the foundress in K. morrisi and marginally so in K. waterhousei, but egg sizes did not differ in K. hamiltoni and K. habrus. When egg size and fecundity data are combined, K. morrisi soldiers invest less in reproduction than the foundress, and in conjunction with other life-history features the species can be considered eusocial. Maximum likelihood analyses reveal relatively low reproductive allocation skew in the ancestral lineages and high skew in the derived lineages, but the trend is not significant when fecundity and egg size are considered separately. Gall size covaried negatively with soldier-to-foundress relative body size-adjusted egg size and reproductive allocation and marginally so with fecundity, suggesting that gall size is a determinant of egg size and fecundity trade-offs in eusocial thrips and providing the strongest support to date that gall size has featured in the social evolution of this clade. This study highlights that data on fecundity alone may be insufficient for assessing reproductive division of labor.