Differential maternal allocation following mixed insemination contributes to variation in oocyte size in a sea squirt

Abstract

In many organisms, offspring fitness is markedly affected by size at hatching. Theoretical models predict offspring from one clutch to have the same optimal size, but empirical studies report within-clutch variation in offspring sizes for a wide range of species. According to the differential allocation hypothesis, this could be explained, at least in part, by multiply mated females selectively provisioning their offspring depending on the sperm donor. We tested this hypothesis in the internally fertilising, colonial sea squirt Diplosoma listerianum, whose larvae are non-feeding so that females' provisioning of oocytes provides all of the energy for larval dispersal and metamorphosis. We show that there is high within-clutch variation in offspring size in a natural D. listerianum population. When we then crossed laboratory clones pairwise in a fully factorial design in the laboratory, we found that the same acting female clone produced differently sized oocytes with different mating partners. This result does not reflect the effect of direct male benefits, as the water-borne sperm cell is the only contact between males and females, but nevertheless indicates that females allocate resources to their offspring in part influenced by the father's identity. This pattern would be expected to contribute to within-clutch variation in offspring size in mixed-paternity broods. © Inter-Research 2011.