The evolution of polyembryony in parasitoid wasps

Polyembryony is a unique mode of development that involves the production of several genetically identical embryos from a single egg through clonal division. This development style appears to carry a high price tag, because it clones an unproven genotype at the expense of genetic diversity in a brood. Polyembryony, nevertheless, has evolved independently in four families of parasitoid wasps. We review three main hypotheses for the selective forces favoring this developmental mode in parasitoids: polyembryony (i) reduces the costs of egg limitation; (ii) reduces the genetic conflict among offspring; and (iii) allows offspring to adjust their numbers to the quality of the host. Using comparative data and verbal and mathematical arguments we evaluate the relative importance of the different selective forces through different evolutionary stages and in the different groups of polyembryonic wasps. We conclude that reducing the cost of egg limitation is especially important when large broods are favored. Reducing the genetic conflict is mostly important when broods are small, thus might have been important during, or immediately following, the initial transition from monoembryony to polyembryony. Empirical data provide little support for the brood-size adjustment hypothesis, although it is likely to interact with other selective forces favoring polyembryony.