Phenotypically plastic traits regulate caste formation and soldier function in polyembryonic wasps

Abstract

Polyembryonic encyrtid wasps are parasitoids that have evolved a clonal form of embryogenesis and a caste system where some progeny become reproducing wasps whereas others develop into a sterile soldier caste. Theory based on the biology of Copidosoma floridanum predicts that the primary role of soldier larvae is to mediate conflict over sex ratio, which also favours female‐biased soldier production. Other data, however, suggest that female‐biased soldier production reflects a developmental constraint. Here, we assessed whether female‐biased soldier function by polyembryonic wasps reflects sex‐specific adaptation or constraint by conducting comparative studies with Copidosoma bakeri, a species that produces clutch sizes similar to C. floridanum yet rarely produces broods associated with sex ratio conflict. Our results indicate that the oviposition behaviour of adults, development of progeny and function of soldier larvae differ greatly between C. bakeri and C. floridanum. These findings indicate that caste formation and soldier function in polyembryonic encyrtid wasps are regulated by phenotypically plastic traits. Our results further suggest that the primary function of the soldier caste in some species is defence of host resources from competitors whereas in others it is the resolution of sex ratio conflict.

Male soldiers are functional in the Japanese strain of a polyembryonic wasp

“…However, in hymenopteran insects, sterile male caste is only known in polyembryonic parasitic wasps. Male caste formation in these polyembryonic parasitoids has been reported well in the genus Copidosoma and other closely related Encyrtid genera12. These species produce larval soldiers in both sexes.…”

“…1B). In a related polyembryonic species Copidosoma bakeri , which produces mostly single-sex broods, the soldiers of both sex are shown to attack heterospecific parasitoid larvae2.…”

“…Thus the copulation success of both male and female wasps emerging from mixed broods is guaranteed. In these mixed broods, functional male soldiers become non-adaptive or even maladaptive, if they attack reproductive female embryos that are the future mates of their own male broods25. This problem of conspecific attack is evident in the Japanese strain, where there are functional male soldiers.…”

The metabolic enzyme phosphoglucose isomerase (Pgi) affects the outcome of intra‐specific competition in a polyembryonic wasp

“…Copidosoma floridanum and other polyembryonic wasps in the family Encyrtidae are fascinating because they have evolved a caste system in which soldiers mediate competition within hosts between offspring from different females (superparasitism) and the sex ratio of mixed-sex broods (Cruz, 1981;Grbic et al, 1992;Harvey et al, 2000;Giron et al, 2004Giron et al, , 2007bSmith et al, 2010). Copidosoma floridanum and other polyembryonic wasps in the family Encyrtidae are fascinating because they have evolved a caste system in which soldiers mediate competition within hosts between offspring from different females (superparasitism) and the sex ratio of mixed-sex broods (Cruz, 1981;Grbic et al, 1992;Harvey et al, 2000;Giron et al, 2004Giron et al, , 2007bSmith et al, 2010).…”

“…Polyembryonic encyrtids are egg-larval parasitoids whose hosts are primarily Lepidoptera. Each egg a female wasp lays undergoes a protracted period of clonal development, which results in the formation of multiple, genetically identical embryos (Harvey et al, 2000;Donnell et al, 2004;Smith et al, 2010). Remarkably, these clonal embryos develop into two morphologically and functionally distinct larval castes called soldier and reproductive larvae.…”