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.

Fig. 1

C. bakeri embryos can be sexed by karyotyping. (a) Phase-contrast micrograph of a C. bakeri polymorula dissected from a fifth instar host. The polymorula consists of thousands of embryos at this stage of development. A majority of embryos are rounded, 80–100 µm in diameter, and will develop into reproductive caste larvae (R). Interspersed among these embryos are embryos in the process of developing into soldier caste larvae (S). Scale bar= 1 mm. (b) Lacto-orcein stained chromosome spreads from a male polymorulae. (c) Lacto-orcein stained chromosome spreads from a female polymorulae. Scale bar in C= 30 µm

Fig. 2

Male and female broods produce similar numbers of soldiers and develop in the same location in hosts. (a) Mean number of soldier larvae per male and female brood (N=30 female and 30 male broods; t=0.43; P=0.67) in the host fifth instar. (b) Location of polymorulae in fifth instar hosts. Location was scored as being either connected to tracheal branch in the thorax or abdomen of the host. The proportion of polymorulae developing in the host thorax did not differ by brood sex (N=30 for each brood type; two-tailed binomial test, P=0.7). (c) Phase-contrast micrograph of a soldier caste larva. (d) Phase contrast micrograph of a newly hatched reproductive caste larva. Scale bar in (d)= 50 µm.