Maternal body size affects sex allocation in a gregarious parasitoid

In parasitoid wasps, body size is a key predictor of reproductive success, as size is often correlated with increased gamete production. In Hymenoptera, due to haplodiploid sex determination, sperm are required to produce daughters but not sons, so sex allocation relies on a female's ability to obtain and manage a finite supply of sperm. Body size may, therefore, affect sex allocation if smaller males provide fewer sperm, or if larger, more fertile females deplete their sperm supplies by fertilizing more eggs. We examined the importance of male and female body size on offspring production and sex allocation in the parasitoid, Nasonia vitripennis (Walker). We predicted larger females would experience constrained sex allocation due to limited sperm, especially if mated with smaller males. We categorized 80 males and 80 females by size (large or small) into 4 mating groups: large females with large or small males, and small females with large or small males. We analyzed brood size and offspring sex ratios after each female oviposited on 12 pupae of Sarcophaga bullata (Parker). Overall, larger females produced more offspring and a greater proportion of males. Sex ratios became more male-biased with increased offspring production, indicating that sperm depletion had affected sex allocation in larger females. In contrast, male size did not affect sex allocation, but exhibited a small positive effect on females' offspring production. Our study highlights a complex relationship between body size, fecundity, and sex allocation, enhancing our understanding of reproductive strategies in haplodiploid organisms.

Sperm Limitation Produces Male Biased Offspring Sex Ratios in the Wasp, Nasonia vitripennis (Hymenoptera: Pteromalidae)

Haplo-diploid sex determination in the parasitoid wasp, Nasonia vitripennis (Walker), allows females to adjust their brood sex ratios. Females influence whether ova are fertilized, producing diploid females, or remain unfertilized, producing haploid males. Females appear to adjust their brood sex ratios to minimize 'local mate competition,' i.e., competition among sons for mates. Because mating occurs between siblings, females may optimize mating opportunities for their offspring by producing only enough sons to inseminate daughters when ovipositing alone, and producing more sons when superparasitism is likely. Although widely accepted, this hypothesis makes no assumptions about gamete limitation in either sex. Because sperm are used to produce daughters, repeated oviposition could reduce sperm supplies, causing females to produce more sons. In contrast, if egg-limited females produce smaller broods, they might use fewer sperm, making sperm limitation less likely. To investigate whether repeated oviposition and female fertility influence gamete limitation within females, we created two treatments of six mated female wasps, which each received a series of six hosts at intervals of 24 or 48 h. All females produced at least one mixed-sex brood (63 total broods; 3,696 offspring). As expected, if females became sperm-limited, in both treatments, brood sex ratios became increasingly male-biased with increasing host number. Interhost interval did not affect brood size, total offspring number, or sex ratio, indicating females did not become egg limited. Our results support earlier studies showing sperm depletion affects sex allocation in N. vitripennis¸ and could limit adaptive sex ratio manipulation in these parasitoid wasps.

Virginity and the clutch size behavior of a parasitoid wasp where mothers mate their sons

Theoretical and empirical research on the evolution of clutch size has proved to be an extremely productive area of evolutionary biology. A general prediction is that individuals should produce a smaller number of offspring when resources are more limited, such as when multiple individuals compete for the same resources for their development. However, we expect that the opposite prediction arises with virgin females of haplodiploid species, which are subject to extreme local mate competition. We test the key assumption and predictions of this theory with the parasitoid wasp Melittobia australica. Our data demonstrate that there is a trade-off between the size of the first and subsequent clutches and that virgin females adjust their production of sons according to the mating status (mated or not) of cofounding females. We also found that mated females facultatively change their offspring sex ratio in response to the mating status of cofoundresses. We discuss the potential mechanisms used to recognize the mating status and the implications of our results in the context of the extremely female-biased sex ratios observed across Melittobia species..

Information use in space and time: sex allocation behaviour in the parasitoid wasp Nasonia vitripennis

Behavioural decisions require the appropriate use of relevant information about the environment. However, individuals may have imperfect information, imposing a constraint on adaptive behaviour. We explored how information use influences the sex allocation behaviour of the parasitoid wasp Nasonia vitripennis in response to local mate competition. Optimal sex ratios under local mate competition require females to estimate the number of other females that contribute eggs to a patch. Females rapidly changed their sex allocation in response to changes in the number of females in the environment, suggesting that they are not constrained by how quickly they can respond to new information. Furthermore, females also showed some response to olfactory cues that indicated oviposition by other females, suggesting that such indirect cues may be part of their information repertoire. Both the absolute and the relative size of the patch were important for sex ratio decisions, with sex ratios declining on larger patches in a way that suggests that large patches in effect become more than one patch, with females on larger patches allocating sex increasingly independently of other females. We conclude by highlighting variation among species in whether particular cues are used for sex allocation.

Characterization of a female-produced courtship pheromone in the parasitoid Nasonia vitripennis

Males of the parasitoid Nasonia vitripennis (Walker) (Hymenoptera: Pteromalidae) show a characteristic courtship behavior. We demonstrate that male arrestment and key behavioral elements of the courtship sequence are mediated by a female-derived contact sex pheromone. Males were arrested on paper disks treated with female extracts but not on those treated with male extracts. Male responsiveness was influenced by the surface to which female extracts were applied. Extracts applied to an extracted beetle elytron arrested males more strongly than those applied to filter paper of comparable size. However, more complex behavioral elements, such as head nodding and copulation attempts, were shown only when extracts were applied to extracted male cadavers, suggesting that tactile or visual cues synergize the male response. The chemicals involved are stable, of low volatility, and nonpolar. Dead females arrested males and elicited courtship behavior for at least 8 d. Males showed no sign of attraction to live females at a distance of 3 cm in an olfactometer. Fractionation of female extracts demonstrated that the activity was exclusively located in the nonpolar fraction. Analysis of the active fraction by gas chromatography-mass spectrometry revealed that cuticular hydrocarbons with chain lengths between 25 and 37 carbon units were present. Comparison of hydrocarbon profiles from males and females showed qualitative and quantitative differences. These results suggest that sex-specific cuticular hydrocarbons are the key signals mediating the male courtship behavior in N. vitripennis.