SEX RATIO IN A PARASITOID WASP SPALANGIA CAMERONI (HYMENOPTERA: PTEROMALIDAE)
Abstract
The haplodiploid sex determination of parasitoid wasps provides mothers a means to control their offspring sex ratio. In this study I review factors affecting offspring sex ratios of parasitoid wasps in general. Then I specifically examine how host size, number of females present, and paternal and maternal age affect offspring sex ratio of Spalangia cameroni, a solitary parasitoid wasp of synanthropic fly pupae. Charnov et al.'s (1981) host size model predicts that mothers should manipulate the sex of their offspring in response to host size by ovipositing daughters in large hosts and sons in small hosts. Data from collections of fly pupae (hosts) from a poultry house support the prediction on a within host species basis for two of three collection dates. Data from laboratory experiments also support the prediction for S. cameroni. The negative relationship between host size and wasp sex ratio (proportion males) was shown to result from offspring sex ratio manipulation by females, rather than from differential mortality of sons and daughters. A major assumption of the host size model is that the relationship between host size and the emerging wasp's reproductive success is more positive for female wasps than for males. Results of this study suggest that if such a relationship occurs for S. cameroni, it may be through an effect of host size on wasp development time rather than through an effect of host size on wasp size. Wasp size increased with increasing host size for both males and females in the laboratory, but in the field only for males and only from one host species. Male development time under laboratory conditions was independent of host size, whereas females developed slightly more quickly in large than in small hosts. The size of males and females and the size of hosts on which they developed did not affect aspects of their reproductive success measured in the laboratory. Whether the number of females present affects offspring sex ratio was examined through laboratory experiments. Local mate competition theory (Hamilton, 1967) predicts that a mother should increase her offspring sex ratio with increasing number of ovipositing females present. This prediction was not supported for S. cameroni. Offspring sex ratios increased with both maternal and paternal age in laboratory experiments.
Degree
Ph.D.
Subject Area
Biology
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