Why do Host-Deprived Seed Beetles 'Dump' Their Eggs?

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Physiological Entomology



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If host seeds are absent, females of the seed beetle Callosobruchus maculatus sometimes ‘dump’ eggs on unsuitable substrates, which causes complete larval mortality and decreases female lifespan. To understand the possible function of this behaviour, the present study examines genetic variation in dumping behaviour between and within populations. When deprived of hosts, females from an African population are much more prone to dump eggs than Asian females, most of which dump no eggs over their lifetimes. Egg dumping therefore cannot be explained as a simple, species-wide constraint imposed by the accumulation of mature oocytes. A transfer experiment tested the hypothesis that dumping eggs improves a female’s ability to exploit the subsequent availability of seeds, perhaps by preventing a disruption of oocyte maturation. Contrary to prediction, the number of eggs laid after females are transferred to seeds is inversely related to the number dumped during deprivation. Two bidirectional selection experiments revealed heritable variation in egg-dumping behaviour within the African population. Deprived females from the dumper line dump more than twice as many eggs as do females from the nondumper line. Pre-adult development time is significantly longer in the nondumper line, which suggests that trade-offs with other fitness components could maintain variation in egg-laying behaviour within populations. The divergent responses to host availability by African and Asian females may represent a pleiotropic effect of similarly divergent responses to host quality.

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