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Competition induces adaptive shifts in caste ratios of a polyembryonic wasp


An important transition in insect life-history evolution was the shift from a solitary existence to living in groups comprising specialized castes. Caste-forming species produce some individuals that reproduce and others with worker functions that have few or no offspring1. Morphologically specialized castes are well known in eusocial species like ants and termites1, but castes have also evolved in less-studied groups like thrips, aphids and polyembryonic wasps2,3,4,5. Because selection acts at both the individual and the colony level, ratios of investment in different castes are predicted to vary with environmental factors like competition and resources6,7,8. However, experimental evidence for adaptive shifts in caste ratios is limited9 owing to the experimental difficulty of manipulating factors thought to influence caste ratios10,11,12,13, and because some species produce behaviourally flexible castes that switch tasks in response to colony needs14,15. Unlike other caste-forming species, the broods of polyembryonic wasps develop clonally, so that increased production of one caste probably results in decreased production of the other16. Here we show that the polyembryonic wasp Copidosoma floridanum alters caste ratios in response to interspecific competition. Our results reveal a distinct trade-off by C. floridanum between reproduction and defence, and show experimentally that caste ratios shift in an adaptive manner.

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Figure 1: Light micrograph of a C. floridanum precocious larva (a) and first instar reproductive larva (b).
Figure 2: The effects of multiparasitism by M. demolitor on caste formation by C. floridanum.
Figure 3: Mean number (± s.e.) of precocious larvae in hosts multiparasitized by M. demolitor and control hosts parasitized by C. floridanum only.

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We thank J. Johnson for assistance during the study, M. Grbic for discussions, and M. Visser, F. Wackers and W. van der Putten for comments on an early draft of the manuscript. This work was supported by the NSF.

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Correspondence to Michael R. Strand.

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Harvey, J., Corley, L. & Strand, M. Competition induces adaptive shifts in caste ratios of a polyembryonic wasp. Nature 406, 183–186 (2000).

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