New Theory Tests Sex Ratios Under Asymmetrical Local Mate Competition Among Parasitoid Wasps
- Date:
- August 17, 2005
- Source:
- University of Chicago Press Journals
- Summary:
- In a new study featured in the September issue of The American Naturalist, David M. Shuker (University of Edinburgh) and colleagues show that females alter the sex ratio they produce on a host by considering whether there are already eggs on the host they are using and if there are other eggs already laid on other hosts in the patch.
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For many species, the ratio of sons to daughters a mother produces canhave a profound effect on her evolutionary success. Too many sons, ortoo many daughters, may mean a female does not pass on as many copiesof her genes to the next generation as another mother. The crucialdeterminant of sex ratio for many species will be how likely relatedindividuals will interact, in particular whether brothers will becompeting amongst themselves to mate with their sisters. This situationbecomes increasingly likely in species with very structuredpopulations, where only one or a few females will produce offspringlocally, and related males and females will be mating with each other.To reduce this local mate competition between sons, mothers should biasthe sex ratio towards daughters, reducing male competition andproviding them each with more females to mate with.
Parasitic wasps that lay their eggs on insect hosts often have sucha population structure, and work has shown that sex ratios vary withthe number of females contributing to a host or group of hosts, asexpected. For example, in the parasitic wasp Nasonia vitripennis,females vary their offspring sex ratios in response to both thepresence of other females on a patch, and the presence of eggs alreadylaid on the host they are about to use themselves. However, oftengroups of hosts will be a mixture of parasitised and unparasitisedhosts, and the mating environment will be influenced by wasps emergingfrom all the parasitised hosts, some related to each other but somenot.
In a new study featured in the September issue of The AmericanNaturalist, David M. Shuker (University of Edinburgh) and colleaguesshow that females alter the sex ratio they produce on a host byconsidering whether there are already eggs on the host they are usingand if there are other eggs already laid on other hosts in the patch.The researchers developed a new theory to explain what the best sexratios should be for different situations and to demonstrate thatfemales qualitatively confirm these novel predictions. This suggeststhat females are incredibly subtle in their use of information from thewhole patch when it comes to making their sex ratio decisions.
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David M. Shuker (University of Edinburgh), Ido Pen (Universityof Groningen), Alison B. Duncan (University of Edinburgh), Sarah E.Reece (University of Edinburgh), and Stuart A. West (University ofEdinburgh), "Sex ratios under asymmetrical local mate competition:theory and a test with parasitoid wasps" 166:3 September 2005.
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