Measuring the heritability of sex ratio in a social insect

Lead Research Organisation: University of East Anglia
Department Name: Biological Sciences


Theories for how social behaviour evolves in animals, which represents a fundamental issue in evolutionary ecology, assume that social behaviour is influenced by genes. This critical assumption has rarely been tested, and very little is known about the genes underlying social traits. Experiments in which researchers subject traits to artificial selection, combined with modern genetic and genomic techniques, can help pinpoint and identify genes for these traits. In future, my group seeks to conduct such experiments in the bumble bee Bombus terrestris, using sex ratio (the ratio of new queens to males) as the social trait of interest. Although the value of the sex ratio in social insects is well predicted by evolutionary models, as with other social traits very little is known about its genetic basis. However, to conduct this work we first need to establish that sex ratio is indeed underpinned by genetic variation, i.e. is heritable, since non-heritable traits cannot respond to either natural or artificial selection. In the proposed work I therefore seek to measure the heritability of sex ratio in B. terrestris. This species is uniquely suited to the proposed work because of its simple, annual societies, its amenability to laboratory rearing and breeding, and the existence of increasingly sophisticated genetic information on its genomic architecture. I will measure heritability by comparing the sex ratios produced by daughter queens with those produced by their mothers. A positive association between daughter and maternal sex ratios will establish heritability (which is formally estimated as twice the regression coefficient of this association). As well as addressing a fundamental issue in evolutionary ecology, this proposal is novel, because the heritability of sex ratio has not previously been measured in any social insect. For this reason, and because it should lead to a future project that would also be novel and fundamental, the proposed research should substantially advance the field.


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Description The bumble bee Bombus terrestris is an important pollinator of wildflowers and field crops and is also reared commercially on a large scale for the pollination of glasshouse crops. Little published research exists on the demography of commercially-reared B. terrestris colonies, which are likely to have been subjected to artificial selection (conscious or unconscious) for queen-biased sex ratios. This is because maximizing queen production maximizes the number of new colonies that can be reared from captive stock. In addition, little is known about the heritability of morphological traits in commercially-reared bumble bees or indeed social insects as a whole, even though heritability is a precondition for any trait to undergo either artificial or natural selection. We investigated the demography of a sample of 37 mature B. terrestris colonies reared from queens supplied by a commercial rearing company. We also investigated the heritability of body size by analyzing the statistical association between the body sizes of mother queens and their workers (daughters) and sons. We found that, demographically, commercial colonies differed very little from colonies reared from wild-caught queens except that male production was significantly lower in commercial colonies. This suggested that there had been artificial selection for queen-biased sex ratios in commercial colonies. We also found that, contrary to a previous study of a North American bumble bee species, body size was not heritable in workers and was negatively associated with queen body size in males (i.e. larger queens had smaller sons). These results suggest that heritability for life-history traits exists in B. terrestris (so permitting artificial selection on the sex ratio) and that there are complex patterns of heritability of size traits meriting further investigation.
Sectors Agriculture/ Food and Drink,Environment