Group selection and inbreeding can explain why fig wasp sex ratios are too female-biased.

Greeff, J. M.*, Labuschagne, T., and De Waal, P. J.

Department of Biochemistry, Genetics and Microbiology, University of Pretoria

Some organisms have very few male offspring. Typically, this is in organisms where one or a few mothers lay their eggs in an isolated patch and where the offspring from a patch mate amongst each other before they disperse. Exactly why selection favours the female-biased sex ratio have been hotly debated. It turns out that everyone was correct and that many factors contribute to this bias. Some modelled it from a group selection perspective, others from an individual perspective and these were shown to both be legitimate descriptions of the same problem. The factors can be summarised as 1) mothers have daughters that compete with unrelated females, in preference to sons that will mostly compete with their brothers, 2) mothers have more daughters to increase the mating opportunities for their sons, and 3) in haplodiploid species, mothers have more daughters because inbreeding causes daughters to have a higher genetic value than sons. Fig wasps are a model taxon to study such highly female-biased sex ratios. However, their sex ratios are frequently too female-biased. We genotyped females from one fig wasp species to estimate the relatedness between competing females to confirm that number 1 is correct, quantified the inbreeding coefficient in order to determine the effect of number 3 accurately, and estimated the population genetic structure of the species. It turns out that there is a significant population structure and in turn, this means there is a true group-selection component that will bias sex ratios more towards daughters. Our estimates also suggest that the biasing effect of inbreeding is substantially more than previously anticipated.

Keywords: inbreeding, population genetics, sex ratios, fig wasps