The genetics and genomics of adaptive sex ratio behaviour

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We will examine the genetic basis of sex ratio behaviour in the parasitoid wasp Nasonia vitripennis. Female N. vitripennis facultatively change their offspring sex ratios in line with Hamilton's theory of Local Mate Competition (LMC). LMC arises from competition between related males (e.g. brothers) for mates, and can occur when mating occurs in localised groups, for instance amongst groups of kin. When LMC is intense (e.g. if all males are brothers), the optimal sex ratio is a female-biased one. This bias reduces competition amongst sons and increases the number of mates for those sons. As LMC declines, so does the predicted sex ratio bias. The degree of LMC depends on how many females lay eggs on a patch of hosts (and how many eggs they lay). Over the last decade, we have explored the cues female Nasonia use when allocating sex under LMC. With a robust theoretical framework, we now have a remarkably good understanding of facultative sex allocation under LMC at the phenotypic level in Nasonia. However, our understanding of the genetics of sex ratio is more rudimentary, especially in terms of the mechanism of sex allocation. Thus far, we have some picture of the quantitative genetics of sex ratio in Nasonia (estimates of heritability, input of new mutations, and the identification of four Quantitative Trait Loci, or QTL). We have also begun to explore what genes are expressed during oviposition. In this proposal, we will build on this work to explore the genetic basis of sex ratio variation and control in Nasonia, using three complementary approaches.First, we will first follow-up our recent QTL study using a Restriction Site Associated DNA sequencing ("RAD-seq") approach and a repeat of the cross between High and Low sex ratio lines drawn from the same natural population. RAD-seq can generate thousands of markers across a genome enabling finer-scale QTL mapping projects. We will also use the data we generate to test for clutch size variation QTL, testing for loci pleiotropically influencing both sex ratio and clutch size.Second, we will follow-up our recent gene expression work to explore changes in gene expression associated with exposure to different LMC environments and different combinations of LMC cues. Back in 2004, Shuker & West experimentally showed that female Nasonia vitripennis responded differentially to "host" versus "social" LMC cues. We will follow a similar protocol, assaying the transcriptomes of the focal females using RNA-seq on the Illumina platform. Our aim is to see whether we can link patterns of gene expression to subtle environmental differences which we know have a big effect on the sex ratio phenotype.Third, we will test whether or not epigenetic modifications of DNA (specifically DNA methylation) are associated with the regulation of sex ratio behaviour. The extent to which epigenetic control of gene expression influences behaviour is currently the focus of much interest, both in humans and other vertebrates, but also increasingly in insects. First, we will look for patterns of differential methylation associated with either mating (as females switch from mate-searching to host-searching) and/or interactions with LMC cues whilst ovipositing. Second, we will disrupt DNA methylation and look for changes in sex allocation. If DNA methylation helps regulate gene networks associated with sex ratio behaviour, then we will see patterns of differential methylation across the treatments in the first experiment and changes in sex allocation across the treatments in the second.Taken together, these approaches will address both the genetic architecture of sex ratio variation and also the genes and gene pathways associated with sex allocation, and whether or not the regulation of those pathways involves DNA methylation. They will provide complementary sets of candidate genes, enabling the functional genomic/molecular evolution studies required to fully realise the genotype-phenotype link.