The genetics and genomics of adaptive sex ratio behaviour

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.

Who will benefit from this research?

We identify a number of groups of beneficiaries. Our project is primary research tackling the nature and control of reproductive behaviour in an insect. As such the primary beneficiaries will be:

(1) Academic beneficiaries (for more details see previous section).

In addition, we identify a set of beneficiaries that includes both academics and possible commercial partners:

(2) The biological control community.

Finally, we also identify a third set of beneficiaries:

(3) The general public.

How will they benefit from this research?

(1) The academic community.

The academic community will benefit from a major advance in our understanding of the genetic basis of a key reproductive phenotype. The advance will be in terms of both the nature and architecture of segregating genetic variation in sex ratio, and also in terms of the mechanistic basis of sex allocation, as explored via the genes and gene pathways associated with sex ratio decision-making (including patterns of differential expression associated with different sex ratio cues). In addition, we will explore the role of epigenetics (specifically DNA methylation) in regulating sex ratio behaviour. There is growing interest in epigenetic modification as a way for a genome to produce phenotypic plasticity, as well as the role of epigenetics in evolution, for instance through trans-generational "genomic imprinting". We will test whether or not DNA methylation is associated with changes in sex ratio behaviour and whether it can be manipulated to alter sex allocation. This will be a major advance in terms of the scope of phenotypes tested for epigenetic effects, and is terrifically exciting.

In addition to these stand-alone data, these approaches will offer up candidate genes associated with segregating variation and/or sex ratio mechanisms. Candidate genes are something we sorely lack, both in Nasonia but also in sex allocation more generally. Really the key advance (and thus benefit) will be made by identifying candidate genes, providing functional tests of those genes (e.g. by RNA-interference), and then scrutinising them with the tools of molecular evolutionary biology. The latter aspects of this advance we expect to be fully realised outside the time-frame of the current proposal. Then we will be in the position of being to take an extremely well-characterised phenotypic adaptation and see what that adaptation looks like at the genetic level.

(2) The biological control community.

Many of the sorts of traits of interest to the biological control community (e.g. that would improve efficacy, specificity, or longevity of the control) are behavioural or life-history traits not previously amenable to molecular genetic study. The Nasonia genome project has provided the opportunity to use the latest techniques to try and dissect relevant behavioural/life-history traits in a species of parasitoid. Success would be a major advance for such a species. Given the speed of developments in whole-genome sequencing, what we can do today in Nasonia, hopefully we will be able to do in other parasitoids tomorrow. The present work will help inform and direct that future work, in terms of techniques and empirical outcomes. Moreover, high profile publications will raise awareness of what can be achieved outside of traditional model organisms.

(3) The general public.

Via a range of outreach activities (detailed in the Pathway to Impact Plan) we will use our work to help the general public understand the role genes play in influencing our behaviour. The genetic basis of human behaviour remains disquieting for some, but the main benefit will be an increasingly informed and scientifically literate society.