Variation in sexual size dimorphism we have created in the lab.
Female (left) and two males, each carrying a different Y chromosome haplotype that affects their size.

Evolution of sexual dimorphism

Sexual dimorphism is central to biodiversity and genome evolution. It is ubiquitous across taxa with two sexes despite the fact that the shared genome imposes a conflict over independent evolution in the sexes. The overall goal in our research is to understand the nature of selection required to overcome this conflict, the mechanisms that create sex differences in the genetic architecture of a shared trait and the wider genomic and phenotypic consequences of the evolution of dimorphism. To address these topics, we combine quantitative genetics, artificial selection and phenotypic laboratory assays with molecular genetics, which we hope to give us a broad overall understanding of what it takes to evolve sexual dimorphism in a microevolutionary scale.

We use body size as the model trait because it is highly relevant to sex-specific fitness in our study species, C. maculatus, as well as across taxa. The fact that the body size is obviously a trait shared by the sexes immediately complicates its evolution compared to reproductive traits present only in one sex. This is because each sex needs to optimise their size leading to a tug-of-war over the shared genes. Body size is a really interesting trait to focus on for other reasons too: it is an outcome of many developmental processes, it is condition dependent, and hence polygenic. This complexity allows testing for many other interesting questions related to the evolution of size dimorphism, including whether sexually antagonistic selection on size can maintain genome-wide genetic variation, and how does this influence the potential for phenotypic plasticity in size in each sex.

This work is funded by the Swedish Research Council (Vetenskapsrådet), Carl Tryggers Foundation, and the Evolutionary Biology program at the Department of Ecology and Genetics (UU)

Our current research program is divided under the following themes.

1. What is the relative effect of sex-limited and sexually antagonistic selection on the evolution of sexual dimorphism?

We are studying this by using artificial selection to mimic a scenario of sexual or fecundity selection acting only on one sex and comparing this to when selection acts antagonistically to increase dimorphism. We started by characterising the quantitative genetic architecture of body size so that we can also test how well we can predict the outcome of each process.

This project is now wrapped up. You can see the results here.

2. How do different forms of sex-specific selection affect genetic variation?

Genetic variation is food for evolution but natural selection should rapidly erode it. Yet many fitness related traits show unexpectedly high amounts of genetic variation. Sexually antagonistic selection could in theory play an important role in maintaining it, by forming an overall balancing selection across the sexes. But can it do so while also driving for an evolutionary change? We are experimentally testing this, by contrasting sexually antagonistic selection on body size with selection operating only on males (which mimics male sexual selection). By comparing the genetic variation before and after selection we can test the quantitative as well as molecular genetic signatures of each form of sex-specific selection, and thereby test in real time what happens to genetic variation when sexual dimorphism evolves. 

3. What is the role of Y chromosome in the evolution of sex-shared traits?

C. maculatus Y chromosome is tiny. Such highly heteromorphic sex chromosomes are not expected to be able to easily maintain adaptive genetic variation that could respond to selection. Sex-limited chromosomes are also not expected to play a big role in regulating traits present in both sexes (Y linked genes in Drosophila are for example only expressed in the gonads). Despite these expectations, our work has discovered a major Y-linked variance component underlying body size variation in males. We are studying this phenomenon further by characterising what kind of genetic variation is present on the Y chromosome, how has it been maintained, how it regulates gene expression underlying size development, and what kinds of pleiotropic effects the variants have on other traits.

4. How do sex-specific selection pressures affect genome and transcriptome evolution in the sexes?

One of the key ways through which phenotypic sexual dimorphism can be achieved is sex- and tissue-specific regulation of gene expression. Recent evidence (including my PhD work with Mike Ritchie and Rhonda Snook) shows how sex-biased gene expression can evolve rapidly under different mating systems. In my postdoc time I also quantified how sex-biased expression in C. maculatus changes in response to mating, to test how each sex utilizes sex-biased genes in reproduction. In this work we predicted that in the reproductive context females would require increased expression of female-biased genes, and vice versa for males, but found the opposite: each sex primarily increased expression of the genes that showed an expression bias towards the opposite sex. Our recent work also shows that female-biased genes evolve under balancing selection maintaining more allelic variation than either male-biased or un-biased genes. Clearly, sex-specific selection has a pervasive influence on transcriptome evolution, but can we predict yet how sex-biased expression should look like for dimorphic traits? 

My group now seeks for new answers to how sex differences in gene expression evolve by experimentally testing how sex-biased gene expression changes as sexual size dimorphism evolves under different forms of sex-specific selection.

Thanks for reading, and if you have any questions do get in touch!

Through a combination of quantitative genetics and artificial selection we discovered that Y chromosome variation causes big differences in male body size. We created chromosome replacement lines where males are otherwise identical but carry two different Y-haplotypes. Here you see the guy with a ‘big-Y’ winning on the top.