Aivars Cirulis

I am interested in sex differences and sexual conflict. During my PhD I am studying sex chromosome evolution. In our lab we use a simultaneously hermaphroditic flatworm Macrostomum lignano containing a GFP marker used as a “sex determining” gene in our experimental evolution setup. By selecting on the GFP marker inherited only through sperm or eggs, we are mimicking XY and ZW chromosome evolution in different male- and female-selected populations. It is expected that sex-specific genes will accumulate next to the GFP marker and the region will cease to recombine, allowing us to observe initial steps of sex chromosome evolution!

The non-recombining sex-determining region later expands by acquiring more sex-specific genes, thus causing sex chromosomes to become heteromorphic. However, as both sexes still share most of the genome, shared genes with opposite fitness effects between the sexes must acquire sex-biased or sex-specific gene expression.

Our results show that there is an evolving divergence between male- and female-selected lines compared to controls in organ size and whole body gene expression. Investigations of sexual behaviour and DNA will also take place.

What happens next, if heteromorphic sex chromosomes are established? They are the only consistent difference between males and females, however this difference is not constant and eternal as degradation of sex-limited chromosomes takes place. Ongoing sexual conflicts still shape old sex chromosomes and maintain genetic and epigenetic variance in phenotypic traits. We are reviewing effects of degenerated sex-limited chromosomes on non-reproductive traits, because the long-held view that degenerated heteromorphic sex-limited chromosomes only affect traits involved in sex determination and sex cell production as well because of the cease of recombination they are not able to sustain standing genetic variation and are doomed to degeneration eventually leading to their loss, recently has been challenged. Thus, we want to make a paradigm shift from the view that these chromosomes are only full of ‘junk’ DNA and code for reproductive traits, like sex cell production, towards the view that sex-limited heteromorphic chromosomes can influence many traits shared between the sexes. Thus, possible non-sexual effects of these chromosomes should not be ignored as they can play an important role in health and disease!