Patterns and processes of sex chromosome evolution

From the spectacular tail of the male peacock to the dazzling courtship dance of male spiders, males and females across the animal kingdom often look and behave very differently. However, the two sexes share an almost identical set of genes, so how do these remarkable sex differences arise? The answer is thought to lie with the sex chromosomes, as they are the only region of the genome to differ between females and males. In humans, where males posses one X and one Y chromosome but females have two X chromosomes, the Y chromosome is the essence of masculinity. However, other animals can have different types of sex chromosomes. For instance, birds have Z and W sex chromosomes. Males have two Z chromosomes and females have one Z and one W chromosome. Furthermore, whereas all mammals are XY and all birds are ZW, geckos are one of the few animal groups that possess both XY and ZW sex chromosomes in different species. My research aims to characterise this diversity of sex chromosomes in birds and geckos and explore their role in generating biological differences between males and females.

Just as the Y chromosome is important for maleness in mammals, the W chromosome is particularly interesting for understanding female traits, such as egg laying in birds. However, the survival of both of these gender-limited chromosomes is under threat. The way for chromosomes to eliminate faulty mutations is by pairing up and swapping genes during reproduction. The single W and Y chromosomes do not have this option because they have no partner to pair with, and so cannot repair genetic mistakes. Over time, these chromosomes have decayed, losing 90% of their original gene content, and are now largely genetic wastelands. But is this degeneration of the Y and W chromosomes terminal? Does this mean the Y chromosome in humans and ultimately males will become extinct?

The key to answering this question lies in the genes that have survived: have they been preserved because their function is so critical or is their survival the result of random chance? My research will explore exactly how genes resist these strong degradative forces by studying bird sex chromosomes, which offer unparalleled insight into the process of degeneration. This is because all birds originally shared an identical W chromosome, but over millions of years the once-identical W chromosome has decayed independently in different bird species. This evolutionary repetition makes it possible to test whether the same sets of genes have survived on the W chromosome across birds and to identify their role in female traits.

However, in order to understand how sex differences are encoded in the genome it is not enough to study only the gender-limited Y and W chromosomes. Studies of the Z and X chromosomes can also help explain how striking differences between males and females evolve. In birds and some gecko species, males have two copies of the Z chromosome whereas females possess only one. Therefore, the Z is thought to encode flashy plumage and elaborate courtship songs in males. Similarly, as the X chromosome is present more often in females than males, it is thought to play a major role in determining female functions. As geckos possess both XY and ZW sex chromosomes they provide an excellent opportunity to explore the roles played by both the X and Z chromosomes in determining sexual differences. I will therefore study this natural variation across geckos and ask how the roles played by the X and Z chromosomes differ from each other.

Together, my research will shed much needed light on the huge diversity by which sex is determined across the animal kingdom. More importantly, by studying the role played by different types of sex chromosomes in multiple animals, my research has the power to greatly advance our understanding of the evolutionary mechanisms responsible for producing the dazzling array of sexual variety exhibited in the animal kingdom.

1. Who will benefit from this research?

Poultry industry
Zoos, aviaries and wildlife parks
Pet owners
General public
Women in Science

2. How will they benefit from this research?

1. Poultry Industry
Egg laying rate, or female fecundity, is a critical component of the poultry industry. The W chromosome is known to be key in female fecundity and this project will generate a catalogue of W-linked genes in three agriculturally important species; turkey, duck and guineafowl. In particular, the turkey is central to the North American poultry industry and production is worth $5.3 billion (USA Department of Agriculture, National Agricultural Statistics Service 2014 values). The dataset of W-linked genes generated during this fellowship will present a useful list of selection targets to breeders to increase productivity.

2. Zoo, aviaries and wildlife parks
Sexual dimorphism is arguably one of the most conspicuous sources of biodiversity in the animal kingdom. The proposed work will shed light on sexual dimorphism at the genomic level and the evolutionary processes associated with sex. Displays and events based on this research would therefore greatly assist wildlife parks in raising awareness of the natural world and igniting the public's interest in evolution. Furthermore, as geckos and zebra finches are common features of aviaries and reptile houses, the educational potential of these establishments would benefit from displays outlining the current scientific research in these species.

3. Pet owners
Lizards and birds are popular domestic pets in the UK and there are a large number of active societies established by enthusiasts and breeders. Knowledge on the mechanisms by which sex is determined in these species, as well as the evolutionary processes shaping biological differences between males and females, will be of particular interest to society members.

4. General public
Sex is a topic of widespread interest and can generate sizeable attention from the general public. Therefore, the charismatic species in this project (lizards, birds), together with the dynamic research topic (evolution, sex) and innovative methods (whole genome sequencing), will spark the general public's interest in evolution and genetics. In doing so, this will inspire young scientists and encourage them to pursue careers in science. I can also act as a female role model to young scientists and promote women in science.

I am ideally positioned to disseminate my research findings effectively to the beneficiaries I have identified. I have already participated in public outreach events and attended a workshop designed to equip attendees with a diverse toolkit of communication skills. Using my skillset and building on my previous experience, I have outlined a range of approaches to transfer knowledge and information to the relevant parties in my Pathways to Impact.