Ever since their discovery, sex chromosomes have captivated researchers because of their involvement in sex determination, sexual form and function, and sexual conflicts. However, despite a long-lasting fascination, central aspects of their evolution remain poorly understood. This is true even for the relatively well-studied mammalian XY and avian ZW sex chromosome systems, because their ancient formation and pronounced Y and W degeneration prevent detailed analyses. In this project, we focus on a unique sex chromosome system in birds, the neo-sex chromosomes in Sylvioidea warblers, that we detected a few years ago. Interestingly, the autosome that fused to create the neo-sex chromosome is the homolog to the mammalian sex chromosomes, which means that ‘mammalian-XY genes’ can now be studied as ZW-linked in birds!
Sex chromosomes experience unique evolutionary conditions, and are affected by drift due to low population size and by both natural and sexual selection. Due to this, sex-linked genes show different functional variation compared to autosomal genes. Sexual selection is a component of natural selection associated with mating success, which often leads to sexual dimorphism. Animal mating systems usually involve strong sexual selection, typically acting on males – males compete and females choose. Sexual dimorphism (i.e. the difference between sexes) is common in nature and can be displayed in various ways including differences in size, colour and behaviour. Sexual dimorphism is particularly visible in birds, where plumage colouration shows huge difference between sexes (dichromatism) and large variation between species. Sex chromosomes are conductive to sexual selection due to their sex-specific inheritance pattern, and theory provides well-supported links between sexual selection, sexual dimorphism, and sex chromosome evolution. This recently gained some empirical support in a study on Galloanserae (game- and waterfowls) that found an association between degree of polygyny and sex chromosome evolution. To be able to draw more firm conclusions, this question needs to be assessed in a wider species group, such as Sylvioidea.
We study molecular evolution in the Sylvioidea neo-sex chromosome system with state-of-the-art analyses on whole-genomes of approximately 30 species. As a bioinformatics resource, we have assembled a reference genome of one Sylvioidea species, the great reed warbler, using PacBio, Chromium and BioNano. Our multi-species whole-genome data now enable us to compare sex chromosome evolution and its driving mechanism in a promising species group. Our aims are:
(i) To evaluate the molecular evolution of more than one hundred sex chromosome gene pairs (ZW gametologs) of each Sylvioidea species, and look for phylogenetically deviating genes.
(ii) To examine whether Sylvioidea species with high degree of sexual dimorphism (indicating strong sexual selection) display enhanced sex chromosome evolution.
(iii) To make use of the fact that the neo-sex chromosome in Sylvioidea is homologous to the sex chromosome in mammals, to study to which degree sex chromosome evolution is repeatable in mammals and birds.