Drivers and dynamics of sex chromosome evolution in African cichlid fishes

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The mechanisms of sex determination are diverse despite their unifying function in defining male and female sex within a species. They range from an array of environmental factors over different genetic sex determination systems to a complex interplay of extrinsic and intrinsic factors. These mechanisms are ultra-conserved in some organismal groups (e.g. all mammals share the same sex chromosomal system), however, in other groups even sister species vary in their way how sex is determined. This project focuses on the exploration of genetic sex determination or -in other words- the investigation of sex chromosomes in a model system of evolutionary biology, the African cichlid fishes. Within the Lake Tanganyika radiation of these fishes, I previously identified an outstandingly high rate of sex chromosome turnover, i.e. change in the actual chromosome used as sex chromosome. This project will investigate sex chromosome evolution from three new perspectives. The first objective submerges deeper into the sex chromosome history of African cichlids with the aim to include representatives of all major African lineages. Identification of sex chromosomes in this pan-african dataset allows investigating if the speed and pattern of sex chromosome evolution varies along the cichlid phylogeny. This will answer if rapid sex chromosome evolution is restricted to radiating lake lineages or a general feature of cichlids.The second project part zooms into particular species of Lake Tanganyika in which I previously identified sex chromosomes. One focus is here on chromosomes, which repeatedly evolved as sex chromosomes in representatives of distinct lineages with the aim to disentangle if changes in the same genes drove sex chromosome evolution convergently. Within this project line, we will also compare genomic sequences and perform crosses between species that have different sex chromosomal systems to identify genomic signatures underlying transitions between sex chromosomes.The third project line investigates the degree of sexual plasticity in correspondence to the presence of sex chromosomes. Within a particular lineage of the Lake Tanganyika cichlid radiation, the so-called Ectodini, we were surprised to see that only a handful of species has sex chromosomes. We suspect that these fish have an increased level of sexual plasticity: Species that do not have sex chromosomes were observed to change sex from female to male. This project line investigates if when raised under the same conditions, species without sex chromosomes change sex more easily than species with sex chromosomes. To understand how sex change is achieved at the molecular level, we will compare gene expression as well as the regulatory signatures activating and silencing gene expression between males and females of species that change sex to those that do not.