The genetics of sexual development in East African cichlid fishes

Sexual development leads to the formation of males and females. This process can be controlled genetically, environmentally or by a combination thereof. Genetic sex determination is based on single- or polygenic actions, either linked to sex chromosomes or independent from them, while environmental sex determination includes factors such as temperature, density or social status. Teleost fishes, as the most species-rich clade of vertebrates, feature an overwhelming diversity of sex-determining mechanisms. In contrast to most mammals, where the presence or absence of one single gene defines the sexual fate, several master sex regulators have been identified in fish. Together with the description of complex polygenic regulations and the discovery of new functions of presumably conserved down-stream factors, fish exhibit a greater plasticity in generating males and females than previously thought.
East African cichlid fishes are extremely diverse in terms of ecology, morphology and breeding behavior and therefore represent a prime model system in evolutionary biology in general, and in the study of sexual development in particular. Several sex determination systems have been described, which differ sometimes even between closely related species in this fish family.
In my thesis, I investigated a set of candidate genes for sexual development in cichlids. More specifically, we studied these genes in adult cichlid species and in one of them also during early male development. Differential gene expression patterns, measured in brain and gonad tissue, were observed between cichlid lineages and within the same species over time. One striking case is the shift in gene expression of the aromatase cyp19a1. In vertebrates, and in all teleosts investigated so far, cyp19a1A and cyp19a1B are expressed in the ovaries and the brain, respectively. In the species rich and extremely diverse haplochromine lineage, we found an overexpression of cyp19a1B in the testis. In another study, we show that Astatotilapia burtoni, a member haplochromine lineage, most likely has an XX-XY sex determination system. Making use of hormonally sex-reversed fish and subsequent crossing experiments, we establish all-male broods, which offered an ideal platform to investigate gene expression of early testis and brain genes. In addition, we identified several promising candidates for genetic sex determiners in this species.