Complex Evolutionary Genetics of Gonadal Differentiation in a Classic Model of Behavior, Astatotilapia burtoni

Proper gonadal differentiation is absolutely fundamental to the fitness of sexually reproducing species, yet surprising diversity exists among species in the mechanisms that determines males vs. females. Genetic sex determination is one common mechanism, yet little is known about how novel sex determination genes arise, interact, and evolve in populations. Historically, genetic sex determination research has largely focused on a handful of single-factor, chromosomal sex determination systems. More recently, genetic sex determination has been mapped in a broader set of species, revealing a diverse catalog of sex determination genes in a variety of chromosomal contexts. Polygenic sex determination (PSD) has also been confirmed in multiple taxa, where multiple genetic factors direct sexual development. PSD challenges traditional thinking regarding genetic sex determination systems, while providing powerful strategies to explore the evolution and development of sex determination, including comparisons of siblings with multiple genetic modes of sex determination. In other words, while individuals of species with PSD develop as male or female, several genetic types of males and females may exist. The cichlid fish species Astatotilapia burtoni uses PSD to determine male vs. female development, providing a model to understand the evolution and development of genetic sex determination. This work aims to provide the most comprehensive analysis of a PSD system to date, using A. burtoni as a model system. The project should reveal novel genes and gene networks driving gonadal differentiation and development, while yielding important evolutionary insights. The research will also produce important genome resources for A. burtoni, a model for neural and behavioral biology. Undergraduate and graduate students and postdoctoral fellows will carry out proposed experiments, preparing them for careers in science. Additionally, K-12 teachers will be given direct research experience through co-mentoring by the researchers and the Kenan Fellows Program, together developing educational materials broadly scaled for state classrooms, regional public outreach venues, and the world wide web.

Multiple sex determination loci segregate in A. burtoni, producing many genotypic sexes and the opportunity to study the interaction of multiple sex determination genes within individuals. The proposed research uses an integrated approach involving genetic mapping, genome assembly and annotation of sex chromosomes, comparative genomics, and gene network analysis during gonadal differentiation. Aims include mapping and functional testing of multiple novel sex determination genes, and tracing the evolutionary impact of novel sex determination alleles on the structure of chromosomes where they reside, including functional evolution of linked genes. Gene expression analyses through sexual development will supplement these mapping and annotation efforts, and provide understanding of how multiple genetic sex switches are integrated during development. Gene expression analyses among different genetic sexes during development will also reveal core genetic networks underlying gonadal development, and yield insight into how genetic networks evolve without negatively impacting indispensable complex traits. Since A. burtoni is a longstanding behavior model, it provides excellent context for analysis of secondary sexual characteristics, including existing behavioral gene expression data, in relation to the various modes of sex determination present in the species.