Evolutionary genetics and dynamics of transitions in sex determination

Sex determination (SD) is an essential part of an individual's development, but the mechanisms controlling SD are incredibly variable and subject to rapid evolutionary change. When new SD genes evolve, this can lead to the formation of sex chromosomes. Sex chromosomes evolve from regular autosomes into a pair of highly-differentiated chromosomes via sex-specific adaptation and genetic degeneration. The evolutionary causes and consequences of SD evolution have been intensively studied using theoretical approaches, but many questions remain. Existing models of SD evolution predict that multiple SD genes generally cannot coexist, while some such systems occur in amongst others insects and amphibians. Here, several models are developed that may help explain why complex SD systems may persist over evolutionary time scales. This is done by assuming that the genes controlling SD do not act in isolation, but instead that SD as a process is integrated into and affected by the biology and ecology of the organism. Early sex chromosome evolution is thought to occur by sex-specific adaptation and the evolution of recombination suppression. Testing this theory has proven difficult as in absence of recombination, sex chromosomes are prone to genetic decay and mutation accumulation. Here, the complex SD system of the housefly Musca domestica is exploited to generate new sex chromosomes, which circumvent the issue of chromosomal decay and may be used to study the early stages of sex chromosome evolution. Further development of the housefly as a model system for evolutionary studies will be needed to bring this potential to full fruition. Altogether, these findings show that SD, and the mechanisms controlling it, is more complex then commonly considered.