Abstract
Sex chromosome evolution is assumed to take place by a gradual arrest of recombination between homologous autosomes, followed by a genetic decay of the non-recombining chromosome. The general model predicts that sex chromosome degeneration is aided by selective forces associated with two different sexes. However, it has proven difficult to disentangle the factors affecting the degeneration: in addition to sex-biased evolutionary forces the process is also affected by reduced recombination per se and a lower effective population size of sex chromosomes than autosomes. In addition, many model organisms have sex chromosomes where degeneration is nearly complete, and these retain few traces of the processes leading to their degeneration. I will talk about our recent work using the filamentous ascomycete Neurospora tetrasperma as a model to study the mechanisms and evolutionary forces leading to degeneration of sex chromosomes. Just as in many animals and plants, N. tetrasperma shows suppressed recombination and substantial divergence of the mating-type chromosome. However, it is hermaphroditic and therefore not exposed to sex specific selection pressure, making it a more specific model. In addition, the relative recent evolutionary origin of N. tetrasperma makes it a suitable model for studies of the ongoing processes underlying the early stages of sex chromosome evolution.