Mechanisms of Homothallism in Fungi and Transitions between Heterothallism and Homothallism

Sexual systems are classified into heterothallism or homothallism in fungi. This chapter focuses on examples of homothallic fungi and how their sexuality is governed. Conversions between heterothallic and homothallic sexual cycles are common evolutionary transitions in fungi, and whether homothallism evolved from heterothallism or vice versa is a controversial topic, and there is evidence supporting both hypotheses. The structural analyses of MAT sequences from homothallic and heterothallic Cochliobolus species support the hypothesis that heterothallism is the ancestral state in this genus. This unique arrangement of the Aspergillus nidulans MAT locus led to the proposal that homothallism is the ancestral state and that a transition from homothallism to heterothallism could have occurred in A. oryzae and A. fumigatus by gene loss, although neither species has a defined sexual cycle. Homothallic species may originate from heterothallic predecessors, and the homothallic lifestyle may have a selective advantage under certain ecological pressures. This hypothesis is consistent with the repeated occurrence of homothallism within numerous genera and the fact that many heterothallic fungi achieve homothallism in the form of pseudohomothallism by mating-type switching or packaging two compatible nuclei into one spore. Homothallic individuals contain all of the necessary genetic information for full sexual expression.