Origin, Evolution, and Extinction of Asexual Fungi: Experimental Tests Using Cryptococcus neoformans

This chapter reviews and discusses recent experimental studies using the model basidiomycetous yeast Cryptococcus neoformans to test the effects of spontaneous mutations and biological interactions that may have contributed to the distribution of asexual fungal strains and species in nature. To begin with, the author first introduces some background information on fungal sexuality and spontaneous mutations. The study of fungal sexuality could be traced back to over 100 years ago when Blakeslee discovered obligatory cross‐fertilization in the Mucorales. At the population level, evidence for clonality and asexual reproduction has been found in many groups of microorganisms, including both sexual and asexual fungi. Spontaneous mutation is the ultimate source of all heritable variations in all organisms. It can occur in both replicating and nonreplicating genetic materials in cells or viral particles. The genome‐wide mutation rate and the average effect per mutation are most commonly estimated using mutation accumulation (MA) experiments. In these experiments, spontaneous mutations are allowed to accumulate in replicate lines in the absence of selection for the trait under investigation. Sexuality in fungi is typically considered a qualitative trait. Two strains are considered either capable or not capable of mating with each other to produce meiotic progeny. The chapter summarizes three recent studies that tested the three hypotheses on fungal asexuality: the loss of sex, the cost of sex, and the fitness consequences of fungal asexual clones in experimental populations of C. neoformans