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Chapter 28 : Origin, Evolution, and Extinction of Asexual Fungi: Experimental Tests Using Cryptococcus neoformans
Category: Fungi and Fungal Pathogenesis
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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.
Key Concept Ranking
- Microbial Ecology
Schematic representation of a typical MA in facultative sexual organisms. (Modified from reference 47 , reprinted with permission from Genetics.)
The mean loss of sex and the among-line divergence over 600 mitotic generations for each of the two parental strains JEC50 and MCC3: (A) mating ability and (B) filamentation ability. In all four graphs, the x axis shows the number of asexual generations, while the y axis represents the mean and standard deviation of losses of mating or filamentation. For each data point, the mean and standard deviation were obtained from eight MA lines (from reference 47 , reprinted with permission from Genetics).
A negative correlation between mating ability (x axis) and the relative vegetative fitness in the presence of active mating partners (y axis, the inverse of cost of sex) for the 16 MA clones at G600 (r = −0.777; P < 0.0001). (From reference 50 , reprinted with permission from Genetics.)
Relative mean fitness of MA lines grown in four different conditions, on the x axis from left to right: (i) 25°C on SD medium; (ii) 25°C on YEPD medium; (iii) 37°C on SD medium; (iv) 37°C on YEPD medium. Only clones from G600 are shown here. Panel A shows MA lines maintained at 25°C on YEPD medium; panel B shows MA lines maintained at 37°C on YEPD medium (from reference 49 , reprinted with permission from Genetics).
Decreases in mating and filamentation abilities in mutation accumulation lines of C. neoformans over 600 asexual generations a
A cost of interacting with active mating partners in C. neoformansa
Reductions in the cost of sex in asexually evolved MA lines after 30 transfers in experimental populations of C. neoformansa
Estimates of mutational parameters on vegetative fitness in C. neoformans using two MA conditions and four testing environments a
Three-way analysis of variance and evidence for significant genotype-environment interactions of spontaneous mutations on vegetative fitness in experimental populations of C. neoformansa
Summary of vegetative fitness comparisons between clinical and environmental samples of C. neoformans at 25 and 37°C a
Two-way analysis of variance table showing a significant interaction between strain source and incubation temperature in vegetative growth of strains of C. neoformansa