Fitness-Associated Sexual Reproduction in a Filamentous Fungus

Abstract

Sex is a long-standing evolutionary enigma. Although the majority of eukaryotes reproduce sexually at least sometimes [1-3], the evolution of sex from an asexual ancestor has been difficult to explain because it requires sexually reproducing lineages to overcome the manifold costs of sex, including the destruction of favorable gene combinations created by selection [4, 5]. Conditions for the evolution of sex are much broader if individuals can reproduce either sexually or asexually (i.e., facultative sex) and allocate disproportionately more resources to sex when their fitness is low (fitness-associated-sex or FAS [6-10]). Although facultatively sexual organisms have been shown to engage in more sex when stressed [11], direct evidence for FAS is lacking. We provide evidence using 53 genotypes of the filamentous fungus Aspergillus nidulans in a reciprocal transplant experiment across three environments. Different genotypes achieved highest fitness in different environments and genotypes invested relatively more in sex in environments in which their fitness was lower, showing that allocation to sexual reproduction is a function of how well-adapted a genotype is to its environment. FAS in A. nidulans is unlikely to have evolved as a strategy to resist or avoid stress because asexual spores are more dispersive and equally resistant [12, 13].

Sexual reproduction and mating-type–mediated strain development in the penicillin-producing fungus Penicillium chrysogenum

“…Sexual reproduction offers an invaluable method to allow targeted crosses to be set up, providing a faster and economically cheaper procedure, without the need for prior knowledge of the genetic basis of traits of interest. Also, continued random mutagenesis and high-throughput screening can lead to undesirable deleterious mutations and genetic instability in producer strains (41). For example, production strains of P. chrysogenum contain multiple point mutations as well as major amplifications and deletions compared with the genome sequence of the progenitor Wisconsin strain (42).…”

Unisexual Reproduction Enhances Fungal Competitiveness by Promoting Habitat Exploration via Hyphal Growth and Sporulation

“…Similarly, the facultatively sexual fungus Aspergillus nidulans has been found to invest more resources in sexual reproduction in environments in which the fitness is lower (16), suggesting that the level of resources invested in sexual reproduction is in part determined by how well adapted the genotype is to a given environment. In C. neoformans, unisexual reproduction often involves increased pheromone expression.…”

Condition-dependent sex: who does it, when and why?

“…A stress signal is also required for sexual differentiation in Schizosaccharomyces pombe [38], and oxidative stress has been shown to increase the percentage of meiotic spores by 4-to 18-fold [39]. Likewise, Aspergillus nidulans fungi produce more of their spores sexually in environments where they are less fit, although asexual spores are more dispersive and equally resistant [40]. The pathogenic fungus Candida albicans does not appear to reproduce sexually, but its parasexual cycle-involving tetraploidy, aneuploidy and genetic variation-has been suggested to be promoted by stress [41,42].…”