Genetics and evolution. Come fly, and leave the baggage behind

Sex and virulence of human pathogenic fungi

Over the past decade, opportunistic fungal infectious diseases have increased in prevalence as the population of immunocompromised individuals escalated due to HIV/AIDS and immunosuppression associated with organ transplantation and cancer therapies. In the three predominant human pathogenic fungi (Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus), a unifying feature is that all three retained the machinery needed for sex, and yet all limit their access to sexual reproduction. While less well characterized, many of the other human pathogenic fungi also appear to have the ability to undergo sexual reproduction. Recent studies with engineered pairs of diploid strains of the model yeast Saccharomyces cerevisiae, one that is sexual and the other an obligate asexual, provide direct experimental validation of the benefits of both sexual and asexual reproduction. The obligate asexual strain had an advantage in response to constant environmental conditions whereas the sexual strain had a competitive edge under stressful conditions (Goddard et al., 2005; Grimberg and Zeyl, 2005). The human pathogenic fungi have gone to great lengths to maintain all of the machinery required for sex, including the mating-type locus and the pheromone response and cell fusion pathways. Yet these pathogens limit their access to sexual or parasexual reproduction in unique and specialized ways. Our hypothesis is that this has enabled the pathogenic fungi to proliferate in their environmental niche, but to also undergo genetic exchange via sexual reproduction in response to stressful conditions such as new environments, different host organisms, or changes in the human host such as antimicrobial therapy. Further study of the sexual nature of the human pathogenic fungi will illuminate how these unique microbes have evolved into successful pathogens in humans.

Evolution of fungal sex chromosomes

Sexual reproduction enables organisms to shuffle two parental genomes to produce recombinant progeny, and to purge the genome of deleterious mutations. Sex is conserved in virtually all organisms, from bacteria and fungi to plants and animals, and yet the mechanisms by which sexual identity are established share both conserved general features and are remarkably diverse. In animals, sexual identity is established by dimorphic sex chromosomes, whereas in fungi a specialized region of the genome, known as the mating-type locus, governs the establishment of cell type identity and differs in DNA sequence between cells of different mating-types. Recent studies on the mating-type loci of fungi and algae reveal features shared with the mammalian X and Y chromosomes, suggesting that these represent early steps in the evolution of sex chromosomes.

Sexual cycle of Cryptococcus neoformans var. grubii and virulence of congenic a and alpha isolates

Cryptococcus neoformans is a human-pathogenic fungus that has evolved into three distinct varieties that infect most prominently the central nervous system. A sexual cycle involving haploid cells of a and alpha mating types has been reported for two varieties (C. neoformans var. neoformans, serotype D, and C. neoformans var. gattii, serotypes B and C), yet the vast majority of infections involve a distinct variety (C. neoformans var. grubii, serotype A) that has been thought to be clonal and restricted to the alpha mating type. We recently identified the first serotype A isolate of the a mating type which had been thought to be extinct (strain 125.91). Here we report that this unusual strain can mate with a subset of pathogenic serotype A strains to produce a filamentous dikaryon with fused clamp connections, basidia, and viable recombinant basidiospores. One meiotic segregant mated poorly with the serotype A reference strain H99 but robustly with a crg1 mutant that lacks a regulator of G protein signaling and is hyperresponsive to mating pheromone. This meiotic segregant was used to create congenic a and alpha mating type serotype A strains. Virulence tests with rabbit and murine models of cryptococcal meningitis showed that the serotype A congenic a and alpha mating type strains had equivalent virulence in animal models, in contrast to previous studies linking the alpha mating type to increased virulence in congenic serotype D strains. Our studies highlight a role for sexual recombination in the evolution of a human fungal pathogen and provide a robust genetic platform to establish the molecular determinants of virulence.

Frequency of Sexual Reproduction by Mycosphaerella graminicola on Partially Resistant Wheat Cultivars

ABSTRACT The frequency of sexual reproduction has a profound effect on the population structure and the adaptive potential of a facultatively sexual parasite. Little is known about the relationship of quantitative host resistance to the frequency of sex in pathogens. We sampled over 5,000 fungal fruiting bodies from eight different wheat cultivars over a 3-year period. The cultivars possessed varying degrees of susceptibility to Mycosphaerella graminicola, a facultatively sexual pathogen that is hetero-thallic and bipolar. The fruiting bodies were classified as M. graminicola pycnidia or ascocarps (asexual and sexual fruiting bodies, respectively), other identifiable fungi, or unidentified. In all 3 years, area under the disease progress curve (AUDPC) explained a significant proportion of the variation in ascocarps as a percentage of M. graminicola fruiting bodies (P < 0.0005). The mean percentage of M. graminicola ascocarps from all cultivars was 63% in 1998, when the epidemic was intense, and 14% in 1999, a year of low disease levels. In 2000, samples were taken at 7-day intervals from 6 June to 27 June from two cultivars with substantially different AUDPCs (788 and 2,185 percentage-days). The less diseased cultivar yielded its first M. graminicola ascocarps 1 week later than the more diseased cultivar, and respective means of ascocarps as a percentage of M. graminicola fruiting bodies across sampling dates were 20.2 and 59.3%. The ratio of sexual to asexual reproduction by M. graminicola is likely to be strongly conditioned by infection density.