The roles of sexual and asexual reproduction in the origin and dissemination of strains causing fungal infectious disease outbreaks

- Selection of pathogenic traits through sexual recombination can initiate fungal outbreaks.
- Asexual reproduction accompanied by mutation or genome rearrangement can generate virulent strains which in turn initiate fungal disease outbreaks.
- The roles sexual and asexual reproduction may differ between the initiation and expansion of disease outbreaks.
- Rapid spread of outbreak strains is often accomplished through asexual reproduction.
- The introduction of novel strains, changes to host populations and changes to environments can all cause outbreaks.

Sexual reproduction commonly refers to the reproductive process in which genomes from two sources are combined into a single cell through mating and then the zygote genomes are partitioned to progeny cells through meiosis. Reproduction in the absence of mating and meiosis is referred to as asexual or clonal reproduction. One major advantage of sexual reproduction is that it generates genetic variation among progeny which may allow for faster adaptation of the population to novel and/or stressful environments. However, adaptation to stressful or new environments can still occur through mutation, in the absence of sex. In this review, we analyzed the relative contributions of sexual and asexual reproduction in the origin and spread of strains causing fungal infectious diseases outbreaks. The necessity of sex and the ability of asexual fungi to initiate outbreaks are discussed. We propose a framework that relates the modes of reproduction to the origin and propagation of fungal disease outbreaks. Our analyses suggest that both sexual and asexual reproduction can play critical roles in the origin of outbreak strains and that the rapid spread of outbreak strains is often accomplished through asexual expansion.