Heterothallism

Heterothallic species have sexes that reside in different individuals. The term is applied particularly to distinguish heterothallic fungi, which require two compatible partners to produce sexual spores, from homothallic ones, which are capable of sexual reproduction from a single organism.

In heterothallic fungi, two different individuals contribute nuclei to form a zygote. Examples of heterothallism are included for Saccharomyces cerevisiae, Aspergillus fumigatus, Aspergillus flavus, Penicillium marneffei and Neurospora crassa. The heterothallic life cycle of N. crassa is given in some detail, since similar life cycles are present in other heterothallic fungi.

The yeast Saccharomyces cerevisiae is heterothallic. This means that each yeast cell is of a certain mating type and can only mate with a cell of the other mating type. During vegetative growth that ordinarily occurs when nutrients are abundant, S. cerevisiae reproduces by mitosis as either haploid or diploid cells. However, when starved, diploid cells undergo meiosis to form haploid spores. Mating occurs when haploid cells of opposite mating type, MATa and MATα, come into contact. Ruderfer et al. pointed out that such contacts are frequent between closely related yeast cells for two reasons. The first is that cells of opposite mating type are present together in the same ascus, the sac that contains the tetrad of cells directly produced by a single meiosis, and these cells can mate with each other. The second reason is that haploid cells of one mating type, upon cell division, often produce cells of the opposite mating type with which they may mate.

Katz Ezov et al. presented evidence that in natural S. cerevisiae populations clonal reproduction and a type of “self-fertilization” (in the form of intratetrad mating) predominate. Ruderfer et al. analyzed the ancestry of natural S. cerevisiae strains and concluded that outcrossing occurs only about once every 50,000 cell divisions. Thus, although S. cerevisiae is heterothallic, it appears that, in nature, mating is most often between closely related yeast cells. The relative rarity in nature of meiotic events that result from outcrossing suggests that the possible long-term benefits of outcrossing (e.g. generation of genetic diversity) are unlikely to be sufficient for generally maintaining sex from one generation to the next. Rather, a short term benefit, such as meiotic recombinational repair of DNA damages caused by stressful conditions such as starvation may be the key to the maintenance of sex in S. cerevisiae.

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