Under most conditions found in nature, genetically diverse populations persist better than those lacking diversity. Indeed, sexual reproduction is widely viewed as a means of promoting genetic diversity for long-term ecological success in variable environments (Chapters 11 and 42). With the exception of early-diverging aquatic groups, fungi do not produce male and female gametes. What prevents an individual from mating with itself?

Fungi have different mating types that are genetically determined and prevent self-fertilization. The mating type of an individual is determined by a mating-type gene. Fertilization can take place only between individuals that have different alleles at the mating-type gene. In some species, there are only two mating-type alleles. In this case, mating patterns are identical to those for species with male and female sexes. If the two mating-type alleles are spread evenly throughout the population, an individual should be able to mate with 50% of the general population.

Some fungi have more than two mating-type alleles. These fungi have a greater likelihood of encountering a compatible genotype in the general population. That likelihood is even higher for fungi in which mating type is determined by two different mating-type genes that each have multiple alleles. Fungi in the group that includes the common mushrooms can have as many as 20,000 different mating-type alleles. In this case, the odds of finding a compatible mate are close to 100%.

Genetic compatibility is a prerequisite for mating. How do fungi go about finding a suitable mate? The answer is that they secrete chemical signals that attract fungi with mating types different from their own. The chemical nature of these signals differs among groups.

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About 20% of fungi appear to lack sexual reproduction altogether. Among these are such well-known groups as Penicillium (the source of the antibiotic penicillin), Aspergillus (the major industrial source of vitamin C), and all the endomycorrhizal fungi. Spores formed by mitosis (asexual reproduction) allow these fungi to proliferate and disperse.

How are asexual species able to persist without a mechanism for generating genetic diversity? The short answer is, they don’t. Fungal species that lack an observable sexual cycle are hypothesized to have another mechanism of generating genetic diversity: the crossing over of DNA during mitosis (Fig. 34.14). Such species are described as parasexual. In parasexual fungi, haploid nuclei within a heterokaryotic cell undergo karyogamy to form a diploid nucleus—so far, a familiar pattern. Next, however, during mitosis, a rare crossing over may occur between the two sets of chromosomes, resulting in novel genotypes. Cells lose excess chromosomes over time, such that the haploid state is restored. Like sexual reproduction, these processes produce novel gene combinations. The key difference between parasexuality and sexual reproduction is that parasexual species do not undergo meiosis.

Entirely asexual species of fungi posed a problem for early taxonomists because these species lack many of the morphological characters such as fruiting bodies or sporangium-bearing stalks previously used to define different groups. With the advent of DNA sequence data, it has become increasingly clear that many asexual species are closely related to sexual forms, indicating that their evolutionary history as asexual species is not long. A major exception are the Glomeromycota, for which fossils as old as 450 million years have been reported.