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Heterothallic species have 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.
Certain heterothallic species (such as Neurospora tetrasperma) are called "pseudo-homothallic". Instead of separating into four individual spores by two meiosis events, only a single meiosis occurs, resulting in two spores, each with two haploid nuclei of different mating types (those of its parents). This results in a spore which can mate with itself (intratetrad mating, automixis).
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.Birdsell JA, Wills C (2003). The evolutionary origin and maintenance of sexual recombination: A review of contemporary models. Evolutionary Biology Series >> Evolutionary Biology, Vol. 33 pp. 27–137. MacIntyre, Ross J.; Clegg, Michael, T (Eds.), Springer. Elvira Hörandl (2013). Meiosis and the Paradox of Sex in Nature, Meiosis, , InTech, DOI: 10.5772/56542
Although A. fumigatus occurs in areas with widely different climates and environments, it displays low genetic variation and lack of population genetic differentiation on a global scale. Thus the capability for heterothallic sex is maintained even though little genetic diversity is produced. As in the case of S. cereviae, above, a short-term benefit of meiosis may be the key to the adaptive maintenance of sex in this species.
Sexuality generates diversity in the aflatoxin gene cluster in A. flavus, suggesting that production of genetic variation may contribute to the maintenance of heterothallism in this species.
Henk et al. concluded that T. marneffei is sexually reproducing, but recombination in natural populations is most likely to occur across spatially and genetically limited distances resulting in a highly clonal population structure. Sex is maintained in this species even though very little genetic variability is produced. Sex may be maintained in T. marneffei by a short-term benefit of meiosis, as in S. cerevisiae and A. fumigatus, discussed above.
The subsequent steps following fusion of ‘A’ and ‘a’ haploid cells, have been outlined by Fincham and Day, and by Wagner and Mitchell. After fusion of the cells, the further fusion of their nuclei is delayed. Instead, a nucleus from the fertilizing cell and a nucleus from the ascogonium become associated and begin to divide synchronously. The products of these nuclear divisions (still in pairs of unlike mating type, i.e. ‘A’ / ‘a’) migrate into numerous ascogenous hyphae, which then begin to grow out of the ascogonium. Each of these ascogenous hypha bends to form a hook (or crozier) at its tip and the ‘A’ and ‘a’ pair of haploid nuclei within the crozier divide synchronously. Next, septa form to divide the crozier into three cells. The central cell in the curve of the hook contains one ‘A’ and one ‘a’ nucleus (see figure, top of §). This binuclear cell initiates ascus formation and is called an “ascus-initial” cell. Next the two uninucleate cells on either side of the first ascus-forming cell fuse with each other to form a binucleate cell that can grow to form a further crozier that can then form its own ascus-initial cell. This process can then be repeated multiple times.
After formation of the ascus-initial cell, the ‘A’ and ‘a’ nucleus fuse with each other to form a diploid nucleus (see figure, top of §). This nucleus is the only diploid nucleus in the entire life cycle of N. crassa. The diploid nucleus has 14 chromosomes formed from the two fused haploid nuclei that had 7 chromosomes each. Formation of the diploid nucleus is immediately followed by meiosis. The two sequential divisions of meiosis lead to four haploid nuclei, two of the ‘A’ mating type and two of the ‘a’ mating type. One further mitotic division leads to four ‘A’ and four ‘a’ nuclei in each ascus. Meiosis is an essential part of the life cycle of all sexually reproducing organisms, and in its main features, meiosis in N. crassa seems typical of meiosis generally.
As the above events are occurring, the mycelial sheath that had enveloped the ascogonium develops as the wall of the perithecium, becomes impregnated with melanin, and blackens. The mature perithecium has a flask-shaped structure.
A mature perithecium may contain as many as 300 asci, each derived from identical fusion diploid nuclei. Ordinarily, in nature, when the perithecia mature the ascospores are ejected rather violently into the air. These ascospores are heat resistant and, in the lab, require heating at 60 °C for 30 minutes to induce germination. For normal strains, the entire sexual cycle takes 10 to 15 days. In a mature ascus containing 8 ascospores, pairs of adjacent spores are identical in genetic constitution, since the last division is mitotic, and since the ascospores are contained in the ascus sac that holds them in a definite order determined by the direction of nuclear segregations during meiosis. Since the four primary products are also arranged in sequence, the pattern of genetic markers from a first-division segregation can be distinguished from the markers from a second-division segregation pattern.
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