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Chapter 3 : Mechanisms of Homothallism in Fungi and Transitions between Heterothallism and Homothallism
Category: Fungi and Fungal Pathogenesis
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Sexual systems are classified into heterothallism or homothallism in fungi. This chapter focuses on examples of homothallic fungi and how their sexuality is governed. Conversions between heterothallic and homothallic sexual cycles are common evolutionary transitions in fungi, and whether homothallism evolved from heterothallism or vice versa is a controversial topic, and there is evidence supporting both hypotheses. The structural analyses of MAT sequences from homothallic and heterothallic Cochliobolus species support the hypothesis that heterothallism is the ancestral state in this genus. This unique arrangement of the Aspergillus nidulans MAT locus led to the proposal that homothallism is the ancestral state and that a transition from homothallism to heterothallism could have occurred in A. oryzae and A. fumigatus by gene loss, although neither species has a defined sexual cycle. Homothallic species may originate from heterothallic predecessors, and the homothallic lifestyle may have a selective advantage under certain ecological pressures. This hypothesis is consistent with the repeated occurrence of homothallism within numerous genera and the fact that many heterothallic fungi achieve homothallism in the form of pseudohomothallism by mating-type switching or packaging two compatible nuclei into one spore. Homothallic individuals contain all of the necessary genetic information for full sexual expression.
Heterothallic sexual reproduction in ascomycetes (upper row) and basidiomycetes (bottom row) shares several conserved features. A cell fusion event creates a dikaryotic state, and then, nuclear fusion occurs followed by meiosis and sporulation. The small circles represent nuclei. The solid and open circles indicate nuclei from different individuals.
Mating-type switching in Saccharomyces cerevisiae. The mother cell (depicted as a) undergoes mating-type switching to α and then mates with the daughter cell (a) to create a heterozygous diploid (a/α). This diploid cell can either amplify by mitosis through budding (not depicted) under rich media conditions or, in response to nitrogen limitation and nonfermentable carbon source, undergo meiosis to produce four meiotic ascospores with two a cells and two α cells. Solid and open circles indicate nuclei of a and α mating type. Only the shaded locus (MAT) is expressed. The thickness of the arrow represents the efficiency of the switching event. Arrows indicate the direction of the switching event.
Mating-type switching in Schizosaccharomyces pombe. The mother cell (P) undergoes a mating-type switch to M and then mates with the daughter cell (P) to create a heterozygous diploid cell (P/M), which undergoes meiosis immediately to produce four meiotic ascospores with two P cells and two M cells. Solid and open small circles indicate nuclei of P and M mating type. H1 and H2 are conserved flanking sequences for all three cassettes. H3 is a conserved element of the mat2P and mat3M silent cassettes. These conserved sequences are involved in regulation of mating-type expression and switching (for details, see the chapter by Nielsen and Egel [chapter 8]). Only the shaded locus (mat1) is expressed. The thickness of the arrow represents the efficiency of the switching event. Arrows indicate the direction of the switching event.
One form of pseudohomothallism involves packaging two nuclei of compatible mating type into a single spore. The dikaryon contains two nuclei of different but compatible mating types (for example, MAT-1 and MAT-2). After nuclear fusion in the basidium/ascus, meiosis occurs and four nuclei are produced. Two nuclei of each mating type are packaged into one spore, which gives rise to a dikaryon competent for sexual reproduction after germination.
Three molecular mechanisms of primary homothallism. The homokaryotic hyphae (a monokaryon in this figure) can undergo nuclear fusion, meiosis, and sporulation in the absence of a partner. The MAT structure in these homothallic fungi can be as follows: two fused idiomorphs are present in a single genome (I); two idiomorphs are present in a single genome but located at different genomic regions (II); and only MAT-1 is present, and MAT-2 is absent (III).
Aspergillus MAT structures and their evolution. Arrangement of the MAT idiomorphs and their flanking genes for A. nidulans, N. fischeri, A. fumigatus, and A. oryzae are shown. Homologous coding regions are color coded and indicated in the corresponding boxes. Arrows indicate the direction of expression. Hypotheses regarding the transition between homothallism and heterothallism in this genus are depicted. See the text and the chapters by Butler (chapter 1) and Dyer (chapter 7) for details.
The sexual cycle of Cryptococcus neoformans. During mating (the heterothallic life cycle), a and α yeast cells undergo cell-cell fusion and produce dikaryotic hyphae. At the stage of basidium development, the two parental nuclei fuse and undergo meiosis to produce four meiotic products that form chains of basidiospores by repeated mitosis and budding. During monokaryotic fruiting (the homothallic life cycle), cells of one mating type, e.g., α cells, become diploid α/α cells, either by endoreplication or by cell fusion followed by nuclear fusion between two α cells. At the stage of basidium development, meiosis occurs and haploid basidiospores of one mating type are produced in four chains.