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Chapter 7 : Sexual Reproduction and Significance of MAT in the Aspergilli
Category: Fungi and Fungal Pathogenesis
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The genus Aspergillus comprises over 180 species that are united by the presence of the “aspergillum,” which consists of a specialized, enlargened conidiophore bearing phialides and characteristic radiating asexual conidia. Of particular relevance to this chapter is the repeated failure to detect the presence of trichogynes or differentiated ascogonia and antheridia in this homothallic species. It has been noted that overexpression of the MAT genes does not result in increased expression of either the pheromone precursor gene ppgA or the pheromone receptor genes preA(gprB) and preB(gprB). It has been suggested that MAT gene expression might be involved in recognition of compatible MAT1 and MAT2 nuclei in heterothallic species. Hülle cells are also formed by three species of “asexual” aspergilli and single mating types of E. heterothallicus, which fail to form fertile cleistothecia. Paoletti and coworkers used RT-PCR to monitor expression of pheromone precursor (ppgA), pheromone receptor (preA and preB), and mating-type (MAT-1 and MAT-2) genes from A. fumigatus during growth on media, under conditions known to induce sexual reproduction in E. nidulans. It is intriguing to note that transposase genes have been found upstream of MAT loci in certain Aspergillus species, which might help to explain the particular ability of the aspergilli to evolve different breeding systems. A long-term goal is now to manipulate the life cycles of medically, economically, and scientifically important species of Aspergillus for the benefit of all.
Massed cleistothecia of E. nidulans (diameter, 150 to 200 μm) visible as dark spheres surrounded by lighter, smaller, glistening Hülle cells. Inset shows higher magnification; note resemblance to certain jeweled eggs produced by Carl Fabergé in the late 19th and early 20th centuries.
Hypothetical genetic model showing pathways involved with control of cleistothecial development in E. nidulans. See the text for details of particular genes encoding regulatory factors. Gene products are enclosed in the same box when the particular order of gene activity (or nature of interaction) is unknown or has not yet been experimentally verified. Cleistothecial initials refer to the very earliest stage of cleistothecial development.
Organization of MAT loci within the aspergilli showing possible evolution of breeding systems from either a homothallic or heterothallic ancestor (not to scale). Adapted in part from reference 20 . Textured blocks indicate mating-type genes (MAT1 α-domain or MAT2 HMG family) or flanking genes (SLA2 or DNA lyase). Dotted lines indicate idiomorph region; heavy bold lines indicate conserved sequence flanking the idiomorph region; the suffix ‘d’ indicates disabled pseudogene. Note that the illustration does not show all genes present in the flanking regions (e.g., an APC gene is also present, but syntenic order varies according to species).
Ascomycetous teleomorphs of Aspergillus subgenera