Conserved Regulators of Mating Are Essential for Aspergillus fumigatus Cleistothecium Formation

Abstract

Sexual reproduction of the human pathogen Aspergillus fumigatus (teleomorph: Neosartorya fumigata) was assumed to be absent or cryptic until recently, when fertile crosses among geographically restricted environmental isolates were described. Here, we provide evidence for mating, fruiting body development, and ascosporogenesis accompanied by genetic recombination between unrelated, clinical isolates of A. fumigatus, and this evidence demonstrates the generality and reproducibility of this long-time-undisclosed phase in the life cycle of this heterothallic fungus. Successful mating requires the presence of both mating-type idiomorphs MAT1-1 and MAT1-2, as does expression of genes encoding factors presumably involved in this process. Moreover, analysis of an A. fumigatus mutant deleted for the nsdD gene suggests a role of this conserved regulator of cleistothecium development in hyphal fusion and hence heterokaryon formation.For decades, aspergilli have served as model organisms in genetic studies, based on their multifaceted life cycle. The characteristic asexual reproductive mode of Aspergillus, during which conidiospores are abundantly formed on highly characteristic structures, the conidiophores (synonym, aspergillum) (2), defines the genus morphologically. Fundamental aspects of the genetics, such as heterokaryosis and parasexuality, were first studied in the model system Aspergillus nidulans (38). Moreover, about one-third of all known Aspergillus species are able to form fruiting bodies, the so-called cleistothecia, in which recombinant ascospores are generated by meiosis (8,15,48). Again, most insights into cleistothecium formation stem from studies on the homothallic ascomycete A. nidulans with its teleomorph Emericella nidulans, for which a variety of environmental as well as genetic determinants for successful fruiting were identified: surface growth in darkness and with restricted aeration generally favors sexual development, which might correspondingly be the dominant reproductive mode in the soil or within substrates. Nuclear identity is presumably achieved by a bipolar mating-type system comprising the highmobility group (HMG) factor MatA and the ␣-box protein MatB (36, 46). Among regulators of cleistothecium formation that have been characterized at the molecular level, the GATA-type transcription factor NsdD plays a prominent role, as it determines the balance between asexual and sexual propagation and positively supports fruiting (18).For Aspergillus fumigatus, the predominant human pathogen among aspergilli, sexuality had been only supposed for a long time, based on several lines of reasoning. The very first hints were deduced from the preliminary genome sequence, in which several genes likely to encode a pheromone and respective pheromone receptors could be identified (37). Moreover, the presence of a putative mating-type locus was pointed out in these primary studies (12,53). A more comprehensive inspection of the complete and annotated genome sequence retrieved several more homologues that ar...

Presence and Functionality of Mating Type Genes in the Supposedly Asexual Filamentous Fungus Aspergillus oryzae

“…In contrast, no significant differences in the transcript levels of putative pheromone receptor genes were observed between the MAT1-1 and MAT1-2 strains by DNA microarray analysis (data not shown). This is consistent with previous reports of analyses of pheromone receptor gene expression, which did not show differential expression in isolates of other heterothallic Pezizomycotina with different mating types (34,49,62).…”

“…Subsequently, the MAT1-1 and MAT1-2 products of A. fumigatus were also shown to be functional by their ability, through heterologous complementation, to restore fertility to MAT deletant strains of A. nidulans, at a time when A. fumigatus was considered asexual (22,57). Following the discovery of a sexual cycle in A. fumigatus (47), the MAT1-1 and MAT1-2 genes were shown to be required for sexual development in A. fumigatus, as evidenced by the sterility of MAT deletant strains (62). However, the genetic mechanisms by which mating type genes regulate sexual reproduction in the aspergilli still remain largely unknown, although a recent study has provided insight into the regulatory mechanisms controlling the expression of A. nidulans MAT2 (a synonym of matA) during the sexual cycle (10).…”

NsdC and NsdD Affect Aspergillus flavus Morphogenesis and Aflatoxin Production

“…The lack of ability to produce sclerotia of the A. flavus ⌬nsdC and ⌬nsdD mutants observed in this study suggests that formation of these structures is controlled similarly to the formation of cleistothecia in A. nidulans (30) and A. fumigatus (40). The absence of sclerotia in the mutants could be caused by defects in hyphal fusion, as was observed previously in A. fumigatus ⌬nsdD mutants (40).…”

“…The absence of sclerotia in the mutants could be caused by defects in hyphal fusion, as was observed previously in A. fumigatus ⌬nsdD mutants (40). This observation suggested that NsdD may be necessary for transcriptional activation of genes required for production of proteins involved in hyphal anastomosis as well as cell wall integrity and maintenance.…”

“…Both the A. flavus ⌬nsdC and ⌬nsdD mutants accumulate a dark pigment during growth on agar media (Fig. 1) that appears to resemble the dark pigments produced by A. nidulans nsdC and nsdD mutants (22) and A. fumigatus nsdD mutants (40). The pigment in the A. flavus mutants appears to resemble melanin, but because its production was not affected by the melanin biosynthesis inhibitor tricyclazole, it does not appear to be the product of the DHN-melanin pathway (7).…”