Genomics reveals sexual secrets of Aspergillus

Breeding systems in the lichen-forming fungal genus Cladonia

The breeding systems of three species of the lichen-forming fungal genus Cladonia were investigated. Cladonia floerkeana, Cladonia galindezii, and Cladonia portentosa were selected due to their contrasting ecologies and reproductive strategies, and because they belong to the Lecanorales, the major lichen-forming order. Sibling single-spore progeny were collected from apothecia and used to establish axenic cultures. Two experimental approaches were used to determine breeding systems. First, RAPD-PCR and AFLP fingerprinting revealed that spores from the same apothecium were not genetically uniform, indicating heterothallism in each of these species. Second, segregation of a MAT-2 mating-type gene was assessed using degenerate PCR primers designed to amplify the high-mobility group region. A MAT-2 gene occurred in 40-60% of progeny, consistent with a heterothallic breeding system. The PCR product from C. galindezii was cloned and sequenced, and confirmed to have the characteristic motifs of a MAT-2 HMG gene. This is thought to be the first report of the use of segregation of a mating-type gene among ascospore progeny to determine the breeding system of a fungal species. The ecological significance of the results is discussed.

Deletion and allelic exchange of the Aspergillus fumigatus veA locus via a novel recyclable marker module

Detailed evaluation of gene functions in an asexual fungus requires advanced methods of molecular biology. For the generation of targeted gene deletions in the opportunistic pathogen Aspergillus fumigatus we designed a novel blaster module allowing dominant selection of transformants due to resistance to phleomycin as well as dominant (counter)selection of a Cre recombinase-mediated marker excision event. For validation purposes we have deleted the A. fumigatus pabaA gene in a wild-type isolate by making use of this cassette. The resulting pabaA::loxP strain served as the recipient for subsequent targeting of the velvet locus. Homologous reconstitution of the deleted gene was performed by an allele whose expression is driven in a nitrogen source-dependent manner, as validated by Northern analyses. Overexpression of the veA locus in A. fumigatus does not result in any obvious phenotype, whereas the sporulation capacities of the veA null mutant are reduced on nitrate-containing medium, a phenotype that is completely restored in the reconstituted strain.

Reproduction inAspergillus fumigatus: sexuality in a supposedly asexual species?

Aspergillus fumigatus has long been considered to reproduce only by asexual means. However, accumulating evidence suggest that a sexual stage for A. fumigatus may yet be identified. We describe results from published and ongoing studies involving population genetic analyses, genome analysis, studies of mating-type gene presence and distribution, expression of sex-related genes, and taxonomic work which support the assertion that A. fumigatus has the potential to reproduce by sexual means. The consequences of sexual reproduction for the population biology and disease management of the species are discussed. The possible mechanisms of evolution of asexuality are then considered. It is proposed that asexual species may arise in one step by mutation or loss of a key gene(s), and/or there may be a 'slow decline' in sexual fertility within the species as a whole. Thus, it is argued that species should not be considered simply as sexual or asexual, but rather as individual isolates being present on a continuum of sexual fertility, with the implications for understanding sexuality/asexuality in A. fumigatus discussed.

The gprA and gprB genes encode putative G protein-coupled receptors required for self-fertilization in Aspergillus nidulans

The filamentous fungus Aspergillus nidulans possesses both asexual and sexual reproductive cycles. Sexual fruiting bodies (cleistothecia) can be formed in both homothallic (self) and heterothallic (outcross) conditions. In this study, we characterized two genes, gprA and gprB, that are predicted to encode putative G protein-coupled receptors (GPCRs) similar to fungal pheromone receptors. Deletion (Delta) of gprA or gprB resulted in the production of a few small cleistothecia carrying a reduced number of ascospores, whereas DeltagprADeltagprB eliminated fruiting body formation in homothallic conditions. However, nullifying gprA and/or gprB did not affect vegetative growth, asexual sporulation, Hülle cell formation or even cleistothecia formation in outcross, indicating that GprA and GprB are specifically required for self-fertilization. The gprA and gprB genes encode two transcripts and, for both genes, larger transcripts are detectable during vegetative growth and asexual development whereas smaller transcripts accumulate during sexual development. Upregulation of nsdD encoding a key sexual developmental activator resulted in the production of barren cleistothecia in the DeltagprADeltagprB mutant, suggesting that NsdD can partially rescue the developmental defects caused by deletion of GPCRs and that GprA/B-mediated signalling may activate other genes necessary for maturation of cleistothecia and ascosporogenesis. Deletion of gprA and/or gprB suppressed growth defects caused by DeltagprD, implying that GprA/B function downstream of GprD-mediated negative control of sexual development.

Comparative genomics in hemiascomycete yeasts: evolution of sex, silencing, and subtelomeres

The recent release of sequences of several unexplored yeast species that cover an evolutionary range comparable to the entire phylum of chordates offers us a unique opportunity to investigate how genes involved in adaptation have been shaped by evolution. We have examined how three different sets of genes, all related to adaptative processes at the genomic level, have evolved in hemiascomycetes: (1) the mating-type genes that govern sexuality, (2) the silencing genes that are connected to regulation of mating-type cassettes and to telomere position effect, and (3) the gene families found repeated in subtelomeric regions. We report new combinations of mating-type genes and cassettes in hemiascomycetous species; we show that silencing proteins diverge rapidly. We have also found that in all species studied, subtelomeric gene families exist and are specific to each species.

The Zn(II)2Cys6 putative Aspergillus nidulans transcription factor repressor of sexual development inhibits sexual development under low-carbon conditions and in submersed culture

Here we have characterized the putative Zn(II)2Cys6 transcription factor RosA from the filamentous fungus Aspergillus nidulans. The rosA gene encodes a protein of 713 aa, which shares 38% sequence similarity to Pro1 from Sordaria macrospora. In contrast to Pro1, which promotes the transition from protoperithecia to perithecia, RosA is a negative regulator of sexual development in A. nidulans. Transcript levels of rosA were usually very low and were only transiently upregulated upon carbon starvation and at 12 hr of asexual development. Deletion of rosA only slightly induced fruiting-body formation under standard culture conditions, but enabled sexual development under low-glucose and high-osmolarity conditions and the production of Hulle cells under submersed growth conditions. Stimulation of fruiting-body formation on agar surfaces was dependent on veA. In delta rosA strains, transcript levels of the sexual developmental regulators nsdD, veA, and stuA were increased. Overexpression of rosA led to a reduction of hyphal growth and to a fluffy phenotype. Post-transcriptional regulation of RosA, with a regulated accumulation in the nucleus, was shown using a RosA-GFP fusion protein. We propose that RosA represses sexual development upon integration of several environmental signals.

Polarity in filamentous fungi: moving beyond the yeast paradigm

Filamentous fungi grow by the polar extension of hyphae. This polar growth requires the specification of sites of germ tube or branch emergence, followed by the recruitment of the morphogenetic machinery to those sites for localized cell wall deposition. Researchers attempting to understand hyphal morphogenesis have relied upon the powerful paradigm of bud emergence in the yeast Saccharomyces cerevisiae. The yeast paradigm has provided a useful framework, however several features of hyphal morphogenesis, such as the ability to maintain multiple axes of polarity and an extremely rapid extension rate, cannot be explained by simple extrapolation from yeast models. We discuss recent polarity research from filamentous fungi focusing on the position of germ tube emergence, the relaying of positional information via RhoGTPase modules, and the recruitment of morphogenetic machinery components including cytoskeleton, polarisome and ARP2/3 complexes, and the vesicle trafficking system.