School of Life and Environmental Sciences, University of Nottingham, University Park, Nottingham NG7 2RD, UK
Correspondence
Paul S. Dyer
(Paul.Dyer{at}Nottingham.ac.uk)
The genus Aspergillus includes fungi of importance in the food and biotechnology industries, and pathogens such as Aspergillus fumigatus. It would therefore be of major economic and medical advantage to be able to study the inheritance of genes of interest and to bring together desirable genetic traits in the aspergilli. Unfortunately, such efforts have been impeded because most Aspergillus species are only known to reproduce asexually, thus the sexual cycle cannot be used for strain improvement and inheritance studies. However, the genomics revolution is now beginning to reveal the sexual secrets of Aspergillus, thereby offering the prospect of understanding reasons for sexuality and asexuality, and the basis of homothallic (selfing) or heterothallic (obligate outcrossing) modes of sexual reproduction in this group of fungi.
The genomes of two species, A. fumigatus and Aspergillus nidulans, have been sequenced recently and the data have been made available to the research community. A subsequent article by Varga (2003) noted that genome analysis revealed the presence of an ORF encoding a fungal mating-type (MAT) gene in A. fumigatus. This was highly significant because A. fumigatus is only known to reproduce by asexual means, yet MAT genes are involved with controlling sexual compatibility. This suggests either that A. fumigatus may have a latent potential for sexuality, as described elsewhere for the supposedly asexual pathogen Candida albicans (Gow et al., 2000
), or that A. fumigatus is derived from a sexual ancestor. The same observation was made by Pöggeler (2002)
, who also used genome analysis to identify other elements of fungal sexual pathways including ORFs encoding putative pheromones and pheromone receptors. In both reports, a MAT-2 family gene was identified encoding a regulatory protein with a high mobility group (HMG) DNA-binding domain (Turgeon & Yoder, 2000
).
We now further explore sexuality in Aspergillus by making observations of sexual pathway genes in the homothallic fungus A. nidulans. Prior to the Aspergillus genome studies, we had already characterized a MAT-2 gene from A. nidulans using classic molecular biological techniques (Dyer, 2002; Dyer et al., 2003
). However, these investigations failed to reveal any MAT-1 family gene encoding a protein with an alpha-box motif (Turgeon & Yoder, 2000
). In heterothallic ascomycete species, complementary MAT-1 and MAT-2 isolates are required for sexual reproduction. The MAT-1 and MAT-2 genes have also been found in homothallic ascomycetes, suggesting that MAT genes may have a role in regulating sex even in selfing fungi. A MAT-1 homologue has been detected in all homothallic fungi investigated so far, with an additional MAT-2 homologue present in many species often linked directly to a MAT-1 gene (Coppin et al., 1997
; Pöggeler, 2001
). Thus, the failure to detect a MAT-1 homologue suggested a novel situation in A. nidulans (Dyer, 2002
; Dyer et al., 2003
). However, by BLAST searching the newly available A. nidulans genome sequence (Aspergillus nidulans Database: http://www-genome.wi.mit.edu/annotation/fungi/aspergillus/), we now report the presence of a MAT-1
-domain homologue in A. nidulans, the first identification of a MAT-1 homologue from a plectomycete fungus (Fig. 1
). The A. nidulans MAT-1 ORF encodes a putative 361 aa polypeptide with at least one intron and a conserved
-domain core which exhibits 59, 52, 59 and 43 % identity to the
-domains of the ascomycetes Pyrenopeziza brassicae, Gibberella fujikuroi, Tapesia yallundae and Podospora anserina, respectively, over a 54 aa region. Intriguingly, the MAT-1 ORF is not linked to the previously identified MAT-2 gene, but is present elsewhere in the genome (on chromosomes 6 and 3, respectively), an unusual organization of MAT genes in homothallic euascomycetes, so far only reported from one other filamentous species, Cochliobolus cymbopogonis (Yun et al., 1999
).
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Finally, there is now the opportunity to study the expression of MAT genes in asexual and homothallic aspergilli, and compare this with heterothallic species in which they have more clearly defined roles. Ongoing research within the Sordariaceae indicates that similar signalling processes are required for sexual development in homothallic as in heterothallic fungi (Pöggeler, 2000). We, and other researchers (Han et al., 2003
), already have functional studies underway assessing sexual pathway genes in A. nidulans. As a whole, these sexual insights' emphasize how genomics may be of use in elucidating biological processes in tandem with post-genomic functional analyses.
Acknowledgements
Research is supported by the Biotechnology and Biological Sciences Research Council (BBSRC, UK). Sequence data were obtained from the Aspergillus Sequencing Project at the Whitehead Institute/MIT Center for Genome Research (http://www-genome.wi.mit.edu).
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