An avid reader pointed out that I was not entirely thorough in describing that we don’t enough about the V8 agar media that is used to induce mating in Cryptococcus. In fact a great deal of work on mating in this fungus had focused on identifying what pathways are induced by V8 agar that induce mating. It was shown that inositol stimulates mating through use of defined media containing inositol (Xue et al, 2007). This paper interestingly explores plant-fungal interactions and Cryptococcus suggesting that mating may occur preferentially on plants in cases where inositol is abundant.
It is also worth noting that V8 media contains a high level of copper ions and it was also pointed out to me that Jef Edman’s lab showed that melanin mutants have mating defects, and both phenotypes are suppressed by copper. And more recently (Lin et al, PLOS Genetics 2006) found that alleles of the Mac1 copper regulated transcription factor are a QTL influencing hyphal growth and melanin production, and showed that copper can enhance hyphal growth.
So the role of copper and interplay with V8 agar media and how this induces mating is actually quite known.
C XUE, Y TADA, X DONG, J HEITMAN (2007). The Human Fungal Pathogen Cryptococcus Can Complete Its Sexual Cycle during a Pathogenic Association with Plants Cell Host & Microbe, 1 (4), 263-273 DOI: 10.1016/j.chom.2007.05.005
A manuscript at Nature AOP details the success of the Dyer lab and collaborators in encouraging Aspergillus fumigatus to complete the sexual cycle under observable (e.g. laboratory) conditions. The authors are the teleomorph (sexual or perfect) stage Neosartorya fumigata for a fungus that had been previously only had an observed anamorphic stage. A. fumigatus can reproduce asexually forming structures called conidiophores which produce asexual spores called conidiospores (or mitospores as they are produced via mitosis) define the anamorph or imperfect stage, but no sexual structures such as cleistothecia that produce the packaged sexual products as ascospores. See a presentation by David Geiser (archived at the Aspergillus website) for more detail on some of the morphological and phylogenetic characters that unify the group of Eurotiales fungi.
Like several other groups of fungi, A. fumigatus was presumed to have a putative cryptic sexual stages inferred from population genetic evidence of sexual recombination, but until no telemorphs had been observed. In addition, an observed perfect stage doesn’t necessarily indicate it is easy to induce mating in laboratory conditions. Complicated media including the ever stressful V8 juice was needed to induce mating in the basidiomycete yeast Cryptococcus neoformans (Erke, J Bacteriol 1976). In fact, Christina Hull’s lab has shown we still don’t even know what ingredients in V8 juice even induce mating (Kent et al, AEM 2008)! Other fungi including Coccidioides have been implicated as cryptically sexual (Burt et al, PNAS 1996) but no one has been able to induce mating in laboratory conditions. In this case a petri plate with a individual of each mating type (since this is a heterothallic fungus), and a series of different media conditions provided an environment suitable for mating to occur.
The work in this paper follows from their previous work identifying isolates of different mating types (Paoletti, Current Biol, 2005). The discovery of sexual stage for Aspergillus fumigatus (which Bret cannot pronounce) is a boon for molecular geneticists in construction of knockout strains and ability to follow recombination. While A. nidulans is a sexual species and model system for genetics, it is useful to have more tools to directly manipulate A. fumigatus and directly test hypotheses about genes involved in pathogenicity.
This observation of meiosis in the laboratory is also is interesting to considered in light of work that RIP is active in other Aspergillus species (and also see this post) suggesting that RIP may be operating under meiotic conditions.
Isolates of different mating types have also been described for the putatively asexual Coccidioiodes (Mandell et al, EC 2007; Fraser et al, EC 2007) so it remains a possibility that we can also induce sexual recombination in laboratory conditions in this fungus.
Céline M. O’Gorman, Hubert T. Fuller, Paul S. Dyer (2008). Discovery of a sexual cycle in the opportunistic fungal pathogen Aspergillus fumigatus Nature DOI: 10.1038/nature07528
A quick post of some recent comparative genomics papers on our desk that are worth a look.
- Khaldi N, Wolfe KH (2008) Elusive Origins of the Extra Genes in Aspergillus oryzae. PLoS ONE 3(8): e3036. doi:10.1371/journal.pone.0003036. This was a cool but somewhat controversal finding presented at Fungal Genetics last year.
- Casselton, LA. Fungal sex genes – searching for the ancestors. doi: 10.1002/bies.20782. A review of recent findings about the Zygomycete MAT locus.
- Soanes DM, Alam I, Cornell M, Wong HM, Hedeler C, et al. (2008) Comparative Genome Analysis of Filamentous Fungi Reveals Gene Family Expansions Associated with Fungal Pathogenesis. PLoS ONE 3(6): e2300. doi:10.1371/journal.pone.0002300
- Lee DW, Freitag M, Selker EU, Aramayo R (2008) A Cytosine Methyltransferase Homologue Is Essential for Sexual Development in Aspergillus nidulans. PLoS ONE 3(6): e2531. doi:10.1371/journal.pone.0002531
On the cover of this week’s Nature is a picture of Phycomyces blakesleeanus highlighting the discovery of the MAT locus in this Zygomycete fungus from Alex Idnurm and Joe Heitman and colleagues. While it was previously known that Zygomycetes (the Orange lineage represented by R. oryzae in the tree below) mate, the specific locus has until now, never been discovered. The authors in this study identified the MAT locus through a sequence search looking for HMG-box genes knowing that these are found the Mating Type locus in Basidiomycetes and Ascomycetes. They confirmed the identity through a through set of experiments that included PCR, sequencing and crosses of (+) and (-) strains of P. blakesleeanus, and Southern blots.
Continue reading Sex in fungi: MAT locus cloned from a Zygomycete