Published in the research journal Proceedings of the National Academy of Sciences, a new study describing the discovery of a sexual life cycle in a mold first identified 100 years ago. Fungi can be broadly classified a number of ways, for instance as “yeasts” or “molds” depending upon whether they have a unicellular or multicellular overall structure. This classification can break down with some fungi; a class of human pathogens are known as dimorphic fungi, and they can switch back and forth between a yeast form and a mold form depending upon environmental conditions.
Another fungal classification contains the imperfect fungi, a large group of species that have not been observed to have a sexual reproductive cycle. The imperfect fungi have therefore only been capable of reproducing asexually, through the process of mitosis. Populations of one of these fungi therefore would best be viewed as populations of clones, where all progeny in a population are essentially genetically identical to the population progenitor. The processes of mating to form diploid cells, and of meiosis to form haploid cells have not been observed in these fungi.
The study linked above describes the identification of a sexual cycle in the mold Penicillium chrysogenum, which has been used in the production of the antibiotic penicillin along with others, as well as a number of enzymes used in industry. The organism has classically thought to reproduce only asexually. Fruiting bodies called conidiophores produce unicellular spores, which are easily dispersed via air currents. The spores germinate upon reaching a favorable environment, and produce another mold. The new mold would be genetically identical to the parent.
Whole genome sequencing of P. chrysogenum has identified the presence of several gene families which greatly resemble genes from other fungi that do possess a sexual reproductive cycle, suggesting that this mold might have a sexual cycle involving mating and meiosis, but had never been observed in nature. The presence of mating type gene families led the researchers to first see if they could turn on this pathway in P. chrysogenum, and second to see if these genes behaved similarly in other fungi.
Mating pathways are frequently turned of in fungi when specific environmental conditions are present. For instance, the process of meiosis in diploid Saccharomyces cerevisiae (baking and brewing yeast) is strongly turned on under conditions when nitrogen sources of nutrition are limiting, and the cells are beginning to be starved. The resulting haploid spores are durable, and can survive until environmental conditions might be more favorable. With P. chrysogenum, the authors found that medium required the addition of the vitamin biotin in order for sexual spores to be formed. The resultant spores furthermore demonstrated that they were haploid, and that genetic recombination between various known genes had occurred.
Additionally, the authors found that production of penicillin by this species was dependent on expression of MAT1-1-1, one of the mating-type genes. Genetic engineering to force P. chrysogenum to express the MAT1-1-1 gene all the time resulted in a mold that had higher levels of penicillin production. This finding has importance for many industrial applications which use enzymes purified from molds, as this will allow these applications to become more efficient. The most significant advantage of this finding though is that researchers will be able to more rapidly improve industrial fungal strains through sexual recombination, as opposed to the previous strategy of inducing random mutations which may introduce deleterious effects.