Life cycle of Neurospora sp. The female element is represented by the protoperithecium. The male element are the conidia, which can supply nuclei to protoperithecium. This results in the formation of asci that bear haploid ascospores generated by sexual fusion of nuclei from two different mating strains. Neurospora may also reproduce asexually by means of conidia.
With it causes bizarre behavior, spontaneous abortion, or even death in humans and other animals. This pathological condition is called ergotism. Many animal diseases are caused by ascomycetes; most of the dimorphic ascomycetes produce systemic diseases of animals.
The genus Neurospora became one of the most important organisms in genetic reseaech when it was used by George Beadle and Edward Tatum in the early 1940s as an experimental model. The two researchers used the fungus to develop their “one gene-one enzyme” hypothesis of gene action, for which they received the Nobel Prize in 1958. This work initiated the era of molecular biology. Neurospora is particularly useful for genetic work because, within each ascus, the four products of meiosis divide once to from eight cells that remain fixed in a row in the order in which they were formed. Each ascospore in an ascus can be determined. This reveals the behavior of chromosomes during a single meiosis and the position of genes on those chromosomes. The life cycle of Neurospora is shown in FIGURE 10.8.
Morphologically, Neurospora produces a loose network of long strands of septate and aerial hyphae. Conidia, usually oval and pink in color, from branched chains at the tips of the aerial hyphae. Some Neurospora species are used in industrial fermentations, and some are responsible for food spoilage, particularly spoilage of starchy food.
Probably the best known of the ascomycetes are the yeasts. Typical of the budding yeasts is Saccharomyces cerevisiae. Strains of this species are used in the fermentation process to make alcoholic beverages. In the presence of oxygen, yeasts oxidize sugars to carbon dioxide, seen as “gas bubbles” in bread. Brewers’ and bakers’ yeasts have been used for thousands of years. Thus S.cerevisiae is a yeast of great and longstanding economic importance.
Cells of S.cerevisiad are elliptical, measuring about 6 to 8 by 5 m. [FIGURE 10.9A]. They reproduce asexually by budding. During budding, the nucleus divides by constriction and a portion of it enters the bud along with other organelles. The cytoplasmic connection is closed by the synthesis of new cell-wall material. Other yeasts can reproduce asexually by transverse binary fission. They are called fission yeasts (as opposed to budding yeasts). FIGURE 10.9B shows the fission yeast Schizosaccharomyces pombe. Note the fission scars on the cells.
The life cycle of S. cerevisiae is diagramed in FIGURE 10.10. In this yeast, both haploid and diploid vegetative stages may be present. Karyogamy (fusion of gametic nuclei) precedes the diploid vegetative stage; meiosis precedes a haploid vegetative stage. It should be noted that yeast life cycles may be quite varied. Variations are due to differences in the timing and location of plasmogamy (fusion of protoplasts), karyogamy, and meiosis.