Unit Four. The Evolution and Diversity of Life
Phylum Ascomycota, the ascomycetes, is the largest of the fungal phyla, with about 32,000 named species and many more being discovered each year. Among the ascomycetes are such familiar and economically important fungi as yeasts, morels, and truffles, as well as molds such as Neurospora (a historically important organism in genetic research) and many plant fungal pathogens, such as those that cause Dutch elm disease and chestnut blight.
Reproduction among the ascomycetes is usually asexual, just as it is among the zygomycetes. The hyphae of ascomycetes possess incomplete septa that separate the cells but have a large central pore in them, so that the flow of cytoplasm up and down the hypha is not impeded. Asexual reproduction occurs when the tips of hyphae become fully isolated from the rest of the mycelium by a complete septum, forming asexual spores called conidia (figure 18.7a, in the enlarged, circled area), each often containing several nuclei. When one of these conidia is released, air currents carry it to another place, where it may germinate to form a new mycelium.
It is important not to get confused by the number of nuclei in conidia. These multinucleate spores are haploid, not diploid, because there is only one version of the genome (one set of ascomycete chromosomes) present, whereas in a diploid cell there are two genetically different sets of chromosomes present. The actual number of nuclei is not what’s important—it’s the number of different genomes.
The ascomycetes are named for their sexual reproductive structure, the ascus (plural, asci), which differentiates within a larger structure called the ascocarp. The morel in figure 18.7b shows an ascocarp. The ascus is a microscopic cell that forms on the tips of the hyphae within the ascocarp, and it is where the zygote forms. The zygote is the only diploid nucleus of the ascomycete life cycle, indicated by the light blue section of the life cycle. The zygote undergoes meiosis, producing haploid spores called ascospores. When a mature ascus bursts, individual ascospores may be thrown as far as 30 centimeters. Considering how small the ascus is, this is truly an amazing distance. This would be like you hitting a baseball 1.25 kilometers, 10 times longer than Babe Ruth’s longest home run!
Figure 18.7. Life cycle of an ascomycete.
(a) Asexual reproduction takes place by means of conidia, spores cut off by septa at the ends of modified hyphae. Sexual reproduction occurs when the female gametangium, or ascogonium, fuses with the male gametangium, or antheridium, through a structure called the trichogyne. The ascocarp (b) develops from the dikaryotic hyphae and sterile hyphae. The nuclei fuse forming the diploid zygote inside the ascus. The zygote undergoes meiosis, leading to the formation of haploid ascospores.
Key Learning Outcome 18.6. Most fungi are ascomycetes, which form zygotes within reproductive structures called asci.