Sac Fungi
Phylum Ascomycota
The sac fungi, or Ascomycetes, are a monophyletic group of fungi, which account for approximately 75% of all described fungi, including many of the known molds and yeasts, as well as the beloved morels and truffles: Saccharomyces cerevisiae, the yeast of the baking and brewing industries, Penicillium chrysogenum, producer of penicillin, and Morchella esculentum, the edible morel. This group is primarily saprotrophic in nature, decomposing leaf litter in forest environments. Some are parasitic on plants, such as the cause of Dutch Elm's disease (Ophiostoma ulmi), Powdery Mildew (Podosphaera xanthii), and Chestnut Blight (Cryphonectria parasitica), which was responsible for the demise of 4 billion chestnut trees in the eastern United States. Others are parasitic on animals, such as Athlete's Foot (Epidermophyton floccosum) and Candida albicans, which is the cause of thrush, diaper rash, and vaginitis in humans. It includes most of the fungi that combine with algae to form lichens, and the majority of fungi that lack morphological evidence of sexual reproduction. Asexual Ascomycota, such as Penicillium or Candida species, used to be classified separately in the Deuteromycota because sexual characters were necessary for classification. However, the comparison of nucleic acid sequence, as well as nonsexual phenotypic characters, have permitted the integration of asexual fungi into the Ascomycota. The Deuteromycota is no longer recognized as a formal taxon in fungal systematics.
Ecology and Form
Decomposers of plant litter
The body of a fungus are aggregated hair-like strands called mycelium (plural = mycelia), which creeps along the ground releasing enzymes and digesting plant material for energy.
Ascomycete mycelium have crosswalls (=septate), and the crosswalls are perforated
In their "vegetative" feeding phase, the mycelia are monokaryotic, in which they have 1 nucleus per cell, which is typical of most eukaryotes. During reproduction, these fungi become dikaryotic, in which they have 2 nuclei per cell. See Life Cycle below for more details.
Below: Wood pattern caused by Scolytus beetles, which are the vectors for Dutch Elm disease (Ophiostoma ulmi)
Above: Leaf with powdery mildew, caused by many fungi species (e.g. Podosphaera xanthii)
Below: Panama disease of banana, Fusarium oxysporum, causes Fusarium wilt
Life Cycle
Ascomycetes exhibit a complex, haplontic life cycle
Sexual reproduction in sac fungi begins when monokaryotic mycelia strands come in contact with mycelia from another individual
The tips of the mycelia, from each organism, fuse together. This creates new cells that combine the protoplasm and organelles of both organisms, but the nuclei do not fuse. This is a process called plasmogamy, and creates cells with two nuclei (=dikaryotic or binucleate)
From these fused cells, these two organisms will begin to produce a large fruiting body, called a ascocarp, such as a morrel. This ascocarp continues to grow and mature creating areas of fertile tissue.
Along the edge of the gills are special cells called asci (singular = ascus). Inside these cells, the two nuclei fuse to form a diploid zygote. This is called karyogamy, and completes fertilization.
The diploid zygote will then go through meiosis to produce four haploid spores, and each spore then divides through mitosis to form 8 spores in total. These spores are then released and dispersed into nature
The spores then germinate into monokaryotic mycelia
Above: Several asci with ascospores
Additional Resources
Oldest ascomycete found in the Rhynie Chert (Taylor & Kerp 1999)
Ergot found on oldest fossil grass floret (Paleoplant Blog 10Feb2015)
Fungomania II. A (not so) new face of Fusarium (Small things Considered 1Apr2021)