A large number of species are Many are saprotrophs, occurring on a number of substrates and occur in diverse is a large and very diverse group that has historically been difficult to characterize.
The unifying that all members whose sexual reproductive stage is known produce ascospores inside of a sac-like cell called an ascus Fig. However, there are also numerous species that are known only from their asexual stage. This has made their classification, based on morphology, within the Ascomycota difficult. However, with utilization of molecular tools, progress in in the proper placement within the Ascomycota has advanced rapidly in recent years.
Traditionally, the classification of the Ascomycota were divided into two major groups: The yeast and the mycelial ascomycetes. The yeast and yeast like species consisted of species that were unicellular, with short hyphal growth and those that have a yeast and mycelium phase, i. Hyphae and mycelium are septate. Their asci and ascospores are borne naked, i.
Species of mycelial ascomycetes have septate mycelium and a yeast stage is absent. During sexual reproduction their asci and ascospores are borne in a complex sporocarp and were divided into four classes, based on the type of sporocarp produced.
However, as is the case with all of the phyla in which recent molecular systematic studies have been carried out, the Ascomycota have under gone major revisions in the classification within this phylum.
A recent, extensive study carried out by Schoch, et al. Earlier studies on the phylogeny of the Ascomycot have also been carried out Lutzoni, et al. There is, presently, agreement that the Ascomycota represents a monophyletic line and that there is 3 subphyla.
We will cover the Saccharomycotina and Taphrinomycotina, and an introduction to the Pezizomycotina, today, and save the topic of phylogeny of the much more complex Pezizomycotina next week. A recent study by Kurtzman and Robnett using 70 type cultures within this group, confirms that the genera recognized in this subphylum represents a monophyletic line.
Species reproduce asexually by budding Fig. It was long suggested that the yeast thallus was found in basal species and that mycelium was derived. However, recent studies now indicate that the yeast stage was derived and that basal species with abundant mycelium, such as in Geotrichum candidumrepresents the earliest evolved species Liu and Hall, ; Landvik, et al.
Most species are unicellular and reproduce asexually by budding. The process of budding begins in a mature yeast cell at predetermined areas of its cell wall. In Saccharomyces cerevisiaefor example, budding takes place at the poles of the cell. In these areas when budding is about to take place, the cell wall is softened and is "blown out" to form the so-called "bud", which will become the new cell Figs. As the bud enlarges, mitosis of the nucleus occurs, with one of the nuclei Fission yeast can reproduce asexually following mitosis by brandon into the newly formed cell.
When the cell reaches the approximate size of the original cell, cell wall material is laid down in the passage between the two cells, which will then shortly separate. Although most species are unicellular in the Saccharomycotina, there are examples of species that have short hyphal growth to those that are mycelial.
We will use Saccharomyces cerevisiae as a representative life cycle of a budding yeast. This species is probably the most studied species of yeast because of its economic importance in the beer and wine industry, as well as in baking. Saccharomyces cerevisiae is a heterothallic species and require the presence of two mating strains of yeast cells that have been designated as "a" and " a ".
Fusion of the two mating strains will produce the zygote. Unlike other species of fungi, the life cycle of this species is not truly zygotic. The diploid cell does not undergo meiosis and will assimilate food and reproduce asexually for a time. Thus, its life cycle is more comparable to a sporic life cycle, but neither the haploid nor diploid stage are multicellular. The life cycle is completed when meiosis occurs in the diploid yeast cells and each of the four nuclei and their adjacent cytoplasm are enclosed with membrane, from the plasma membrane.
Cell walls forming around the nuclei and membrane completes the formation of the ascospore, with the original cell wall of the diploid yeast cell serving as the ascus Fig. Two of the ascospores will be of the "a" and the other two will be of the "Fission yeast can reproduce asexually following mitosis by brandon" a " mating strains. A summary of the life cycle is illustrated below Fig.
Although most species of the sub phylum are unicellular yeast that bud, there are examples of species that have short hyphal growth to one that have extensive mycelium.
A common species with short hyphal that is found on the slime flux of various trees is Dipodascopsis uninucleata Fig. A common species that has extensive mycelial growth is Geotrichum candidum Fig. It is a plant pathogen that can cause sour rot on peaches, nectarines tomatoes, carrots and lemons. It may also cause a lung disease, geotrichosis, in humans.
It has recently been suggested that the Taphrinomycotina is the most basal of the three sub phyla and to be monophyletic Schoch, et al. The sub phylum is more diverse than the Saccharomycotina and 4 classes are recognized: We will use Schizosaccharomyces octosporusas an example of a fission yeast and Taphrina deformanas a representative of a dimorphic species. Schizosaccharomyces octosporus is an example of Fission yeast can reproduce asexually following mitosis by brandon fission yeast, which also readily produce asci and ascospores in vitro Fig.
Thus, any two yeast cells of this species can fuse to produce a zygote.
Once the zygote is formed, it immediately undergoes meiosis. The old zygote cell wall becomes the ascus. The life cycle is summarized below Fig. Taphrina deforman is a dimorphic species that is a pathogen on peach trees.
The specific epithetic describes the deformation of the leaves by hypertrophic growth of cells Fig. The asci and ascospores are borne naked on the surface of the leaves Fig. The yeast phase is saprotrophic and persists in the soil and reproduces asexually by budding. When contact with the host cell occurs, the yeast germinates and undergoes synchronous mitotic division of the nucleus to give rise to a dikaryotic binucleate mycelium.
The mycelial stage is the phase of the life cycle in which sexual reproduction occurs. Karyogamy occurs in the ascogenous cells that then undergoes an unequal mitotic division, giving rise to a small, rectangular stalk-cell that anchors the larger ascus cell that will give rise to the ascus and ascospores.
Ascospore formation occurs following meiosis and Fission yeast can reproduce asexually following mitosis by brandon mitotic division that will give rise to 8 nuclei that will be walled off with the cytoplasm adjacent to the nuclei to give rise to 8 ascospores. The Pezizomycotina is the largest of the sub phyla, consisting of more than 32, species.
It is characterized by septate mycelium that will give rise to complex sporocarps on or in which asci and ascospores will be borne. Mycelial cells have incomplete septa that has a single pore that is often associated with Woronin body Fig These structures are vesicles that function in sealing the septal pore when cellular damage occurs.