Ascomycota has been shown to be the largest phylum of fungi, as compared to the other phyla (Chytridiomycota, Zygomycota, Basidiomycota and Deuteromycota), with well over 33,000 species identified and named while many others are yet to be described.
This phylum is also morphologically diverse with species that range from single celled organism to multicellular cup fungi.
While a majority of Ascomycota species reproduce asexually, sexual reproduction has also been identified in some phases of reproduction.
Some of the organisms that belong to this phylum include:
Apart from being the largest phylum, the Phylum Ascomycota is also diverse. This diversity has been identified in their morphology, life cycle as well as their habitats.
With regards to morphology, some species are unicellular while others are more complex. Yeast is a good example of unicellular Ascomycota.
With other yeast-like fungi in the phylum, yeast is a structurally simple Ascomycota fungi that can exist as single cells. The shape of yeast varies depending on where they are grown and the type of nutrients available. For this reason, yeast cells can assume different structures.
Some of the other traits of these cells include; a cell wall, large vacuole, granular cytoplasm, a nucleus and nucleolus. While yeast can exist as single cells, they can also transform to multicellular organisms in a process known as dimorphism.
In such cases, yeast cells (Candida yeast) produce hyphae and pseudo-hyphae and thus appear more complex than the single celled yeast - In this state, the organisms have been shown to be pathogenic.
On the other hand, Some like Aleuria aurantia (also known as Orange peel fungus) are good examples of multicellular organisms. As such, they are more complex.
A good number of multicellular Ascomycota, such as A. aurantia, have a fruiting body and relatively large spores. When compared, these multicellular organisms will also be seen to vary in color, shape and size in addition to inhabiting different habitats.
Like Basiodiomycetes, some can also exist through a partnership with algae or cyanobacteria. In this symbiotic relationship, the algae or cyanobacteria benefits the fungi by synthesizing carbon compounds while the fungi host the organism.
This relationship results in the formation of lichens, which are a symbiotic relationship between the fungi and cynabacteria/algae. One of the other common relationships is found in mycorrhizal species where the fungi find shelter in the roots of plants.
* Based on morphology, Ascomycota is divided into saccharomycetes and mycelial ascomycetes. Whereas saccharomycetes are largely single celled organisms, including true yeasts among other unicellular organisms, mycelial ascomycetes are those that tend to form fruit bodies.
Some of the other traits of this group include; Woronin bodies, dikaryotic phase during their life cycle as well as septal pores.
Being a diverse phylum, various species can be found in a variety of habitats. For instance, whereas some of the yeast (Candida yeast) can be found in the body of the host (human beings) where they cause diseases, other more complex Ascomycota like Aleuria aurantia are terrestrial and can be found growing on habitats with warmth, moisture and decaying matter.
Species of Ascomycota that can be found in marine habitats include Decorospora gaudefroyi, Julella avicenniae and Massarina among others while types like mycorrhizal species can be found living in plant roots.
For Ascomycota species (filamentous species), the life cycle starts with the germination of the spores (haploid spores) to produce mycelia. Mycelia then grow vegetatively and mature to repeat the cycle. Once they are mature, the mycelia form conidia that produce spores. Once the spores are released, the life cycle begins.
Sexual reproduction takes place when gametes are produced. Essentially, these (gametes) are nuclei that are produced in the hyphae of the organism or within the spores and are capable of cross-fertilization with other gametes.
Here, the spores and hyphae of the filamentous Ascomycota acts as the gametangia that is responsible for the production of gametes for sexual reproduction.
For filamentous fungi, an organ referred to as a fruiting body is produced and develops on the mycelia of the organism. As soon as it is mature, it's fertilized by the male gametes (nucleus) that is produced from the conidium of another mycelia if they are compatible.
Following fertilization, the fruiting body develops further to produce a zygote (ascospores develop to zygote), which in turn develop into the mycelia of male/female nuclei. These nuclei can then divide and develop further for the cycle to start again.
* Because male and female parts are produced in these organisms, they have been described as hermaphrodites.
For the single celled species such as yeast, asexual reproduction involves cell division through mitosis. The fruiting body is therefore not produced in these single celled organisms.
Rather, fusion takes place between two different cells. Following budding, the spores (meiotic) germinate and ultimately fuse with a compatible resulting in the formation of a diploid cell from the two haploid cells. As soon as the cell has matured, budding begins resulting in the production of two daughter cells and the cycle continues.
Fruiting bodies are common among the mycelial ascomycetes.
Also referred to as ascomata or ascocarps, they are complex structures that are made up of different types of cells. The asci, which the nucleus develops is found within this complex structure.
There are four types of fruiting bodies that include:
While fruiting bodies are largely produced by mycelial ascomycetes, this has been shown to depend on a number of external factors including:
In favorable conditions, and once the vegetative mycelium are at a given phase of competence, they start to differentiate to form the fruiting bodies, which are important in sexual reproduction of some of the Ascomycota species.
The ascus (plural asci) is the uniting characteristic among all species of Ascomycota. These cells (asci) are contained in the ascomata/fruiting body and play an important role of producing ascomycete sexual spores (ascospores) that are involved in sexual reproduction among some of this species .
Asci vary in both shape and size from one species to another and also release spores that vary in shape and size.
Ascomycota is a large and diverse phylum. As such, it consists of many different types of species that can be found in different habitats. These organisms obtain their nutrition from different sources ranging from dead and decaying matter to nutrition from compounds synthesized by other organisms such as cynabacteria as is evident in their lichen symbiotic relationship.
Some of the species like entomopathogenic Ascomycota have evolved to infect and affect their hosts, but then switch back to saprophytisism. This allows them to survive in different environments.
Studies have also shown some members of phylum Ascomycota practice carnivorism by using specialized structures to trap their prey that include nematodes.
To trap and capture these prey, the species use such traps as constricting rings and adhesive traps like stalked adhesive knobs and sessile adhesive knobs among a few others. This allows them to capture, kill and obtain their nutrients from their prey.
Yanyan Wang et al. (2016) Lichen-Associated Fungal Community in Hypogymnia hypotrypa (Parmeliaceae, Ascomycota) Affected by Geographic Distribution and Altitude.
Ence Yanga et al. (2011) Origin and evolution of carnivorism in the
S. Pöggeler, M. Nowrousian and U. Küc. (2006) Fruiting-Body Development in Ascomycetes.
Satinee Suetrong, Sayanh Somrithipol and Ka-Lai Pang (2009) Classification of marine Ascomycota, anamorphic taxa and Basidiomycota.