MCAT Biology Review: Fungi

FUNGI

The Kingdom Fungi includes eukaryotic, generally multicellular organisms with an unusual form of multicellularity. Their cells are not entirely divided by cell walls, so cytoplasm and even nuclei can flow between individual cells. The cell walls are made of chitin, a protein also found in the exoskeletons of arthropods, which reflects the close relationship between the Fungi and Animalia kingdoms. They are more closely related to each other than they are to plants. Fungi are split between acting as parasites, which prey on insects, plants, and animals, and saprophytes, which primarily aid in the breakdown of dead organic material. Fungi can also live in symbiotic relationships with other organisms. Lichens are the result of a symbiotic relationship between a fungus and a photosynthesizing organism. Other fungi live in symbiosis with plant roots to increase the root surface area.Fungi can be divided into three groups based on relatedness and shared reproductive habits: Basidiomycota, Ascomycota, and Zygomycota. Basidiomycota are characterized by the presence of basidia and dikaryotic fruiting bodies and include the mushrooms, puffballs, and shelf fungi. Ascomycota are characterized by the presence of asci and include such diverse organisms as the cup fungi and mildews as well as the unicellular fungi, yeast. Zygomycota are characterized by the presence of a thick-coated zygospore and include bread and other molds, plant and animal parasites, as well as many plant-root symbionts.

Basidiomycota: The Club Fungi

The division of fungi known as the club fungi, Basidiomycota, includes some of the most familiar fungi. Mushrooms, puffballs, and shelf fungi are all members of this group, as are the plant rusts and smuts. This group, which contains approximately 15,000 known species, is distinguished by the presence of a club- shaped reproductive organ called the basidium. This organ is most likely derived from the ascus found in Ascomycota (see Ascomycota Structure), with which it shares several characteristics. Both originate as a binucleate, dikaryotic structure and serve as a site for karyogamy and meiosis. They differ in that the basidium bears its spores outside while the ascus retains them inside the structure. The role of the basidium in sexual reproduction is discussed in heading.

StructureHaploid spores grow into cottony tangles of hyphae called mycelia. These mycelia usually grow under the surface until they meet up with another mycelium. The two join (plasmogamy) and produce a series of binucleate, dikaryotic hyphae that reach above the ground and form the fruiting body or basidioma. The cells of the basidioma cannot divide by normal mitosis because they have must produce two daughter cells each with a copy of both parental nuclei. This is accomplished through the formation of a clamp connection.

In this process, a bulging pocket forms in the hyphal cell wall at a point between the two nuclei. This pocket will eventually form the clamp. Both nuclei (a and b) then divide mitotically. These divisions are oriented such that the a' nucleus is positioned in the clamp pocket and both the a" and b'nuclei are toward the tip of the hypha, while the b" nucleus takes a posterior position. Next, a cell wall forms between the clamp, the posterior cell and the tip of the hypha. The tip now has a complete cell with two nuclei, but the posterior cell and the clamp each have only one nucleus. This is remedied when the clamp curves back toward the hypha and merges with the posterior cell.A feature used to identify Basidiomycota, aside from the presence of basidia, is the degree of separation between individual cells. Basidiomycota have more septate hyphae than Zygomycota, though their septae are perforated, allowing cytoplasm to flow freely between cells.

Asexual ReproductionBasidiomycota reproduce asexually by either budding or asexual spore formation. Budding occurs when an outgrowth of the parent cell is separated into a new cell. Any cell in the organism can bud. Asexual spore formation, however, most often takes place at the ends of specialized structures called conidiophores. The septae of terminal cells become fully defined, dividing a random number of nuclei into individual cells. The cell walls then thicken into a protective coat. The protected spores break off and are disbursed.

Sexual ReproductionSexual reproduction in Basidiomycota takes place in the fruiting body, in specialized structures called basidia. The basidia is itself formed by plasmogamy between mycelia from two different spores. Plasmogamy results in binucleate hyphae, that is, hyphae with two types of nuclei, one from each parent. In the gills of the fruiting body, some cells undergo fusion of these two nuclei. These now diploid cells are the basidia. The diploid phase is very brief. Soon after fusion, meiosis takes place, resulting in four haploid nuclei. The nuclei then migrate to the terminus of the basidium and form four individual projections. These projections are then separated by cell walls to become spores.

Overview: The Ascomycota are morphologically diverse. The group includes organisms from unicellular yeasts to complex cup fungi. There are 2000 identified genera and 30,000 species of Ascomycota. The unifying characteristic among these diverse groups is the presence of a reproductive structure known as the ascus, though in some cases it has a reduced role in the life cycle. As a group, they are economically important. Many are tree diseases, such as dutch elm disease and apple blights. The yeasts are used to produce alcoholic beverages and breads. The mold Penacillium is used to produce the anit-biotic penicillin.

Structure:Like basidiomycota, most ascomycota sprout from spores into hapliod mycelia. These mycelia can produce two types of reproductive structures. First, they can produce conidiophores for asexual reproduction. Conidiaphores may simply branch off from the mycelia or they may be formed in fruiting bodies. Secondly, ascomycota produce structures for sexual reproduction called gametangia. These structures are either male or female. The male gametangia may be anything from a detached cell (called a spermatium) to a differentiated region called an antheridium. The female structure is always a differentiated region known as the ascogonium. Many Ascomycota form a fruiting body, or ascoma, similar to that of the Basidiomycota, but with an important difference. The ascomycota fruiting body is composed mainly of entangled monokaryotic hyphae from the male and female mycelia rather than of dikaryotic hyphae formed from the joining of hyphae from the two mycelia, as in the basidiomycota. The only dikaryotic structures in the fruiting body are those produced by the gametangia after plasmogamy.The exceptions to the above discussion of structure are the unicellular ascomycota or yeasts. These organisms are non-motile single cells with chitinous cell walls that earn them classification as fungi. Though they mainly reproduce by budding and fission, yeasts also engage in sexual reproduction that results in the production of an ascus, placing them in the Ascomycota. Most varieties of yeast do not form multicellular filiments like the mycelia and hyphae of other fungi, though they do live in massive groupings called colonies.

Sexual Reproduction:Sexual reproduction in the Ascomycota differs from that in the Basidiomycota and Zygomycota because Ascomycota have male and female gametangia in their haploid stage. These structures, discussed in Heading , form on the mycelia. Plasmogamy, or the transfer of cytoplasm and nuclei, takes place when a part of the ascogonium, the trichogyne, fuses with the antheridium. This produces a binucleate, dikaryotic condition in the ascogonium. This phase is prolonged and a series of dikaryotic cells called an ascogonius hypha is produced. At the tip of this hypha, karyogamy or nuclear fussion takes place, resulting in the formation of a diploid ascus. Within this structure, the diploid nucleus undergoes meiosis, producing four haploid nuclei. These nuclei then undergo mitosis to form eight haploid ascospores. Notice that this is twice as many spores as produced in the basidium.

Ye

Yeast Reproduction:The different kinds of yeast most commonly reproduce by budding and fission, both forms of asexual reproduction. Budding occurs when a small portion of the cytoplasm of the parent cell becomes separated into small daughter cell. Fission involves an equal division of the cytoplasm into two daughter cells.Yeast can also reproduce sexually, and usually do so under starvation conditions. Most yeast have two mating types. In the most commonly studied species,Saccharomyces cerevisiae, these are designated a and alpha. When yeast of opposite mating types meet, the cells fuse (plasmogamy), followed by the fusion of their nuclei (karyogamy). This diploid cell can produce more diploid cells by budding. Eventually, a diploid cell will become and ascus and enter meiosis. This produces four haploid nuclei that are then surrounded by thick protective coats and become spores. These spores are released and become new haploid cells.

ASCOMYCOTA

Intro

Basidiomycota

Ascomycota