Ascomycota is the largest phylum of fungi
Ascomycota and Basidiomycota share a number of characters:
Primary morphological character that distinguishes members of the Ascomycota are the ascus a sac-like cell containing the ascospores cleaved from within by free cell formation after karyogamy and meiosis. Eight ascospores typically are formed within the ascus, but this number may vary from one to over a thousand according to the species.
(IMAGE) Mycelial ascomycetes are characterized by a compartmentalized mycelium with distinctive walls, septa having simple pores, and the presence of Woronin bodies
saprobes - biotrophic
masters of symbioses: mutualists - commensals - parasitic
terrestrial - aquatic - marine
Many ascomycetes are closely associated with insects. Some, such as Ophiostoma, Ambrosiella, Raffaelea, and Symbiotaphrina, are found in insect mycangia and provide or detoxify the food of the insects.
Ascomycetes produce most of the known plant growth regulators; disease symptoms called leaf curls and witches' brooms, seeing the effects of these fungal products that induce the plants to differentiate in unusual ways. Gibberellin, the growth regulator involved in stem elongation, was first discovered in rice infected by Gibberella fujikuroi, the cause of foolish seedling disease.
Fungal secondary products may act as pheromones, some of which provide signals to fungus-associated insects or mammals. E.g., truffles
Ascomycetes may have two distinct reproductive phases, one sexual involving the formation of the asci and ascospores mentioned earlier, and the other asexual, with spore production occurring at different times on the same mycelium.
(IMAGE - Asco life cycle)
Ascomycetes are delimited and classified by their sexual reproductive structures; vast numbers of ascomycetes known only by their asexual stages can be difficult, and the practice has been to place these taxa in the artificial group called Deuteromycota or Fungi Imperfecti.
Somatic StructuresSomatic stages of ascomycetes may be single-celled, mycelial or dimorphic A large proportion of the cell walls of filamentous ascomycetes is chitin In the yeasts (Saccharomycetales) mannans and ß-1,3 glucans are the principal wall polysaccharides, and only limited amounts of chitin are present, often restricted to bud scars
In ascomycetes the hyphal wall appears two-layered, with a thick translucent inner layer and a dense, thin outer layer. Some basidiomycete walls are characterized by several interspersed dense and translucent layers, but this is not universal. Hyphae of ascomycetes are divided into compartments by septa that form from the hyphal periphery and advance toward the center, thus invaginating the plasma membrane. In most ascomycetes a small circular opening or pore is left near the center of the septum through which the plasma membrane and cytoplasm extend from one hyphal compartment to the next
In the filamentous ascomycete septum, the potential exists for cytoplasmic continuity between all parts of the mycelium; septal pores may be plugged or blocked by various types of membrane-bound structures
Woronin bodies are spherical, hexagonal, or rectangular membrane bound structures with a crystalline protein matrix that usually are associated with the septum; elevated levels of nitrogen, sulfur, and phosphorus in Woronin bodies when they were compared to the adjacent cell walls of the species they studied. Woronin bodies frequently plug the septal pores of hyphae and it is believed that they serve to separate aging or damaged hyphae from the rest of the mycelium, but details of their exact function remain unknown.
In addition to the Woronin bodies that may plug septal pores, a more complex structure occurs in some filamentous ascomycetes;septal pore organelles, often shaped like pulley wheels, that are distributed in parts of the mycelium so that structures involved in sexual reproduction routinely are isolated from other regions of the mycelium. They usually are found at the base of asci and sterile parts of the hymenium of the ascocarp.
Hyphal compartments often are uninucleate, but mycelia consisting of multinucleate cells are well known. The perforations in the hyphal septa permit nuclei to migrate from one compartment of a hypha to another; the ability of nuclei to migrate throughout the mycelium is important in the phenomenom of heterokaryosis Ascomycete mycelium may be organized into fungal tissues (plectenchyma); If such a tissue is loosely woven and the mycelial strands are more or less evident, it is known as prosenchyma. If, however, the hyphae have lost their individuality and the cells are more or less isodiametric, closely resembling the parenchyma of plants, it is known as pseudoparenchyma.
Sexual Reproduction - unifactorial
Two compatible nuclei are brought together in the same cell by one of several methods:
In the ascomycetes, with yeasts being the major exception, the two nuclei may remain in close association and undergo successive divisions that result in a number of dikaryotic cells.
Nuclear fusion eventually takes place in the young ascus. Meiosis in the diploid zygote nucleus occurs almost immediately after fusion, and results in the production of four haploid nuclei. These four nuclei then divide mitotically, resulting in the formation of eight nuclei that will become incorporated into the eight ascospores during ascosporogenesis.
Homogenic incompatibility- is a process that promotes outcrossing in sexual fusions and is controlled by mating type genes.
Heterogenic incompatibility- the process that governs fusion of like somatic or vegetative hyphae; referred to as somatic or vegetative incompatibility.
Sexual reproduction in heterothallic ascomycetes requires the participation of genetically different strains. The mating system is controlled by a single genetic locus (MAT) which specifies one of two alternative mating types, and is termed unifactorial or bipolar.
The first mating type locus cloned was from Saccharomyces cerevisae. Haploid cells are designated either MAT A or MAT a. Each mating type signals its presence to a cell of the opposite mating type by producing specific peptide pheromones. The a cells secrete a factor that is recognized by a receptor protein in A cells, and conversely A cells produce A factor that binds to a receptor in a type cells. The binding of both pheromones to the receptors results in a transient arrest of the cell cycle and induces the production of cell surface molecules that facilitate conjugation and fusion of the two cells
In the ascomycetes that have been studied, the genes at the MAT loci are completely dissimilar and cannot be regarded as homologous alleles. The term "idiomorph" has been coined to describe such alternative forms of a locus that lack significant sequence homology, but occupy the same site in the genome
Other points about ascomycete mating loci.
Mating type switching - occurs when a cell of the opposite mating type is not present and is due to "silent copies" of the second mating gene that are present at other loci in the genome of a single cell. Some ascomycetes are homothallic.
Vegetative or somatic incompatibility in ascomycetes prevents the fusion of genetically different mycelia (heterogenetic incompatibility) and is usually under multigenic control by a series of bi- or multiallelic genes at vegetative incompatibility (het or vic) loci. When two mycelia in different vegetative compatibility (vc) groups meet in a substrate, they may interact with varying degrees of antagonism, as would be expected of a phenotype under multigenic control. In some cases barrage reactions may be recognized by a clear zone between the two mycelial fronts due to the lysis of the interacting cells. Sometimes incomplete interactions occur at the margins and unstable dikaryons may exist for a short time. In some cases pigments or enhanced conidium production indicates the meeting of the two mycelia
How do heterothallic ascomycetes in different VC groups undergo sexual fusions. In the species that have been studied, separate individuals are maintained until the time of sexual reproduction when morphologically and physiologically differentiated sex organs, the ascogonia and antheridia, develop and apparently are not usually under the control of the vegetative incompatibility system.
Life Cycle Pattern
No "typical" life cycle to describe
Pyronema omphaloides as an example that illustrates the pattern of both sexual and asexual reproduction in a single life cycle
In many species of filamentous ascomycetes each ascogenous hypha branches and rebranches in various ways, and produces a cluster of asci. This often is accomplished as follows: The crook cell elongates into a new hook instead of developing directly into an ascus, and the tip and basal hook cells fuse and form another hook by the side of the first. This process may be repeated several times, forming a cluster of hooks, the crook cells of which finally develop asci.
Ascospores are formed within the ascus as a result of free cell formation or ascosporogenesis.
Two mechanisms involved:
In most of the filamentous ascomycetes examined these envelopes initially are part of a discontinuous cylinder that forms around the extreme periphery of the ascus very near the plasma membrane and surrounds all of the nuclei of the ascus. This cylinder, which consists of two closely spaced unit membranes is called the ascus vesicle or the enveloping membrane system (EMS).
The EMS invaginates and fragments into sheets that then cleave out the young ascospore; not all the cytoplasm within an ascus is incorporated into the ascospore initials. The portion that remains outside the spores is called epiplasm and possibly serves to nourish the developing spores and deposit the external ornamentation on the spores.
Once an ascospore initial has been delimited by the EMS, the inner membrane of the EMS becomes the spore plasma membrane while the outer membrane becomes what has been termed the "ascospore investing membrane". The investing membrane is displaced progressively from the plasma membrane of the spore as the spore wall develops between these two membranes. Although it appears that the inner portion of the ascospore wall is deposited by the spore itself, in some species at least part of the outer wall layer(s) as well as the spore ornamentation appear to be deposited by the epiplasm of the ascus.
Most common number of ascospores produced per ascus in most species is eight. However, some species routinely produce asci containing smaller numbers of spores including one, two, three, or four, while larger numbers of spores, up to possibly as many as a thousand, are produced in a few species. Ascospores come in a variety of sizes and shapes ranging from long, thin, and thread-like to globose and even hat-shaped in appearance. Ascospores may be one-celled or septate.
Croziers and clamp connections:
Crozier and clamp connections are homologous?
Ascus mother cell - basidium
Endospores vs. exospores
(IMAGE - See p81)
Asci may be spherical to elongated with cylindrical, ovoid, or globose forms
Asci may be stalked or sessile; they may arise at various levels within the ascocarp or from a single level.
A definite layer of asci, whether naked or enclosed in an ascocarp is called a hymenium.
Developmental studies have shown that sometimes asci may develop in a hymenium, but become rearranged at maturity so that they appear to be scattered.
Three basic types of asci can be defined from light microscopy studies: prototunicate, unitunicate, and bitunicate.
The prototunicate asci have a thin, delicate wall and release their spores by deliquescing.
The wall in both a unitunicate and a bitunicate ascus is said to consist of two layers: exotunica and endotunica. In the so-called unitunicate ascus these layers adhere closely throughout the life of the ascus, and the spores are released through a terminal pore, slit, or hinged cap (operculum).
In the bitunicate ascus the endotunica expands up to twice or more its original length, separating from the ruptured exotunica at the time of spore release. Spores are released through a pore in the endotunica. Because of this behavior, the bitunicate ascus has been called the Jack-in-the-box ascus or the fissitunicate ascus by lichen specialists.
In general there are five ways that ascomycetes can be separated according to the way they bear their asci:
Ascocarps may be formed singly or in groups. They may be superficial, erumpent, or deeply embedded in the substrate. In some cases ascocarps with true walls may form in a stroma. The stroma may be composed of both fungal and host tissue in some plant pathogenic species, or a stroma may be made entirely of fungal tissue
Hamathecium- the sterile cells and hyphae that are interspersed among asci or project into the locule or ostiole of the ascocarp.
Release, Dispersal and Germination of Ascospores.
In some cases the method of release is passive with physical forces or animals breaking the asci.
Many ascomycetes ascospores are released by forcible ejection and the second event, dispersal, is by another agent.
In species that do not form ascocarps, the release of the spores generally takes place by the breaking or the deliquescence of the asci formed on the substrate. The spores are then free to be dispersed by wind, water, animals, or other agents.
In some groups the ascocarp is completely closed and the ascospores are liberated only on the partial or complete disintegration of the ascocarp. The release process may be hastened by ingestion by animals that also may be dispersal agents; e.g. truffles
In some ascocarps there is a pore through which the ascospores usually are aimed and forcibly ejected.
Ascocarps with the entire hymenia exposed at maturity also have ascospores that are discharged forcibly into the air. In the instances when the spores are shot into the air, some may be carried by air currents for comparatively long dispersal distances, although the majority fall in the vicinity of the place where they were produced.
In a large number of ascomycetes, ascospores are ejected forcibly from the ascus by a puffing action. In some species such as the morels, puffing is accompanied by a hissing sound. Ascospores are ejected either through the bursting of the ascus at the top or through a natural pore, slit, or cap-like operculum hinged at one end.
In certain yeasts and filamentous ascomycetes, ascospores may multiply by budding or conidium formation (repetitive or iterative germination) instead of germinating by germ tubes. Depending upon the environmental conditions, some ascospores have the ability to germinate by either method.
Sclerotia are considered to be resistant or resting structures and their formation may serve to help a fungus survive conditions that are unfavorable to growth. They often are characterized by thick walls, and this may be the only criterion applied to assess their "resistant" function.
Stromata have a similar function, but rather than initiating mycelial growth directly, they give rise to conidia or ascocarps
may be carried out by fission, fragmentation, or formation of chlamydospores or conidia according to species and environmental conditions it is in the ascomycetes that conidium development has reached its zenith
Conidia often are important in propagating and disseminating species throughout the spring and summer with several generations being produced in a growing season.
Conidia may arise either directly from the somatic hyphae or from specialized conidiogenous cells, often borne on hyphal branches known as conidiophores.
Conidiophores vary from short hyphal branches to those that are long and intricately branched. In some species the conidiophores may be produced free from each other without any evident organization, while in other species they are joined together to form complex structures
Blastic - growth of conidium occurs after septation; young conidium is recognizable prior to septation
Thallic - growth of conidium occurs prior to septation; young conidium is not recognizable prior to septation
schizolytic - halves of dle septum split apart by breakdown of middle lamella
rhexolytic - outer wall of cell beneath or between condia breakdown
blastic acropetal - conidia develop in chains by apical budding;Cladosporium
blastic synchronous - conidia develop in chains synchronously;Botrytis
blastic retrogressive - conidia develop chains basipetally
blastic sympodial - alternate branching;Beuveria
blastic annelidic - possess annular scars; conidiogenous locus moves;Scopulariopsis
blastic phialidic - produce conidia in rapid succession from a specialized cell, the phialide;Penicillium, Aspergillus, Fusarium, Verticillium
thallic arthric - hyphae stops growing and becomes divided into arthrospores;Geotrichum
thallic solitary - hyphae stops growing and terminal cell becomes conidium;Microsporum
amerospores - aseptate
didymospores - single septum
phragmospores - several horizontal septa
dictyospores - muriform septation
helicospores - coiled spores
staurospores - stellate
scoleospores - curved, filiform
simple - complex (penicillate) - synnema - sporodochium
pycnidium - walls of structure are of fungal origin
acervular - walls of structure are of host origin
Ascomycetes are taxonomically difficult, and over the last decade mycologists have concentrated on delimiting monophyletic orders rather than grouping orders in higher taxa 45 orders unplaced in higher taxa in the Systema Ascomycetum (Eriksson and Hawksworth, 1993).
Characters that distinguish ascomycetes
3 major groups of Ascomycetes based upon molecular data:
Archiascomycetes-Taphrina, Pneumocystis, Schizosaccharomyces
Saccharomycetales are characterized by the loss of simple septal in all except a few taxa, restriction of chitin primarily to bud scars in the cell walls in most; mannans and ß-1,3 glucans are the primary wall polysaccharides; EMS in most yeasts examined appears to be derived from membranes that are associated with individual nuclei, rather than as a cylinder initially enclosing all of the nuclei.
Euascomycetes- the filamentous ascomycetes; mycelium with a simple septal pore, an ascus vesicle, and forcibly discharged ascospores. Woronin bodies are associated with the septal pores; production of ascogenous hyphae with many asci resulting from a single mating and the formation of an ascocarp. This group appears to have diverged rapidly and is marked by a diversity of mycelial types, ascus structure and function, and ascocarp morphology. In addition filamentous ascomycetes are notable for their elaboration of conidium structure and function.
Hymenoascomycetes and Loculoascomycetes
Ascomycetes usually are classified on the basis of sexual reproduction. Forms that do not reproduce sexually have been placed in an artificial group, either Deuteromycota or Fungi Imperfecti; the application of molecular techniques provides a means to incorporate the two.