Asexual reproduction Asexual reproduction is the dominant form of propagation in the Ascomycota, and is responsible for the rapid spread of these fungi into new areas. It occurs through vegetative reproductive spores, the
conidia. The conidiospores commonly contain one nucleus and are products of
mitotic cell divisions and thus are sometimes called mitospores, which are genetically identical to the mycelium from which they originate. They are typically formed at the ends of specialized
hyphae, the
conidiophores. Depending on the species they may be
dispersed by wind or water, or by animals.
Asexual spores Different types of asexual spores can be identified by colour, shape, and how they are released as individual spores. Spore types can be used as taxonomic characters in the classification within the Ascomycota. The most frequent types are the single-celled spores, which are designated
amerospores. If the spore is divided into two by a cross-wall (
septum), it is called a
didymospore. When there are two or more cross-walls, the classification depends on spore shape. If the septae are
transversal, like the rungs of a ladder, it is a
phragmospore, and if they possess a net-like structure it is a
dictyospore. In
staurospores ray-like arms radiate from a central body; in others (
helicospores) the entire spore is wound up in a spiral like a spring. Very long worm-like spores with a length-to-diameter ratio of more than 15:1, are called
scolecospores.
Conidiogenesis and dehiscence Important characteristics of the anamorphs of the Ascomycota are
conidiogenesis, which includes spore formation and dehiscence (separation from the parent structure). Conidiogenesis corresponds to
Embryology in animals and plants and can be divided into two fundamental forms of development:
blastic conidiogenesis, where the spore is already evident before it separates from the conidiogenic hypha, and
thallic conidiogenesis, during which a cross-wall forms and the newly created cell develops into a spore. The spores may or may not be generated in a large-scale specialized structure that helps to spread them. These two basic types can be further classified as follows: •
blastic-acropetal (repeated budding at the tip of the conidiogenic hypha, so that a chain of spores is formed with the youngest spores at the tip), •
blastic-synchronous (simultaneous spore formation from a central cell, sometimes with secondary acropetal chains forming from the initial spores), •
blastic-sympodial (repeated sideways spore formation from behind the leading spore, so that the oldest spore is at the main tip), •
blastic-annellidic (each spore separates and leaves a ring-shaped scar inside the scar left by the previous spore), •
blastic-phialidic (the spores arise and are ejected from the open ends of special conidiogenic cells called
phialides, which remain constant in length), •
basauxic (where a chain of conidia, in successively younger stages of development, is emitted from the mother cell), •
blastic-retrogressive (spores separate by formation of crosswalls near the tip of the conidiogenic hypha, which thus becomes progressively shorter), •
thallic-arthric (double cell walls split the conidiogenic hypha into cells that develop into short, cylindrical spores called
arthroconidia; sometimes every second cell dies off, leaving the arthroconidia free), •
thallic-solitary (a large bulging cell separates from the conidiogenic hypha, forms internal walls, and develops to a
phragmospore). Sometimes the conidia are produced in structures visible to the naked eye, which help to distribute the spores. These structures are called "conidiomata" (singular:
conidioma), and may take the form of
pycnidia (which are flask-shaped and arise in the fungal tissue) or
acervuli (which are cushion-shaped and arise in host tissue). Dehiscence happens in two ways. In
schizolytic dehiscence, a double-dividing wall with a central lamella (layer) forms between the cells; the central layer then breaks down thereby releasing the spores. In
rhexolytic dehiscence, the cell wall that joins the spores on the outside degenerates and releases the conidia.
Heterokaryosis and parasexuality Several Ascomycota species are not known to have a sexual cycle. Such asexual species may be able to undergo genetic recombination between individuals by processes involving
heterokaryosis and
parasexual events. Parasexuality refers to the process of heterokaryosis, caused by merging of two hyphae belonging to different individuals, by a process called
anastomosis, followed by a series of events resulting in genetically different
cell nuclei in the
mycelium. The merging of nuclei is not followed by
meiotic events, such as
gamete formation and results in an increased number of
chromosomes per nuclei.
Mitotic crossover may enable
recombination, i.e., an exchange of genetic material between
homologous chromosomes. The chromosome number may then be restored to its
haploid state by
nuclear division, with each daughter nuclei being genetically different from the original parent nuclei. Alternatively, nuclei may lose some chromosomes, resulting in
aneuploid cells.
Candida albicans (class Saccharomycetes) is an example of a fungus that has a parasexual cycle (see
Candida albicans and
Parasexual cycle).
Sexual reproduction Sexual reproduction in the Ascomycota leads to the formation of the
ascus, the structure that defines this fungal group and distinguishes it from other fungal phyla. The ascus is a tube-shaped vessel, a
meiosporangium, which contains the sexual spores produced by
meiosis and which are called
ascospores. Apart from a few exceptions, such as
Candida albicans, most ascomycetes are
haploid, i.e., they contain one set of
chromosomes per nucleus. During sexual reproduction there is a
diploid phase, which commonly is very short, and meiosis restores the haploid state. The sexual cycle of one well-studied representative species of Ascomycota is described in greater detail in
Neurospora crassa. Also, the adaptive basis for the maintenance of sexual reproduction in the Ascomycota
fungi was reviewed by Wallen and Perlin. They concluded that the most plausible reason for the maintenance of this capability is the benefit of
repairing DNA damage by using
recombination that occurs during
meiosis. From the fertilized ascogonium,
dinucleate hyphae emerge in which each cell contains two nuclei. These hyphae are called
ascogenous or fertile hyphae. They are supported by the vegetative mycelium containing uni– (or mono–) nucleate hyphae, which are sterile. The mycelium containing both sterile and fertile hyphae may grow into fruiting body, the
ascocarp, which may contain millions of fertile hyphae. An ascocarp is the fruiting body of the sexual phase in Ascomycota. There are five morphologically different types of ascocarp, namely: • Naked asci: these occur in simple ascomycetes; asci are produced on the organism's surface. •
Perithecia: Asci are in flask-shaped ascoma (perithecium) with a pore (ostiole) at the top. •
Cleistothecia: The ascocarp (a cleistothecium) is spherical and closed. •
Apothecia: The asci are in a bowl shaped ascoma (apothecium). These are sometimes called the "cup fungi". •
Pseudothecia: Asci with two layers, produced in pseudothecia that look like perithecia. The ascospores are arranged irregularly. The sexual structures are formed in the fruiting layer of the ascocarp, the
hymenium. At one end of ascogenous hyphae, characteristic U-shaped hooks develop, which curve back opposite to the growth direction of the hyphae. The two nuclei contained in the apical part of each hypha divide in such a way that the threads of their
mitotic spindles run parallel, creating two pairs of genetically different nuclei. One daughter nucleus migrates close to the hook, while the other daughter nucleus locates to the basal part of the hypha. The formation of two parallel cross-walls then divides the hypha into three sections: one at the hook with one nucleus, one at the basal of the original hypha that contains one nucleus, and one that separates the U-shaped part, which contains the other two nuclei. (the typical cup-like reproductive structure of ascomycetes) showing sterile tissues as well as developing and mature asci. Fusion of the nuclei (karyogamy) takes place in the U-shaped cells in the hymenium, and results in the formation of a diploid
zygote. The zygote grows into the
ascus, an elongated tube-shaped or cylinder-shaped capsule. Meiosis then gives rise to four
haploid nuclei, usually followed by a further mitotic division that results in eight nuclei in each ascus. The nuclei along with some cytoplasma become enclosed within membranes and a cell wall to give rise to ascospores that are aligned inside the ascus like peas in a pod. Upon opening of the ascus, ascospores may be dispersed by the wind, while in some cases the spores are forcibly ejected form the ascus; certain species have evolved spore cannons, which can eject ascospores up to 30 cm. away. When the spores reach a suitable substrate, they germinate, form new hyphae, which restarts the fungal life cycle. The form of the ascus is important for classification and is divided into four basic types: unitunicate-operculate, unitunicate-inoperculate, bitunicate, or prototunicate. See the article on
asci for further details. == Ecology ==