Mating in fungi

Fungi are a diverse group of organisms that employ a huge variety of reproductive strategies, ranging from fully asexual to almost exclusively sexual species.

This contrasts with most multicellular eukaryotes, such as mammals, where the adults are usually diploid and produce haploid gametes which combine to form the next generation.

[3][4] Not all fungi reproduce sexually and many that do are isogamous; thus, for many members of the fungal kingdom, the terms "male" and "female" do not apply.

A zygomycete hypha grows towards a compatible mate and they both form a bridge, called a progametangia, by joining at the hyphal tips via plasmogamy.

The zygosporangium is a unique structure to the Zygomycota and is easily recognizable in microscopy due to its characteristic dark color and spiky shape.

The nuclei join in a process called karyogamy to form a zygote, which grows into a mature diploid zygomycete.

[5] As it approaches a mate, a haploid sac fungus develops one of two complementary organs, a "female" ascogonium or a "male" antheridium.

N. crassa is used as a model organism because it is easy to grow and has a haploid life cycle: this makes genetic analysis simple, since recessive traits will show up in the offspring.

Analysis of genetic recombination is facilitated by the ordered arrangement of the products of meiosis within a sac-like structure called an ascus (pl.

Neurospora was used by Edward Tatum and George Wells Beadle in the experiments for which they won the Nobel Prize in Physiology or Medicine in 1958.

Protoperithecia are formed most readily in the laboratory when growth occurs on solid (agar) synthetic medium with a relatively low source of nitrogen.

The sexual cycle is initiated (i.e. fertilization occurs) when a cell, usually a conidium, of opposite mating type contacts a part of the trichogyne (see Figure).

The products of these nuclear divisions (still in pairs of unlike mating type, i.e. A/a) migrate into numerous ascogenous hyphae, which then begin to grow out of the ascogonium.

As the above events are occurring, the mycelial sheath that had enveloped the ascogonium develops as the wall of the perithecium becomes impregnated with melanin, and blackens.

In a mature ascus containing eight ascospores, pairs of adjacent spores are identical in genetic constitution, since the last division is mitotic, and since the ascospores are contained in the ascus sac that holds them in a definite order determined by the direction of nuclear segregations during meiosis.

In haploid multicellular fungi, such as N. crassa, meiosis occurring in the brief diploid stage is one of their most complex processes.

In N. crassa, recessive mutations affecting the diploid stage of the life cycle are quite frequent in natural populations.

Thus, outcrossing, promoted by the necessity for union of opposite mating types, likely provides the benefit of masking recessive mutations that would otherwise be deleterious to sexual spore formation (see Complementation (genetics)).

During vegetative growth that ordinarily occurs when nutrients are abundant, S. cerevisiae reproduces by mitosis as either haploid or diploid cells.

Katz Ezov et al.[15] presented evidence that in natural S. cerevisiae populations clonal reproduction and a type of “self-fertilization” (in the form of intratetrad mating) predominate.

Ruderfer et al.[14] analyzed the ancestry of natural S. cerevisiae strains and concluded that outcrossing occurs only about once every 50,000 cell divisions.

[citation needed] Instead, a short-term benefit, such as meiotic recombinational repair of DNA damages caused by stressful conditions such as starvation, may be the key to the maintenance of sex in S.

Since sexual reproduction takes place in haploid organisms, it cannot proceed until complementary genes are provided by a suitable partner through cell or hyphal fusion.

When a receptor on one haploid detects a pheromone from a complementary mating type, it approaches the source through chemotropic growth or chemotactic movement if it is a gamete.

The combination of A and B (or b and a) alleles, termed mating type, determine the "specificity" or sexual identity of the individual harboring them.

Dikaryotic hyphae, under the appropriate environmental conditions will give rise to the fruiting body which contains the basidia – specialized cells in which sexual recombination via karyogamy and meiosis occurs.

Examples of tetrapolar organisms are the smuts Ustilago maydis and U. longissima,[23][24] and the mushrooms Coprinopsis cinerea, Schizophyllum commune, Pleurotus djamor and Laccaria bicolor.

In Agaricomycotina the two types of homeodomain transcription factors are termed HD1 and HD2; so the HD1 and HD2 proteins from an individual interacts with the HD2 and HD1 proteins from the other partner, respectively, generating heterodimers able to activate the A transcriptional regulated pathway, which involves formation of clamp cells, coordinated nuclear division and septation.

[35] Finally, the fungus causing witches' broom in cacao, Moniliophthora perniciosa, has a primarily homothallic biology despite having A and B mating type-like genes in its genome.