Mating in fungi is a complex process governed by mating types. Research on fungal mating has focused on only a few model species. Since not all of the fungi reproduce sexually and many that do are isogamous, the terms male and female do not apply to this kingdom. Homothallic species are able to mate with themselves, while in heterothallic species only isolates of opposite mating types can mate.
Mating between isogamous fungi may consist only of a transfer of nuclei from one cell to another. Vegetative incompatibility within species often prevent a fungal isolate from mating with another isolate. Isolates of the same incompatibility group do not mate or mating does not lead to successful offspring. High variation has been reported including same chemotype mating, sporophyte to gametophyte mating and biparental transfer of mitochondria.
A picture of the mating type mechanism has begun to emerge from studies of particular fungi such as yeast. The mating type genes are located in homeobox and encode enzymes for production of pheromones and pheromone receptors. Sexual reproduction thereby depends on pheromones produced from variant alleles of the same gene. 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. The number of mating types depends on the number of genes and the number of alleles for each.
Depending of the species, sexual reproduction takes place through gametes 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.
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. A pair of septa forms around the merged tips, enclosing nuclei from both isolates. A second pair of septa forms two adjacent cells, one on each side. These adjacent cells, called suspensors provide structural support. The central cell is destined to become a spore. The nuclei join in a process called karyogamy to form a zygote.
As it approaches a mate, a haploid sac fungus develops one of two complementary organs, a "female" ascogonium or a "male" antheridium. These organs resemble gametangia except that they contain only nuclei. A bridge, the trichogyne forms, that provides a passage for nuclei to travel from the antheridium to the ascogonium. A dikaryote grows from the ascogonium, and karyogamy occurs in the fruiting body.
In club fungi, cells from compatible hyphae fuse upon contact. The donor nuclei divide and travel from cell to cell of the receiver hypha. Septa open to allow the passage. The exchange may or may not be reciprocal. As with sac fungi, karyogamy is deferred until a fruiting body forms.