Mating in fungi
is, like a fungal mycelium
, a complex tangle. There are several sources of confusion. Mycologists
must content themselves with discovering rough generalities because the rules of fungal mating are fraught with exceptions. A further limitation is that in the vast kingdom of fungi, research has focused on only a few model species. A third issue is the definition of sex. In the traditional usage the female is the donor of the larger gamete
. Since not all of the fungi reproduce sexually and many that do are isogamous
, the traditional terms male and female do not apply.
Based on detailed studies of cellular mechanisms, British biochemist Nick Lane has suggested another definition in which the female is the donor of mitochondria. For most other eukaryotes the two definitions are one and the same. But a mating transaction between isogamous fungi may consist only of a transfer of nuclei from one cell to another. The offspring would have the mitochondria of the receiver. The donor may be a receiver in another transaction or in a two way exchange, both parents could simultaneously play the role of donor and receiver. To add to the complexity, fungal mating types are also often referred to as sexes. They are a chemical signature that functions to prevent a fungus from mating with another closely related fungus. Typically fungi with the same signature do not mate or the mating does not lead to successful offspring. Exuberant 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. The mating genes appear in the homeobox
and in the genes for pheromones
and pheromone receptors
. The products are protein fragments that also act as molecular signatures. Sexual reproduction depends on biomolecules that contain two signatures 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 can take place through gametes or through hyphal fusion. The pheromones act as chemical signals. When a receptor on one haploid picks up a pheromone from a complementary mating type it approaches the source through growth or motion if it is a gamete.
Of the four types of fungi, there is little detail about the mating patterns of Chytrids. Some species produce gamatangias with male or female gametes.
hypha grows towards a compatible mate and they both form a bridge, called a progametangia
, by joining at the hyphal tips in a process called plasmogamy
. A pair of septa
forms around the merged tips, enclosing nuclei of both types. 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" arcogonium or a "male" antheridium. These organs resemble gametangia except that they contain no gametes, only nuclei. A bridge, the trichogyne
forms, that provides a passage nuclei to travel from the antheridium to the ascogonium. A dikaryote
grows from the ascogonium. Karyogamy
is deferred until a fruiting body
In club fungi
, cells from compatible hyphae fuse where they touch. 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