Sexual Reproduction of Fungi

As in other species, variability, adaptation  and evolution in fungi depends on sexual reproduction. It often occurs in response to adverse environmental conditions.  

When fungi uses sexual reproduction, two mating types are produced.  Then, there are variations: when both mating types are present in the same mycelium,  the event is called homothallic, or in other words, the fungi is self-fertile. On the other hand, when
the mycelia requires two different but compatible mycelia to reproduce sexually, the event is called Heterothallic. 

As in many other species Pheromones are released into the environment to  attract different mating types. These sexual signaling molecules then bind to receptors on the different mating type. 

At this point, the cytoplasms of the two mycelia fuse together, but the nuclei do not fuse. Fusion of the cytoplasm occurs by a process called anastomosis, in which the hyphae of the two mycelia fuse to form an interconnected network. The fusion of cytoplasm is called plasmogamy, and marks the beginning of sexual reproduction.

Soon after, a secondary mycelium forms, made of dikaryotic cells which contain one haploid nucleus from each of the parent mycelia. 
This new stage is called a heterokaryon because the genetic material of the two co-existing nuclei in the mycelium are not identical.
Different species take different time, but at some point,  the two nuclei in the dikaryotic cells fuse to form a diploid nucleus. This new process is called karyogamy. 

Something remarkable is that it may occur relatively quickly or the dikaryotic state may last for years, even centuries.

 In cases where the dikaryotic state persists as is the case for the fungi groups of Basidiomycota and Ascomycota, the nuclei divide synchronously during cell division, so that each succeeding generation maintains two nuclei, one descended from each of the parent mycelia. 

After karyogamy occurs, the zygotes formed by this process undergo meiosis, forming haploid spores, which are released into the environment.

Sexual reproduction with meiosis exists in all fungi and differs in many aspects from sexual reproduction in animals or plants. 

Differences such as sexual structures and reproductive strategies can be used to distinguish fungal species.  
Vocabulary
Homothallic. Refers to the possession, within a single organism, of the resources to reproduce sexually.
Heterothallic. Heterothallic fungi,  requires two compatible partners to produce sexual spores
Pheromones. Chemical substance produced and released into the environment by an animal, esp. a mammal or an insect, affecting the behavior or physiology of others of its species.
Cytoplasms. The material or protoplasm within a living cell, excluding the nucleus.
Anastomosis. a cross-connection between adjacent channels, tubes, fibers,or other parts of a network.
Plasmogamy.  Is a stage in the sexual reproduction of fungi. In this stage, the cytoplasm  of two parent mycelia  fuse together without the fusion of nuclei, as occurs in higher terrestrial fungi. After plasmogamy occurs, the secondary mycelium forms. The secondary mycelium consists of dikaryotic  cells, one nucleus from each of the parent mycelia.
Dikaryotic.  Two compatible nuclei of two cells pair off and cohabit without karyogamy within the cells of the hyphae, synchronously dividing so that pairs are maintained in the older cells while newer cells or hyphal tips are also binucleate.
Karyogamy.  is the fusion of pronuclei of two cells, as part of fertilization in eukaryotes
Heterokaryon.  A cell that contains multiple, genetically different nuclei.
Diploid. Containing two complete sets of chromosomes, one from each parent
Meiosis. A type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell, as in the production of gametes and plant spores.
Zygote. A diploid cell resulting from the fusion of two haploid gametes; a fertilized ovum
Haploid. Having a single set of unpaired chromosomes.