Conidia, sometimes termed conidiospores, are asexual, non-motile spores of a fungus; they are also called mitospores due to the way they are generated through the cellular process of mitosis. They are haploid cells genetically identical to the haploid parent, can develop into a new organism if conditions are favorable, and serve in biological dispersal.
Asexual reproduction in Ascomycetes (the Phylum Ascomycota) is by the formation of conidia, which are borne on specialized stalks called conidiophores. The morphology of these specialized conidiophores is often distinctive of a specific species and can therefore be used in identification of the species. There are two main types of conidium development:
A conidium may form germ tubes (germination tubes) and conidial anastomosis tubes (CATs) in specific conditions. These two are some of the specialized hyphae that are formed by fungal conidia. Conidial anastomosis tubes are morphologically and physiologically distinct from germ tubes and are under separate genetic control. The biology of conidial anastomosis tubes is not completely understood. After conidia are induced to form conidial anastomosis tubes, they grow homing toward each other, and eventually they fuse. Once fusion happens, the nuclei can pass through fused CATs. These are events of fungal vegetative growth and not sexual reproduction. Fusion between these cells seems to be important for some fungi during early stages of colony establishment. The production of these cells has been suggested to occur in 73 different species of fungi.
Conidia are often the method by which some normally harmless but heat-tolerating (thermotolerant), common fungi establish infection in certain types of severely immunocompromised patients (usually acute leukemia patients on induction chemotherapy, AIDS patients with superimposed B-cell lymphoma, bone marrow transplantation patients, or major organ transplant patients suffering from graft versus host disease). Their immune system is not strong enough to fight off the fungus, and it may, for example, colonise the lung, resulting in a pulmonary infection.