Conidiospores, commonly known as conidia, are asexual reproductive structures. The word is derived from the Greek konidion, a diminutive of konis, meaning dust. Conidia are found in many different groups of fungi, but especially within Ascomycota and Basidiomycota.
The term conidium has, unfortunately, been used in a number of different ways, so that it no longer has any precise meaning. It has been defined by Kirk et al. (2001) as ‘a specialized non-motile (zoospore) asexual spore, usually caducous (i.e. detached), not developed by cytoplasmic cleavage (sporangiospore) or free cell formation (ascospore); in certain Oomycota produced through the incomplete development of zoosporangia which fall off and germinate to produce a germination tube’. In many fungi, conidia represent a means of rapid spread and colonization from an initial focus of infection.
In general, conidia are dispersed passively, but in a few cases discharge is violent. For instance, in Nigrospora the conidia are discharged by a squirt mechanism, and in Epicoccum discharge is brought about by the rounding-off of a two-ply septum separating the conidium from its conidiogenous cell.
In the Helminthosporium conidial state of Trichometasphaeria turcica, drying and shrinkage of the conidiophore is associated with the sudden development of a gas phase, causing a jolt sufficient to project the conidium There is great variation in conidial ontogeny.
This topic will be dealt with more fully later when considering the conidial states of Ascomycota, and at this stage it is sufficient to distinguish between the major types of conidial development, which may be either thallic or blastic.
Cells which produce conidia are conidiogenous cells. The term thallic is used to describe development where there is no enlargement of the conidium initial, i.e. the conidium arises by conversion of a pre-existing segment of the fungal thallus.
An example of this kind is Galactomyces candidus, in which the conidia are formed by the dissolution of septa along a hypha. In most conidia, development is blastic, i.e. there is an enlargement of the conidium initial before it is delimited by a septum. Two main kinds of blastic development have been distinguished:
1. Holoblastic, in which both the inner and outer wall layers of the conidiogenous cell contribute to conidium formation. An example of this kind of development is shown by the conidia of Sclerotinia fructigena.
2. Enteroblastic, in which only the inner wall layers of the conidiogenous cell are involved in conidium formation. Where the inner wall layer balloons out through a narrow pore or channel in the outer wall layer, development is described as tretic.
Examples of enteroblastic tretic development are found in Helminthosporium velutinum and Pleospora herbarum. Another important method of enteroblastic development is termed phialidic development.
Here the conidiogenous cell is a specialized cell termed the phialide. During the expansion of the first-formed conidium, the tip of the phialide is ruptured. Further conidia develop by the extension of the cytoplasm enclosed by a new wall layer which is laid down in the neck of the phialide and is distinct from the phialide wall.
The protoplast of the conidium is pinched off by the formation of an inwardly growing flange which closes to form a septum. New conidia develop beneath the earlier ones, so that a chain may develop with the oldest conidium at its apex and the youngest at its base.
Details of phialidic development are discussed more fully in relation to Aspergillus and Penicillium, which reproduce by means of chains of dry phialoconidia dispersed by wind. Sticky phialospores that accumulate in slimy droplets at the tips of the phialides are common in many genera; they are usually dispersed by insects, rain splash or other agencies.
The term conidium is sometimes used for structures that are probably homologous to sporangia. A series can be erected in the Peronosporales in which there are forms with deciduous sporangia which release zoospores when in contact with water (e.g. Phytophthora), and other forms which germinate directly, i.e. by the formation of a germ tube (e.g. Peronospora).
A similar series can be erected in the Mucorales wherein some forms the number of sporangiospores per sporangium is reduced to several or even one. One-spored sporangia may be distinguished from conidia by being surrounded by two walls, i.e. that of the sporangium and that of the spore itself. There are numerous other kinds of spore found in fungi.