In this article we will discuss about the general features and development of deuteromycetes.
General Features of Deuteromycetes:
This class comprises a group of fungi in which only the asexual or imperfect stage is known. The sexual stage also called the perfect stage is unknown. The latter is either non-existent or has not been discovered so far.
The life cycle of these forms is thus incompletely known. Such fungi with imperfectly known life cycles are included in the class Deuteromycetes or Fungi Imperfecti. Their number goes up to thousands. Many of them live as saprophytes and many more as parasites.
The latter are the causative agents of diseases in plants and animals including man. Some cause spoilage of stored products.
The somatic phase in the majority of these fungi consists only of the haploid mycelium. It is septate and profusely branched. Reproduction takes place chiefly by the formation of exogeneously developed asexual spores.
They are of the nature of conidia. Formation of oidia and chlamydospores has also been reported. Sporangiospores, meiospores such as ascospores and basidiospores, and sexual organs are either non-existent or have not been observed so far.
In many fungi which were formerly described as fungi imperfecti, sexual stages have been found. One such example is furnished by Aspergillus or Eurotium.
The sexual stage of this fungus was called Eurotium and the asexual or the conidial stage as Aspergillus. These two stages were considered to belong to two different fungi.
Whenever the perfect stage of the imperfect fungus is known it is transferred to its systematic position among the fungi in the classes Ascomycetes and Basidiomycetes. The name applied to the perfect stage has priority.
The conidial stage is very variable in the different members of this class. The classification is itself based on the peculiarities of conidial development. Hence, before taking up the classification we describe the various methods of conidial development.
Conidia and Conidial Development of Deuteromycetes:
The conidia are exogenously produced spores. They are non-motile and are borne externally or vertically growing hyphae called the conidiophores. Generally, the condiophores are simple or branched hyphae with special region of conidia formation.
The apical region of conidiophore may produce a single conidium or a chain of conidia usually in basipetal succession.
Ellis (1971) has recognised two types of conidial developments:
1. Blastic:
In this type of conidial development marked enlargement of the recognisable conidial initial takes place before it is delimited by a septum.
It may be of two types:
(i) Holoblastic:
In the formation of conidia both the inner and outer walls of the conidia forming cell of the conidiophores take part. In Cladosporium the conidial chain is branched due to formation of two buds at the apex of a conidium.
(ii) Enteroblastic:
Only the inner wall of the conidiogenous cell takes part in the formation of conidia.
It may be of two types:
(a) Tretic:
The conidium develops when the inner wall protrudes through a channel in the outer wall. Helminthosporium velutinum is an example.
(b) Phialidic:
Conidial development takes place from a specialised conidiogenous cell known as phialide conidia are formed in basipetal succession. Aspergillus and Penicillium are examples.
2. Thallic:
In this type of conidial development there is no enlargement of conidial initial, or when such development does occur, it takes place after the initial has been delimited by a septum or septa. Geotrichum candidum is an example of thallic development of conidia.