Parasexual Cycle in Deuteromycotina
DEUTEROMYCOTINA : THE FUNGI IMPERFECTI
•• Parasexual cycle was first discovered by Pontecorvo and Roper of University of Glasgow in 1952 in Aspergillus nidulans the, imperfect stage of
The Subdivision Deuteromycotina includes those fungi in which only asexual or imperfect stages are known and sexual or perfect stages are not yet recorded. They are also called Fungi imperfecti. This subdivision includes about 600 genera and over 20,000 species, but this number is now decreasing as members previously included in Deuteromycotina are being transferred to appropriate groups as soon as their perfect sexual stages are discovered. Thus, this subdivision is a purely artificial and temporary assemblage of fungal species waiting to be included in appropriate groups after the discovery of their perfect stages.
General Characteristics of Deuteromycotina •• The members belonging to this group are saprophytes or parasites. •• The mycelium is well developed and profusely branched with perforated septa. •• The hyphae may be inter or intracellular and the cells are multinucleate. •• The reproduction takes place by asexual means only and sexual stages are not known. •• Asexual reproduction takes place by oidia or conidia formation. The conidia are non-motile structures which develop exogenously on conidiophores. •• Conidiophores are either free or are formed in some special types of fruiting bodies such as synnemata, acervuli, sporodochia and pycnidia. •• Some members show parasexuality.
Fruiting Bodies Synnema feather cluster or a brush. •• Branched or unbranched conidiophores arise very close to each other and are often united along a greater part of their length to form dense fascicles. •• This type of arrangement of conidiophores is given the name corenium or synnema. •• Towards the upper end of synnema the conidiophores become free and diverge to give rise to conidia.
Sporodochium
•• Sporodochium is
a hemispherical or barrel-shaped asexual fruiting body. Its lower or the basal part is a cushioned stoma like mass of hyphae. •• Conidiophores constitute the upper part of sporodochium bearing conidia. Conidia
Conidia
Pycnidium
•• It is a flask shaped fruiting
body whose cavity is lined by a sterile pseudo-parenchymatous tissue comprising its wall, called pycnidial wall. •• Fertile layer consists of very short conidiophores which bear pycniospores or pycnidiospores at their tips, lies in the cavity. The whole structure, i.e., cavity, wall and fertile layer constitute pycnidium. Ostiole
Acervulus
•• It is a saucer shaped
structure consisting of stromatic mass of hyphae and fertile layer of conidiophores. •• Conidiophores arise from a mat of closely crowded hyphae forming a small disc called stroma. •• The conidia are abstricted from the tips of conidiophores.
Conidia
Conidiophores
Conidia
Host epidermis
Hyphal mat
Sporodochium
Pycnidium
Acervulus
The classification of Deuteromycetes is artificial, hence various taxa are known as form - order, form-family, form-genus, etc. On the basis of structure of the asexual fruiting body and type of conidia, the group has been divided into following four orders.
•• This order is represented by about 10,000
form-species. The mycelium is colourless or black in colour. •• Mycelia are septate and branched. •• Conidia are produced directly on hyphal cell or specialised hyphal cells called conidiophores. •• The conidiophores are usually free but sometimes they are assembled into sporodochium or synnemata. E.g., Cercospora, Fusarium, etc.
Anastomosis Step (i)
pathogenic to plants. Many are responsible for the degradation of foods, including decay from rots on vegetables and fruits. Some of the common diseases caused by them are as follows. –• Red rot of sugarcane caused by Colletotrichum falcatum. –• Early blight of potato caused by Alternaria solani. –• Wilt of cotton caused by Fusarium oxysporum. –• Tikka disease of groundnut caused by Cercospora personata. –• Leaf stripe of barley is caused by Helminthosporium gramineum. –• Black point disease in wheat caused by Alternaria tenuis, Fusarium, Helminthosporium, etc.
Strain 2
Order : Sphaeropsidales
•• They produce
conidia in pycnidia. •• The pycnidia may be separate or joined by stromatic tissue. E.g., Phyllosticta, Macrophoma, etc.
Order : Melanoconiales
•• The mycelium is internal and asexual fruiting bodies are acervulus type.
•• The acervuli are sub-epidermal and
develop beneath the host cuticle or epidermis. •• Usually they burst through the host surface as the pigmented conidia mature. •• Conidiophores are aggregated in a palisade like layer on the surface of stroma and conidia are abstricted singly or in chains. E.g., Colletotrichum, Gloeosporium, etc.
Order : Mycelia Sterilia
•• It includes 20
genera and 200 form-species. •• The mycelium is sterile and conidia are not formed. •• They reproduce by sclerotia, rhizomorphs and other vegetative means. E.g., Rhizoctonia sterilia.
Step (ii)
Heterokaryon
Unlike nuclei fuse
Step (vi) Sorting out of new haploid nuclei through conidia
Haploid conidia
Diploid conidia
Haploid conidia Step (v) Haploidisation Recombinant diploid with yellow or white conidia Mitotic crossing over
Step (iii)
Sorting out of diploid nuclei through diploid conidia
Step (iv) Fig.: Pontecorvo’s idea of parasexual cycle
•• Several members, which were previously included in Deuteromycotina due to unknown sexual (perfect) stages, were later transferred to an appropriate group when their perfect stages were recorded in nature or in artificial culture media. The following are few examples : Imperfect stage
Classification
Order : Moniliales
Deuteromycotina Members as Potent Pathogens
Perfect Stages of Imperfect Fungi
Conidiophores Synnema
Strain 1
•• Many species of Deuteromycotina are
•• Conidiophores get assembled in a variety of ways to form different types of asexual fruiting bodies. These are:
•• The whole fructification resembles a long
Emericella nidulans. Parasexuality can be defined as a phenomenon in which the three processes, e.g., plasmogamy, karyogamy and meiosis occurs at an unspecified time and stages in the life of a fungus. Several steps involved in the process of parasexuality: •• Formation of heterokaryotic mycelium - Existence of nuclei of different strains in the protoplasm of the cells of a hypha is termed as heterokaryosis and such hypha is called ‘heterokaryotic’. Heterokaryosis generally occurs in the thallus of a fungus by following ways : by anastomosis (plasmogamy between two genetically different hyphae), by mutation of one or more nuclei in a homokaryotic mycelium by diploidisation, etc. •• Karyogamy - The fusion of haploid nuclei of similar or dissimlar genotypes results in the formation of homozygous or heterozygous diploid nucleus respectively. If the genotype of unlike nuclei present in the heterokaryotic mycelium are A and B, then five different types of nuclei may be formed (haploid A and B,homozygous diploid AA and BB and heterozygous diploid nuclei AB). •• Multiplication of diploid nuclei - These five types of nuclei multiply, but the diploid nuclei are present in much smaller number than the haploid nuclei. •• Occasional mitotic crossing over - Mitotic crossing over takes place during multiplication of diploid nuclei and new gene combinations are formed. These recombintions which are dependent on the existence of heterokaryosis and give the fungus some of the advantages of sexuality within the parasexual cycle. •• Sorting out of diploid nuclei - In those fungi which produce uninucleate conidia, sorting out of the diploid nucleus occurs by their incorporation into conidia. Conidium which contains diploid nucleus germinates to produce diploid mycelium. Diploid strains of several important imperfect fungi have been isolated. •• Haploidisation of diploid nuclei - Occasionally, some hyphae of diploid mycelium form haploid conidia which form haploid mycelia on germination. The formation of haploid conidia by diploid mycelium indicates that haploidisation occurs in some diploid nuclei. •• Sorting of new haploid strains - Some diploid nuclei undergo haploidisation in the mycelium and are sorted out by incorporation of haploid nuclei in the uninucleate conidia. Some of these haploid strains are genotypically different from their parents because of mitotic recombinations.
Perfect stage
Alternaria solani
Pleospora infectoria
Cercospora cerasella
Mycosphaerella cerasella
Cercospora personata
Mycosphaerella berkeleyi
Helminthosporium gramineum
Pyrenophora graminea
Helminthosporium oryzae
Cochliobolus miyabeanus
Colletotrichum falcatum
Glomerella tucumanensis
Economic Importance of Deuteromycotina •• The member of Genus Penicillium are cultivated for the industrial manufacture of cheese (Penicillium roqueforti, Penicillium camemberti), antibiotics (Penicillium notatum, P. chrysogenum) and gluconic acid (Penicillium purpurogenum).
•• Arthrobotrys oligospora and Dactylella cionopaga trap nematodes (microscopic roundworms that often infest the roots of crop plants) by forming hyphal
rings which constrict the nematode when stimulated by contact. Thereby, these Deuteromycotina reduce the disease incidence due to nematode of the crop plant.