Biology Today - May 2018

Strain 1

Strain 2

Step (i)

Step (vi)

Step (ii)

Step (iii)

Step (iv)

Mitotic

crossing over

Fig.: Pontecorvo’s idea of parasexual cycle

Step (v)

Haploidisation

Sorting out of

new haploid

nuclei through

conidia

Sorting out of

diploid nuclei

through diploid

conidia

Haploid conidia

Recombinant diploid with

yellow or white conidia

Anastomosis

Heterokaryon

Unlike

nuclei

fuse

Haploid

conidia Diploid

conidia

• •Parasexual cycle was first discovered by Pontecorvo and Roper of University of Glasgow in 1952 in Aspergillus nidulans the, imperfect stage of
  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.

Parasexual Cycle in Deuteromycotina

• •Many species of Deuteromycotina are
  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.
  
  
  
  

Deuteromycotina Members

as Potent Pathogens

• •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.

Economic Importance of Deuteromycotina

• •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 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

Perfect Stages of Imperfect Fungi

DEUTEROMYCOTINA : THE

FUNGI IMPERFECTI

• •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.
  
  
  • •Conidiophores get assembled in a variety of ways to form different types of asexual fruiting bodies. These are:
  
  
  
  

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

Fruiting Bodies

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.

Classification

Order : Moniliales

• •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 spo-
  rodochium or synnemata.
  E.g., Cercospora, Fusarium, 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 :

Sphaeropsidales

• •They produce
  conidia in
  pycnidia.


• •The pycnidia
  may be separate
  or joined by
  stromatic tissue.
  E.g., Phyllosticta,
  Macrophoma,
  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, rhizo-
  morphs and other
  vegetative means.
  E.g., Rhizoctonia
  sterilia.

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.

Sporodochium

Conidiophores

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 Conidia

Pycnidium

Synnema

• •The whole fructification resembles a long
  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.

Conidiophores

Synnema

Conidia

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

Acervulus

Host

epidermis

Hyphal

mat