Lecture - 9 Ascomycotina: Chaetomium

Sub-division: Ascomycotina:- It includes 1 class -

Class - Ascomycetes:-

> Most of the members are terrestrial, although a large number lives in fresh and marine waters. The majority of ascomycetes are saprophytic, some are parasites of insects and other animals, and some are responsible for causing destructive plant diseases. Some ascomycetes characteristically grow on dung and are popularly called coprophilous fungi (Peziza).

> The members vary in their form and structure. Yeasts and other a few members (Eg. Taphrina) are unicellular, but almost all other members of this group have a well-developed, profusely branched, and septate mycelium with uni- or multinucleate cells and perforated septa.

> In unicellular forms, the cell wall is composed of glucans and mannans, whereas in septate forms it consists of chitin and glucans.

> Asexual reproduction takes place by various types of non-motile spores, such as oidia, chlamydospores, and conidia. In unicellular forms, fission, fragmentation, and budding are the most common methods of propagation.

> They are homothallic or heterothallic. In some heterothallic species, though male (antheridium) and female (ascogonium) sex organs develop on the thallus of the same strain, they are self-incompatible. In these species, male gamete of one mating type fertilises ascogonium of other mating type. This process is known as physiological heterothallism.

> Sexual reproduction takes place by gametangial copulation (Eg., yeast), gametangial contact (Eg., Aspergillus, Penicillium, Erysiphe), somatogamy (Eg., Peziza, Morchella) or spermatization (Eg., Polystigma).

> The sexual spore is haploid called ascospore, which is formed endogenously by free cell formation after karyogamy and meiosis within a sac-like or cylindrical structure referred to as an ascus. If the sexual spore of a fungus is an ascospore, the fungus is an Ascomycete regardless of any other character. This one character distinguishes Ascomycetes from all other fungi.

> They show the phenomenon of heterokaryosis, i.e., the nuclei of two different genotypes are present in the same mycelium.

> The fruiting bodies are known as ascomata (sing. ascoma; earlier called ascocarp). The ascomata are of four types cleistothecium (cleistothecial ascoma), perithecium (perithecial ascoma), apothecium (apothecial ascoma), and ascostroma (stromatic stroma) or pseudothecium.

> Classification:- It includes 5 sub-classes -

1. Hemiascomycetidae:- Karyogamy takes place immediately after plasmogamy. It means dikaryophase is absent here.

3 Orders:-

a. Protomycetales:- Asci fused to form synascus. Parasites on vascular plants.

Eg.- Protomyces (On Umbelliferae)

b. Endomycetales:- Zygote or single cell transformed directly into ascus or a diploid ascophore.

Eg.- Endomyces, Saccharomyces (Yeast)

c. Taphrinales:- Asci form binucleated ascogenous cells developed from dikaryotic hyphae.

Eg.- Taphrina

2. Plectomycetidae:-

5 Orders:-

a. Ascosphaerales:- Asci borne in spore balls formed inside a trophocyst.

Eg.- Ascosphaera

b. Elaphomycetales:- Ascocarps are found below the ground or on the soil, usually 0.5 - 5 cm in diameter.

Eg.- Elaphomyces

c. Onygenales:- Arthroconidia are commonly found.

Eg.- Onygena

d. Eurotiales:- Phialoconidia are commonly found.

Eg.- Penicillium, Aspergillus

e. Microascales:- Ascospores are dextrinoid.

Eg.- Microascus

3. Hymenoascomycetidae:-

7 Orders:-

a. Erysiphales (Powdery mildews):- Eg.- Erysiphe, Phyllactinia

b. Meliolales (Black mildews):- Eg.- Meliola

c. Xylariales:- Eg.- Xylaria, Chaetomium

d. Clavicipitales:- Eg.- Claviceps

e. Pezizales:- Eg.- Morchella, Peziza, Verpa, Helvella

f. Tuberales (Truffles):- Eg.- Tuber

g. Sordariales:- Eg.- Neurospora

4. Laboulbeniomycetidae:-

2 Orders:-

a. Laboulbeniales:- Eg.- Laboulbenia

b. Spathulosporales:- Eg.- Spathulospora

5. Loculoascomycetidae:-

5 Orders:-

a. Myriangiales:- Eg.- Myriangium

b. Dothideales:- Eg.- Dothidea

c. Pleosporales:- Eg.- Pleospora

d. Hysteriales:- Eg.- Hysterium

e. Hemisphaeriales:- Eg.- Microthyrium

Chaetomium:-

1. Classification:-

Kingdom:- Myceteae

Division:- Amastigomycota

Sub division:- Ascomycotina

Class:- Ascomycetes

Sub-class:- Hymenoascomycetidae

Order:- Xylariales (Sphaeriales)

Family:- Chaetomiaceae

Genus:- Chaetomium

2. Habit and Habitat:-

> It is an obligate saprophyte which is found on decaying plant material such as logs and stumps, dead twigs and branches of trees, dead leaves and stems of herbaceous plants.

> Large genus with 96 species.

i. C. hispidum

ii. C. tetrasporum

iii. C. globosum

iv. C. convolutum

3. Thallus structure:-

> The mycelium is composed of septate hyphae.

> The hyphae are branched and composed of uninucleated cells.

4. Reproduction:-

> Only sexual reproduction takes place.

> Most of the species are homothallic, but some are heterothallic.

> Plasmogamy takes place by gametangial contact and by trichogyne contact.