Genus- Pilobotus kleinii

Contents in the Article

Pilobotus

Taxonomic position

According to Alexopulos (1962)

  • Division– Mycota
  • Sub-division– Eumycotina
  • Class- Zygomycetes
  • Order– Mucorales
  • Family– Pilobolaceas
  • Genus– Pilobotus

Family: Pilobolaceae

The family is represented by three genera : Pilbolus, Pilaira and Ulharomyces. All the three genera are comprophilous and heterothallic. The phototropic sporangiophores bear dark sporangia with columella which is not very conspicuous.

Genus Piloblus is described in detail as a representative of the family.

Genus- Pilobolus kleinii

Occurrence

Genus Pilobolus is represented by 25 species out of which about 10 species are reported from India. Of these ten species P. kleinii, P. longipes and P. crystallinus are most common. Pilobolus- a coprophilous fungus grows in laboratory only on media containing dung decoctions. According to Hesseltine and coworkers (1952) dung contains a factor named as corpogen which is necessary for growth of the fungus. It is the only genus of the order Mucorales exhibiting violent spore discharge.

It is the only genus of the order Mucorales exhibiting violent spore discharge.

Somatic phase

The somatic phase of the fungus is a eucarpic thallus which consists of mycelium. The hyphae are coenocytic, aseptate, hyaline branched and remain submerged in the substratum, feeding on dung.

Reproductive phase

The fungus reproduce both by asexual and sexual methods.

  1. Asexual reproductive-

    It occurs mostly by aplanospores produced within the sporangium. Some hyphae which are to produce sporangia become upright and come out of the substratum. Erect sporangiophores develop from a trophocyst a caroftene rich swelling embedded in the substratum. The trophocyst is ovate, globose napiform or cylindrical in shape and is delimited from the mycelium by one or two septa. The trophocyst may be terminal or intercalary in position. The sporangiophores is 0.5 to 1.0 cm long and bears apically a black button shaped columellate sporangium subtended by a much larger crystal clear sub-sporangial vesicle. The sporangiophores are phototropic. Each sporangiophores at maturity consists of a trophocyst, embedded in the substratum, a long straight stalk which is enlarged at its tip to form a sub- sporangial vesicle capped by a small black-columellate sporangium containing 15-30 thousand spores.

Dispersal of spores in Pilobolus is an interesting phenomenon in which the vesicle plays an active role. The large vacuole in the vesicle has cell sap which has an atmospheric pressure as high as 7 atmospheres. Drops of excreted liquid commonly adhere to the sporangiophore. The vesicle below the sporangium explodes transversely at a line of weakness just beneath the columella. This is accompanied by sudden contraction of the walls of the sporangiophore, the vesicle and trophocyst resulting in the squirting out of the cell sap from the open mouth of the vesicle. The squirting out of the cell sap is in the form of a jet that propels the sporangium alongwith columella to a distance of 2.5 metres. When the sporangium strikes an object it gets attached with the help of a mucilaginous ring and is surrounded by the cell sap. After some time the cell sap dries and the sporangium gets glued to the object say grass leaves. The sporangia reach the alimentary canal of the herbivore animal alongwith grass leaves and only there the spores are released. The spores get proper nutrition and stimulation to germinate in the alimentary canal which later takes place in the freshly deposited horse dung.

Pilobolus is aptly known as Hat- Thrower or shotgun fungus because of the peculiar mechanism of discharge of sporangium.

  1. Sexual Reproduction-

    Sexual Reproduction rarely occurs in Pilobolus and takes place in adverse conditions. The genus has both homothallic and heterothallic species. Fusion of two multinucleate similar gametangia of opposite strains leads to the formation of thick walled Zygospores. The suspensors are twisted around each other and are then separated like two jaws of a pair of tongs. The Zygospore is held by their tips. The Zygospore undergoes a period of dormancy and rest after which it germinates and forms a sporangiophore and sporangium. Meiosis takes place during germination and the spores are haploid. However, the Zygospore can also germinate by a germ tube directly.

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