In this article we will discuss about the reproduction in ascobolus with the help of suitable diagrams.
Mycelium of Ascobolus:
The mycelium of the fungus is well developed and ramifies in the substratum forming complex hyphal masses. The mycelium is septate with multinucleate cells and branched.
The complex hyphal mass absorbs the food material from the substratum and supplies to fruiting bodies. Thus it acts as an absorptive organ for the aerial apothecia developing on the substratum.
Reproduction in Ascobolus:
1. Asexual Reproduction:
Asexual reproduction in Ascobolus is not of common occurrence. In some species like A. furfuraceus, asexual reproduction takes place by oidia or arthrospores produced in chains on small hyphal branches.
The oidia are thin walled structures and germmate to form new mycelium. The oidia are also reported to play an active role in sexual reproduction.
2. Sexual Reproduction:
Species of Ascobolus may be heterothallic or homothallic. Common heterothallic species are A. magnificus, A. furfuraceus, and A. immersas while A. crenulatus is homothallic.
In A magnificus male and female sex organs i.e. ascogonia and antheridia develop on two compatible mycelia. The sex organs develop on different branches. Usually the two sex organs i.e. antheridia and ascogonia develop on both the strains i.e. each strain being bisexual.
But the antheridia of one strain do not fertilise the ascogonia of the same strain meaning thereby that each strain is self-incompatible.
While the antheridium is clavate or cylindrical and erect, the female element i.e. ascogonium consists of a coiled-hypha with a stalk of few cells, with swollen oogonial region and an apical trichogyne. Both the sex organs are multinucleate.
The trichogyne elongates till it reaches the tip of the antheridium and coils around the body. The wall of contact between the two dissolves and the male nuclei pass into the apical cell of the trichogyne and ultimately reach the ascogonmm where plasmogamy occurs.
Female nuclei also divide and male and female nuclei form pairs moving towards the periphery of the ascogonium. Soon the ascogenous hyphae are given out from the periphery of the ascogonium.
In usual way, asci develop, each containing eight ascospores. The ascospores are ellipsoidal or spherical, one celled, uninucleate, and purple to dark brown in colour.
In another heterothallic species, A. furfuraceus the antheridium is absent and the function of the male component is performed by oidia produced in chains. The oidium from one strain fertilises the ascogonium of another strain.
The ascogonium develops only when the oidium comes in contact with the mycelium of the opposite strain. The development of ascogonium takes place within 10-12 hours of contact.
The ascogonium is made up of a broad coiled base and a long narrow trichogyne which shows chemotropic growth towards the oidium and finally fuses with it. The development of apothecia takes place in the usual way within 10 days of the fertilisation.
In the apothecium asci are surrounded by paraphyses. Each ascus is long, clavate with eight ascospores of which four belong to one strain and the remaining four belong to the opposite stram.
Generally two types of apothecial development in Ascobolus has been recognised:
(i) Cleistohymenial in which the hymenium is enclosed in its early development;
(ii) Gymnohymenial in which the hymenium is exposed from the very beginning. A. furfuraceus is an example of cleistohymenial type of apothecial development while A. magnificus is an example of gymnohymenial type.
In A. citrinus both the antheridia and oidia are absent. The development of apothecium from the female branch is parthenogenetic with copulation.
The apothecia are generally yellowish in colour, saucer shaped and about 5 mm in diameter. At maturity the surface of the apothecium is studded with purple dots which signify the ripe asci. As the asci mature, they elongate above the general level of hymenium.
Asci are long cylindrical structures intermixed with paraphyses. Each ascus has an operculum at its apical end. The ascospores are ellipsoid with mucilaginous epispore.
In A. immersus, the ascospores are very large measuring 70µ x 30µ. On germination, each ascospore gives rise to new mycelium.