In this article we will discuss about the reproduction in ascomycetes. This will also help you to draw the structure and diagram of reproduction in ascomycetes.
Asexual Reproduction in Ascomycetes:
The Ascomycetes reproduce asexually by fission, budding, fragmentation, arthrospores, chlamydospores or conidia. A new individual may be produced directly by budding or by budding spores known as blastospores which on germination give rise to new individuals.
The development of conidia is rather common, especially among pathogenic ones. The conidia are usually produced on conidiophores which may be branched or un-branched and are developed from the somatic hyphae.
The conidiophores may be solitary or in groups forming an elongated spore- bearing structure, synnema or structures, synnemata. They may also be developed in a pyenidium. Inside the pyenidium is lined with conidiophores from which conidia are produced.
Again the conidiophores may be developed in the peripheral layer of an erumpent, cushion-like mass of hyphae, the acervulus. Conidia and conidiophores may also be borne around a sporodochium.
The conidia developed profusely, are disseminated by wind or insects or by some other agencies suitable for quick and wide spreading of the organism. They are usually thin-walled and cannot stand extreme climatic conditions. Sometimes the conidia can also survive seasonal variations.
Sexual Reproduction in Ascomycetes:
In comparison with the Phycomycetes the mode of sexual reproduction in the Ascomycetes is much more elaborate. In the Phycomycetes there is almost no interval between plasmogamy and karyogamy, one follows immediately the other. Whereas, in majority of the Ascomycetes there is an interval between plasmogamy and karyogamy.
The duration of this interval is again extremely variable. During this interval the fusing nuclei pair forming one or more dikaryons.
Where the dikaryotic condition is very much prolonged, the paired nuclei divide by conjugate division. In the Ascomycetes the dikaryotic phase is dependent on the monokaryotic mycelium for nutrition. Whereas, in the Basidiomycetes it is completely independent. The three phases clearly encountered in the sexual cycle of the Ascomycetes are haplophase, dikaryophase, and diplophase.
Just like all fungi the diplophasic condition in the Ascomycetes is very brief. Meiosis follows immediately after karyogamy producing haploid nuclei and reinstating haploid condition. There is wide diversity in the methods by which the compatible nuclei are brought together leading to the development of ascus and ascospores in the members of the Ascomycetes.
The most common methods are indicated below:
a. Gametangial Copulation:
This method has great similarity with what is encountered in some members of the Phycomycetes. The copulating gametangia are morphologically similar and are developed either from mycelial soma or unicellular uninucleate somatic bodies which behave directly as gametangia.
Their entities are lost during the sexual process producing an unicellular structure in which karyogamy follows immediately after plasmogamy and dikaryotic condition is almost absent. An unicellular zygote is formed. The unicellular zygote so formed is transformed directly into an ascus in which ascospores are developed.
The variations in the method of gametangial copulation are as follows:
(i) In the genus Dipodascus gametangia are separated from the branches of the same or different hyphae by septa. The gametangia soon come into contact and fuse with each other by the dissolution of the intervening walls (Fig. 188A). The protoplasts of the two gametangia intermingle. Two nuclei, one from each gametangium, fuse (Fig. 188B).
The zygote nucleus then undergoes several divisions of which the first one is reductional. The zygote cell elongates into a tapering saccate ascus. Numerous ascospores are developed in the ascus by free cell formation (Fig. 188C to E).
(ii) A similar condition is also found in the genus Eremascus. But here the ascospore number is strictly eight. In Eremascus albidus, two copulating gametangia arise simultaneously, wound spirally, their tips touch, the walls at the point of contact dissolve and there arises a globular ascus, in which eight ascospores are developed (Fig. 189).
(iii) In Schizosaccharomyces octosporus any somatic cell is a potential gametangium. During sexual reproduction, cells copulate in pairs by short tubes which ultimately develop into a copulation canal by the dissolution of the common walls. Two nuclei migrate into the copulation canal and fuse (Fig. 190A to E). The copulation canal then broadens to form a barrel-shaped structure.
Here three nuclear divisions take place, of which the first one is reductional resulting in the formation of eight nuclei. The barrel-shaped structure then becomes an ascus in which eight ascospores are formed (Fig. 190F to I).
b. Gametangial Contact:
In this method two morphologically distinguishable gametangia known as antheridium and ascogonium come in contact with each other and their entities are not lost during the sexual process. This method has variations shown below.
(ii) In Sphaerotheca humuli the uninucleate antheridium is applied to the uninucleate ascogonium and at the point of contact a broad pore is developed through which the male nucleus with cytoplasm passes into the ascogonium (Fig. 191A to D).
The antheridium then collapses. The two nuclei in the ascogonium then divide mitotically, and the ascogonium grows giving rise to a row of cells, the penultimate of which is binucleate and behaves as an ascogenous cell (Fig. 191E) producing the single ascus in which ascospores are developed (Fig. 191F & G).
(ii) In Pyronema confluens club-shaped multinucleate antheridium arises near the multinucleate ascogonium surmounted by a curved tube-like structure, the trichogyne (Fig. 192A), which affects the union between the ascogonium and the antheridium. During sexual reproduction the trichogyne grows on towards a neighbouring antheridium, and presses firmly against it.
The walls at the point of contact are dissolved forming an aperture through which the multinucleate protoplast of the antheridium passes, by way of the trichogyne, into the ascogonium; plasmogamy takes place.
Plasmogamy is followed by an increase in size of the ascogonium, and by the formation of protuberances which ultimately develop into thick short-celled ascogenou hyphae (Fig. 192B) from the penultimate Cells of whose recurved rips the asci are developed (Fig. 192C).
c. Aplanogametic Union:
In Spermophthora gossypii, two non-motile fusiform gametes come in contact with each other and unite by a conjugation tube within which two nuclei fuse with each other (Fig. 193A & B). From the conjugation tube rather limited branched septate hyphae of uninucleate cells are developed which behave as ascogenous hyphae.
The tips of the ascogenous hyphae become spherical asci containing eight ascospores (Fig. 193G & D).
In some Ascomycetes (Pleurage anserina and Neurospora sp.) antheridium is not developed. Specialized sex cells known as spermatia (sing, spermatium) are developed from hyphae. The spermatia are brought in contact with the trichogyne of the ascogonium or with hyphae where there is no ascogonium, usually through some external agency like insects, water, wind, etc.
Dikaryotic condition is achieved by the dissolution of common walls which is followed by the development of asci and ascospores (Fig. 149F to H) and Fig. 250.
Here the dikaryotic condition is attained by the union of somatic hyphae of opposite strains which ultimately leads to the development of asci and ascospores (Fig. 149 I), encountered in Ascophanus sp. and Sordaria flmicola.