Sexuality in fungi is diverse and fascinating. In fungi, gradual degeneration of sexuality can be observed.
Degeneration of sexuality means functional and structural degeneration of sex organs, leading to the progressive simplification of the process.
Before discussing the degeneration of sexuality, here is a brief explanation of how sexual reproduction is accomplished in fungi.
Sexual Reproduction
Sexual reproduction in fungi involves the union of two nuclei of different percentages. So sexual reproduction comprises distinct processes which occur in regular sequence. Which are-
- Plasmogamy
- Karyogamy
- Meiosis
Plasmogamy
Union between the two protoplasts without the fusion of nuclei, which results in bringing two haploid nuclei close together in the same cell.
Karyogamy
A fusion of two nuclei that are brought together by plasmogamy. The fusion between nuclei of different percentages results in the establishment of a diploid condition.
Meiosis
Reduction division follows immediately after karyogamy, resulting in the re-establishment of the number of chromosomes in haploid conditions.
The advanced or common method by which the nuclei of different parents are brought close together through plasmogamy is of the following types:
- Gametic Copulation: This is the simplest type of sexual reproduction and involves the fusion of two well-organized gametes. The gametes may be isogametes or anisogametes.
- Gametangial Contact: In this process, the gametangia come in contact with each other. The nuclei migrate from one gametangium to another, either through a passage formed by the dissolution of the common walls at the point of contact or fertilization tubes.
- Gametangial Copulation: This process in which the fusion of entire contents of two gametangia.
The above method of sexual reproduction takes place through the development of morphologically distinguishable male and female gametangia and gametes.
But in some fungi, there is a complete absence of the development of morphologically distinguishable gametangia and gametes. In such cases, sexuality is at the nuclear level only by spermatization, somatogamy, or the auto-gamous method. And here we found the statement, i.e., the degeneration of sexuality in fungi.
Degeneration of Sexuality
Now we describe briefly how the gradual degeneration is followed among the higher groups of Phycomycetes, Ascomycetes and Basidiomycetes.
According to Gwynne Vaughan and Barnes System of classification (1926):
Subclass Oomycetes of Phycomycetes
The fungi of this class represent a fairly advanced type of sexual reproduction, i.e., oogamy.
In Oomycetes, oogonia (female sex organs) and antheridia (male sex organs) are formed. The result of fertilization (sexual fusion) is the formation of a zygote or oospore, which is covered by a thick wall.
For example, in Saprolegnia sp., the gametangial contact takes place through a fertilization tube.
In other members of Phycomycetes, where the gametic copulation occurs by isogamy, i.e., the fusion between two morphologically indistinguishable gametes. E.g., Olpidium sp.
Whereas in Allomyces arbusculus, in the order Blastocladiales of subclass Oomycetes, among the class Phycomycetes, anisogamous takes place.
Subclass Zygomycetes of Phycomycetes
In the subclass Zygomycetes of Phycomycetes, sexual reproduction occurs through gametangial copulation. The sexual reproduction in Mucorales takes place by means of two similar coenogametangia of different physiological strains (+ and -), still exhibiting the dioecious nature of the hyphae.
Ascomycetes
In Ascomycetes, typical oogamous reproduction is observed in some members (e.g., Pyronema sp.) with the help of gametangial contact.
Here, ascogonium (equivalent to oogonium) and antheridium are developed. Ascus and, within ascus, ascospores are also formed.
But in different members of the order Discomycetes among Ascomycetes, there is a gradual and progressive deterioration of the sexual process, leading to a simple method of sexual reproduction.
In Aspergillus, the antheridium is functionless. This gradual minimization of the disintegration of antheridium, i.e., the concept of male structure, is followed by spermatization, which occurs in some members of Ascomycetes and in Basidiomycetes.
- Spermatization: This is the process by which spore-like, small, uninucleate, unicellular, non-motile structures called spermatia (serve as male sex organs) are carried by various agencies like wind, water, insects, etc. to the female gametangia, to special reproductive hyphae, or sometimes to somatic hyphae and get fused. The contents of the spermatia enter those reproductive structures (function as female sex organs) through pores formed by the dissolution of walls at the point of contact. E.g., Puccinia sp. of Basidiomycetes, and Podospora sp of Ascomycetes.
- Somatogamy: In most of the higher fungi, sex organs are totally absent. In such fungi, somatic cells take over sexual reproduction. The compatible nucleus of one cell passes into the corresponding somatic cell. Thus, a dikaryotic condition is established. E.g., Peniophora sp.
Conclusion
The above idea shows that the degeneration of sexuality in fungi is due to the gradual minimization of the sex organs from Phycomycetes to Basidiomycetes.
