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In this article we will discuss about:- 1. Introduction of Phycomycetes 2. Occurrence and Economic Importance of Phycomycetes 3. Somatic Structures 4. Behaviour of Compatible Nuclei 5. Life Cycle Pattern.
Contents:
- Introduction to Phycomycetes
- Occurrence and Economic Importance of Phycomycetes
- Somatic Structures of Phycomycetes
- Behaviour of Compatible Nuclei of Phycomycetes
- Life Cycle Pattern in Phycomycetes
1. Introduction to Phycomycetes:
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The Phycomycetes are ordinarily considered to be the most primitive fungi. They constitute a very diversified assemblage of individuals ranging from a microscopic cell to those with a conspicuous, filamentous branched form. Some show relationships with the flagellates, others closely resemble algae and still others being true molds.
Because of their morphological similarity with algae they were for long time known as ‘algal-fungi’ and were regarded as degenerate algae by loss of chlorophyll and consequent loss of ability to elaborate food have become fungi. The supporters of this view emphasized only on the morphological resemblances between algae and fungi and ignored the most important physiological differences between them.
2. Occurrence and Economic Importance of Phycomycetes:
The members of this class of fungi occur on a wide variety of substrata. Some are aquatic (both fresh-water and marine) being parasitic on plants and animals including fish and insects, or are saprophytic on submerged vegetable and animal debris. Very few are mycorrhizal.
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The aquatic ones are known a water molds. Whereas, others are amphibious, and still others terrestrial. Many of the advanced species attack leaves and shoots of angiospermous plants and induce various plant diseases.
Some of the terrestrial Phycomycetes are the most destructive parasites of crop plants and certain insects. Again others often cause spoilage of foodstuff, various fruits and vegetables. Some others are utilized in industrial fermentations. There are also species which inhabit damp soil.
Many of these soil-inhabiting species cause serious root troubles of economic plants and others destroy seedlings in the seed bed. Most of the aquatic species are very susceptible to hydrogen-ion concentration of the substratum. They decrease in number with the decrease in hydrogen-ion concentration along with the rise in temperature and falling of water-level.
Again some of the aquatic fungi are reported to play the role of natural purification of water.
3. Somatic Structures of Phycomycetes:
Among the Phycomycetes the most primitive ones have unicellular uninucleate uniflagellate holocarpic thalli with or without any surrounding membrane. In some- what advanced species the somatic phase is a rhizomycelium, and in more advanced forms it is represented by a scanty mycelium consisting of a few short hyphal branches.
The majority of the Phycomycetes, however, have a profusely branched well-developed, filmentous aseptate mycelial thallus. Most of them are eucarpic.
Even in the aseptate mycelium the septa appear in old hyphae of some species, and during the development of reproductive structures, septum being formed at the base of each reproductive organ. The septa of the Phycomycetes are different from other fungi being solid and are not perforated in the centre.
In a profusely developed mycelium the reproductive bodies are less conspicuous. In the group as a whole the development of fruiting bodies, comparable to those of other groups of fungi is absent, but in a few, a tendency toward the formation of such structures is present, encountered in’ the genus Endogone (Fig. 151).
4. Behaviour of Compatible Nuclei in Phycomycetes:
In the Phycomycetes there is no interval between plasmogamy and karyogamy which occur almost simultaneously. But in most of them the post-sexual stages are not clearly known.
In general, meiosis takes place before the development of new individuals. In a small number of Phycomycetes, for example, species of Allomyces, however, meiosis of the zygotic nucleus is delayed resulting in the development of a diploid structure which may be equally well developed like the haploid vegetative body indicating the presence of an alternation of generations.
The post-sexual stages so far known are extremely variable in this group of fungi.
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In general, they follow along the lines as indicated below:
i. The Fusion Product of Two Well-Organized Free Gametes is Usually a Flagellate Zygote:
The flagella are shed and the zygote then forms doploid structure which, depending on the nature of fungi, behaves in a different manner as outlined below.
(a) In case of parasitic fungi, e.g., Synchytrium endobioticum, Olpidium viciae, the zygote infects the host tissue and ultimately develops into sporangia or gametangia as the case may be, and meiosis takes place during the formation of spores and gametes.
(b) In some water molds like the genera Allomyces, Blastocladiella, the zygote directly germinates to form a conspicuous diploid structure called sporothallus which bears two types of sporangia. Some of them produce diploid zoospores which on germination give rise to sporothalli, whereas, others bear haploid zoospores, meiosis takes place during their development. These haploid zoospores give rise to the somatic body.
ii. The Oospore Formed as a Result of Gametangial Contact or Otherwise Behaves in any One of the Following Ways:
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(a) The oospore directly germinates to produce germ tube or hypha of the new individual, exhibited by the genera Monoblepharis, Saprolegnia, Pythium. Meiosis takes place during the germination of oospore.
(b) The oospore germinates by a germ tube which terminates in a sporangium. Meiosis takes place in the oospore at the time of germination. The sporangium produces secondary zoospores which encyst and ultimately germinate to produce germ tubes the germ tubes give rise to the hyphae of new individuals. Examples are the species of Phytophthora and Plasmopara.
(c) The entire content of the oospore, on germination is transformed directly into secondary zoospores from which new individuals are formed, found in the species of Albugo. Meiosis takes place during the germination of oospore.
3. Where gametangia are not distinguishable as male and female, the zygospore that is formed by gametangial copulation germinates producing a sporangiophore bearing a germ sporangium at its tip. The germ sporangium produces sporangiospores which give rise to hyphae of new individuals. Meiosis takes place during the zygospore germination, exhibited by the general Mucor and Rhizopus.
5. Life Cycle Pattern of Phycomycetes:
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The life cycle pattern in the Phycomycetes is rather simple in comparison with that of the Ascomycetes and the Basidiomycetes. It consists of well-differentiated asexual cycle and sexual cycle.
The somatic body may directly take part in the asexual reproduction or may produce sporangium from which spores are developed asexually. Each of these spores on germination produces somatic body. Thereby asexual cycle is completed. This process is repeated to enable the development of new individuals.
The sexual cycle is rather less elaborate than what is encountered in the Ascomycetes and the Basidiomycetes. Here also the somatic body, depending on surrounding conditions, may directly take part in the sexual reproduction or may produce gametangia from which gametes are developed and by gametic union zygote is formed.
Again the gametangia without producing well-organized gametes may also take part directly in the sexual process. As soon as the compatible nuclei are brought together by plasmogamy, almost immediately after that zygote is formed by karyogamy resulting in the establishment of diplophase. The diplophase is followed by meiosis leading to the development of spores or germ tube from which somatic body is developed.
The zygote may also be thick-walled and becomes active only after a period of rest. Meiosis may thus be delayed. The major portion of the sexual cycle in the Phycomycetes comprises of haplophase. Whereas, the dikaryophase is particularly very short or may often be absent. Life cycle pattern is indicated in Figure 156.
