Consortium for Educational Communication

Transcription

1 FrequentlyAsked Questions (FAQs) 1. Briefly summarize the progressive degeneration of sex in Fungi? The existence of some sort of sexual reproduction in Fungi has long been suspected, although in earlier instances upon insufficient grounds; but of late observations have multiplied and facts accumulated which leave no doubt of its existence. However, studies on the major groups within the fungal kingdom have provided significant and wide-ranging clues regarding the gradual loss in sexuality and sexual identity. In the flagellate Phycomycetes (Chytridiales, Blastocladiales, Monoblepharidales, Saprolegniales and Peronosporales) the sex organs are quite conspicuous, and called the antheridia (male) and the oogonia (female). The resultants of fusion are oospores. In the series Aplanatae (aflagellate Phycomycetes, class Zygomycetes) the gametangial copulation takes place. The gametangia are represented by + and - signs. The resultants of gametangial union are zygospores. In Ascomycetes, the sex organs are not so prominent, and there is a regular phenomenon of reduction of sexuality. The phenomenon of nuclear fusion is not completely understood as yet. In some of the Ascomycetes (e.g., Morchella) no sex organs are produced and the somatic cells take part in sexual fusion. This process is called the somatogamy. In the Basidiomycetes there are no sex organs and only the fusion of the nuclei of a dikaryon represents the process of sexuality. In the members of Fungi Imperfecti or Deuteromycetes, the sexual process is altogether absent and they reproduce asexually by conidia.

2 It is clear that many fungi have no sexual stage or true sexual nuclear fusion in their life history, and many others develop parthenogenetically. 2. Describe Planogametic copulation and Gametangial contact in Phycomycetes? The sexuality in the primitive fungi or Phycomycetes ranges from isogamy through anisogamy to oogamy. Plasmogamy is achieved through one of the following methods: Planogametic copulation: This type of sexual reproduction involves the fusion of two naked gametes where one or both of them are motile. The motile gametes are known as planogametes. The most primitive fungi produce isogamous planogametes, e.g. Synchytrium, Olpidium, Plasmodiophora, etc. The anisogamous planogametes are only found in the genus Allomyces of order Blastocladiales. Monoblepharis illustrates the most advanced type of planogametic copulation. It produces small opisthocont male gametes (sperms) in large numbers in the sex organ called antheridium. The oogonium produces a single large immobile female gamete, the egg or ovum. The male gamete enters the oogonium and fertilizes the egg. Fig 1: Planogametic copulation in Phycomycetes

3 Gametangial contact: A group of aplanogametic lower fungi (Oomycetes) produce nonmotile gametes (aplanogametes), they are never released. The male gametangium is called the antheridium and the female oogonium. No sperms are organised. The gametangia donot actually fuse and do not lose their identity. The antheridium puts out a slender, tubular outgrowth the fertilization tube at the point of contact with the oogonium. Meiosis is gametangial. Examples are Saprolegnia, Achlya, Pythium, Phytophthora, etc. Fig 2: Gametangial contact in Phycomycetes 3. Write a note on Gametangial copulation in Phycomycetes In terrestrial Mucorales Mucor, Rhizopus, Entomophthora, etc, gametic union is brought about by the fusion between the gametangia. The uniting gametangia lose their identity in the sexual act. The intervening walls between the two gametangia dissolve, forming a common fusion cell. Plasmogamy is immediately followed by karyogamy. The diploid contents of the zygospore or the diploid nuclei undergo zygotic meiosis, the meiospores or meiozoospores on liberation

4 germinate to produce a new haplomycelium. It is obvious that sexual reproduction arose several times in evolutionary development of lower fungi. There is no linear series to suggest that all the lower forms are isogamous and higher forms oogamous. Sexual reproduction becomes less frequent than asexual and is resorted to towards the end of the growing season when conditions unfavourable for growth are to set in. Zygospores help to tide over unfavourable period. Fig 3: Gametangial copulation in Zygomycetes The gradual transition from the aquatic forms to purely terrestrial species noticeable in the phycomycetes culminates in Ascomycetes which are fungi of drier terrestrial habitats completely lacking motile cells in their life cycle. From the primitive to the more advanced zygomycetous phycomycetes, there is a tendency for the sporangia to be transformed into sporangioles and these into conidia the typical asexual

5 spores of Ascomycetes that have played significant role in their spread. In fact, in some species of Ascomycetes conidia alone appear to be sufficient for survival. 4. Describe briefly the Sexuality in Basidiomycetes? In basidiomycetes, manifestations of sexuality, as specially differentiated gametes and gametangia, are usually absent except rusts. But there are other ways of coming together of the two compatible nuclei. These may be: 1) Somatogamy or mycelial fusions 2) Spermatization or oidization i.e. fusion between oidia and hyphal cells and 3) Fusion between two oidia which are small, hyaline one celled and uninucleate spores formed on oidiophore tip. Thus sexuality is said to be reduced in Basidiomycetes. The primary manifestations of sexuality i.e. the union of cells and nuclei and the association of chromosomal and gene complements are obviously present in the basidiomycetes, in spite of the absence of such secondary criteria as specialized gametes and gametangia. The compatibility mechanism in fact, is much more highly developed than the other fungi. Some important representative examples like Ustilago and Agaricus illustrate the range in variation of sexual mechanism among Basidomycetes. In Ustilago No sex organs are developed and the sexual process is represented by three fundamental phenomena characteristic of it, namely plasmogamy, karyogamy and meiosis. a) Plasmogamy: Heterothallism is common in the genus

6 Ustilago. The mycelia though morphologically alike are different physiologically. They are physiologically unisexual. However there is no distinction into male and female mycelia. They are different only in their sexual behaviour. The difference of sex is thus very rudimentary. It is denoted by the signs plus and minus. Plasmogamy in such species is brought about by different methods of diploidization that results in binucleate condition also called dikaryotic. Plasmogamy thus initiates dikaryophase in the life cycle. b) Karyogamy: Here the two nuclei fuse and this fusion may be regarded as the culmination of sexual process that started at the time of diploidization. It is equivalent of fertilization. The diploid nucleus formed is called a synkaryon. The smut spore with a synkaryon is the probasidium or hypobasidium that represents a transitory diplophase in the life cycle of smuts. c) Meiosis: The diploid smut spore germinates to form the promycelium or epibasidium. The synkaryon in the epibasidium undergoes meiosis to form four haploid daughter nuclei. After walls are formed, the epibasidium converts into a four celled structure, each cell of which bears a haploid basidiospore. In Agaricus and other mushrooms the sexual apparatus in the form of sex organs is completely lacking. Their function has been taken over by the somatic hyphae which are heterothallic. The fusion between two somatic hyphae of + & - strains represents the first stage. Plasmogamy is accomplished by somatogamy or somatogamous copulation. This is followed by karyogamy and meiosis.

7 5. Explain the degenerative Sexuality in Ascomycetes with reference to Morchella In Morchella, both the sex organs i.e. antheridia and ascogonia are absent. Consequently, the sexual process is extremely simplified. It involves two distinct processes, namely Plasmogamy and Karyogamy. The later is immediately followed by meiosis. a) Plasmogamy: It consists in the union of cytoplasmic contents of two cells without nuclear fusion, which is delayed. This results in a sequence of binucleate cell generations, constituting the Dikaryophase. Plasmogamy occurs at a very late stage during the development of the ascocarp in the subhymenial layer. It takes place just before the formation of asci. Plasmogamy in Morchella takes place by the following two methods: i) Somatogamous Copulation: Two vegetative hyphae of the subhymenium region of the pileus come in contact. The intervening walls between the copulating cells dissolve at the point of contact. The two multinucleate protoplasts intermingle in the fusion cell. Two functional nuclei, one from each copulating cell form a dikaryon. All other nuclei in the fusion cell disappear. Later ascogenous hyphae arise from the fusion cell containing the dikaryon. Each young ascogenous hypha receives a pair of nuclei. These are the derivatives of the parent dikaryon. The ascogenous hyphae afterwards become septate. The terminal cells of these hyphae function as ascus mother cells. They are

8 binucleate. The nuclei are the derivatives of the parent dikaryon. ii) Autogamous pairing: It has been reported in Morchella elata. In this species, any vegetative cell of the sub-hymenium (Hypothecium) may establish a dikaryon. It is accomplished by pairing of two of its own vegetative nuclei. It is called autogamous pairing. The remaining nuclei in the cell degenerate. The cells with dikaryons (dikaryotic cells) develop ascogenous hyphae as in somatogamous copulation. b) Karyogamy: The two nuclei in the ascus mother cell fuse. The fusion cell with a diploid nucleus is called the young ascus. It represents the transitory diplophase in the life cycle. The asci in Morchella thus develop from the tip cells of ascogenous hyphae without the formation of crosiers. c) Meiosis: The young ascus cell elongates. The synkaryon in the ascus undergoes two successive divisions. These constitute meiosis. During meiosis the number of chromosomes in the resultant four daughter nuclei is reduced to half the diploid number. The four haploid nuclei undergo the third division. It is mitotic. In this way eight haploid daughter nuclei are formed in each ascus cell. Each of these is fashioned into an ascospore. With the formation of ascospores, the haploid or gametophyte phase is again initiated in the life cycle of Morchella. 6. Describe the mode of sexuality in Yeasts?

9 Of course no sex organs like antheridia and oogonia are produced in Yeasts. Instead the sexual process is extremely simplified. It consists of three phenomena characteristic of the sexual process, namely plasmogamy (sexual fusion), karyogamy (fusion of nuclei) and meiosis. In the culture of brewer s or baker s yeast (Saccharomyces cerevisiae) there occur intermixed two kinds of somatic cells, namely small dwarf strain and large strain cells. Under normal conditions, but absence of cells of the opposite mating type, these multiply by budding. Under normal conditions and presence of small haploid cells of the opposite mating strain, they function as gametangia and resort to sexual or gametangial conjugation. During conjugation + and strains of haploid dwarf yeast cells agglutinize to form clusters. In this condition each mating type secretes a sex hormone (Tkacz and Mackay 1977) which induces the haploid spherical cells A and B of the opposite strains to elongate to a pear shaped form (A 2 & B 2 ) and also causes alterations in the cell walls of the newly formed regions. These surface alterations facilitate fusion. The zygotes formed as a result of sexual fusions between two haploid yeast cells of opposite mating types are, in fact the large strain yeast cells. During unfavourable conditions, these diploid large strain yeast cells resort to ascospore formation. In Saccharomycodes ludwigii, the ascospores directly function as gametangia and copulate. Sexual fusion takes place within the ascus between the two adjacent ascospores of opposite mating types A and a or + and strains. Consequently two diploid cells or zygotes are formed within the wall of the ascus. Each zygote germinates in situ to form a small germ-tube which grows and emerges through the ascal wall and functions

10 as a sprout mycelium. The cells of the sprout mycelium bud off diploid yeast cells. The latter separate from the sprout mycelium not by a constriction but by the formation of a septum at the base. Soon the diploid sprout cell gets severed from the parent cell. This detached diploid somatic or sprout cell functions as ascus mother cell. Each of these becomes an ascus after its diploid nucleus undergoes meiosis to produce ascospores. It is obvious that the haplophase (gametophyte) in Saccharomycodes ludwigii is extremely reduced, represented only by ascospores. In fission yeast, Schizosaccharomyces octosporous, each haploid somatic cell is a potential gametangium. At the time of sexual reproduction two somatic cells come to lie side by side and each sends out a short narrow beak like process, the two meet, intervening walls dissolve to form a conjugation tube later functioning directly as ascus mother cell. octosporus Fig 4: Sexuality in Schizosaccharomyces 7. Briefly explain Gametangial contact in Ascomycetes with special reference to progressive degeneration of

11 sex in Aspergillus? Gametangial contact in Ascomycetes though similar to the oogamous type in its initial stages, has many evolutionary and distinct features. In this method well-developed uninucleate or multinucleate gametangia develop. The males are designated antheridia and the females ascogonia, which are provided with a specialized receptive hypha, the trichogyne, not formed in the oomycetes. The male nucleus or nuclei migrate into the female via the trichogyne, do not fuse but remain associated, thus undergoing dikaryophase. Then one or more ascogenous hyphae arise from the ascogonium, and ultimately asci are formed by the crozier or hook method, characteristic of the ascomycetes. This type of sexuality occurs commonly in taxa belonging to several groups such as Plectomycetes, Pyrenomycetes and Discomycetes. Fig 5: Gametangial contact in Ascomycetes The sexual or perfect stage is rather rare. Possibly the species lacking it have lost it during the course of evolution. This view is supported by the fact that even in species that form asci there is evidence of sexual degeneration. Different species of this genus show variation in their sexual behaviour. These different species of Aspergillus illustrate the way in which

12 elimination of male sex organ (antheridium) in the Ascomycetes has taken place. These species can be arranged in series. The series indicates the stages or steps in which the progressive degeneration of the male sex organ (antheridium) has been accomplished. These steps are: i) In some there is a degeneration of male nuclei, whereas in others the antheridium is rudimentary, non-functional or even absent. ii) iii) iv) In Aspergillus herbariorum the pollinodium cuts off a terminal antheridium. The tip of the antheridium fuses with that of trichogyne but there is no migration of contents of the antheridium towards the trichogyne. In some species the antheridium may develop late. By that time the ascogenous hyphae have already been developed from the ascogonium. e.g. A. nidulans In a few species, the male nuclei in the antheridium are reported to degenerate before the antheridium has reached maturity. v) The antheridium in some species, such as A. flavus, A. fisheri and A. fumigatus does not develop at all. Only the ascogonia are seen to be developing. 8. Give a brief not on the Gametangial copulation in Ascomycetes? Gametangial copulation in Ascomycetes may be regarded similar to that found in the zygomycotina. This persists among the primitive ascomycetes (i.e. Hemiascomycetes). In this

13 method two morphologically similar gametangia copulate in the same manner as those of the zygomycetes. Plasmogamy takes place, a zygote is immediately formed at the fusion bridge and the zygote gets converted into an ascus. No dikaryotic phase is developed because plasmogamy is immediately followed by karyogamy. This is exemplified by members of the family Dipodascaceae. But interestingly enough, the hyphal cells or gametangia may be multinucleate as in Dipodascopis albidus or uninucleate as in Dipodascus aggregatus and Dipodascopis uninucleatus The fusion of the multinucleate gametangia of D. albidus is reminiscent of gametangial copulation in some of the zygosmycetes and the multispored ascus has been compared to the germ sporangium. The life cycle of Dipodascus aggregatus and Dipodascopsis uninucleatus are a step ahead of that of Dipodascus albidus which have become simplified by the elimination of the nonfunctional supernumerary nuclei from the very beginning. 9. Describe briefly Autogamy? Autogamy: This is a stage in the retrogressive evolution of sexuality, where the female sex organ is retained and sexual development proceeds without any direct fertilization (plasmogamy) from an antheridium, (antheridium is nonfunctional as there is no migration of the male nuclei). The contact stimulus of the male gametangium is enough to initiate further development and the dikaryon stage is attained by pairing between the ascogonial nuclei only. This type of development is seen in certain aspergillii and pencillia, such as species of Eurotium like E. amestelodam, E. repens (Aspergillus repens) and E. glaucus (Aspergillus glaucus); Talaromyces vermiculatus (Pencillium vermiculatum), in powdery mildews,

14 such as Phyllactinia corylea, Sphaerotheca mors-uvae, in many Sphaeriales and Pezizales (e.g. Lachnea stercorea). 10. Write a brief note on Parthenogenesis in Ascomycetes? This is further modification of the above process (autogamy) where there is a complete elimination of the male gametangium. Dikaryotization and the production of the ascogenous hyphae occur in one of the following ways: i) Humaria granulata (Discomycetes): This is a heterothallic fungus. The ascogonia are produced in the normal way. The compatible nuclei of the opposite strain enter one of the nearby (connected with ascogonium) hypha by means of mycelial fusion. This enables the opposite strain to migrate into the ascogonium, which then gives rise to ascogenous hyphae in a normal way. If contact with the mycelium of the opposite strain is denied, the ascogonia die without developing any further. ii) Ascobolus citrinus (Discomycetes) In this fungus ascogonia are produced normally. No outside nuclei are introduced from other hyphae but nuclei from one of the adjoining cells of the ascogonium (usually the next cell of the stalk) enter the ascogonium by the dissolution of the partition wall. The ascogonium then gives rise to ascogenous hyphae and proceeds to develop asci. iii) Sartorya (Plectomycetes): In Sartorya which is the perfect state of a number of aspergillii e.g. A. funigatus, the ascocarp is exclusively initiated by a coiled ascogonium and no antheridium is produced. 11. Describe Spermatization of Trichogynes in

15 Ascomycetes? Spermatization of trichogynes is a highly specialized type of sexual process in which no antheridia are produced and the ascogonia provided with trichogyne are fertilized by means of spermatia, microconidia, or conidia. It is found in many higher ascomycetes, such as in the members of Sphaeriales, Pseudosphaerales (Pyrenomycetes), Laboulbeniales (Laboulbeniomycetes), Sclerotiniaceae and Pezizales (Discomycetes), Dothideaceae (Laculoascomycetes) and Lecanorales(Disclichens).This can be illustrated by the following examples: Fig 6: Spermatization of Trichogynes i) Mycosphaerella (Loculoascomycetes): Spermatia are formed in special ostiolate spermogonia. These are produced internally in basal spermogonial cells or sperm mother cells and are pushed out through a sterigma-like phialide of the parent cell. In perithecial primordia, coiled ascogonia are formed each with a basal ascogonial cell and a long trichogyne. After fertilization, the ascogonium throws off short ascogenous hyphae which develop asci by hook or crozier method.

16 ii) Collema (=Collemodes) (Lecanorales): The ascogonium consists of one to three coils of cells which terminate in a filament (the trichogyne). The male organs are branched hyphae that project into a conceptacle-like spermogonium. Minute, uninucleate non-motile sperms bud off the branches externally. Fertilization is accomplished and asci are formed by hook method. iii) Neurospora sitophila- This is another step towards degeneration of sexuality. The ascogonia are more degenerate and consists only of a coiled hypha which give out a number of hyphae, each acting as a trichogyne. No special spermatia are produced here and the spermatial function is either performed by compatible (since it is a heterothallic fungus) conidia and/or microconidia, or a germ-tube from the conidium supplies nuclei to the receptive trichogynes. Fusion between the trichogyne and the fertilising cell is followed by migration of one or more nuclei down the trichogyne into the ascogonium. After fertilization, ascogenous hyphae emerge and asci are formed by the crozier method. 12. Write a brief note on Sexuality of Deuteromycetes? Fungi Imperfecti or Deuteromycetes comprises of a group of fungi in which only the asexual or imperfect stage is known. The sexual stage also called the perfect stage is completely unknown; the sexual spores like sporangiospores, meiospores (ascospores and basidiospores) are either non existent or have not been observed or discovered so far. Reproduction takes place chiefly by the formation of exogenously developed

17 asexual spores called conidia. They are the most highly evolved group of fungi which have lost the sexual character during the progressive process of evolution. 13. Write a short note on sexuality in Ustilago and Agaricus. In Ustilago, no sex organs are developed. The sexual process is represented by three fundamental phenomena characteristic of it, namely plasmogamy, karyogamy and meiosis. d) Plasmogamy: In Ustilago, there is no distinction into male and female mycelia. They are different only in their sexual behaviour. The difference of sex is thus very rudimentary. It is denoted by the signs plus and minus. Such mycelia are said to be heterothallic. Plasmogamy in such species is brought about by different methods of diploidization that results in binucleate or dikaryotic condition. Plasmogamy thus initiates dikaryophase in the life cycle. e) Karyogamy: Here the two nuclei fuse and this fusion may be regarded as the equivalent of fertilization. The diploid nucleus formed is called a synkaryon. The smut spore with a synkaryon is the probasidium or hypobasidium that represents a transitory diplophase in the life cycle of smuts. f) Meiosis: The diploid smut spore germinates to form the promycelium or epibasidium. The synkaryon in the epibasidium undergoes meiosis to form four haploid daughter nuclei. After walls are formed, the epibasidium converts into a four celled structure, each cell of which bears a haploid basidiospore. In Agaricus and other mushrooms the sexual apparatus in the

18 form of sex organs is completely lacking. Their function has been taken over by the somatic hyphae which are heterothallic. The fusion between two somatic hyphae of + & - strains represents the first stage. Plasmogamy is accomplished by somatogamy or somatogamous copulation. This is followed by karyogamy and meiosis. 14. Differentiate between somatogamous copulation and autogamous pairing of Ascomycetes. Answer: In Somatogamous Copulation, two vegetative hyphae come in contact. The intervening walls between the copulating cells dissolve at the point of contact. The two multinucleate protoplasts intermingle in the fusion cell. Two functional nuclei, one from each copulating cell form a dikaryon. All other nuclei in the fusion cell disappear. Later ascogenous hyphae arise from the fusion cell containing the dikaryon. Each young ascogenous hypha receives a pair of nuclei. These are the derivatives of the parent dikaryon. The ascogenous hyphae afterwards become septate. The terminal cells of these hyphae function as ascus mother cells. They are binucleate. In autogamous pairing, any vegetative cell of the subhymenium (Hypothecium) may establish a dikaryon. It is accomplished by pairing of two of its own vegetative nuclei. The remaining nuclei in the cell degenerate. The cells with dikaryons (dikaryotic cells) develop ascogenous hyphae as in somatogamous copulation. 15. Write a short note on Somatogamy in Ascomycetes. Somatogamy is a reduced type of sexuality and represents the final stage in the elimination of both the antheridium and ascogonium. The sexual process is extremely simplified. It takes

19 place by the copulation between the cells of two compatible hyphae at any point along the mycelium through anastomosis. Two strains of the opposite sex, however, are necessary. After dikaryon formation, ascogenous hyphae and finally asci and ascospores are formed normally in the ascocarp. This type of reproduction occurs in Gelasinospora tetrasperma, Humaria rutilans, some species of Penicillium [e.g. P. brefeldianum (Carpenteles brefeldianum), P. egyptiacum (Carpenteles egyptiacum)] and aspergillus that have their perfect state in Emericella, e.g. Emericella nidulans (Aspergillus nidulans). 16. Differentiate between Gametangial Contact of Phycomycetes and Gametangial Contact found in Ascomycetes. The lower fungi particularly Oomycetes produce nonmotile gametes which are never released. The male gametangium is called the antheridium and the female oogonium. No sperms are organised. The gametangia do not actually fuse and do not lose their identity. The antheridium puts out a slender, tubular outgrowth the fertilization tube at the point of contact with the oogonium. Meiosis is gametangial. Examples are Saprolegnia, Achlya, Pythium, Phytophthora, etc. In Ascomycetes, the process though similar to the oogamous type in its initial stages, has many evolutionary and distinct features. In this method well-developed uninucleate or multinucleate gametangia develop. The males are designated antheridia and the females ascogonia, which are provided with a specialized receptive hypha, the trichogyne, not formed in the oomycetes. The male nucleus or nuclei migrate into the female via the trichogyne, do not fuse but remain associated, thus undergoing dikaryophase. Then one or more ascogenous

20 hyphae arise from the ascogonium, and ultimately asci are formed by the crozier or hook method. This type of sexuality occurs commonly in taxa belonging to several groups such as Plectomycetes, Pyrenomycetes and Discomycetes.