Dr. Mudassir Ahmad. Consortium for Educational Communication. Fungi: Degeneration of Sex. Lecturer in Botany Mobile No.:

Transcription

1 Consortium for Educational Communication Module on Fungi: Degeneration of Sex By Dr. Mudassir Ahmad Lecturer in Botany Mobile No.:

2 Text Fungi are eukaryotic, spore-bearing achlorophyllous organisms that generally reproduce asexually and sexually, and whose usually filamentous branched somatic structures are typically surrounded by cell walls containing chitin and/or cellulose. The lower fungi or Phycomycetes ( phycos meaning alga, and mycetes meaning mushrooms) comprise the simplest and most primitive group of fungi, represented by about 1500 species. Sexual process ranges from isogamy to oogamy through anisogamy. Bringing together male and female protoplasts or nuclei, that is plasmogamy is achieved by planogametic copulation (in Chytridiomycetes, Plasmodiophoromycetes), gametangial contact (Oomycetes) and gametangial copulation (Mucorales). 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.

3 Fig 1: Planogametic copulation in Phycomycetes Gametangial contact: A group of aplanogametic lower fungi (Oomycetes) produce nonmotile gametes (aplanogametes) that 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. Fig 2: Gametangial contact in Phycomycetes Gametangial copulation: In another group of aplanogametic

4 lower fungi (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 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.

5 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 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. Degeneration of Sex in Ascomycetes The ascomycetes are characterized by the presence of an ascus. The ascus is the product of sexual act and the ascospores produced inside it are thus sexual spores. The young ascus is always binucleate (n + n). These two nuclei fuse (Karyogamy) to form the diploid zygote nucleus (2n). This is followed by 3 successive nuclear divisions, the first division is normally reductional, the others equational forming a definite no of ascospores (usually 8) by free cell formation. Some important features of the sexual apparatus of the ascomycetes are: (i) (ii) (iii) (iv) (v) The presence of receptive hyphae, the trichogyne. Formation of dikaryophase Maintenance of dikaryophase through conjugate divisions of nuclei. Formation of ascogenous cells and hyphae. Karyogamy in the ascus mother cell.

6 (vi) Formation of ascospores by free cell formation. These are the basic processes found in the group. The differences, however, arise in the form and the type of sexual apparatus, the manner and the extent of formation of ascogenous hyphae and the type and form of fructifications produced. Moreover, the various methods and mechanisms that bring the compatible nuclei together in the progressive process of reduction in sexuality vary widely in different groups of fungi and are discussed below: A) Gametangial copulation: This process may be regarded similar to that found in the zygomycotina. This persists among the primitive ascomycetes (i.e. Hemiascomycetes). In this 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 (Endomycetales). 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 (Alexopoulos and Mims, 1979). 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.

7 In the family Saccharomycetaceae, however, modification of the above process can be seen as complete suppression of the specialized gametangia and copulation takes place between vegetative cells, a process known as somatogamy. The gradual suppression of the sexual phenomenon is characteristic of yeast fungi. This shows an increasing trend in the degeneration of the process. The formation of ascospores in yeasts is contingent upon a sexual process. Of course no sex organs like antheridia and oogonia are produced. 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

8 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 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.

9 Fig 4: Sexuality in Schizosaccharomyces octosporus Further modification of the sexual process is seen in the Taphrinales, where any (dikaryotic) vegetative cell becomes an ascus. Moreover the ascospores are capable of budding; copulation occurs between blastospores giving rise to binucleate mycelium in the host. Thus the intervention of the dikaryophase ultimately leads to karyogamy in the ascus mother cell followed by meiosis and the production of a definite number of ascospores in the ascus. The type of sexuality exemplified by Taphrinales has several interesting phases, both of degenerative and evolutionary character, such as suppression of specialized gametangia, copulation between blastospores, development of dikaryophase, formation of ascogenous cells, delayed karyogamy, alternation of haplophase with dikaryophase and production of many asci and ascopores with a single plasmogamous fusion. B). Gametangial contact: This 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

10 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 Class Plectomycetes includes the Ascomycotina in which the asci are irregularly arranged within the ascocarp. Many members show degeneration of sex organs especially of male gametangium. Let us illustrate sexuality in Aspergillus (from Plectomycetes) and see how it throws light on the degeneration of sex 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

11 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. In Eurotium, the antheridia and ascogonia are produced close to each other on the aerial somatic hyphae. The antheridium may or may not be functional but pairing of the nuclei takes place in ascogonium. When the antheridium is functional, its nuclei enter the ascogonium and pair with its nuclei. However,

12 when the antheridium is non-functional, the ascogonial nuclei form pairs. No sex organs have been found in Elaphomyces, but Bessey (1950) and Gaumann, (1952) reported a multinucleate ascogonium coiled around an antheridium in the closely related Ascoscleroderma. He believed, however that the antheridium does not function. The ascogonium produces ascogenous hyphae the tips of which elongate after karyogamy and form asci directly without crosiers. It appears that in Elaphomyces, however, crosiers are formed. Over 100 species of Penicillium are known and in most of them ascocarp formation is not known. Like Aspergillus, Penicillium is also a form genus based on conidial morphology. Only a few species are known to reproduce sexually. The sexual or perfect state of Penicillium has been assigned to different genera, such as Talaromyces, Eupenicillium and Carpenteles (Webster, 1980) A few species that show sexual reproduction are Penicillium vermiculatum (=Talaromyces vermiculatus) and P. stipitatum. Here the mycelium contains uninucleate cells from which develops a uninucleate ascogonium. The ascogonium elongates and its nucleus divides several times to produce as many as 64 nuclei. Simultaneously, a uninucleate antheridium also develops and coils spirally around the multinucleate ascogonium. The antheridial nucleus does not migrate into the ascogonium. It remains very much there in the antheridium, even after the development of ascogenous hyphae. This indicates that antheridia are formed but remain non-functional. In Sartorya, the ascoma originates exclusively from the ascogonium and the antheridium is not formed at all. Emericella

13 with 15 species bears no sex organs-neither ascogonia nor antheridia and the ascoma appears to originate from a loose hyphal knot. Pyrenomycetes is a class of sub division Ascomycotina (Ainsworth, 1971) and includes the fungi in which the ascocarp is usually of perithecium type and the asci are inoperculate with an apical pore or slit. Here again the actual process of nuclear fusion and fertilization are still not clearly known in most of the species of Erysiphe and others, even the actual process of ascosporogenesis is still not clearly known in most of the species. In Sphaeriales, sexual reproduction takes place by spermatia uniting with the trichogyne of ascogonia. Many species bear antheridia but are generally non functional. In Neurospora crassa the spermatia or conidia act as male cells, female sex organ is represented by protoperithecium or bulbil (Webster, 1980) Discomycetes are the ascomycetes with apothecium type of fruiting bodies which are cup, saucer or disc-shaped. Members show progressive reduction in their sexuality. The antheridia and ascogonia are both functional in Pyronema. In Lachnea stercorea, the antheridia are non-functional and in L. cretea the antheridia are totally absent. Along with the total absence of antheridia, Humaria granulata shows the disappearance of trichogyne from the ascogonium. Morchella shows the total absence of both antheridia and ascogonia. The sex organs which constitute the accessory parts of sexual process are completely suppressed in Morchella. Consequently, the sexual process is extremely simplified. It involves two distinct processes, namely Plasmogamy and Karyogamy. The later is immediately

14 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 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

15 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. It is speculated that the over wintering stage in the life cycle of Morchella is the sclerotium. The sexual apparatus is wholly lacking in Peziza vesiculosa. This does not prevent the development of fructification or the apothecium. The sexual process does take place, however it is extremely simplified and consists in the association of two purely vegetative nuclei in a pair to form a dikaryon by somatogamous copulation of adjacent hyphae or by autogamous pairing. Sexual Reproduction in Coprobia (syn: Humaria) varies

16 widely. In C. granulatus only the female sex organ is developed and is called ascogonium. The antheridium is lacking. It is only when the fusion between two vegetative hyphae of opposite strains of mycelia takes place that the ascogonium develops further. Coprobia rutilans develops no sex organs. The entire sexual apparatus is absent. The sexual process is extremely simplified. Sexual fusion takes place between any two vegetative cells of the hyphae of opposite strains. In Ascobolus furfuraceus, the antheridium is absent and the function of the male component is performed by oidia produced in chains. The ascogonium develops only when the oidium comes in contact with the mycelium of opposite strain. In Ascobolus citrinus both the antheridia and oidia are absent. The development of apothecium from the female branch is parthenogenetic with copulation. C). 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, such as Phyllactinia corylea, Sphaerotheca mors-uvae, in many Sphaeriales and Pezizales (e.g. Lachnea stercorea). D) Parthenogenesis: This is further modification of the above

17 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. E) Spermatization of trichogynes: This 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),

18 Laboulbeniales (Laboulbeniomycetes), Sclerotiniaceae and Pezizales (Discomycetes), Dothideaceae (Laculoascomycetes) and Lecanorales (Disclichens). Fig 6: Spermatization of Trichogynes This can be illustrated by the following examples: 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. 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.

19 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. From the above, it is clear that various modifications of the sexual processes take place and that the sexual apparatus has degenerated by various degrees. F) Somatogamy: The reduced type of sexuality is the final stage in the elimination of both the antheridium and ascogonium. The sexual process becomes extremely simplified. It takes 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). G) Even this impulse towards sexuality finally disappears.

20 The two vegetative nuclei of adjacent cells of the same homothallic mycelium come together to establish a dikaryon. The two nuclei of the dikaryon fuse in the ascus mother cell to form a synkaryon. The latter by meiosis, followed by mitosis, gives rise to four or eight haploid ascospores within the ascus. Hence the final stage in the development of the sexual apparatus in the life cycle is totally abandoned. H) Total absence of sexuality: In the cases discussed above, though various modifications have taken place, the pairing of nuclei is still essential. Total apomixis (absence of sex) has been reported in some cases. In Ascobolus equinus although solitary ascogonia are produced, karyogamy is totally absent. Only haploid nuclei are found everywhere. And since karyogamy does not occur, neither does meiosis, only a single haploid phase occurs in the life cycle, in which all nuclei are alike. This is indeed a rare and exceptional phenomenon. Thus it becomes clear that among the lower ascomycetes, the sexual apparatus is well-developed. Both the sex organs, antheridia and oogonia or ascogonia are well-marked and functional. They are usually developed on the same mycelium. In few cases as in the family Laboulbeniaceae, they occur on distinct individuals. There is however, a gradual and progressive degeneration of the sex organs as we proceed from the lower to the higher ascomycetes. In the most advanced ascomycetes, there is complete suppression of the sex organs. In them the sexual apparatus is wholly lacking. The progressive reduction on sexuality can be well illustrated by a comparison of species. They can be arranged in a series illustrating the gradual reduction and gradual elimination of the sexual apparatus. Firstly the antheridium ceases to function. The different species of

21 Aspergillus illustrate the way in which elimination of male sex organ (antheridium) in the Ascomyceltes 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: 1. In some, there is a degeneration of male nuclei, whereas in others the antheridium is rudimentary, non-functional or even absent. 2. 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. 3. 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 4. In a few species the male nuclei in the antheridium are reported to degenerate before the antheridium has reached maturity. 5. 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. In some species of Aspergillus, both the sex organs are well-developed. Sexual fusion takes place. Same is the case in Lachnea stercorea. There is, however, no migration of contents of the antheridium into the oogonium in this species. It shows that the antheridium has become non-functional.

22 There is another species of the same genus, which goes a step further. It is Lachnea cretea. In this species the antheridium is absent. The ascogonium is well developed and functional. Coprobia (Humaria) granulatus illustrates another step towards degeneration of sexuality. It is characterized by the absence of trichogyne from the ascogonium. The antheridium is also lacking. Coprobia (Humaria) rutilans illustrates the final stage in the elimination of sex organs. The antheridia are absent and so are the ascogonia or the oogonia. The sexual process in these advanced forms becomes extremely simplified. It takes place by the copulation between the cells of two vegetative hyphae. Finally even this impulse towards sexuality disappears. The two vegetative nuclei of adjacent cells of the same homothallic mycelium comes together to establish a dikaryon. The two nuclei of the dikaryon fuse in the ascus mother cell to form a synkaryon. The latter by meiosis followed by mitosis gives rise to 8 haploid ascospores within the ascus. Hence the final stage the development of the sexual apparatus in the life is totally abandoned. It is clear that many fungi have no sexual stage or true sexual nuclear fusion in their life history, and many others develop parthenogenetically. These must have developed from types with a normal sexual stage. No evidence exists as to how and when this partial or complete loss of sexuality occurred. In culture, however, it is common for fungi to show a gradual loss in intensity of sexual reproduction over a number of transplants. This may show itself in a reduction in the number of fruit bodies formed or in the number of viable spores per fruit body. In a number of fully investigated examples, such an apparently gradual loss of sexuality has been shown to be

23 due to the unconscious selection of sterile or relatively sterile mutants during sub-culturing. Mohendra and Mitra (1930) showed that, with repeated subculturing, a strain of Sphaeropsis malorum changed from a dark coloured colony with abundant pycnidia, containing viable spores, to a white one which was almost sterile. When spores from a fertile type of pycnidia were plated out they gave rise to both black and white colonies. Those giving rise to sterile white colonies germinated more quickly than those producing black colonies and hence with mass inoculum of spores the white colonies had greater chance of developing. Sexuality in Basidiomycetes Basidiomycetes are the most advanced of all fungal classes. The development of basidium is a unifying character common to all members of the class. The somatic phase consists of a well-developed septate filamentous mycelium which passes chiefly through two important phases primary mycelium and the secondary mycelium. The primary mycelium is monokaryotic, bears neither sex organs and is short lived. The monokaryotic mycelium originates with the germination of a basidiospore and is often called primary mycelium. The secondary mycelium (which is dikaryotic) originates variously by fusion between two monokaryotic structures (hyphae, oidia, sporedia etc) and is binucleate. The binucleate condition begins when the protoplasts of two uninucleate cells fuse i.e. plasmogamy without karyogamy. Obvious manifestations of sexuality, as specially differentiated gametes and gametangia, are usually absent in the basidiomycetes (excepting rusts). But there are other ways of coming together of the two compatible nuclei. These may be:

24 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 in basids than the other fungi. In view of the diversified nature of the processes formed in different groups of these fungi we would discuss some important representative examples to illustrate the range in variation of sexual mechanism. Sexuality in Ustilago No sex organs are developed in Ustilago. 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 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. Such mycelia are said to be heterothallic. Plasmogamy in such species is brought about by different methods of diploidization that results in binucleate condition also called dikaryotic. Plasmogamy thus initiates dikaryophase

25 in the life cycle. b) Karyogamy: With karyogamy, the dikaryophase ends. 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. 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 asexual spores called conidia. Thus it is clear that sexuality reduces as we proceed from

26 primitive fungi i.e. Phycomycetes to Basidiomycetes through Ascomycetes and the perfect stage is completely lacking in Deuteromycetes.