Sexual Reproduction in Rhizopus stolonifer (The Bread mould)

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Sexual Reproduction in Rhizopus 

Rhizopus displays a sexual process which is of conjugation type. It consists in the fusion of entire cell contents of the two gametangia. In many respects, it is similar to that of Spirogyra. As in Spirogyra the gametes are non-motile, isogamous and are produced singly but differ in being multinucleate. Sexual reproduction generally takes place towards the end of the growing season. Some species of Rhizopus are homothallic (R. sexualis) and others are heterothallic. To the latter category belong R. stonifer.

In the heterothallic species, the mycelia are of two distinct types. They are similar outwardly but function differently in reproduction. They belong to two different mating types. One of these is labeled as plus strain and the other minus strain. Sexual reproduction takes place only when a plus strain hyphae and minus strain hyphae come in contact. Sexual reproduction in R. stolonifer is thus possible only when mycelia of plus strain and minus strain grow in the same culture. This of course, is not of common occurrence and thus zygospore formation in the heterothallic species is rare.

Sexual reproduction takes place by gametangial copulation The fusing gametangia are multinucleate and similar in every respect but are distinct genetically. The sexual processs starts with the formation of two special hyphae termed zygophores. The zygophores arise from near the apices of the actively growing somatic hyphae of two neighbouring mycelia of+and – strains. They are chemically induced. According to Burgeff (1924), the processs is initiated by the secretion of a diffusible

sexual hormone. He showed hat zygophore development in each strain beings only when the opposite strain is close enough for the stimulus to reach the receptive hyphae. On the secretion of another diffusible metabolite, the zygophores of each strai bend toward those of the opposite strain preparatory to contact. The zygophores meet and adhere in pairs at their tips to form the fusion septum. Thereafter the tips of paired zygophores begin to swell due to the flow of cytoplasm and nuclei into them. The swollentips are now called the progametangia. A transverse septum known as the gametangial septum then appears in each progametangium near its tip. It cuts off a small terminal cell, the gametangium from the proximal part which is now termed the suspensor. The densely granular, multinucleate protoplast of the gametangium constitutes the aplanogamete, The suspensor contains vacuolated cytoplasm with fewer nuclei. In some other species the two gametangia in the pair are markedly unequal in size and suggest anisogamy. When the gametangia are mature, the intervening fusion septum between them dissolves. The adjoining aplanogametes intermingle in the fusion cell which is technically known as the prozygosporangium. The majority of the nuclei in the combined protoplast fuse in pairs. The two nuclei in the fusing pair belong to opposite strains. The nuclei that do not fuse and are left over, later degenerate. The combined protoplast in the prozygosporangium containing a number of diploid nuclei is called the zygophores. The pro-zygosporangium containing the young zygophore increases in size and secretes a thick multilayered wall around it to become the zygosporangium. The outermost layer is thick and warty. The single zygophore develops within the zygosporangium.

When mature the suspensors wither and the the zygosporangium containing the zygophore is set free. It enters upon a period of rest which may extend over several (5.9) months.

Meiosis

It takes place after the resting period prior to germination of zygophore. During meiosis segregation of strains takes place. According to Cutter (1942), half of the resultant nuclei are of + strain and the other half of- strain. If both the strains of nuclei migrate to the germ sporangium, the latter will produce both kinds of germ spores + as well as – . In case only one kind of nuclei migrate to the germ sporangium it will produce either + germ spores or – germ spores.

Germination of zygophore

The zygospore germinates under suitable conditions. The early nuclear divisions are meiotic. So at the time of germination it has numerous haploid nuclei of + and – strains. The haploid zygospore protoplast absorbs water and swells. As a result the thick zygosporangium wall and exospores split open. The contents of the zygospore still surrounded by the endospore emerge through the split and grow out into a small, stout vertical hyphae. It is called the germ tube or the promycelium. The promycelium grows to a limited extent. It soon bears a typical, terminal sporangium. It is called the germ sporangium. In many cases the germ tube rows horizontally for a short distance over the substratum to form sparsely branched or unbranched vegetative hyphae. From the latter arises a vertically growing branch bearing the germ sporangium. The latter has the structure of a typical sporangium. The protoplast of the germ sporangium undergoes cleavage to produce non-motile meiospores which are often called the germ spores.

According to Gauger (1961), the germ spores in Rhizopus stolonifer are either all (+) or (-) type or of both types (+) and (-) In the letter case half of them contain (+) nuclei and half (-) nuclei. How it happens has ben explained by Cutter (1942). After meiosis half of the nuclei are of + strain and the other half of- strain. The germ sporangium containing both types of spores receives both (+) and (-) nuclei. The promycelium dies after liberation of the germ spores. In some other Mocorales (Mucor mucedo). All the spores in the germ sporangium are of one kind (+) or (-). In still others (Phycomyces nitens) each zygosporangium contains three kinds of spores + , – , + (diploid).

Sometimes the gametangia fail to fuse. Each such gametangium functions as a parthenospore. It develops into a thick-walled zygospore-like structure called the a zygospore.

Cytology of the Germ Sporangium

There are two views with regard to the occurrence of meiosis in the life cycle of Mucorales. According to some authorities, meiosis occurs prior to the germination of the zygospore. Each diploid zygospore nucleus undergoes meiosis. As a result four haploid daughter nuclei are formed from each diploid nucleus. This is a condition similar to that of Spirogyra. Two of these haploid nuclei are of plus and two of minus strain.

During this division the plus and minus strains are segregated. According to this view, the zygospore is the only diploid structure in the life cycle. The promycelium, germ or zygosporangium, germ spores, mycelium, gametangia, sporangiophores, sporangia and the sporangiospores all belong to the haplophase or gametophyte, generation. The consensus of opinion favours this view.

The other hypothesis is that reduction takes place at the time of the differentiation of spores in the germ sporangium. Taking this hypothesis to be correct the zygospore, promycelium, germ sporangium up to the spore mother cell stage constitute the diplophase or the sporophyte phase. The spores of the germ sporangium are the first haploid structures. They together with the mycelium, gametangia, sporangiophores, sporangia and generation. These two phases alternate in the life cycle. This view has been practically discarded by the modern mycologists.

Life cycle of R. stolonifer

The life history of Rhizopus consists of two phases, asexual and sexual. The structures comprising the asexual phase are the mycelium, sporangiophores, sporangia and the sporangiospores. This phase in the life cycle of Rhizopus serves to propagate the haplophase of the fungus during the favourable season.

The sexual phase comprises the plus and minus mycelia, plus and minus gametangia and their protoplasts (aplanogametes), zygospore, promycelium, germ sporangium and the germ spores. This cycle is so designed as to enable fungus to pass uninjured the period which is adverse for vegetative growth. In the sexual phase zygospore is the only diploid structure. All others are haploid. Such a sexual cycle is termed haploid or haplontic. It is characterized by zygotic meiosis and haploid mycelium as the only adult plant.

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