In this article we will discuss about the asexual and sexual modes of reproduction in Monoblepharis with the help of suitable diagrams.
Occurrence of Monoblepharis:
Monoblepharis is a saprophytic fungus commonly found in still water in the form of small pustules on decaying plant remains such as twigs submerged in the clear water of ponds and tanks. The growth of the fungus is favoured by low temperatures of about 3°C.
Mycelium (Fig. 4.16 A):
The mycelium is filamentous and hypha-like. It is usually unbranched and attached to the substratum by tuft of rhizoidal hyphae. The hypha is rigid and coenocytic. The hyphal wall gives reactions of cellulose. According to Hawker (1967), the wall contains chitin.
The protoplast is vacuolated and presents a foamy appearance—a feature characteristic of the entire order. The vacuolation is very regular. The cytoplasm forms a network. The nets enclose vacuoles of regular size and form. The nuclei lie at equal distance from one another in a single row. The septa remain supressed during the vegetative phase.
Reproduction in Monoblepharis:
The reproductive organs (sporangia and sex organs) are developed at the tips of hyphae on the same plant.
(i) Asxeual Reproduction (Fig. 4.16 B-E):
It occurs within the range of 8° to 12° C and takes place by means of uniflagellate, opisthocont zoospores produced endogenously within elongated sporangia in large numbers.
The sporangium is a narrow, cylindrical or club-shaped terminal cell delimitied from the parent hyphae by a cross wall. The sporangia occur singly. At the time of sporangium development, the terminal portion of a hypha becomes slightly swollen—not in any way much larger than the diameter of a parent hypha.
It is multinucleate and contains homogeneous protoplast. Soon the terminal slightly inflated portion is cut off by a basal wall to function as a sporangium (B). The multinucleate protoplast of the sporangium becomes differentiated, into uninucleated daughter protoplasts by cleavage (C). Each protoplast rounds off to form a nearly spherical or pyriform zoospore.
The zoospores move about in an amoeboid manner within the sporangium. The tip of the mature sporangium dissolves to form a circular apical aperture (D) through which the zoospores finally escape one by one. A second sporangium may, sometimes, be formed immediately below the first. The liberated zoospore is a spherical or pyriform uniflagellate cell (E).
The single flagellum is much longer than the diameter of the cell. It is of a whiplash type with a painted tip and is inserted at the posterior end. The zoospore with a flagellum attached at the posterior end is opisthocont. The zoospore of Monoblepharis is thus uniflagellate and opisthocont.
On liberation, the zoospores swim about for a while and then come to rest on a suitable substratum. In the quiescent state, the zoospore retracts the flagellum, becomes rounded and secretes a wall around it. The encysted zoospore (F) germinates by two germ tubes (G).
One of these grows into a rhizoidal system which fixes the germling to the substratum. The second germ tube forms the mycelium which grows out into the water.
(ii) Sexual Reproduction (Fig. 4.17):
It is oogamous. The same mycelium that produces the sporangia bears the sex organs (antheridia and oogonia) at a higher temperature (20°C) at the tips of the hyphae (A). Prior to the formation of sex organs, the hypha gets swollen at the tip. The inflated terminal portion is cut off by a basal wall.
The cell thus formed matures into an antheridium (B) or oogonium depending upon the species. The position of sex organs thus varies in different species. For example, in M. polymorphia, M. fasciculata and M. insignis, the antheridium is terminal and the oogonium lies below it on the same hypha (B).
During further development, the oogonium increases in size and becomes distended in such a way as to project laterally at right angles to the antheridium.
The antheridium becomes pushed to one side and appears to be arising from the surface of the oogonium (C). M. sphaerica and M. hypogyna have a terminal oogonium (Fig. 4.18 A). The antheridium develops below it (Fig. 4.18 B). M. macrandra has the two kinds of sex organs borne on the tips of separate hyphae.
Antheridium (Fig. 4.17A):
The antheridium is an elongate, narrow, somewhat club-shaped structure situated at the tip of a hypha. It contains a multinucleate protoplast and is separated from the supporting hypha by a septum (Fig. 4.17 B). Towards maturity its protoplast becomes differentiated into 4- 8 posteriorly uniflagellate male gametes or antherozoids (B).
The single flagellum is of whiplash type and several times longer than the cell itself. When mature the antherozoids are released through a small apical aperture (Fig. 4.17 C). The motile male gametes are smaller in size than the zoospores and show a greater tendency to amoeboid movements, otherwise they resemble the zoospores.
Oogonium (Fig. 4.17):
The portion of the hypha immediately below the antheridial septum puts out a small hyphal outgrowth (B). The latter swells into a club-shaped structure which projects to one side are right angles to the antheridium (C). The clavate outgrowth is separated by a septum. It is the young oogonium which is uninucleate.
The oogonium increases in size and becomes globose at maturity. The antheridium appears to be borne on it. The mature oogonium is much larger than the antheridium. Later the oogonial protoplast recedes from the oogonial wall to form a single, central, spherical, uninucleate oosphere which contains large oil drops (E). The mature oogonium develops a pore at its apex (D).
Fertilisation in Monoblepharis:
The liberated sperms swim to the oogonium and creep over the wall to reach apex (E). One of them crawls through the apical pore (E) and fuses with the oosphere. In some cases the fusion of the male and female nuclei is delayed until oospore is partially clothed with a thick wall.
Oospore (Fig. 4.17 F-G):
The fertilized egg, in some species, remains inside the oogonium. In others (M. polymorpha), a few minutes after plasmogamy it moves to the tip of the oogonium and comes out to undergo maturation (F). However, it remains attached to the pore in the oogonial wall by a hyaline collar.
The male and the female nuclei fuse. The fusion of the male and female nuclei is termed karyogamy. After karyogamy, it enters the oogonium, secretes Oosphere Antherozoid a thick wall around it and becomes the oospore. Characteristic blister-like projections develop on the oospore wall (G).
Soon it is shed (M. polymorpha) and enters upon a period of rest which coincides with summer drought or winter cold. In M. fasciculata, the oospore remains attached to the oogonium for a considerable time. Occasionally parthenogenesis takes place. Parthenospores are smooth-walled. They remain within the oogonium.
Under suitable conditions (cooler temperature) the oospore germinates. The thick oospore wall cracks. A hypha emerges through the rupture and grows into a new haploid mycelium (H). The first two divisions of the oospore nucleus are considered to be meiotic. Each of the resultant four daughter nuclei may divide once or twice.