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In this article we will discuss about the vegetative and sexual methods of reproduction that occur in the life cycle of Navicula.
Vegetative Reproduction:
It takes place by the mitotic cell division or fission. Successive cell division takes place very rapidly at night. Presence of aluminium-silicate in water is essential for cell division to occur. As the cell division starts, the cell protoplast increases in diameter. The cell also increases in size.
The diploid nucleus divides mitotically and produces two daughter nuclei (Fig. 3A-C). Two chromatophores divide. The single chromatophore spits longitudinary in such a manner that one chromatophore comes to lie in each half. Now the protoplasm cleaves into two uninucleate portions by division in longitudinal plane parallel to valve surface (Fig. 3C).
One daughter protoplast now lies in epitheca and the other in hypotheca.
Now both the daughter protoplasts with one daughter nucleus secrete the new siliceous wall on the two fresh protoplasmic surfaces exposed along the cleavage plane. The new valves developed always become hypotheca while the older theca (which may be epitheca or hypotheca of the parent cell) becomes the epitheca of the new or daughter cell.
When this cell again divides, it produces a daughter ceil which is again smaller than the present parent. Thus, in a population of diatom cells during successive divisions there is normally a progressive decrease in the average cell size (Fig. 4). It is called Macdonald-Pfitzer law. The smaller cells of later series of division loose their vitality and capacity of division.
Sexual Reproduction:
It takes place by the formation of auxospores. The successive decrease of cell size in vegetative reproduction is prevented by the auxospore formation. The auxospore formation is actually a ‘restorative process’ because the reduction in the original size of the cells, during the cell division is restored. During the process only those cells which have diminished sufficiently in size can act as ‘sex cells’ or conjugating cells.
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Those cells which do not decrease in size by cell division apparently do not show sexual reproduction. Majority of the species of Navicula are monoecious but, N. haplophila is dioecious. Two sex cells come together, pair up longitudinally (called gamontogamy) and secrete a common mucilaginous envelope (Fig. 5A, B).
The diploid nucleous of each cell undergoes meiosis to form 4 haploid nuclei. Out of these two nuclei degenerate and only two remain functional (Fig. 5C). The protoplasm of each cell now cleaves into two portions each obtaining one haploid nucleus. The functional nuclei ultimately metamorphose into gametes (Fig. 5B-D).
The parent cell fuses (cytogamy) and the fusion of gametes occurs in a copulatory jelly. In N. haplophila the two gametes formed in one cell (conjugant) are amoeboid and the two gametes formed in the other are passive or immobile. The amoeboid gametes emerge through the open valves of the parent frustule and dip into open shell of the other to fuse with opposite gametes to form two zygotes (Fig. 5 E. F) in one shell.
The other is empty. Thus, N. haplopila shows physiological anisogamy. Two diploid fusion cells or zygotes escape from the enclosing pustules. They remain doranant for some time. Later the zygote elongates (more in the longitudinal plane) and functions as auxospore (Fig. 5G-I), which develops a silicified membrane called perizonium around its protoplasm.
It may be secreted by the auxospore or by the remains of the zygotic membrane. The auxospore secretes new pustules around itself around the perizonium. The reconstituted new cell is of normal size and after sometime begins to divide vegetatively to form new generations. The valves of the old pustules are often seen attached to the newly formed pustules (Fig. 5G).
