In this article we will discuss about the reproduction in basillariophyta. This will also help you to draw the structure and diagram of reproduction in bacillariophyta.
Diatom cells multiply usually by a peculiar method of cell division. An increase in the volume of the frustule results in a slight separation of the valves which is followed by mitotic division of the nucleus with the axis of the spindle being parallel to the short axis of the cell to ensure longitudinal division of the protoplast (Fig. 99E).
The division of the protoplast along the median line of the long axis follows immediately after the nuclear division resulting in the formation of two daughter protoplasts and ultimately the cell divides into two, with each half having half of the old wall and the side along the plane of cell division is bounded by a protoplasmic membrane, the surface of which is covered by ‘a new half Wall secreted by the protoplast (Fig. 99F to H).
The new half wall fits inside the original old wall in both the newly formed cells. The old wall of each of the two newly produced diatom cells forms the epitheca no matter whether it was the epitheca or hypotheca of the parent cell and the new wall forms the hypotheca. Of the two cells that are formed one has the same size as the parent cell and the other slightly smaller.
The process of cell division usually takes place during night. The size of some of the diatom cells continues to diminish with each succeeding division. Theoretically it is possible that as new generations are produced, the cell will decrease more and more. This theoretical assumption was first laid down by Macdonald Pfitzer and was later known as Macdonald Pfitzer’s law.
This law, however, does not hold good, because, it has been experimentally demonstrated that the diatom cell wall being elastic, the progressive diminution in size is not a regular feature. But this process of diminution in size does not continue indefinitely.
With the attainment of minimal size the cells die off. But this ‘wasting away’ process is prevented by the appearance of certain re-juvenescent eel’s, or auxospores by which restoration of normal size and physiological re-juvenescence follow.
The re-establishment of normal vegetative cell size is by the formation of auxospore. Until 1907 the auxospore formation was considered to be solely by asexual means. In recent decades it has been shown that auxospores result from a sexual process which may be autogamous, isogamous, anisogamous or oogamous. Auxospore formation by asexual means is by mere an enlargement of the vegetative cell.
In the asexual process the protoplast escaping from the frustule enlarges several times its former size and surrounds itself with two siliceous valves.
The sexual process involved in the auxospore formation is accomplished in several different ways with considerable variation in the nuclear behaviour. In both pennatc and centric diatoms the zygote begins to grow immediately after formation and develops into an auxospore. Two mitoses, at each of which only one daughter nucleus survives, ultimately giving rise to the vegetative cell which has the largest dimension.
The process of sexual reproduction is therefore linked to reduction in cell size and, for a given species, there appears to be distinct size range (usually 30-40 per cent, of maximum) within which the cells are capable of becoming sexual. In those diatoms which do not decrease in size sexual reproduction does not apparently occur.
The vegetative cells of both Pennales and Centrales are diploid and gametes are formed directly following meiosis.
However, the sexual process shows a striking difference between the centric and the pennate diatoms. In the Pennales the process is isogamous and the gametes are amoeboid, whereas in the Centrales the process is oogamous and there is presence of spermatozoids.
Auxospore Formation in the Pennales:
In the Pennales, the auxospore is formed mainly by isogamy. Cases of anisogamy and oogamy are rather rare. During isogamy, cells aggregate in pairs (gamonto gamy), nuclear division (meiotic) produces 4 nuclei, and of which only 1 or 2 normally develop into gametes.
The parent cells fuse (cystogamy) and the fusion of gametes (generally derived from different cells) occurs in a copulatory jelly. In most species, fusion is isogamous and the gametes move towards each other. However, some species show physiological anisogamy, in which, although the gametes are morphologically identical, one is stationary whereas the other is motile.
There are four different ways of auxospore formation:
(i) Each cell produces two gametes but gametic fusion takes place between gametes of two different cells (Fig. 100A & B);
(ii) One gamete formed by each cell and hence fusion of two gametes from two different cells (Fig. 100C & D);
(iii) Two gametes formed in each cell and they fuse with each other (Fig. 100G & H);
(iv) The entire protoplast of a cell is transformed into an auxospore parthenogenetically (Fig. 100E & F).
The auxospore formed by any one of the above processes elongates considerably after which siliceous wall is formed producing a vegetative cell. In general, the vegetative cell of diatom is a diploid structure and meiosis is gametic (Fig. 101).
In anisogamy, one gamete is amoeboid while the other is non-motile. The amoeboid gamete (male) travels through a mucilage matrix, or a fusion tube, toward the female and nuclear fusion takes place there. Oogamy is rather very rare in the Pennales.
Auxospore Formation in the Centrales:
In the Centrales, the auxospore formation is usually by oogamous sexual process. Isogamy has also been recorded in some species. Oogonia occur as slightly extended cells with an elongated nucleus.
Normally, each oogonium contains only one egg nucleus. The egg cell normally remains enclosed in the oogonium or attached to it until fertilization. Four uniflagellate spermatozoids are formed by division of the contents of the male sex organ, the spermatogonium, also designated as a spermatocyte.
However, the formation of the spermatogonium itself is variable. The vegetative cell either acts directly as a spermatogonium, or divides to form 4-8 spermatogonia. The spermatogonia are dispersed passively through water, before becoming attached by mucilage pads to the oogonium. They then produce spermatozoids. At fertilization only the spermatozoid nucleus is injected into the egg cell.
Besides the above process of oogamy an intracellular autogamy may also occur in some species. In isogamy, one to two gametes are produced in a cell and are freed from each mother cell. They fuse to form an auxospore. Asexual auxospore formation has also been reported in the Centrales.
Some marine diatoms produce flagellated cells which are interpreted by some as microspores and others as zoogametes according to their behaviour (Fig. 99J & K).