Describe Auxospore formation in Pennales
Isogamy is primarily responsible for the formation of auxospore in the Pennales. Anisogamy and oogamy cases are rather uncommon. During isogamy, cells aggregate in pairs (gamontogamy), and nuclear division (meiotic) produces 4 nuclei, 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.
Four separate steps result in the formation of auxospores:
(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).
