I B.Sc Diatoms notes

2. Sexual Reproduction:

The pattern of sexual reproduction differs in both orders — Pennales and Centrales. During this process, auxospore is formed in both the groups. During cell division, those cells become reduced in size, are able to regain their normal size through the formation of auxospore, so it is a “restorative process” rather than multiplication.

Auxospore Formation in Pennales:

It takes place through gametic union, auto­gamy and parthenogenesis.

These are of the following types:

1. Production of one auxospores by two conjugating cells. In this process two uniting cells come very close to each other (Fig. 3.103) and become covered by a mucilaginous sheath. The diploid nucleus of each cell undergoes meiosis.

Out of four nuclei, three degenerate and only one survives. The surviving nucleus behaves as gamete (n). The gametes come out from the parent frustules and unite together, to form a zygote (2n).

After a short period of rest the zygote elongates considerably and functions as an aux­ospore. The auxospore projects out from the par­ent frustules along with mucilage and elongates in a plane parallel to the long axis of the parent diatom.

The auxospore is enclosed in a pectic membrane, the perizonium. The auxospore then develops new frustule inside the perizonium. Thus new diatom cell is formed which regains the normal size. It is found in Cocconis placen­tula, Surirella saxonica etc.

2. Production of Two Auxospores by Two Conjugating Cells:

This is a very common process of auxospore formation. In this process the conju­gating cells come very close to each other and get enclosed by mucilage (Fig. 3.104). The nucleus (2n) of each cell undergoes meiotic division and forms four nuclei.

Out of four nuclei, two degene­rate, the rest two survive. The cytoplasm then divides either equally or unequally and along with one nucleus they behave as gametes. Thus two gametes are formed in each cell.