Reproduction in Unicellular Alga - Chlamydomonas

Chlamydomonas is a haploid unicellular alga found in fresh water ponds. The plant body is pear-shaped with two flagella attached at the narrow end. On one side of the cell, a light sensitive eye spot is present. A large cup-shaped chloroplast is present. Towards the centre, a definite nucleus is present. Chloroplast contains a single pyrenoid.

1. Asexual Reproduction

Asexual reproduction takes place with the help of zoospores, aplanospores or hypnospores depending upon the availability of water for swimming.

Asexual Reproduction by Zoospores

If plenty of water is available for free swimming, Chlamydomonas reproduces by flagellate thin-walled spores, called zoospores. Chlamydomonas cell loses flagella and becomes non-motile. Its protoplasm (cytoplasm and nucleus) divides mitotically and forms 2-16 daughter protoplasts, each of which develops flagella, and is called a zoospore.

The parent cell wall is ruptured and zoospores are released. Each zoospore develops a cell wall and grows into an adult cell. After release of zoospores the parent cell does not exist, any more.

Asexual Reproduction by Aplanospores and Hypnospores

If a thin-film of water is available where swimming is not possible, Chlamydomonas produces thin-walled, non-flagellate daughter protoplasts, called aplanospores. The parent cell loses flagella and becomes highly extended. Its protoplast divides repeatedly to produce 100 or more daughter protoplasts, each of which is called an aplanospore.

The whole structure containing groups of non-motile aplanospores resembles a non-motile Colonial alga, called Palmella, and so this is called palmella stage of Chlamydomonas. If plamella-stage is flooded with water, each aplanospore develops flagella, comes out of the parent cell wall and grows into a normal independent plant.

If water suddenly dries up, some of the aplanospores develop thick-wall, each of which becomes dark brown or black, and is called a hypnospore. When favourable conditions are present and water is available for swimming, each hypnospore ruptures to release protoplast that develops flagella, becomes a zoospore and grows into normal Chlamydomonas-plant.

2. Sexual Reproduction

Chlamydomonas reproduces sexually by isogamy, anisogamy or Oogamy depending upon the species.

Sexual Reproduction by Isogamy

Isogamy is exhibited by Chlamydomonas eugametos and C. eherenburgii. The male and female cells become non-motile by losing their flagella. The protoplasm of each cell divides mitotically into 32-64 daughter cells. Each daughter cell develops flagella and is released in water by the rupture of mother cell wall. Each of these cells acts as a gamete.

The gametes are morphologically identical in structure but differ physiologically or chemically. Gametes released in water from two different mother cells fuse in pairs forming quadriflagellate zygotes. When the contents of the two gametes fuse, they form a zygote (diploid). This is the only diploid stage in the life cycle of Chlamydomonas.

The zygote develops a thick wall around itself and develops brown to black coloured pigmentation to tide over unfavourable conditions (zygospores). On the return of favourable conditions (temperature, food and water) the diploid nucleus of the zygote divides by meiosis and forms four haploid zoospores. Each zoospore grows into a new adult Chlamydomonas plant.

Sexual Reproduction by Anisogamy

Anisogany is exhibited by Chalamydomonas braunii. Male and female cells lose flagella and become non-motile. In male cell, protoplast divides repeatedly to produce 32-64 biflagellate gametes but in female cell, protoplast divides to produce 8 to 16 biflagellate gametes. Both male and female gametes are released in water.

When larger female gametes lose flagella and become non-motile, each one is fertilized by a smaller motile male gamete. After fertilization, the fusion product loses flagella, becomes spherical and develops thick wall to become a resting zygote.

On return of favourable conditions of water, temperature and light, the zygote undergoes meiosis and produces four haploid zoospores each of which grows into an independent Chlamydomonas plant.

Sexual Reproduction by Oogamy

Oogamy is exhibited in Chlamydomonas coccifera and C.ooganum. Here, female and male cells lose flagella and become non-motile. All the contents of female cell act as female gamete or egg, but the protoplasm of male cell divides to produce 32-64 biflagellate gametes. The biflagellate gametes are liberated in water and swim around in search of female gamete.

Two or more flagellate gametes enter each female cell having nonmotile egg but only one fertilizes the egg and others degenerate, contributing nutrition to the young zygote. The fusion product of egg and a motile gamete is called zygote that develops Heredity a thick, pigmented wall to enter into resting phase.

On return of favourable conditions of water, temperature and light, the zygote undergoes meiosis to produce four haploid biflagellate zoospores, each of which on liberation from zygote, grows into an independent plant of Chlamydomonas.