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Chlamydomonas reproduce by asexual and sexual means. Asexual reproduction occurs during the favourable as well as unfavourable condition. The sexual reproduction takes only under favourable condition.

1. Asexual reproduction

Chlamydomonas under favourable season withdraws flagella and it comes a resting stage. During this stage the protoplasm withdraws inwards from the cell wall and thus the contractile vacuole disappears. It will be soon followed by repeated longitudinal division of the protoplasm giving rise to 8- 16 daughter protoplasts. Around each daughter protoplast cell wall is developed followed by the formation of two flagella and becomes parent like organism. These newly formed daughter cells are called zoospores. Zoospores are held inside the parent cell wall, later it gets ruptured. Each of the zoospore then develops to new organism.

Chalamydomonas under unfavourable season also the protoplasm divide repeatedly forming number of daughter protoplast. This daughter protoplast does not develop flagella and are known as endospore. These endospores remain in the gelatinous matrix. This stage very much resembles the alga Palmella, so known as the Palmella stage. Under the onset of favourable season endospore become zoospores.

2. Sexual reproduction

The Chlamydomonas during sexual reproduction involves three stages. They are the;

a. Gametogenesis: The process of gametogenesis occurs by repeated division of the protoplast giving rise to 16-32 gametes from a single cell. The gametes produced are similar to zoospore which motile and biflagellate but functions as gametes. These are later released to the environment where the syngamy is supposed to take place.

b. Syngamy: It is the process of fusion of gametes as a result in the formation of zygote. These zygotes formed will then transforms into zygospores. On basis of the nature of the gametes fusing and mode of fusion three kinds of syngamy are recognized. They are;

i. isogamy: The fusing gametes are identical in nature so known a isogametes.

ii. Anisogamy: The fusing gametes are dissimilar and motile so known as anisogametes.

iii. Oogamy: The fusing gamete in which the female is non-motile and male is motile gamete.

c. Zygospore formation: Zygospore are the spores which have thick wall to protect from the extreme environment. Under favourable condition the zygospore germinates. The diploid nucleus meiotically produces four haploid daughter nuclei. Then wall formation occurs which is followed by the development of two flagella. The wall of the zygospore open and releases the daughter zoospores which then give rise to vegetative organisms.

The lifecycle of chlamydomonas is known as haplobiontic lifecycle.