Chlamydomonas: Classification, Structure and Life Cycle

Table of Contents

• Classification of Chalamydomonas
• Morphology
• Reproduction and Life Cycle of Chlamydomonas
• Asexual Reproduction in Chlamydomonas
• Sexual Reproduction in Chlamydomonas
• Zygote or Zygospores
• Frequently Asked Questions

Chlamydomonas is a green alga that is found in both freshwater and seawater. About 150 species of the genus are known that are all unicellular and biflagellated. It can sometimes become so abundant that the whole water body appears green. It is a model organism that is studied in molecular biology.

Classification of Chlamydomonas

Morphology

• Chlamydomonas are motile and unicellular green algae.
• They can be oblong, spherical, oval, pyriform or ellipsoidal in shape.
• A pyriform or pear-shaped thallus is common in Chlamydomonas, such that they have a narrow anterior region and broad posterior region.
• The cell wall of the organisms is thin and firm and is made up of cellulose.
• The cytoplasm inside the cell wall possesses a nucleus, mitochondria, dictyosomes, ribosomes and endoplasmic reticulum.
• The nucleus is present inside the cavity-shaped chloroplast in the cytoplasm.
• There are two vacuoles in the cytoplasm that perform osmoregulatory and excretory functions.
• Two whiplash flagella are found at the anterior end of the organism.
• The cup-shaped parietal chloroplast is an identifying feature of the organism. However, it can be ‘H’ shaped in C. bicilliata, stellate in C. arachne, reticulate in C. reticulata and so on.
• The pyrenoids are smaller microcompartments within the chloroplast that are responsible for the synthesis of starch. There can be one to many pyrenoids in Chlamydomonas.
• The anterior portion of the chloroplast has an eyespot. It is an orange or red-coloured tiny spot that is photoreceptive in nature. It is concerned with the movement of flagella.

Reproduction and Life Cycle of Chlamydomonas

In the life cycle of Chlamydomonas, both sexual and asexual reprodction has been observed. Let us look at them one by one.

Asexual Reproduction in Chlamydomonas

1. Zoospores

Zoospores are formed in favourable conditions. The flagella of the organism contracts, and the protoplast separates from the cell wall. The protoplast, at first, divides longitudinally to form two daughter cells. Next, it divides at a right angle to the previous longitudinal division, forming four daughter cells. Likewise, Chlamydomonas form 2-16 daughter cells by simple mitotic division. The daughter cells grow their own cell wall and flagella. These daughter cells are known as zoospores. The zoosporangium or the parent cell ruptures to release the daughter cells that grow into separate individual cells.

2. Aplanospores

Aplanospores are formed in Chlamydomonas in unfavourable conditions. The parent cell loses flagella, and the protoplast separates from the cell wall. The protoplast divides into 2-16 daughter protoplasts that do not have flagella. They secrete a thin layer around the protoplast and are known as aplanospores. On arrival of favourable conditions, either the aplanospore may germinate or convert into zoospores.

3. Hypnospores

Hypnospores are formed in unfavourable conditions by secreting thick walls around the protoplast. The spores germinate into separate individuals on the arrival of favourable conditions.

4. Palmella Stage

This stage is showcased by the Chlamydomonas in unfavourable conditions. The protoplast divides to form daughter cells but does not form zoospores. Each daughter protoplast secretes a gelatinous layer around itself but does not develop flagella. These daughter protoplasts are known as palmello spores. The protoplast divides slowly to form an indefinite number of spores known as the palmella stage. When favourable conditions are restored, the gelatinous layer dissolves, and each spore can germinate into a new thallus.

Sexual Reproduction in Chlamydomonas

1. Isogamy

In isogamy, Chlamydomonas produces gametes that are morphologically similar but physiologically different. The vegetative thallus of the organism, referred to as gametangium, loses its flagella, and the protoplast is separated from the cell wall. The separated protoplast divides into 6-32 daughter protoplast. Each daughter protoplast grows flagella and turns into a gamete. The gametes are released from the gametangium.

Each gamete is covered by agglutinins that secrete a hormone called gamone. This hormone helps in the recognition of gametes of the opposite strain. In heterothallic species, (+) and (-) strains are attracted to each other. The gametes of the opposite strains come in contact, and a fusion of their cytoplasm and nucleus takes place. A quadriflagellate zygote is formed, which later on loses the extra flagella, secretes a thick wall and is known as a zygospore.

2. Anisogamy

In anisogamy, gametes of two different sizes are produced. The male gametes, also known as microgametes, are smaller and are produced in the male gametangium. The female gametes, also known as macrogamete, are larger and are produced in the female gametangium. The two gametes come together to form a diploid zygote which is later converted into zygospores.

3. Oogamy

In oogamy, the vegetative thallus of the Chlamydomonas becomes non-motile and acts as a female gamete or macrogamete or egg. The male gamete is formed by the division of protoplasts and is known as microgametes. The microgamete and macrogamete come together, and a zygote is formed.

Zygote or Zygospores

The zygote is a resting diploid spore that secretes thick, ornamented walls around itself. It accumulates a large amount of starches and oils. The zygospores tolerate a long stretch of unfavourable conditions and start to germinate as soon as favourable conditions are restored. It divides by meiosis to form four haploid nuclei that develop into separate individuals.

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