3 Types of Reproduction found in Chlorophyceae

Three main types of reproduction are found in Chlorophyceae, i.e., (1) Vegetative, (2) Asexual and (3) Sexual.

1. Vegetative reproduction:

This type of reproduction takes place vegetatively by several means.

(i) By fragmentation, e.g., in Spirogyra, Ulothrix, Oedogonium and several others.

(ii) By amylum stars. Such structures are found in Chara which help in vegetative propagation.

(iii) By tubers. They are found in Chara. Such bodies are full of starch.

(iv) By secondary protonema. Such structures are found in Chara which help in vegetative propagation.

(v) By bulbils. Such vegetative structures develop in Chara which develop into new plants.

(vi) By cell division. Here the algal form divides vegetatively, e.g., Protococcus.

(vii) By akinetes. The akinetes are vegetative structures. Here the vegetative cells develop into spore-like stages with much thicker walls and more food material. Akinetes have been reported from several Chlorophyceae, e.g., Oedogonium, Ulothrix, etc.

2. Asexual reproduction:

This type of reproduction is a common feature in Chlorophyceae and takes place in several ways.

(i) By zoospores:

This is the most important type of asexual reproduction. The zoospores are motile and naked protoplasts. They may be biflagellate or quadriflagellate. In Oedogonium, stephanokontean types of zoospores are found. In such cases, many flagella are present at the anterior end of the zoospore in circle.

(ii) By aplanospores:

They are non-flagellate (non-motile) spores and sometimes interpreted as abortive zoospores. They are found in many Chlorophyceae and usually develop in unfavourable conditions.

(iii) Palmella stage:

Hundreds and thousands of cells are embedded within a common gelatinous matrix. In Chlamydomonas, the Palmella stage occurs temporarily whereas this is permanent feature in Palmella.

(iv) Hypnospores:

They are very thick-walled aplanospores. They are perennating bodies. They are found in many green algae, e.g., Pediastrum.

(v) Autospores:

The autospores are aplanospores with the shape of mother cells. They are present in order Chlorococcales. When autospores are found in the colonies they are called autocolonies.

(vi) Daughter colonies:

In Volvox, daughter colonies and sometimes even granddaughter colonies are developed asexually.

3. Sexual reproduction:

This may be divided into four main sub-types, i.e., (i) isogamy, (ii) Anisogamy, (iii) aplanogamy and (iv) oogamy.

(i) Isogamy:

This is the simplest and primitive type of sexual reproduction. Two morphologically identical flagellated zoogametes take part in fusion. Usually such gametes come from two different individuals. The resultants are zygospores. Isogamy is found in many Chlorophyceae, e.g., species of Chlamydomonas, Ulothrix, etc.

(ii) Anisogamy:

Here the gametes taking part in fusion are not identical. The gametes are flagellate. One gamete is smaller and the other larger in size. The smaller one is supposed to be male and the larger female. The resultants are zygospores. The examples of Anisogamy are found in several Chlorophyceae, e.g., Chlamydomonas braunii.

(iii) Oogamy:

This is the highest evolved type. There is union of small flagellate antherozoid with a large non-motile egg. The resultants are called oospores, e.g., Volvox, Oedogonium, etc. and several other Chlorophyceae.

The species may be homothallic (monoecious), i.e., both male and female sex organs are present on the same plant, or heterothallic (dioecious), i.e., male and female sex organs develop on two different thalli.

(iv) Aplanogamy:

This is an unusual type found in order Conjugales. Here the amoeboid gametes (aplanogametes) fuse to each other and zygospores are resulted, e.g., Zygnema, Spirogyra, etc.

Parthenogenesis:

Sometimes the perfect spores are attained without fusion and are called azygospores or parthenospores. Here the female gamete may give rise to a new plant without fusing with male gamete. The process is called parthenogenesis. This process has been recorded from several Chlorophyceae. Sometimes one species of the genus reproduces parthenogenetically and the other species does not, e.g., Ulva lactuca is parthenogenetic species and U. lobata is not.

Zygote and its germination:

The zygote is a very resistant perennating body which usually faces the adverse environmental conditions for its survival. The zygotes of Chlorophyceae may be thin-walled or thick-walled. The thin-walled zygotes germinate within a day or two after their formation. The thick-walled zygotes remain dormant for a short or long period and then germinate.

Iso – and anisogamous species form thick-walled zygotes which possess flagella on them for a short time and the zygote remains motile during this period. Later on, these flagella are withdrawn and the zygote secretes a thick wall around it.

All oogamous species possess non-motile zygotes from the very beginning.

The rest period of the zygotes of various Chlorophyceae varies from genus to genus. The zygotes of Chlamydomonas germinate in ten days; those of Ulothrix in 5 to 9 months, and the zygotes of Oedogonium take the largest period in their germination, i.e., 12 to 14 months.

At the time of the germination of the zygote, meiosis occurs to bring the haploid phase again.