VOLVOX - STRUCTURE OF VEGETATIVE BODY AND REPRODUCTION

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A. CLASSIFICATION:
Division – Chlorophyta
     Class – Chlorophyceae
          Order – Volvocales
                Family – Volvocaceae
                    Genus – Volvox
The genus Volvox comprises about 20 species which is world-wide in distribution. The genus is found to occur in both temporary and fresh water tanks, ponds, pools, etc. It appears as a minute floating ball, floating and moving on the surface of water. During and rainy season, the surface of water in which Volvox occurs look green in colour because of the abundance of Volvox colony. During early summer Volvox abruptly disappears and it remains in a resting zygote condition during rest of the year.
B. STRUCTURE OF THE VEGETATIVE BODY:
Volvox occurs in colony i.e., it is coenobial form. Each colony i.e., coenobium is free swimming and appears as a small pinhead-like (about 0.5mm in diameter) spherical or ovoid, hollow sphere of mucillagenous mass in which definite number of cells are arranged in its periphery in a single layer – the number of which may vary from 500 to 60,000 or more in different species.
Each cell of the coenobium is biflagellate and is provided with individual gelatinous sheath or mucilage envelope. The cells remain interconnected with each other by cytoplasmic connections. The cells of most species are oval in shape. Sometimes the cells are pear-shaped (V. aureus).
              Each cell has a central nucleus, one cup shaped or a laminate chloroplast with one or more pyrenoids towards the posterior pole of the cell, the reddish eye spot at the anterior end, two flagella of equal length at the anterior end of the cell and 2-5 contractile vacuoles near the base of the flagella. The flagella of the peripheral cells penetrate through the pores of the outer sheath of the coenobium.

C. REPRODUCTION: Volvox reproduces both by asexual and sexual methods of reproduction. A coenobium has all its reproductive cells either entirely asexually when the seaseon is favourable for growth but sexually towards the end of growing season.
             1. Asexual Reproduction:- A few cells (2 to about 50) in the posterior half of the coenobium take part in asexual reproduction. These special reproductive cells gradually push back into the colony, withdraw their flagella, increase ten or more times the size of the vegetative cells and become more or less rounded in shape. They are recognized by their well defined nucleus and dense granular cytoplasm. Such reproductive cells are called parthenogonidia or gonidia. The protoplast of each gonidium by successive longitudinal divisions forms a daughter coenobium within a parent cell wall. This is a tightly packed ball of cells.

2. Sexual Reproduction:- Sexual reproduction is of oogamous type. Some of the species are monoecious (V. globater) while others are dioecious (V. aureus). Certain cells in the posterior region of the mature coenobium enlarge, withdraw their flagella and behave as gametangia. The gametangia are large, round cells with many pyrenoids but no flagella. The male gametangia are called antheridia or androgonia and the female oogonia.
(a) Antheridium:- The biflagellate cell destined to form the antheridium enlarges, withdraw its flagella and pushes back into the colony but keeps its connection with the adjacent vegetative cells by cytoplasmic strands. The protoplast of the antheridium undergoes successive mitotic divisions and forms 64-128 small conical sperm cells. They are arranged in a bowl shaped plate or hollow sphere phialopore. The sperm cells develop their flagella in the anterior ends directed towards the inside. As the bowl or sphere matures, the sperm cells undergo inversions so that flagella remain in the exterior side.
Each sperm is biflagellate elongated, conical or fusiform structure. It has a small, yellow, green or pale green chloroplast. The flagella are inserted apically or sub-apically at the long, pointed anterior end. The sperms are liberated by the rupture of the antheridial wall.
             (b) Oogonium:- The cell destined to form oogonium enlarges considerably many times the size of the ordinary vegetative cell. It becomes rounded or flask-shaped and projects inwards into the colony and withdraws its flagella. The entire protoplast of the oogonium gets metamorphosed into a single, non-flagellated, green, spherical egg or oosphere. It has a large central nucleus, and a parietal chloroplast containing numerous pyrenoids. It also has a reserve food stored absorbed from the surrounding vegetative cells through cytoplasmic connections. The oosphere often develops beak like protrusion which marks the point of entrance of the sperms.

(c) Fertilization:- The mass of antherozoids after liberation from the antheridium swims about as a colonial unit, until it reaches the vicinity of the egg. At the time of fertilization, the individual antherozoids swim through the gelatinous sheath around an egg and enter through the protrusion of the oogonium. Finally, one of the antherozoid fuses with the egg to complete the fertilization. As a result of fertilization a diploid zygote or oospore is produced.
With the return of the conditions favourable to growth, the oospore germinates. Prior to germination the zygote nucleus undergoes meiotic division to form four haploid daughter nuclei. Of these three degenerates and only one remains functional which later develop into new individual coenobium.
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Location: Assam, India

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