Structure and Reproduction in Volvox (Green Algae)

The Volvox is a green, flagellated, colonial algae found in both temporary and permanent water ponds, pools and ditches. It usually grows in spring and summer but abundant growth takes place during rainy season. The colonies are found in the form of small balls of the size of a pin-head which are visible to the naked eye. It passes winter in resting condition as oospore or zygospore.

Taxonomic Position

Division: Chlorophyta

Class: Chlorophyceae

Order: Volvocales

Family: Volvocaceae

Genus: Volvox

Structure of Volvox

Colony

The Volvox colony is in the form of a hollow sphere having a definite number of cells arranged at its periphery in a single layer. The interior of the colony is filled with watery mucilage. Such a colony is called coenobium. The number of cells vary 500-50,000. Each cell is biflagellate and the flagella project outward the surface of colony. Each cell of the colony is surrounded by a mucilaginous sheath of its own and all the cells of the colony are connected to each other with the help of a series of protoplasmic strands.

Cell Structure

Each cell is surrounded by a mucilaginous sheath. All the cells in the colony are similar in shape. The structure of each cell is resembled with the Chlamydomonas. The cells are pear shaped containing a cup shaped chloroplast. Only one pyrenoid is present on the chloroplast. There are two contractile vacuoles near the pointed end. The single nucleus is located in the center. Each cell has a single eyespot towards the outer face. The eyespot of the cells located at the anterior of the colony is better developed then the posterior one. The cells of the colony are joined together by delicate cytoplasmic strands

Reproduction

The Volvox is a Chlorophytic Green Algae that reproduces asexually when the season is favorable and sexually when the growing conditions become unfavorable.

Asexual Reproduction

Few posterior cells of a mature colony enlarge in size, become more or less rounded, withdraw their flagella and are pushed back into the colony. The nuclei and protoplasmic contents of these cells are more prominent than the surrounding cells. These cells are called gonidia and are responsible for the development of the daughter colony.

The following stages are involved in the development of a daughter colony.

1. The gonidium divides by two vertical divisions at right angles to each other t form four daughter cells. These cells divide further by a vertical division to form an eight-celled plakea stage. The cells are arranged in the form of an incurved plate with concave surface outside.
2. Another vertical division results in the formation of 16 cells arranged in the form of a hemisphere. Further divisions in the same plane result in the formation of hollow sphere with a pore, the phialopore.
3. The vertical divisions of daughter cells continue till the specific number of cells for a species is produced. Up till this stage all the cells are without mucilaginous sheaths and their anterior ends face towards the center of the sphere.
4. Now the inversion of the daughter colony takes place. A slight constriction just opposite phialopore marks the beginning of inversion. This invagination moves inward into the anterior half of the young coenobium and the philaopore enlarges in size greatly and its edges curl backwards. The movement continues until the whole structure is inverted through the phialopore.
5. At the end of inversion, the anterior ends of the cells face towards the outer side. All the cells now acquire a cell membrane, develop flagella and secrete their own mucilaginous sheaths to become separated from each other.

Several daughter colonies develop within a parent colony. These colonies are set free by the disintegration of the cells of the parent colony.

Sexual Reproduction

The sexual reproduction in Volvox is strictly oogamous. Some species are monoecious (Volvox globator) while the others are dioecious. Most of the monoecious species are protandrous, i. e., the antheridia develop first. Some cells in the posterior region of the coenobium enlarge in size, withdraw their flagella and their nuclei and cytoplasmic contents become more conspicuous. These cells are called androgonidia and gynogonidia and are responsible for the development of antheridia and oogonia respectively.

Developmental Stages of Volvox

Development of Antheridium

The protoplast of the androgonidial cell divides by successive vertical divisions to form a group of 64-128 cells. The anterior ends of the cells face inward the hollow sphere. This group of cells undergoes inversion and after the inversion the anterior ends of the cells face outside. Each cell of the group is uninucleate; long, narrow, conical or fusiform; and without mucilaginous sheath. The cells develop two flagella at their anterior ends to become antherozoids.

Development of Oogonium

The gynogonidial cell becomes rounded or flask-shaped and functions as oogonium directly. The protoplast of the oogonium metamorphoses into a single, non- motile, spherical, uninucleate egg. It has a parietal chloroplast with many pyrenoids embedded in it and contains a large amount of reserve food. The egg usually develops a beak-like protrusion, the receptive spot, which probably marks of place of entry of the male sperms.

Fertilization

The entire mass of the male sperms is released from the antheridium and swim towards the oogonium due to chemotactic stimulation. A few male sperms enter the oogonium but only one succeeds in entering the egg through the receptive spot.

Development of Zygote

The diploid zygote secretes a three-layered smooth or spiny wall around it. It is liberated from the parent colony after its disintegration and germinate at the return of favorable conditions. The nucleus of the zygote divides meiotically into four haploid nuclei out of which three disintegrate. The inner wall of the zygote protrudes out in the form a vesicle. The haploid protoplast of the zygote migrates into this vesicle and develops two flagella to become a meiospore. This meiospore undergoes repeated divisions till the formation of characteristics number of cells of a new colony in the same manner as in the case of formation of asexual daughter colony. The coenobia formed as a result of sexual reproduction are of small size.

Alternation of Generations

The zygote is the only diploid phase in the life cycle of the

and the main plant body is haploid, therefore it exhibits haplontic life cycle.

Microscopic View of Volvox

The microscopic view of Volvox is as follow