Volvox

Chlorophyceae

The class chlorophyceae is represented by about 425 genera and6500 species they are commonly called as green algae because of the presence of the pigment chlorophyll a and b in their plastids

1. They have a wide distribution in aquatic (both fresh water and marine and terrestrial habits)

2. Cell wall compassed of two layers, inner layer made of cellulose and an outer layer consisting pectin

3. The conspicuous part of protoplast is chloroplast which varies in number and shape from genus to genus

Chloroplast may be cup shaped(clamydomonas) discoid(vauchert) stellare(zygnema) or reticulate (oedogonium)

4. The main pigment are chlorophyll a and chlorophyll b  α, β,γ, carotene, astaxanthin, leutein, neoxanthin, siphonein etc.

5. Associated with the chloroplast are bodies known pyrenoids.

6. Generally cells are uninucleate, but multinucleate conducts is also not uncomman members of siphonales and cladophorales are coenocyctic.

7. Both flagellated and non-flagellate cells are present. Number of flagella may be two, four or more. Flagella are whiplash type.

8. Vegetative propagation takes places by fragmentation and cell division.

9. Asexual reproduction takes place through the formation of Zoospores, aplanospores, akinites and hypnospores.

10. Sexual reproduction may be isogamous, anisogamous or oogamous.

Fritsch (1935) recognized nine orders in Chlorophyceae

1) Volvocales 2) Chlorococcales 3) Ultotrichales 4) Cladophorales 5) Chaetophorales

6) Oedogoniales 7) Conjugales 8) Siphonales 9) Charales

Smith (1955) included the green algae in one division Chlorophyta and divided it into two classes – Chlorophyceae and Charophyceae.

The class Chlorophyceae was divided into 12 orders, and only a single order was included in Charophyceae.

Volvacales

The volvacales include chiefly the microscopic genera of the green algae in which the thallus is one-celled and generally motile throughout life. The unicell is furnished with 2 or 4 whiplash type flagella.

This is the only order of the green algae in which the vegetative cell is actively motile. In some genera, the motile cells are joined into groups to form an colony. A colony is aggregation of individuals mechanically held together generally in a gelatinous sheath. The individuals in the colony have little or no dependence upon others.

The order volvocales, include both unicellular and colonial forms which occur widely in fresh water plankton. They are absent from sea. Many of them prefer water rich in nitrogen and organic substances tne his occur in quiet pools.

The order comprises 6 families with about 60 genera and 500 species.

Volvox

Class – Chlorophyceae

Order – Volvacales

Sub-order – Chlamydomonadinae

Family –Sphaerellaceae

(Volvocaceae)

Genus – Volvox

Distribution and Habitat:

Volvox is green, flagellate, colonial, planktonic algae. The genus has 20 species, which occur both in temporary and freshwaters of ponds, pools and ditches. They appear like spherical balls of pin head size, just visible to the naked eye.

The colonies multiply so rapidly during rainy season that the water of small ponds may be coloured green by their presence.

The Volvox includes about 20 species, which are worldwide in distribution. The genus is represented in India by 6 species, of these V. glabatox, V. aureus, V. prolificus, V. africanus are more common.

Thallus Structure:

The volvox is coenobium which is largest and highly differentiated. The coenobia is of the size of a pin-head and is spherical or ovoid in outline, hollow and only one cell thick. The size varies from 0.5 to 2.8 mm in diameter.

Each colony consists of 500 to 60,000 cells. The number of cells in each colony is in multiple of two, and each species has a definite number of cells. A colony consisting of a definite number of cells arranged in a specific manner and forming an integrated whole is called a coenobium.

The numerous cells of the coenobium are embedded in the gelatinous or mucilaginous matrix and are arranged in a single layer in the periphery. The coenobium is thus a hollow sphere with the interior showing no cellular organization. The central part of the colony is filled with a water (e.g., V. glabator) or gelatinous substances (V. aureus).

Each cell in the coenobium is connected to its neighbor cell by broad protoplasmic strands which are formed during the cell division. These strands from a continuous spherical network. In some species e.g., V. monoae and V. tertius protoplasmic strands are absent.

Cell Structure:

Each cell of the colony is pyriform in shape with a broad posterior and narrow anterior end. Each cell of the colony has its own gelatinous sheath or mucilaginous envelope and thus is separated from its neighbouring cell. This shows that Volvox is not an individual but an association of a number of independent cells.

The cell has a thick cell wall differentiated into outer firm and inner mucilaginous layer. Each cell has two flagella at its anterior end. The flagella are acronematic type and project beyond the surface of the colony into the water. The flagella arise from the basal bodies. Due to the concerted movement of flagella, the colony rolls along rather rapidly in water behaving like a single organism.

Each cell has a single large, cup-shaped chloroplast situated at the posterior side of the cell. Chloroplast has 1 or 2 pyrenoids in its centre. A single nucleus is present in the cytoplasm which fills the concavity of the chloroplast. The nucleus is connected with the neuromotor apparatus consisting of bhlepharoplast, rhizoplast and centrome.

The reddish brown eye spot is located in the anterior region of the chloroplast. Volvox being ovoid or a sphere, the end which contains few or no reproductive cells is regarded as anterior end of the plant and opposite the posterior end. The cells at the anterior end have larger eye-spots than those at the posterior end.

In the young Volvox colonies the cells are similar in size and shape and are purely vegetative in function. As the colony grows older the cells get differentiated. A certain number, especially in the posterior half of the colony, the cells become considerably enlarged and function either as asexual or sexual reproductive cells. The reproductive cells are recongnisible by their large size, definite large nuclei, more denser granular protoplasmic contents, numerous pyrenoids.

Volvox is an assemblage of similar and independent cells. Each cell functions like an individual carrying out its own nutrition, respiration and excretion. There is no co-operation between the cells in these functions.

Movement of the colony:

Volvox is a motile alga. By concerted action of the flagella of the cells, the coenobium rotates above its axis, thus making a forward movement. The colony does not roll forward like a ball but rotates about its axis, with one end of the colony always leading in progression. Hence, Clamydomonas is also called as rolling alga.  This shows that the beat of flagella of all the cells in the colony is co-ordinated.

Reproduction:

In young Volvox colonies all the cells are similar and vegetative in function. A certain number, especially in the posterior half of the colony, become considerably enlarged and function either as asexual or sexual reproductive cells.

The reproductive cells are recognizable by their large size, definite larger nuclei and more denser granular protoplasmic contents. They divide and form new colonies.

Asexual Reproduction:

Asexual reproduction occur during favourable conditions in spring and early summer months.

Once the young coenobium attains maturity, a few cells (2-50) in the posterior half of the colony, are pushed back into the hollow cavity. These cells withdraw their flagella, increase ten or more times the size of the vegetative cells and become round in shape. Such reproductive cells are called parthenogonidia or gonidia. The cells have well – defined, central nucleus and dense granular cytoplasm with numerous pyrenoids within their chloroplast.

Development of Daughter Colony:

The first division of the gonidium is longitudinal with respect to the parent coenobium as a whole. The second longitudinal division is at right angles to the first. The four resulting cells divide lengthwise so that a curved plate-like eight-celled plakea stage is formed. The cells are arranged in such a manner that their concave surface faces towards outside.

Another longitudinal division in the plakea stage results in the formation of 16 celled stage. At the 16-celled stage the cells in the young daughter colonies become arranged in form of a hemisphere. Finally, a hollow sphere with a small aperture is formed. This aperture is called the phialophore. It is directed towards the exterior of the mother coenobium.

The cells continue to divide longitudinally, until a specific number of cells are formed specific to a particular species.

At the end of division stage all the cells are naked and in contact with one another. The anterior flagellar end of each cell is directed inwards or towards the center of the sphere.

Now, the young daughter colony turns inside out through the phialophore to bring flagella from its interior to its outer surface. This is called the inversion. Due to this inversion, the inside portion of the young daughter coenobium turns towards outer side. The phialophore is ultimately becomes closed.

Each cell of the daughter coenobium acquires a cell wall and separate form one another. The flagella elongate rapidly. The daughter colony is still retain within the parent or gonidial cell wall. The latter develops into a mucilaginous membrane surrounding the young volvox colony.

Several daughter colonies may be developed simultaneously in the parent colony. Finally, the young colonies escape either by the disintegration of the parent colony or through a pore at the position of the original gonidium. In the former case, the parent colony immediately perishes.

After releasing from the parent colony, increase in size of the colony is partly due to cell enlargement and mainly due to the swelling of the mucilaginous cell walls.

Sometimes, due to long time for disintegration of the parent colony, we may find colony containing daughter colony which in turn may contain granddaughter colony e.g., V. Africana.

Sexual Reproduction:

Sexual reproduction takes place at the end of the growing season. It is oogamous. Certain special cells in the posterior region of the colony enlarge, retract the flagella to become gametangia. The male gametangia are called antheridium and female gametangia are called oogonium.

Many species of Volvox are homothallic or monoecious, they produce male and female sex organs in the same colony, e.g., V. globator. Some species are heterothallic or dioecious, they produce male and female sex organs in two different colonies, e.g., V. aureus.

The monoecious species are protandrous i.e., antheridia mature before the oogonia.

Antheridia or Androgonidia:

The cell destined to form the antheridium enlarges, retracts its flagella and pushes back into the colony. It keeps connections with the adjacent vegetative cells by cytoplasmic strands.

The protoplast of an antheridium undergoes mitotic divisions and form 16-512 small, elongated rod like cells. These are male reproductive cells – sperms or antherozoids.

They are arranged in form of bowl-shaped plate. The sperm cells have their anterior ends directed towards the inside, where flagella begin to develop. As the bowls matures, they undergo inversion just like the asexual colony, so that, the flagella are on the exterior (convex) side of the bowl.

Each antherozoid is a biflagellated, elongated, conical or spindle shaped, pale yellow body. The antherozoids are liberated as a unit, swim around as a unit. They do not separate until they reach the egg.

Oogonium:

Also called as gynogonidia.  The cell predestined to form oogonium enlarges, looses its flagella, and become rounded or flask-shaped and projects into the anterior of the colony.

The entire content 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 has large amount of reserve food materials.

The oosphere often shows a beak-like protrusion which serves as the receptive spot, where the sperm enters during fertilization.

Fertilisation:

Before fertilization, the entire mass of antherozoids swims as an unit. Due to chaemotactic stimulation the packet of antherozoids break up in the vicinity of the egg. A single antherozoid swim through the gelatinous oogonial wall, enter the egg through the beak-like receptive spot and fuses with egg nucleus to form a diploid zygote.

Zygote:

Soon after fertilization, the zygote secretes a thick, three-layered, smooth or spiny wall around it and becomes an oospore. It accumulates haematochrome pigment due to which the protoplast becomes orange red.

The oospore undergoes a period of dormancy and is retained in the parent colony until the latter decay or disintegrates. Thus, oospore constitutes the perennating stage in the life history of Volvox. Eventually, the parent colony disintegrates and the oospere fall to the bottom of the pool, where it ripens and may remain viable for several years.

Germination of Oospore:

The oospore germinates during favourable conditions. Prior to germination, the zygote nucleus undergoes meiosis to form four haploid daughter nuclei. Out of 4 nuclei, three nuclei degenerate.

At the time of germination, the outer most layer of the oospere splits. The middle layer them splits and the delicate inner layer protrudes to form a vesicle. The haploid protoplast containing the functional daughter nucleus migrates into the vesicle.

The zygote protoplast develops as biflagellate zoospore or meiospores inside the vesicle. The zoospores rarely escapes from the vesicle. It divides to form a hollow sphere of cells. This sphere is of about 125 to 500 cells, undergoes the process of inversion and develops into a new colony or coenobium.

In V. aureus and V. minor the two thick outer layers of the zygote wall split. The haploid protoplast surrounded by the inner layer escapes. By repeated division it directly develops into a colony in the same manner as in asexual reproduction.

The young colony contains small number of cells. The maximum size constant for the species is attained after it has passed through several asexual cycle. With each cycle or generation, the daughter coenobium becomes longer n size and has increased number of cells until a fully developed colony is formed.

Life-Cycle:

The life cycle of Volvox is haplontic or haploid type. The plant body belongs to haploid generation. It reproduces asexually by daughter colonies. Sexually, it reproduces by haploid antherozoids or sperms and egg. The diploid zygote is formed by the fusion of a sperm and egg. The zygote undergoes meiosis and forms the colony of haploid daughter cells.

The haploid phase is dominate and diploid phase is represented only by the diploid zygote.