Volvox Structure and Reproduction // Notes By Rohit Jirobe //
Volvox
Class : ChlorophyceaeOrder : VlvocalesFamily :VlvocaceaeGenus: Vlvox
Volvox is a green alga. It is included in the class Chlorophyceae. Linnaeus named the genus, Volvox.
Occurrence
Volvox is a colonial green alga. It is a freshwater alga. It is found in ponds, lakes, tanks, pools, ditches, etc. During favourable season, it grows vigorously to form water blooms and gives green colour to the
water. Volvox colony floats during bright light and goes deep during night.
Volvox includes about 20 species. There are five species of Volvox in India.
Volvox africanus, Volvox prolificus, Volvox merelli, Volvox globator , Volvox rousseletii
Structure of Volvox
Volvox is a colonial green alga. It is included in the class Chlorophyceae. It is a freshwater alga.
It is a free-floating form. It is spherical in shape.
It is macroscopic. It is visible to the naked eye in the size of a pin-head.
Volvox colony is 1to1.5mm in diameter. The colony of Volvox consists of 500 to 50,000 Chlamydomonas-like cells. Volvox is an assemblage of similar and independent cells.
It is a free-floating form. It is spherical in shape.
It is macroscopic. It is visible to the naked eye in the size of a pin-head.
Volvox colony is 1to1.5mm in diameter. The colony of Volvox consists of 500 to 50,000 Chlamydomonas-like cells. Volvox is an assemblage of similar and independent cells.
Each cell functions like an individual, carrying out its own nutrition, respiration and excretion.
Volvox is a haploid gametophyte. It is a colony. The colony is spherical in shape.
Volvox is a haploid gametophyte. It is a colony. The colony is spherical in shape.
The colony is made up of Chlamydomonas-like cells . The number and arrangement of cells in a particular colony are definite . Hence the colony is called a coenobium.
The cells of the colony are embedded in a mass of mucilage. Each cell is also surrounded by a mucilage sheath. The cells are interconnected by protoplasmic strands called plasmodes-mata
The cells are arranged as a single layer along the periphery of the colony. The centre of the colony is hollow and it is filled with mucilage.
The colony has a polarity. It has an anterior side and a posterior side.
At the posterior side of the colony, there is a perforation called phialopore.
The cells of Volvox resemble a Chlamydomonas. They are green in colour. Each cell is oval in shape. Its anterior end is pointed and the posterior end is rounded. The anterior end faces outwards.
The cell is surrounded by a cell wall made up of cellulose. A thin plasma membrane lies below the cell wall. The plasma membrane surrounds the protoplasm.
The anterior end has a thickening of cell-wall called papilla. The anterior end bears two flagella. The flagella arise from basal granules. The flagella are equal in length. They bring out the movement of the colony.
The protoplasm contains a pair of contractile vacuoles near the anterior end. A nucleus is present in the centre. The nucleus is eukaryotic and haploid.
A large chloroplast is situated at the posterior side of the protoplasm. It is cup-shaped The chloroplast has an eye-spot anteriorly and a pyrenoid in the centre. Starch is stored in the The chloroplast contains pigments like chlorophyll-a and- b, carotenes and xanthophyll, pyrenoid in the form of starch plates.
Reproduction:
Volvox reproduces by two methods: 1. Asexual reproduction 2. Sexual reproduction.
1. Asexual reproduction
In Volvox, asexual reproduction takes place by formation of daughter colonies from gonidia.
It takes place during favorable season. At the posterior part of mature colony , 2-20 cells enlarge and function as reproductive cells called gonidia.
The gonidia lack eye-spots and flagella. They have well defined nuclei and dense granular cytoplasm.
Each gonidial cell divides 3times into 2, 4 and then into 8 daughter cells. The 8 celled stage is called octant stage. The eight daughter cells are arranged in the form of a curved plate. This stage is known as plakea stage.
The cells of the plakea stage divide into 16 cells. These 16 cells rearrange to form a hollow sphere. This stage is called cruciate plakea stage. It has a layer of cells, a terminal pore and a central cavity. The pore is called phialopore. The cells are arranged in such a way that the anterior ends point inwards.
Then the number of cells increases by repeated mitotic divisions.
The sphere of cells turn completely inside out through the phialopore. This process is called inversion.
At this stage, the anterior ends of cells point outwards.
Each cell then develops an eye-spot and two flagella at its anterior end. In this way, a daughter colony is formed inside the gonidial wall.
The daughter colonies come out by the rupture of parent colony and live as independent colonies.
Sexual Reproduction:
The Sexual reproduction in Volvox is oogamous type. It takes place during unfavorable season.
Many species of Volvox are homothallic or monoecious. They produce male and female sex organs in the same colony. Eg. Volvox globator
Some species are heterothallic or dioecious. They produce male and female sex organs in two different colonies. Eg. Volvox aureus. The plant is a haploid gametophyte. The male sex organ is called antheridium and the female sex organ is called oogonium.
Antheridium
The male sex organs of Volvox are called antheridia or androgonidia. They are produced at the posterior part of the colony.
Each antheridium develops from a vegetative cell.
The vegetative cell first enlarges in size and loses its flagella to become an antheridial cell.
The antheridial cell divides into 2, 4 and then into 8 cells. The eight daughter cells are arranged in the form of a curved plate. This stage is known as plakea stage or octant stage.
The cells of the plakea stage divides into 16 cells. These 16 cells rearrange to form a hollow sphere. This stage is called cruciate plakea stage. It has a central cavity, a layer of cells, a terminal pore and a central cavity. The pore is called phialopore. The cells are arranged in such a way that the anterior ends point inwards.
16 to 128 cells are formed by mitotic divisions.
The sphere of cells turn completely inside out through the phialopore. This process is called inversion.
At this stage, the anterior ends of the cells point outwards.
Each cell then develops two flagella at its anterior end to become a sperm or antherozoid Mature sperms are liberated in water in a cluster or singly.
The cluster of sperms gets separated, when they reach near the egg.
The sperms or male gametes are narrow and spindle-shaped. They have two anterio flagella. They are haploid. They have a large nucleus and dense cytoplasm. They are yellow in colour. They swim freely in water towards the oogonium.
Oogonium
The female sex organs in Polvos are known as oogonia of gynogonidia
A few oogonia develop in a colony. They are flask shaped
They are formed in the posterior part of the colony Each oogonium develops from a vegetative cell.
The vegetative cell enlarges in size, loses its flagella and eye-spot. The protoplast of the
cell gets rounded to form a single egg of ovum. The egg contains a haploid nucleus and dense cytoplasm.
Fertilization
The sperms liberated from the antheridium swim towards the oogonium and enter the gonium. One of the sperms fuses with the egg to form a diploid zygote. (2N)
The zygote secretes a thick three layered, smooth or spiny wall around it to form an oospore. The thick-walled oospores remain in the parent coenobium. The oospore is released by t rupture of the colony.
Germination of Oospore
The oospore germinates after a period of rest. The diploid nucleus undergoes reduction division or meiosis to form four haploid nuclei. Of these three nuclei get disorganized and the remaining one is functional. At this stage, the oospore wall ruptures and the protoplast comes out in the form of a vesicle. This protoplast develops into a biflagellate zoospore.
The zoospore swims in water and divides mitotically into 2,4,8 and then into a mass of cells. These cells are arranged in a single layer to form a hollow sphere. The anterior end of the cell faces inwards. The hollow sphere has a pore called phialopore.
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