Volvox

Volvox is a genus of green algae that includes about 20 species of freshwater algae. Each colony is made up of thousands of individual cells, with colony sizes ranging from 500 to 60,000 cells. The first observation of Volvox was made by Leeuwenhoek in 1700. These algae are commonly studied to explore the process of morphogenesis. Found in freshwater environments like ponds and ditches, the rapid growth of Volvox often gives the water a greenish tint.

Volvox Classification

Volvox belongs to the class Chlorophyceae because it contains chlorophyll.

• Domain- Eukaryota

• Kingdom- Plantae

• Phylum- Chlorophyta

• Class- Chlorophyceae

• Order- Chlamydomonadales

• Family- Volvocaceae

• Genus- Volvox

Some common species of Volvox include Volvox aureus, Volvox globator, V. carteri, and V. barberi.

Volvox Diagram

Volvox Characteristics

• A single Volvox colony appears as a ball approximately 0.5 mm in diameter.

• The body is a hollow sphere called a coenobium, with thousands of cells arranged on its outer surface.

• The cells in the coenobium are of two types-

• Germ cells

• Flagellated somatic cells

• The extracellular matrix of the coenobium is composed of glycoproteins.

• Individual cells are spherical and contain a cup-shaped chloroplast with pyrenoids.

• Each cell also has a nucleus, vacuoles, and an eyespot.

• Two flagella of equal size are present in the anterior part of each cell, allowing the colony to move through the water.

• Each cell performs independent metabolic functions like respiration, photosynthesis, and excretion.

• Cytoplasmic strands form during cell division, connecting adjacent cells.

• There is polarity in the coenobium-

• Anterior cells have larger eyespots than posterior cells.

• Eyespots help with light reception and contribute to phototaxis.

• Volvox contributes significantly to oxygen production and serves as a food source for various aquatic organisms.

• Volvox aureus can multiply rapidly, leading to harmful algal blooms (HABs) in warm waters with high nitrogen content.

Volvox Life Cycle

The life cycle of Volvox is haplontic, meaning the dominant stage is the free-living haploid (n) gametophyte, and the diploid zygote (2n) is the only sporophyte.

• Cell differentiation occurs, creating reproductive and somatic cells.

• Volvox reproduces by both-

• Asexual reproduction under favorable conditions.

• Sexual reproduction under unfavorable conditions.

Asexual Reproduction-

• Gonidia (reproductive cells) form daughter colonies, which mature and are released from the parent.

• The gonidium undergoes multiple divisions to form a colony of around 3,200 cells.

Sexual Reproduction-

• Volvox exhibits oogamous sexual reproduction, with male and female reproductive cells-

• Spermatozoa (male) and ova (female).

• Ova-

• Produced inside the oogonium.

• Large, non-motile, green (due to chloroplasts), and contain pyrenoids for food storage.

• Spermatozoa-

• Produced inside the antheridium.

• Spindle-shaped, with apical cilia.

• Released in groups or individually.

• Chemotactic movement attracts spermatozoa to the oogonium.

• Fertilisation occurs when one spermatozoid fertilizes the egg, forming a diploid zygote.

• The zygote forms a cyst and becomes red due to the accumulation of hematochrome.

• The zygote detaches from the parent and remains dormant for a longer duration.

• Upon germination, the zygote undergoes meiosis to form four haploid cells.

• The haploid cells undergo multiple mitotic divisions to form a new colony, continuing the cycle.

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