Volvox Reproductive Biology

Study Notes

The greater part of the coenobium is vegetative (somatic)
Only a few cells located on the posterior side of the coenobium can give rise to a new colony or reproductive organs
These cells are termed Gonidia and are characterized by their large size (ten times more than vegetative cells or even more)

Asexual reproduction: Gonidium is responsible for the formation of autocoenobium
Autocoenobia formation is the mode of asexual reproduction in Volvox
Sexual reproduction: is Oogamous, taking place by the formation of antheridia and oogonia in the posterior half of the colony

Volvox is the most advanced colony in this group because it exhibits a considerable degree of complexity
Number of cells ranges from 1000 to 50 000 cells
All cells of the colony are arranged in a single layer within the periphery of the colonial matrix
The colony is surrounded by a marked mucilaginous lamella
The colony moves and rotates slowly by the flagella and behaves like a single individual

Chlorophyta is considered the largest algal division
It comprises a great variety of species which vary in form, size, habitat, reproduction, and life cycle
Pigments of these algae are much similar to those of higher plants
They are composed of chlorophyll a, and b, and several carotenes and xanthophylls
Pigments are carried in well-organized chloroplasts that resemble those of higher plants

Most chloroplasts have one or more pyrenoids
Motile forms contain light-sensitive structures, the stigma
Motile reproductive units of non-motile green algae contain stigma as well
Chloroplasts have many shapes, including cup-shaped, parietal, spiral, reticulate, and ribbon-like
Cells of green algae are surrounded by a firm wall composed mainly of polysaccharides (usually cellulose) and pectins

This division comprises only one class; Chlorophyceae, which is divided into 15 orders
The basis of classification includes:
Form of the thallus (unicellular, coenobic, filamentous, foliose, or siphonous)
Motility of both vegetative and reproductive cells
Number of nuclei in the cell (uninucleate or multinucleate)

This order includes two families: Chlamydomonaceae and Volvocaceae
Family 1: Chlamydomonaceae, which includes unicellular forms
Family 2: Volvocaceae, which includes colonial forms with common gelatinous matrices

Chlamydomonas is one of the largest algal genera (more than three hundred species)
It occurs in both freshwater and saline water
The cell wall is chemically composed of glycoprotein
The cell contains a single chloroplast, which is mostly cup-shaped, but other shapes are known
It contains one or more pyrenoids and a stigma, which is embedded in it
The cell is provided with a pair of contractile vacuoles
The nucleus is embedded in the cavity of the chloroplast and is connected with the peripheral cytoplasm with cytoplasmic strands

It takes place by zoospores, which is common and achieved when the cell becomes immotile, rests, and undergoes one or more mitotic divisions
The mother cell wall ruptures, and the cells acquire two flagella and swim again, repeating an asexual life cycle
Under certain conditions, these cells fail to release from mother cells and form a mucilaginous material, known as the palmella stage

It takes place by Isogamy, anisogamy, and Oogamy
Isogamy: it takes place between two gametes, morphologically similar but physiologically different
Anisogamy: it takes place between two gametes which are morphologically and physiologically different
Oogamy: it occurs when a cell becomes immotile and rests, acting as an ogonium, and a motile gamete adheres to it, and fusion takes place to yield a zygote