Structure and life cycle of Volvox & Oedogonium Ms. Inderveena Sharma H.O.D. Botany Department P.G.G.C.G. 11, Chd.

Volvox

Habitat Volvox (chlorophytes), is a green, free floating algae. Occurs in fresh water habitats (ponds, ditches, shallow puddles) Occurs as green rolling balls of pin head sized coenobia It forms spherical colonies of up to 50,000 cells. The colony is comprised of many single, bi-flagellated cells connected together by protoplasmic strands.

Vegetative structure Plant body is multicellular motile coenobium (colony with fixed number of cells arranged in a definite manner) 2. The central hollow cavity is filled with mucilage 3. Each cell is biflagellate 4. Alga rolls on water surface by joined action of flagella 5. Each cell of coenobium performs its own function of nutrition, respiration & excretion

Individual cell of Volvox is spherical, elliptical or oval & biflagellate. The flagella are attached at anterior end. The cell has outer mucilaginous layer A number of central vacuole are distributed near the surface of protoplast single cup shaped or curved plate like chloroplast chloroplast is associated with 1-2 pyrenoids single nucleus associated with nueromotor apparatus (blepharoplast + rhizoplast + centromere) each cell has single eye spot at anterior end

Asexual reproduction Occurs during growing season at maturity, few cells in posterior half are pushed back into hollow cavity cells withdraw their flagella, increase in size, become round shaped reproductive cells are called gonidia The protoplasm of cells divides by successive longitudinal divisions & forms daughter coenobium

Sexual Reproduction (oogamous type) Male sex organs: Antheridia (androgonia) Female sex organs: oogonia (Gynogonidia) Some cells at posterior half of coenobium retract flagella, enlarge, & become gametangia Male gametangium give rise to antheridium and female gametangium oogonium

Antheridia Produced inside male gametangia in large numbers Antheridia Produced inside male gametangia in large numbers. The protoplasm of gametangium divides by successive & simultaneous longitudinal divisions forming bowl shaped plate of 64-128 male cells. Each protoplasmic piece is differentiated into conicle motile antherozoid (spermatozoid) each antherozoid is biflagellated.

Oogonia (gynogonidia) The female gametangia enlarges in size & becomes flask shaped the protoplasm does not divide and changes to egg or oosphere & called oogonium oogonium possesses beak like opening for entry of antherozoids Fertilization oogonium secretes some chemicals which attract antherozoids. Only one antherozoid fuses with egg and forms zygote

Volvox colony with Zygote

Germination of zygote and development of coenobium

Life cycle of Volvox

Oedogonium

Habitat An unbranched, multicellular, filamentous green alga, Grows in fresh water e.g. ponds, pools, shallow lakes Filament attached to substratum by basal cell, called rhizoidal cell which is modified to hold fast The apical cell is rounded Except hold fast and apical cell, all other cell (intercalary vegetative cells) are arranged end to end Cells with distinctive rings, (called caps or scars) at the apical ends are called cap cells. Cell structure Uninucleate, nucleus lies in periphery of cytoplasm Single, large reticulate chloroplast, parietal in position with several pyrenoids & encloses big central vacuole

Oedogonium Monoecious filament

Asexual Reproduction Zoospore: under favorable conditions, multifagellate zoospores are formed inside zoosprangium. Akinete: under unfavorable conditions, formation of red walled, thick akinetes are produced

Sexual reproduction (oogamous) 1 Sexual reproduction (oogamous) 1. Macrandrous species: male and female filaments are morphologically similar. These are (a) monocious species when oogonia and antheridia develop on the same filament or (b) Dioecious species when these are borne on different filaments. Thus filaments are unisexual 2. Nanadrous species (dioecious species): male & female filaments are morphologically distinct & exhibit dimorphism. Anteridia are borne on Nannadrium (dwarf male). A. Macrandrous monoecious filament showing antheridia and oogoniaon on same filaments B-C. Macrandrous diecious showing antheridia and oogonia on different filaments

Antheridia Terminal or intercalary in position A cap cell acts as antheridal mother cell and divides into 2 cells i.e antheridia cell and sister cell. The antheridial cell give rise to 2 daughter protoplasts and give rise to antherozoid (spermatozoid)

Oogonium Terminal or intercalary in position Develops from actively growing cap cell called oogonial mother cell. Oogonial mother cell divides into upper cap cell and lower basal cell. The upper cap cell differentiates into an oogonium while the lower basal cell becomes supporting cell.

Zygote Production Zoospores In addition to being able to reproduce asexually, the oedogonium is capable of reproducing sexually. In sexual reproduction, the filament portion of the oedogonium, called the antheridium, produces and releases sperm. The sperm sinks through the water until it meets with the filament portion of the oedogonium, called the oogonium, which contains a large egg. If the sperm successfully fertilizes the egg, a zygote is produced. Zoospores After the zygote is produced, it is released into the water and develops zoospores. The zoospores are similar to those created through asexual reproduction. Once the zoospores mature, the zygote splits open, releasing the zoospores. The zoospores then attach to the bottom of the sea and the cells in the zoospores divide to produce a new filament of oedogonium.

Life cycle of Oedogonium

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