Oedogonium

 

                                                                                                      Classification        

                                                                                                (Fritsch1931)

                                                                                                                  Class   – Chlorophyceae
                                                                                                                  Order -  Oedogoniales
                                                                                                                  Family -  Oedogoniaceae
                                                                                                                  Genus – Oedogonium
Occurrence
Oedogonium (Gr Oedos-swelling, gonos – reproductive bodies) is a freshwater, filamentous algae. This genus was named by Link. The genus has 400 species.
It grows in ponds, lakes and shallow tanks. The filaments are attached to rocks, logs or epiphytic on aquatic plants ( e.g., Hydrilla) and other algae. Some species are terrestrial, growing on moist soils e.g., O. terrestris, O. randhawa.

Structure of the Thallus:
The thallus is multicellular and filamentous. The filaments are unbranched and uniseriate. The filament consists of a single row of elongated, cylindrical cells arranged end to end. All the cells of a filament appear more or less similar except the basal cell and the distal or apical cell.
The basal cell is modified to form a holdfast or hapteron. It is devoid of chloroplast. The holdfast is expanded into a flattened disc with finger like outgrowths or projections. It helps in attachment of the filament to the substratum.
The terminal or apical cell of the filament may be rounded, acuminate or sometimes prolonged to hair-like structure (e.g., O.ciliate). Thus, the filament show clear apical-basal polarity.

Cell Structure:
The cells are elongated and cylindrical with a slightly swollen or dilated upper end. The green cell consists of a thick, rigid cell wall enclosing the protoplasts.
The cell wall is made up of three layers; an outer chitinous, a middle layer of pectin and an inner layer of cellulose. There is no mucilage on the surface of the filament.
The centre of the cell is occupied by a large vacuole containing the cell sap. The protoplast forms a thin layer in between the central vacuole and the cellulose layer of the cell wall.
There is a single, large, reticulate chloroplast present in the protoplasm. The chloroplast extends from one end of the cell to the other. The strands of the chloroplast are parallel to the long axis of the cell. A number of pyrenoids are characteristically present in the chloroplast particularly in the region of intersections.
There is a single large parietal nucleus which lies near the middle of the cell embedded in the cytoplasm just within the chloroplast. The nucleus is haploid.

Growth and cell division :
Growth in Oedogonium takes places as a result of cell division which is largely intercalary. Only certain cells in the filament divide.
Every cell division is accompanied by the formation of a specialized structure called the cap cell. The cell division in Oedogonium is peculiar and unique.
The cell division starts with the movement of the peripheral nucleus to the centre of the cell. A ring like thickening of hemicellulose develops at the apical end of the cell. The ring gradually increase in thickness and becomes grooved.
By this time, the nucleus divide mitotically. The division of the nucleus is followed by the formation of a floating cytoplasmic strand between the two daughter nuclei. For sometime, the cytoplasmic strand remains unconnected with the later wall.
The outer and the middle layers external to the groove then ruptures allowing the free elongation of the ring. Consequently, the cell elongates with the distal half having a new membrane formed from the stretched new wall material of the thickened ring. It is intercalated between the upper and the lower ends of the old rupture wall.
At the same time, the septum is pushed upwards and finally becomes fixed near the lower end of the intercalated membrane. The upper daughter cell thus formed has now a new bounding wall consisting mainly of the intercalated membrane formed from the thickened ring and the newly synthesized piece. There is however, a portion of the ruptured parent cell wall fitting like a cap at its upper end. This forms a characteristic ring-like mark, the apical ring or apical ring.
The upper daughter cell with its apical cell is called cap cell and the lower daughter cell is called the sheath cell. Only the cells with cap divide again. After each division a new cap is formed. Thus, the number of apical rings the cap cells contains denotes the number of divisions the cell has undergone.

Reproduction
All three types, Vegetative, Asexual and Sexual reproduction are seen in Oedogonium.

Vegetative Reproduction
The vegetative reproduction takes place by means of fragmentation. The fragmentation takes place by (i) Dying out of some cell here and there in the filament, (ii) through accidental breaking, (iii) formation of zoospores or gametes and liberation because of wall splitting. Each fragment by cell division and growth develops into a new filament.
Asexual Reproduction
In Oedogonium, asexual reproduction takes place by three kinds of asexual spores (i) Zoospores, (ii) Akinites and (iii) Aplanospores

Zoospores
Zoospore formation is the most common and effective method of asexual reproduction. They are produced singly in the specialized cell called zoosporangium. Any cap cell usually , the recently divided onw which contains abundant reserved food material may become zoosporangium.
The entire protoplast of the zoosporangium contracts from the cell wall and becomes a rounded mass. The nucleus moves towards one side of the protoplast.
A semi-circular, colourless area appears on one side due to receding of the chloroplast towards other side. A single or double row of blepharoplast granules then appear at the base of the hyaline area. The basal granules are connected by fibrous strands to form complete circular ring.
From each granule arise a single flagellum. In this way a ring of flagella is formed around the base of colourless beak like area of the protoplast.
With the formation of zoospore, the cell wall near the upper end splits transversely and the upper portion of cell is lifted off like a lid. The mature zoospores enclosed in a delicate mucilage vesicle, slips out through the aperture.
The mucilaginous vesicle soon disappears and makes the zoospore swim freely in the water. The liberated zoospore is a deep green, spherical or pear shaped. It has a ring of short flagella at the base of colourless, beak like anterior end. This kind of flagellar arrangement is called stephanokont.
The zoospore possesses an eye spot, a chloroplast, a haploid nucleus and numerous vacuoles. The liberated zoospore remains motile for about an hour. Then it settles down, attaches itself to the substratum by colourless flagellar end. It withdraws the flagella and secrets a cell wall.
The one celled zoospore divides transversely forming an basal cell and a apical cell. The basal cell remains colourless and doesnot divide again. It develops into a cylindrical hapteron or holdfast. The upper cell by the normal methods of division and redivision of its daughter cells forms the new filament.

Akinete Formation
In some species of Oedogonium, during unfavourable conditions, resting cells or akinetes are formed. They are thick walled, reddish brown, rounded or oval structures. They are formed in chains, each inside an inflated cell resembling an oogonium.
The akinetes are rich in starch as reserve food material and reddish orange oil. On returning of favourable condition each akinete germinates into a new filament.
Aplanospore formation
In some species of Oedogonium aplanospores are also formed. They are slightly oblong or spherical and formed one or two in each cell.

 Sexual Reproduction
The sexual reproduction in Oedogonium is Oogamous type. The sexual cells or the gametes are structurally and physiologically different. They are produced in highly specialized reproductive organs, the gametangia.
The male gametangia is called the antheridium and the female gametangia is called the oogonium. Depending on the distribution of sex organs, species of Oedogonium are grouped into two categories: 1. Macrandrous species and 2. Nannandrous species.
Macrandrous species:
The species of Oedogonium that produces antheridia and oogonia in normal filament is called Macrandrous species. They may be monoecious (O.fragile) producing antheridia and oogonia on the same filament or dioecious (O. crissum, O, aquaticum) producing antheridia and oogonia on different filaments.
Nannandrous species:
The species of Oedogonium that produce oogonium in normal filament and antheridium in small, dwarf filament or nannadria is called Nannadrous species.
Sexual Reproduction in Macrandous forms:
The development of antheridia and oogonia is similar in both monoecious and dioecious species.
Antheridia
Antheridia are formed in either terminal or intercalary cell of the filament. Any cap cell may function as antheridial mother cell. The antheridial mother cell divides into two unequal cells, the upper small antheridium and the lower larger sister cell.
The sister cell divides again repeatedly, so as to give rise a series of 2 to 40 antheridia. The protoplast of each antheridia divides mitotically, by transverse or vertical wall into two haploid daughter protoplasts. Each protoplast becomes pear-shaped and develops a ring of flagella around a colourless portion at its one end and metamorphoses into an antherozoid or sperm. Thus two antherozoids are produced in each antheridium.
 The pattern of division decides the arrangement of antherozoids i.e., they may lie superimoposed (one above the other) or side by sied in an antheridium.
The wall of antheridium ruptures transversely and two antherozoids are freed into a thin vesicle. Soon, the vesicles dissolves and two antherozoids swim freely in the water. The liberated sperms are pale green, yellowish green spherical bodies. Each has a sub-apical ring of short flagella at the base of colourless, beak-like antherior end.
The morphology, flagellation and liberation mechanism of the antherozoids is similar to the zoospores. However, antherozoids are smaller in size and have fewer flagella. The liberated antherozoids swim freely and finally reach the oogonia.
Oogonia
The oogonia are highly differentiated female gametangia. Each oogonium develops from an actively growing cap cell called the oogonial mother cell.
The oogonial mother cell divides by a transverse wall into two cells. The upper cell is riches in cytoplasm, has larger nucleus than the lower cell. It functions as an oogonium. It has one or more caps at the upper end.
The lower cell supports the oogonial cell and is called suffultory cell or supporting cell. It often remains undivided. In some species, it again functions as an oogonium mother cell and undergoes further divisions to form a chain of two or three or four oogonia.
The oogonium becomes prominently enlarged and is filled with reserve food material. The oogonia is always  much broader than filaments a characteristic feature of order Oedogoniales.
The entire protoplast is converted into a single ovum. Owing to presence of chlorophyll the ovum is green. It is non-motile and retained within the oogonium. Prior to fertilization, the ovum or egg protoplast slightly receds from the oogonial wall to form a small clear patch, the receptive spot. The oogonial wall develops a small pore or transverse slit near the anterior end above the receptive spot. It forms a sort of a conduit leading down to the ovum and provides a passage to antherozoids entrance during fertilization.
Thus, a mature oogonium complex consists of an oogonium and a supporting cell. Enclosed inside the wall are a large nucleus and an anterior receptive spot. The ovum remains laden with reserve food material.

Sexual Reproduction in Nannadrous forms:
The nannandrous species are dioecious. They exhibit a curious dimorphism of sexual plants. The oogonia are produced on normal, large filaments. The antheridia are produced by special, short, few celled filament called the dwarf or nannadria.
The dwarf male are produced by the germination of peculiar type of motile spores called the androspores. The androspores are produced within cells called the androsporangia.
If oogonia and androsporangia are present in the same plant, the condition is called gynandrosporous, e.g., O. concatenatum. The species in which androsporangia are borne in a separate filament, it is named as idioandrosporous, e.g., O. iyengarii.

Structure and germination of Androspores
Androsporangia are similar to antheridia in development and are produced by unequal division of mother cell but are rather larger. The androsporangia are flat, discoid cells. Its protoplast metamorphoses into a single androspore.
The androspore are motile and are provided with a sub-polar crown of flagella. They are larger than antherozoids but smaller than zoospores. On liberation, the androspore is surrounded by the mucilaginous vesicle. This vesicle soon vanishes and andropsore swims freely till it reaches and attaches itself to the wall of the oogonium or supporting cell.
The attached androspore germinates on the oogonium or supporting cell producing 2-3 celled filament called the dwarf male or nannadrium. The lower, rhizoid-like elongated cell of the dwarf male is called the stalk. The stalk cell cuts off one or more cells at its tip. These are antheridia.
The protoplast of antheridium divides mitotically into two daughter protoplasts. Each daughter protoplast develops a ring of many flagella at one end and forms a multiflagellated sperm or antherozoid.
The antherozoids are liberated either by the disorganization of the antheridial cell or by the separation of a cap like lid at the top.
Oogonium:
Oogonial development in Nannadrous species is similar to that of Macrandrous species.
Fertilisation:
It is similar in both Nannarous and Macrandrous forms. The antherozoids swimming in water enter the oogonium through a pore or tansverse slit in the oogonial wall. There is a chaemotatic attraction between the antherozoids and oogonium (Hoffman 1973)
One of the antherozoid, probably the first to arrive enters the egg at the receptive spot. The male and female nuclei in the egg fuse to form the diploid nucleus and a diploid zygote is formed.
Oospore:
The zygote secretes a 2-3 layered thick wall to form an oospore. Due to accumulation of reddish oil oospore appears red in colour. The oospore is liberated from the filament by the decay of the oogonial wall and rests on the mud at the bottom of the pond where it enters into a period of rest.
Germination of Oospore:
After a period of rest, the oospore germinates. Prior to germination, the diploid oospore nucleus undergoes zygotic meiosis to form four haploid nuclei. The protoplast loses, its red colour and turns green. The haploid nuclei are organized into four uninucleate daughter protoplasts by cleavage of the oospore protoplast.
Soon, each haploid daughter protoplasts furnishes itself with a crown of flagella to become a motile spore resembling the zoospore of the asexual stage. It may be called a meiozoospore.
The oospore wall ruptures to liberate the mature, motile meiozoospores. They are at first surrounded by a delicate vesicle. The vesicle soon disappears. The liberated meiozoospores swims about for a while and then settles down to germinate. They grow into haploid Oedogonium plants. In dioecious species, two zoospores develop into male and two into female filaments, eg., O.plagiostomum.
Life Cycle:
The life cycle is known as haplontic because the plant is haploid. The only diploid stage is the zygote which soon undergoes meiosis resulting in 4 haploid zoospores. The zoospores germinate into new haploid filaments.