Oedogonium

BSc Semester One Algae

1. From Greek oidos, " swelling " + gonos, “ Reproductive Structure " Oedogonium Oedogonium is an unbranched, filamentous green, fresh water alga that is easily identified by the distinctive rings at the apical ends of certain cells. Each cellular division creates a new ring on the cap cell . The cells are cylindrical , contain a parietal , netlike chloroplast with several pyrenoids . DR. SAURABH KUMAR, HOD BOTANY KCMT

2. Occurrence of Oedogonium: Oedogonium is an exclusively fresh water alga . They are very common in pools, ponds, lakes etc. The filamentous plant body may get attached with the stone, wood, leaves of aquatic plants, small branches of dead plant remain in water etc. by their basal cell the holdfast. Some species like O. terrestris are terrestrial. Plant Body(Thallus) of Oedogonium : The thalloid plant body is green, multi­cellular and filamentous. The filaments are unbranched and cells of each filament are attached end to end and form uniseriate row. The filament is differentiated into 3 types of cells: Basal cell, Apical cell Middle cells. Filaments of Oedogonium Oedogonium in Fresh water DR. SAURABH KUMAR, HOD BOTANY KCMT

3. Cell Structure: The cells are elongated and cylindrical with a slightly swollen or dilated upper end. The cell wall is made up of three layers ; an outer chitinous , a middle layer of pectin and an inner layer of cellulose . The center of the cell is occupied by a large vacuole . There is a single, large, reticulate chloroplast present in the protoplasm. The strands of the chloroplast are parallel to the long axis of the cell. Many pyrenoids are present. There is a single large parietal nucleus .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. DR. SAURABH KUMAR, HOD BOTANY KCMT

4. 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 Dying out of some cell here and there in the filament, through accidental breaking, 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 They are produced singly in the specialized cell called zoosporangium . Any cap cell usually , the recently divided one 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 basal granules are connected by fibrous strands to form complete circular ring . DR. SAURABH KUMAR, HOD BOTANY KCMT

5. From each granule arise a single flagellum . In this way a ring of flagella is formed around beak (stephanokont). 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, slips out through the aperture . The zoospore swim freely in the water . 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. 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 colorless and does not 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 . Stages in Asexual Reproduction in Oedogonium DR. SAURABH KUMAR, HOD BOTANY KCMT

6. Akinete Formation In some species of Oedogonium, during unfavorable 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 favorable condition each akinete germinates into a new filament . Aplanospore formation Aplanospore are non-motile, non flagellate, Uninucleate cells They are slightly oblong or spherical and formed one or two in each cell. The cell of the filament producing Aplanospore is called Aplanosporangium Aplanospore are produce during dry season When favorable season comes, the aplanospore comes out of the aplanosporangium and germinate into new plants Akinetes Formation in Oedogonium Aplanospore Formation DR. SAURABH KUMAR, HOD BOTANY KCMT

7. Sexual Reproduction The sexual reproduction in Oedogonium is Advanced 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: Macrandrous species Nannandrous species . Macrandrous species: The species of Oedogonium that produces antheridia and oogonia in normal filament is called Macrandrous species. They may be monoecious producing antheridia and oogonia on the same filament or dioecious 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. DR. SAURABH KUMAR, HOD BOTANY KCMT

8. Macrandrous Species Nannandrous Species Antheridia develops on the filaments of normal size Male filament which develop Antheridia are smaller than female filament on which oogonium is develop Antheridia develop on 2-4 celled filament known as Dwarf Male (these filament develop from Androspores inside androsporangia) Antheridia and Oogonia develop on same filament are known as Macrandrous Monoecious or Homothallic Nannandrous species are always Dioecious i.e. Antheridia and Oogonia develop on different filaments Example: O. nodulosum Antheridia and Oogonia develop on different filament are known as Macrandrous Dioecious or Heterothallic Androsporangia and Oogonia develop on same filament are called Gynandrosporous Example: O. aquaticum Example: O. concatenatum Androsporangia and Oogonia develop on different filaments are called Idioandrosporous Example: O. conferatum DR. SAURABH KUMAR, HOD BOTANY KCMT

9. 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 cel ls, 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 colorless portion at its one end and metamorphoses into an antherozoid or sperm . Thus two antherozoids are produced in each 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. Antherozoids are smaller in size and have fewer flagella. The liberated antherozoids swim freely and finally reach the oogonia. DR. SAURABH KUMAR, HOD BOTANY KCMT

10. Oogonia The oogonia is 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 (suffultory cell or supporting cell). It functions as an oogonium . 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 . It is non-motile and retained within the oogonium. Prior to fertilization, the ovum or egg form a small clear patch, the receptive spot and provides a passage to antherozoids entrance during fertilization. DR. SAURABH KUMAR, HOD BOTANY KCMT

11. Sexual Reproduction in Nannadrous forms: The nannandrous species are dioecious . 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 . The species in which androsporangia are borne in a separate filament, it is named as idioandrosporous . 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. DR. SAURABH KUMAR, HOD BOTANY KCMT

12. 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 nannandrium. 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. DR. SAURABH KUMAR, HOD BOTANY KCMT

13. Fertilization: It is similar in both Nannandrous and Macrandrous forms. The antherozoids swimming in water enter the oogonium through a pore or transverse slit in the oogonial wall. There is a chemotactic attraction between the antherozoids and oogonium. 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 color. 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. DR. SAURABH KUMAR, HOD BOTANY KCMT

14. 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 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. 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. DR. SAURABH KUMAR, HOD BOTANY KCMT