Describe the structure and life cycle of Oedogonium.
Ans: Oedogonium is a freshwater alga belonging to the family Chlorophyceae. It is a thallus with a green, filamentous, and unbranched plant body with a holdfast that attaches the plant body to a substratum. The structure and life cycle of Oedogonium have been discussed below:
Systematic Position:
Kingdom: Plantae
Division: Chlorophyta
Class: Chlorophyceae
Order: Oedogoniales
Family: Oedogoniaceae
Genus: Oedogonium
Occurrence:
There are more than 400 species of Oedogonium, of which Gonzalves and Sonad reported 114 species from Karnataka only. Oedogonium is generally observed in freshwater habitats, such as ponds, tanks, rivers, etc. Besides this, there are also terrestrial species, such as O. terrestris, from moist soil, aquatic epiphytes, and some species attached to the substratum, like wood, stone, stems, leaves, etc.
Thallus Structure:
The thallus of Oedogonium is green, multicellular, and filamentous. Numerous cylindrical cells, varying in shape and size, constitute the filaments. Each filament has a cylindrical shape and reticulates chloroplasts and vacuoles. The cells can be differentiated into three types: basal cells, or holdfast cells, intercalary cells, and apical cells. Basal cells are the lowermost cells, having poorly developed chloroplasts that help in attachment to a substratum. They may be simple, with multiple lobes, or in the shape of a disc or finger. The apical cells are green, rounded at the top, and broad. The cells in between the apical and basal cells are called intercalary cells. The distal cells have annular striations, or cap cells.
Cell Division:
Oedogonium exhibits a peculiar cell division. It occurs through the ring-like cap cells present in some of the filaments, and their number indicates the number of divisions a cell has undergone. Before the beginning of mitosis, below the apical end of the cell, the inner cell wall layer gives rise to a ring structure. The nucleus moves towards the distal ends and undergoes mitotic division. During mitotic division, the ring grows, thickens, and forms a groove-like structure. After the division is completed, a floating septum develops between the two daughter nuclei. Followed by the rupture of the outer and middle layers associated with the ring, the ring disintegrates, and the ring elongates to form a new cell wall. The floating septum settles at the end of the old cell wall, and the ruptured cell wall near the daughter nuclei becomes the cap. The cells with caps are called cap cells.
Cell Structure:
The Oedogonium cell comprises a three-layered cell wall composed of cellulose, pectin, and chitin. Next to this lies the cell membrane, within which there is cytoplasm and the nucleus. The cells are uninucleated with a central nucleus. They also have a pyrenoid-containing reticulate chloroplast, a large central vacuole filled with cell sap, and other eukaryotic cell organelles, such as the endoplasmic reticulum, mitochondria, Golgi bodies, ribosomes, etc.
Reproduction:
Oedogonium exhibits vegetative, asexual, and sexual modes of reproduction.
Vegetative Reproduction:
Oedogonium reproduces vegetatively through fragmentation and akinete formation.
Through fragmentation, the plant body breaks into filaments due to injury and the drying or rupture of intercalary cells. Later, each fragment grows into a new plant body.
Through akinete formation, the thick-walled akinetes, abundant with reserve food material formed during unfavorable conditions to withstand the extreme harsh conditions, germinate into new plants on the return of favorable conditions in some species.
Asexual Reproduction:
Oedogonium reproduces asexually through zoospores formed in cap cells or newly divided cells. The formation of zoospores begins with the contraction of the protoplast in place and the movement of the nucleus towards one side of a cap cell. In this way, the cap cells attain an oval shape and function as zoosporangium. Near the nucleus, a semicircular and colorless area develops, which gets surrounded by a ring of blepharoplast granules. A flagellum develops from each granule, thus forming a crown of flagella. The zoospores formed in this way consist of a nucleus, a chloroplast, and a ring of flagella around the beak-like anterior end. One of the mature zoospores ruptures the cell wall of the zoosporangium and remains temporarily inside the mucilaginous vesicle. After the dissolution of the vesicles, the zoospore becomes free and starts swimming. It then loses flagella and increases in length after coming into contact with a substratum. The lower part develops into a holdfast of different shapes and sizes. The upper part undergoes subsequent transverse division to produce a new filament.
Sexual Reproduction:
Oedogonium undergoes sexual reproduction through oogamy. This mode of reproduction is promoted by alkaline pH and a deficiency of nitrogen in the environment. The majority of species are heterothallic, except for a few homothallic species. The male and female gametes are produced in antheridium and oogonium, respectively.
Oedogonium sp. can be macrandrous or nannandrous, depending upon the size of the male filaments, the filaments that bear antheridium. The species having normal-sized male filaments are called Macrandrous, while those having male filaments with sizes smaller than those of female filaments are called Nannandrous species or dwarf males. Macrandrous species can be monoecious or dioecious, while Nannandrous species are always dioecious.
Sexual Reproduction in Macrandrous Species: The Macrandrous monoecious and Macrandrous dioecious species produce antheridium and oogonium in the same and different filaments, respectively, following similar development stages.
Development of Antheridia:
The vegetative cap cells of the filament function as the antheridial mother cell. It undergoes a series of transverse divisions to produce a row of flat and compartment-like uninucleate cells, called antheridia. The nucleus of antheridia undergoes a mitotic division to produce two nuclei, which develop their cytoplasmic content. In this way, two uninucleated and multi-flagellated antherozoids are produced from each antheridium. They have comparatively less chlorophyll content, a lesser number of flagella, and a smaller size than the zoospores. Similar to the zoospores, the antherozoids move out of the antheridium by rupturing the wall and later get surrounded by a thin vesicle for some time.
Development of Oogonium:
The development of oogonium begins in an actively growing cap cell, which functions as the oogonial mother cell. It undergoes transverse division to produce two daughter cells. The upper and lower daughter cells produce the oogonium proper and supporting cell, or suffultory cell, respectively. Each oogonium is uninucleate and spherical, ellipsoidal, or oval with a prominent nucleus and one or more caps at the upper end. Some species, such as O. americanum, lack a supporting cell, while others give rise to another oogonium. Each oogonium develops into one non-motile female gamete, called an egg. A young egg is green and contains a central nucleus, while in a well-developed egg, the nucleus is at the periphery.
In the oogonial wall, a small pore or transverse slit develops near the anterior end of the oogonium. The species having pores are called poriferous species, while those having a transverse slit are called operculate species. Opposite to this structure, there's a colorless patch, called a receptive spot, in each egg.
Sexual Reproduction in Nannandrous Species:
All nannandrous species, antheridia and oogonia, develop on nannandria and normal-sized, large filaments, respectively. There are two types of nannandrous species. In gynandrosporous species, androsporangia and oogonia develop on the same filament, e.g., O. concatenatum, while in idicandrosporous species, they develop on separate filaments, e.g., O. iyengarii.
Development of Androsporangia and Androspores:
The development of androsporangia in nannandrous species is similar to that of antheridia in macrandrous species, i.e., through repeated transverse divisions in vegetative cells. There are slight differences, such as that the androsporangia are larger than antheridia and arranged as flat discoid cells, and there's no division in the androsporangium nucleus. The androsporangia develop their cytoplasmic content and transform into a uninucleate and multiflagellate androspore. It remains inside a mucilaginous bladder after its release for a while and then separates to swim freely in the water.
When the androspore attaches to oogonium or a supporting cell, it secretes a wall, increases in length, and cuts off some flat cells called antheridia on short filaments. The antheridium nucleus undergoes mitotic division to produce two nuclei, each of which develops its cytoplasmic content and transforms into antherozoids.
Fertilization:
Most Oedogonium species exhibit the same mechanism for fertilization. The antherozoids swim in the water to reach oogonium. The chemotactic attraction brings the antherozoids towards the oogonium. The antherozoid enters the oogonium through an oogonial wall pore; one of them penetrates the egg membrane and fuses with the nucleus to produce a diploid zygote or oospore.
The zygote picks up its content from the oogonium, surrounds itself with a two- or three-layered thick wall, and enters the resting phase or germinates directly. The green-colored zygote attains a red color during the resting period due to the buildup of reddish oil. The oogonial wall degrades, and the zygote is released.
Germination of Zygote:
The zygote in the resting period germinates during the onset of favorable conditions. The diploid nucleus produces four haploid nuclei upon meiosis. These nuclei develop into four daughter protoplasts, which produce the flagella. These structures are also called zoomeiospores or meiospores. They exit by rupturing the wall and getting temporarily enclosed within a vesicle for a while. They swim freely in water after the dissolution of the vesicle for some time. Later on, they reach a substratum, lose their flagella, and germinate into new filaments. In heterothallic species, the male and female filaments develop from the germination of two zoomeiospores.
Life cycle:
Oedogonium exhibits a haploid life, as the plant body is always haploid. The zygote is the only diploid phase, which undergoes meiosis to form haploid zoomeiospores. These structures develop into new haploid filaments.
What is Oedogonium?
Oedogonium is a freshwater alga belonging to the family Chlorophyceae, characterized by its green, filamentous, and unbranched thallus structure.
What is the Systematic Position of Oedogonium?
Oedogonium is classified under the following taxa: Kingdom: Plantae Division: Chlorophyta Class: Chlorophyceae Order: Oedogoniales Family: Oedogoniaceae Genus: Oedogonium
Where does Oedogonium Occur?
Oedogonium is commonly found in freshwater habitats like ponds, tanks, rivers, and even in moist soil as terrestrial species. It can also be observed as aquatic epiphytes and attached to substrata such as wood, stone, stems, and leaves.
What is the Structure of Oedogonium Thallus?
The thallus of Oedogonium is multicellular and filamentous, composed of cylindrical cells with reticulate chloroplasts and vacuoles. It consists of basal cells, intercalary cells, and apical cells, with cap cells at the distal ends.
How does Cell Division occur in Oedogonium?
Oedogonium exhibits a peculiar cell division mechanism involving ring-like cap cells. The process includes the formation of a ring structure, mitotic division, septum development, and cell wall elongation.
Describe the Cell Structure of Oedogonium.
Oedogonium cells possess a three-layered cell wall, cell membrane, cytoplasm, nucleus, pyrenoid-containing reticulate chloroplasts, vacuoles, and various organelles such as endoplasmic reticulum, mitochondria, Golgi bodies, and ribosomes.
What are the Reproduction Modes of Oedogonium?
Oedogonium reproduces vegetatively, asexually, and sexually.
How does Vegetative Reproduction occur in Oedogonium?
Vegetative reproduction involves fragmentation and akinete formation, enabling the regeneration of new plant bodies.
Describe Asexual Reproduction in Oedogonium.
Asexual reproduction in Oedogonium involves the formation of zoospores in cap cells or newly divided cells, leading to the development of new filaments.
Explain the Sexual Reproduction of Oedogonium.
Sexual reproduction occurs through oogamy, with male and female gametes produced in antheridia and oogonia, respectively.
What are Macrandrous and Nannandrous species in Oedogonium?
Macrandrous species have normal-sized male filaments, while Nannandrous species have smaller male filaments compared to female filaments.
How does Sexual Reproduction occur in Macrandrous Species?
Macrandrous species produce antheridia and oogonia in the same or different filaments, depending on whether they are monoecious or dioecious.
Describe the Development of Antheridia and Oogonium in Oedogonium.
Antheridia and oogonia develop from cap cells, undergo transverse divisions, and produce uninucleate cells. Antheridia produce antherozoids, while oogonia develop into eggs.
What is the Fertilization process in Oedogonium?
Fertilization involves the fusion of antherozoids with eggs to form a diploid zygote or oospore.
Explain the Life Cycle of Oedogonium.
Oedogonium exhibits a haploid life cycle, with the zygote being the only diploid phase, which undergoes meiosis to form haploid zoomeiospores, ensuring the continuity of haploid generations.