Oedogonium: Salient Features, Occurrence, Thallus Structure, Reproduction

Oedogonium is a multicellular, filamentous, fresh water green alga of the class Chlorophyceae. It can be found in freshwater such as pools, tanks, ditches, etc.

The young filaments of Oedogonium are found to be attached to some substrate.

Salient Features of Oedogonium

The salient features of Oedogonium are as follows:

  • The plant body is green, multicellular, unbranched, and filamentous.
  • The mature filament is differentiated at the apex and base.
  • A reticulate chloroplast is present in each cell with many pyrenoids.
  • In the dividing cells, caps are present.
  • Cell division in the vegetative cells is much more elaborate.
  • Vegetative reproduction occurs through the formation of akinetes.
  • Asexual reproduction takes place by multiflagellate zoospores.
  • In each zoospore, flagella are arranged around the colorless beak-like region.
  • Sexual reproduction is of the advanced oogamous type.
  • Based on the distribution of the sex organs, the plants are divided into two groups: the Macrandous and the Nannandrous type.
  •  In the macrandrous type, the antheridia develop into filaments of normal size. But in the nannandrous type, the antheridia grow on a small and thin male filament, the dwarf male or nannandrium.

Occurrence of Oedogonium

Oedogonium is a genus of about 400 species. It is exclusively fresh water in habit and mainly occurs in permanent and temporary freshwater bodies like ponds, lakes, and shallow tanks.

The filaments are attached to various substrates ( such as rocks, wood, and small branches of dead plants) underwater, or epiphytic on aquatic plants (e.g., Hydrilla) and other algae.

A few species are terrestrial (e.g., O. terrestris, O. randhawa), growing on moist soils.

Thallus Structure of Oedogonium

The plant body of Oedogonium is multicellular, filamentous, and green in color. The filaments are unbranched and uniseriate (cells are arranged in a single row or attached end to end).

Oedogonium thallus
Figure: Oedogonium thallus showing its different parts

The mature filament is differentiated into three types of cells.

  • The lower-most basal cell or holdfast
  • The intercalary cells
  • The upper-most apical cell

Basal Cell or Holdfast

The lowermost cell of the filament is the basal cell. It is modified to form a holdfast or hapteron.

The holdfast is mostly found in aquatic species, and it rarely occurs in terrestrial forms. In terrestrial species, it may produce rhizoid-like outgrowths.

Oedogonium cell with holdfast
Figure: Oedogonium cell with a holdfast

The holdfast is long, gradually narrows, and towards the basal part, it expands to form a simple, disc-like, multilobed, or finger-shaped outgrowth or projection.

Basal cells are usually colorless and do not take part in photosynthesis due to chloroplasts being absent or poorly developed. It helps in the attachment of the filament to the substratum.

Intercalary Cells

All cells of the filament lying in between the apical cell and the basal cell are intercalary cells. The cells are similar in structure and longer than their breadth (rectangular in shape).

Some cells contain a ring-like structure in their upper part called a cap or apical cap. Such cells are called “cap cells.” The number of caps on a cell shows how many times it has been divided.

Apical Cell

The terminal cell or distal cell of the filament is called the apical cell. It is usually rounded or acuminate towards the apex. In some species (e.g., O. ciliate), the apical cell prolongs into a hair-like structure.

The apical cell is green in color due to the presence of chloroplasts and takes part in photosynthesis.

Cell Structure of Oedogonium

The vegetative cells are elongated and cylindrical with a slightly swollen or inflated upper portion. The cells are surrounded by a thick, rough, and rigid cell wall.

The cell wall is composed of three concentric layers: an outer chitinous layer; a middle layer of pectin; and an inner layer of cellulose. However, electron microscopy revealed that there was no cellulose layer. Mucilage is absent on the surface of the filament.

A single Oedogonium cell
Figure: A single Oedogonium cell

A thin plasma membrane encloses the protoplasm. The protoplasm contains cytoplasm, the central vacuole, reticulate chloroplasts, and the nucleus.

A large vacuole is present in the center of the cell, which remains filled with the cell sap. The cell sap consists of excretions, secretions, and inorganic compounds.

The chloroplast is a single, large sheet-like reticulate which remains embedded in the cytoplasm. It extends from one end of the cell to the other and completely encircles the protoplast. The strands of the chloroplast are broad or narrow and are parallel to the long axis of the cell.

Many pyrenoids are present in the chloroplast, particularly in the region of intersections. Pyrenoids are surrounded by a sheath of starch granules.

There is a single, large, biscuit-shaped or biconvex nucleus that lies near the middle of the cell, internal to the chloroplast. The nucleus contains one or more nucleoli and thread-like or elongated chromosomes.

Reproduction in Oedogonium

Oedogonium reproduces in all three methods: vegetative, asexual, and sexual.

Vegetative Reproduction

Oedogonium reproduces vegetatively through fragmentation.

Fragmentation

Oedogonium filament can break into small fragments. The broken fragments, which are detached from the thallus, develop into new filaments under favorable conditions.

Fragmentation takes place due to:

  • Accidental breakage of filaments (mechanical injury or insect bites).
  • Dying or dehydration of intercalary cells.
  • The disintegration of intercalary cells due to conversion of zoospores or gametes.
  • Change in the environmental conditions.

Asexual Reproduction

Asexual reproduction in Oedogonium takes place by the formation of various types of asexual spores, such as zoospores, aplanospores, and akinetes.

By Zoospores

Zoospore formation is the most common method of reproduction in aquatic algae. Zoospores are formed singly within the specialized cell (intercalary cap cell) called zoosporangium. Usually, a recently formed cap cell functions as the zoosporangium.

Several factors that control zoospore formation, including the high pH and CO2 concentration of the medium, as well as the diurnal rhythm of light and darkness, are significant.

During favorable conditions, the zoospore formation begins in a newly formed cap cell of the filament. The zoosporangium gets filled with abundant reserve food material. The entire protoplast of the zoosporangium contracts from the cell wall and becomes a rounded mass or oval-shaped structure. The nucleus moves towards one end of the protoplast.

Formation of zoospore
Figure: Formation of zoospore in Oedogonium

The central vacuole disappears. The chloroplast frees itself from one side of the cell and becomes conical. A colorless, semi-circular hyaline area appears on one side, close to the nucleus. This hyaline balding area later forms the anterior region of the zoospore.

At the base of the hyaline area, a single or double row of blepharoplast granules or basal granules develops. The blepharoplast granules are connected by fibrous strands to form a complete circular ring (Ringo, 1967). Later on, a single flagellum arises from each blepharoplast granule. In this way, a crown of about 30 flagella is formed around the base of the colorless beak-like area of the protoplast.

When the zoospore is mature, the wall of the zoosporangium splits transversely near the apical cap region, and the upper portion of the cell is lifted off like a lid. During the liberation, the mucilage substance is secreted at the base of the zoospore, which helps in the liberation of the zoospore. The zoospore remains enclosed in a delicate mucilaginous vesicle for 3–10 minutes. The mucilaginous vesicle soon disappears and the zoospores are liberated into the water.

The zoospore is a deep green, oval, spherical, or pear-shaped structure. It contains a single nucleus, an eye spot, a chloroplast, and numerous vacuoles. A subapical ring of flagella is present at the base of the colorless, beak-like anterior end.  Such a flagellar arrangement is called a stephanokont or a stephanokontic type.

Germination of Zoospore

After liberation, the zoospore swims for about an hour or more. Then it settles down and attaches itself to the substratum by its anterior end.

Liberation and germination of zoospore
Figure: Liberation and germination of zoospore

After attachment, the flagella are withdrawn and the zoospore begins to elongate. The lower hyaline part elongates to form a holdfast and the upper part divides repeatedly by transverse division, forming a new filament of Oedogonium.

By Akinetes

Akinetes (modified vegetative cells) are formed during unfavorable conditions in some species of Oedogonium. They are swollen thick-walled, reddish-brown, rounded, or oval structures.

Akinetes are formed in chains and are rich in starch as a reserve food material and in reddish-orange oil.

Under favorable conditions, the akinetes germinate into new filaments.

By Aplanospores

In unfavorable conditions, instead of zoospores, aplanospores are formed in some species. Aplanospores are thin-walled, non-flagellated, non-motile spores that are slightly oblong or spherical.

The aplanospore comes out of the parent cell and forms a new plant with the onset of favorable environmental conditions.

Sexual Reproduction

In Oedogonium, sexual reproduction is an advanced oogamous type. Sexual reproduction is more frequent in stagnant water than in running water. Alkaline medium, deficiency of nutrition, light and dark periods, and increased temperature are influencing sexual reproduction.

The gametes are produced in highly specialized cells called gametangia. The male gametes, or antherozoids, are produced in antheridium (male gametangium), and the female gamete, or egg, is produced in oogonium (female gametangium). The male gametes and the female gametes are structurally and physiologically different.

Distribution of Sex Organs

Depending on the distribution of sex organs (size of the male filaments), Oedogonium species are divided into two groups:

  • Macrandrous Species
  • Nannandrous Species

Macrandrous Species

In the macrandrous Species, the antheridium is produced on the filament of normal size. Macrandrous species are of two types: monoecious type and dioecious type.

  • Monoecious (homothallic) Type: In the monoecious macrandrous type, antheridia and oogonia are produced on the same filament, e.g., O. fragile, O. kurzii, O. nodulosum, and O. hirnii.
  • Dioecious (heterothallic) Type: In the dioecious macrandrous type, antheridia and oogonia are produced on different filaments, e.g., O. aquaticum, O. cardiacum, and O. gracilius.

Nannandrous Species

In nannandrous species, oogonium is produced on the normal-sized filament, but antheridium is developed on a small or dwarf filament called the dwarf male, or nannandrium. The dwarf males are formed by androspores which are produced in androsporangia.

Nannandrous species are more primitive than macrandrous, according to some algologists.

Nannandrous species are of two types: gynandrosporous type and idioandrosporous type.

  • Gynandrosporous Type: In the gynandrosporous nannandrous type, androsporangia and oogonia are formed on the same filament, e.g., O. concatinatum.
  • Idioandrosporous Type: In the idioandrosporous nannandrous type, androsporangia and oogonia are formed on different filaments, e.g., O. setigerum, O. iyengarii, and O. confertum.

Antheridium in Macrandrous Species

The structure and development of antheri­dium and oogonium are similar in both monoecious and dioecious type macrandrous species.

In macrandrous species, antheridium is developed in either the terminal or intercalary cell of the normal-sized filament. The initial cell that gives rise to antheridium is called the antheridial mother cell.

Any cap cell of the vegetative filament may function as an antheridial mother cell. The antheridial mother cell divides transversely into two unequal cells; the upper small antheridium and the lower, larger sister cell. The sister cell then divides transversely to form a uniseriate row of 2–40 antheridia. Rarely, the antheridia are produced singly.

Oedogonium filament bearing antheridia
Figure: Oedogonium filament bearing antheridia in chain

The antheridium is a uninucleate, broad, flat, short-cylindrical cell. The nucleus of each antheridium is divided mitotically by a transverse or vertical wall into two haploid nuclei. Each nucleus gets surrounded by some cytoplasmic content and becomes pear-shaped. It 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 two antherozoids are positioned side-by-side or one above the other (i.e., they lie superimposed) in the antheridium.

The antherozoids or spermatozoids or sperms are unicellular, uninucleate, multiflagellate, oval or pear-shaped, and yellowish in color. Morphologically, they are similar to zoospores but are relatively smaller in size and have fewer flagella.

Antheridia and liberation of antherozoids
Figure: Antheridia and liberation of antherozoids

The libe­ration of antherozoids from antheridium is similar to zoospore formation during the asexual process. The wall of the antheridium ruptures transversely, and two antherozoids are freed into a thin hyaline vesicle. Soon, the vesicles dissolve and two antherozoids swim freely in the water. The liberated antherozoids swim freely in water before they reach oogonia and participate in fertilization.

Antheridium in Nannandrous Species

In nannandrous species, the antheridium is developed on a short or dwarf male filament called the dwarf male or nannandrium. The dwarf male filament is produced by the germination of a peculiar type (antherozoid-like zoospore) of spore called an androspore.

The androspores are produced singly within the androsporangium. Androporangia are more or less similar in appearance to the antheridia of macrandrous species and are produced in a similar manner from the mother cell by unequal division.

The androsporangia are flat, discoid cells that are slightly larger than the antheridia of macrandrous species. The protoplast of each androsporangium metamorphoses into a single androspore, just as in the case of the zoospore.

The androspore is a unicellular, uninucleate, motile structure that has a sub-polar crown of flagella. It is larger than an antherozoid but smaller than a zoospore.

During the liberation, the androspore is surrounded by a mucilaginous vesicle which helps in the liberation of the androspore. The mucilaginous vesicle soon disappears and the androspores are liberated into the water.

Nannadrous species showing dwarf male
Figure: Nannadrous species showing dwarf male with oogonium

After swimming about for some time, the androspore settles on the oogonial wall (e.g., O. ciliatum)
or on the suffultory cell (e.g., O. concatenatum). The androspore germinates into a dwarf male filament or nannandrium. The nannandrium, or dwarf male, can be a few cells (2–3 cells) long. The lower, rhizoid-like, elongated cell of the dwarf male is known as the stalk. The stalk cell cuts off one or more cells at its tip and forms antheridia.

The protoplast of each antheridial cell divides to form two sperms or antherozoids, which are similar to the antherozoids of macrandrous species. Each antherozoid is uninucleate, unicellular, oval, or pear-shaped and has a sub-apical ring of short flagella at the base of a colorless, beak-like anterior end.

The antherozoids are released by the separation of a cap-like lid at the top or by the disorganization of the antheridial cell.

Oogonium

In Oedogonium, the oogonium is a highly differentiated female gametangium (female sex organ). It is generally intercalary but sometimes can be terminal, e.g., O. palaiense.

The structure and development of oogonium are similar in both macrandrous and nannandrous species. Like antheridium, any actively growing cap cell of the vegetative filament may function as the oogonial mother cell. The oogonial mother cell divides transversely into two unequal cells, the upper larger cell, and the lower smaller cell.

Oogonium associate with dwarf male
Figure: Oogonium associated with dwarf male

The upper cell is rich in the cytoplasm and forms an oogonium. The lower cell has a smaller nucleus than the upper cell and functions as a supporting cell or suffultory cell. It supports the oogonial cell.

The lower cell may again function as an oogonial mother cell and undergo similar divisions in repeated sequence to form two or three or more oogonia with a lower supporting cell. In some species of Oedogonium, the oogonial mother cell directly develops the oogonium. Supporting cell is absent in O. americanum.

The oogonium is an oval, spherical, or elliptical structure. It enlarges prominently and is filled with reserved food material.

The entire protoplast in an oogonium metamorphosis into a single non-motile, egg or ovum, or oosphere. The egg is green in color due to the presence of chlorophyll.

Oedogonium mature oogonium
Figure: Oedogonium mature oogonium

As the oosphere matures, the nucleus moves to the periphery. The protoplast of the oosphere retracts slightly from the oogonial wall and develops a hyaline, receptive spot just outside the nucleus. The oogonial wall forms a small pore or transverse slit near the anterior end of the oosphere above the receptive spot. It develops a sort of conduit leading down to the oosphere and provides a passage for the entrance of antherozoids during fertilization.

In some species, a mucilage drop is discharged through an opening to attract antherozoids.

Fertilization

The process of fertilization is similar in both macrandrous and nannandrous species. The antherozoids are attracted by the mature oosphere through chemical substances or mucilage. The antherozoids swim through the opening of the oogonial wall. Only one antherozoid enters the egg at the hyaline receptive spot and fertilizes the egg.

After plasmogamy and karyogamy, the male and female nuclei in the egg fuse to form a diploid zygote.

Zygote

After fertilization, the zygote secretes a thick 2-3-layered wall around itself to form an oospore. The oospore is a spherical, reddish-brown structure.

Zygote within oogonium
Figure: Zygote within oogonium

Initially, the oospore remains green but due to the accumulation of reddish oil, it becomes red in color.

The oospore is liberated from the oogonium by the degeneration of the oogonial wall. It rests on the mud at the bottom of the pond and remains dormant for a long period.

Germination of Oospore

During favorable conditions (the oospore may require chilling before germination; Mainx, 1931), the oospore germinates into a new thallus.

The diploid oospore nucleus undergoes zygotic meiosis to form four haploid daughter nuclei. The nuclei accumulate some cytoplasm to form four haploid daughter protoplasts.

Each daughter protoplast develops a crown of flagella around itself and metamorphoses into a zoospore, often called a meiospore or meiozoospore. The meiospores are liberated by rupturing the oospore wall.

Zygote germination and zoospores liberation in Oedogonium
Figure: Zygote germination and zoospores liberation

After liberation, the meiospores are at first surrounded by a delicate vesicle. The vesicle soon disappears and the meiozoospores get free into the water. After swimming for some time in the water, they settle on some substratum.

The meiospores withdraw their flagella and germinate into new haploid Oedogonium filaments. In some macrandrous dioecious species, out of four meiozoospores, two develop into male and the other two develop into female filaments, e.g., O. plagiostomum.

Under certain conditions, meioaplanospores (i.e., non-flagellated, non-motile, thick-walled haploid spores) are formed instead of meiozoospores.

In some cases, out of four nuclei (n), only one or two remain viable and the rest degenerate.

Taxonomic Position of Oedogonium

  • Class: Chlorophyceae
  • Order: Oedogoniales
  • Family: Oedogoniaceae
  • Genus: Oedogonium

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