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Oedogonium

Oedogonium belongs to Oedogoniaceae, the lone

family in the order Oedogoniales. Oedogoniales are

essentially a group of fresh-water green algae, which

prefer growing in quiet situations, and usually avoid

flowing waters. They often occur attached by a special

holdfast cell. Oedogoniaceae comprise 3 genera

(Oedocladium, Bulbochaete and Oedogonium) with about

400 species. The distinctive features of the family are:

Oedogonium: Systematic position

Kingdom Plantae

Division Chlorophyta

Class Chlorophyceae

Order Oedogoniales

Family Oedogoniaceae

Genus Oedogonium

Salient features of Oedogonium are as:

(i) The thallus is a branched (Oedocladium and

Bulbochaete), or unbranched (Oedogonium) filament,

made up of uninucleate, cylindrical cells. The nucleus is

parietal.

(ii) The vegetative cells are usually broader near the

anterior end and, thus, exhibit apical-basal polarity.

(iii)The chloroplast is a parietal network (close or loose),

with the padded portions enclosing pyrenoids.

(iv)The method of cell-division is unique, by the annular

splitting of the lateral cell wall.

(v) There is a subapical ring of numerous flagella around

the anterior end of the reproductive cells (zoospores,

androspores and male gametes).

(vi)Asexual reproduction is by means of multiflagellate

zoospores formed singly in zoosporangia.

(vii)Sexual reproduction is by advanced type of oogamy,

comprising specialized sex organs (oogonium and

antheridium), formed on the same, or on different

filaments.

(viii) Some species produce dwarf male plants.

Of the three genera, Oedocladium is mainly

terrestrial and the thallus is a heterotrichous filament.

The other two genera are aquatic. Oedogonium is the

only genus with an un-branched filament. It is by far the

commonest in the family with about 285 species. In

India, about 114 species of Oedogonium were recorded

by Gonzalves and Sonad (1961) in the Karnataka State

alone. The genus is best known and considered as type of

the group.

Habitat. It is exclusively fresh water in habitat. The

filaments are found to be attached to some substrate.

Often it grows epiphytically on the larger green algae, or

upon the leaves, petioles and stems of aquatic

angiosperms in fresh-water ponds, tanks, lakes and quiet

streams. The mature filaments are free floating, but the

younger ones are attached. It is less common in the

running water. The attaching organ is the basal cell

differentiated especially for this purpose.

Thallus structure (Fig. 1). Thallus is a long,

unbranched thread called the filament. The filament

consists of a single row of elongated, cylindrical cells

arranged end to end. The filament usually occurs

attached at the lower end by means of a basal cell, the

rhizoidal cell or holdfast. The holdfast is expanded into a

flattened disc with outgrowths. In the mature condition,

however, the filaments float in yellowish-green mats. The

free end of the distal cell of the filament is broadly

rounded in case of majority of the species. In a few

cases, however, it ends in a fine, slender, hairlike

process (O. ciliata).

Cell structure (Fig. 2). The cells are elongated and

cylindrical with a more or less dilated upper end in some

species. The vegetative cell consists of conspicuous or

inconspicuously-thickened, rigid cell wall enclosing the

protoplast. The cell wall is differentiated into two

concentric layers. The inner layer which is next to the

protoplast is cellulose in nature. The outer layer

according to Tiffany (1930) consists of pectic substance

(pectose). External to the pectic layer in Oedogonium is a

surface investment of chitin. It is often referred to as the

third layer. It prevents the dissolving away of the pectic

layer so that the filaments feel like wet threads. The use

of electron microscopy has, however; revealed the

absence of cellulose in the cell wall.

The protoplast is differentiated into a thin plasma

membrane and the, cytoplast. The cytoplast, covered by

the plasma membrane, is closely adherent to the cell

wall. lt encloses a large central vacuole containing the

cell sap. There is a single large, reticulate parietal

chloroplast with a number of pyrenoids, embedded in the

cytoplasm. The chloroplast extends from one end of the

cell to the other. It is parietal in position and lies in the

primordial utricle (lining layer of cytoplasm), which it

completely encircles. The chloroplast has the form of a

hollow-cylindrical network, with narrow or broad sub-

parallel meshes; numerous pyrenoids lie at the

intersections of the reticulum. There is a single large

parietal nucleus. It lies near the middle of the cell,

embedded in the cytoplasm just within the chloroplast.

There is generally no slime around the filament. Certain

cells in every filament possess one or more ring-like

markings of hemicellulose, the so-called apical caps, at

their distal ends. Such cells are called the cap cells. The

presence of cap cells at intervals in the filament of

Oedogonium is a safe criterion for its recognition from

other unbranched green algae.

Growth. Growth occurs by the increase in the number of cells in the filament. The new cells arise by cell division which is largely intercalary. Only certain cells in the filament divide, these have one or more ring-like striations at their apical ends and are called the cap cells; the latter arise at variable intervals in the filament. Cell division. The mode of cell division is unique. It

occurs in the following steps (fig.3).

i. Ring formation: Just before the cell division, a ring-

like scar is developed near the anterior end of the

cell. The process starts with the movement of the

peripheral nucleus to the centre of the cell and the

appearance of a thickened transverse ring of wall

material towards the upper end of the cell on the

inner face of lateral wall. The ring gradually in-

creases in thickness and becomes grooved with the

open part covered by the outer layer.

ii. Nuclear division: The nucleus of the parent cell

divides into two. Its division starts with the upward

migration of the nucleus. It lies at about one-third the

distance from the upper end of the cell. Here it

divides mitotically. The division of the nucleus is

followed by the formation of a cytoplasmic strand

across the vacuole between the two daughter nuclei,

which remain unconnected with longitudinal cell walls

for some time.

iii. Formation of new cell: At the level of the ring, the

outer and the middle layers external to the groove

rupture all round, permitting the thickened portion to

be stretched out. Consequently, the cell elongates to

about double its normal length. 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 septum. There is, however, a

portion of the ruptured parent cell wall fitting like a

cap at its upper end, and a portion forming a bottom

sheath at the other end. The former produces a

characteristic ring-like mark, the apical ring. A new

cell thus formed always interposed between the two

old portions of the parent cell wall. Only the cell

possessing the apical ring divides again. It is called

the cap cell. As successive divisions always occur at

the same place, a number of apical rings develop

there, giving a characteristic striated appearance to

the cap cells. Thus the number of apical rings the cap

cell contains denotes

the number of

divisions the cell

has undergone.

Asexual reproduction

It takes place by the vegetative method of fragmentation

and sporulation.

Fig. 3. Oedogonium: stages in cell division.

(a) Fragmentation. It consists in the breaking of the

filament into small segments of living cells called

fragments. Each fragment by cell division and growth

develops into a new filament. Fragmentation of the

filament may take place by any of the following

methods:-

(i) Dying out of some cells here and there in the

filament.

(ii) Through accidental breaking

(iii) Through the formation of zoospores or gametes

here and there in the filament

(b) Sporulation: It takes place by means of large

zoospores and sometimes by akinetes.

(i) Zoospore formation: It is the normal and most

effective method of asexual reproduction (Fig.4). The

formation of zoospores is said to depend on the presence

of a certain amount of free carbon dioxide in the

surrounding water (Gussewa, 1931). Any cap cell of the

filament, usually the recently divided one at the terminus

of the filament, may become a zoosporangium (A).

Generally only a few cap cells in the filament produce

zoospores. The zoospores are produced singly and in the

cells containing abundant food reserves. In the formation

of the zoospore, the entire protoplast of the

zoosporangium withdraws somewhat from the wall. The

nucleus moves towards one side of the protoplast. The

protoplast then rounds up. At the same time a

semicircular colourless area appears on one side adjacent

to the nucleus. This is caused by the receding of the

chloroplast from this end of the protoplast. A single or

double row of blepharoplast granules then appears at the

base of the hyaline area. The basal granules are

connected by fibrous strands to form complete circular

ring. From each granule arises a single flagellum. In this

way, a ring of flagella is formed around the base of the

colourless beak-like area of protoplast. It is the anterior

pole. With the formation of zoospore, the cell wall

ruptures transversely in the region of the apical cap. The

two halves gap apart. The mature zoospore, surrounded

by a delicate mucilaginous vesicle, slips out through the

aperture. The vesicle soon dissolves, allowing the typical

Oedogonium swarmer to escape and swim about. The

liberated zoospore is a deep green spherical or pear-

shaped structure. It has a ring of short flagella at the

base of a colourless, beak-like forward end. This kind of

flagellation is called stephanokont. The zoospore

possesses an eye spot, a chloroplast and numerous

contractile vacuoles near the periphery.

Germination of zoospore: The liberated zoospores

remain motile for about an hour. Finally it settles down

on some solid object, with the colourless, flagellar

(anterior) end downwards. In this state it withdraws its

flagella and secretes a cell wall, which lacks the

superficial chitinous material. The colourless anterior end

of the quiescent zoospore develops into a simple or

branched hotdfast. The smooth surface of the substratum

induces a simple holdfast, while the rough one induces a

branched holdfast. The development of the one-celled,

sessile germiling, depending on the species, takes place

in the following two ways:

(i). In most of the species, the one-celled germiling

divides transversely by an apical ring (normal method

of cell division). The basal cell cut off by the first

division remains colourless and does not divide again(

Fig.5) .

(ii) In some species, the quiescent zoospore flattens to

become hemispherical. From the free convex surface

of this one-celled hemispherical germling arises a

cylindrical outgrowth. The cylindrical outgrowth

subsequently grows into a new filament by the normal

method of cell division described above.

Emerging zoospore

Fig. 5. (A-D) Oedogonium: Steps in the germination of the zoospore to form a filament

Fig. 4. (A-D) Oedogonium. Development and liberation of zoospores in O. concatenatum

A

D

A

C

(ii) Akinete Formation. Wille (1909) reported the

formation of resting cells (akinetes) in certain species of

Oedogonium. The akinetes are thick-walled, reddish

brown, more or less rounded structures. They are found

in chains, each inside an inflated cell resembling an

oogonium. The akinetes are developed with the

approach of unfavourable period for vegetative growth

and contain abundant starch as reserve food material,

and reddish orange oil. Each akinete under suitable

conditions directly produces a new filament.

Sexual Reproduction: Oedogonium shows marked

advance over Ulothrix in the method of sexual

reproduction. It is distinctly an advanced type of oogamy.

The sexual cells or the gametes are not only different

physiologicalIy, but also structurally. They are produced

not in vegetative cells as in Ulothrix but in highly

specialized reproductive organs, the gametangia. The

latter are, of course, specially modified cells of the

filament. They are differentiated from the vegetative

cells. The male gametangium is called the antheridium,

and the female oogonium. Sexual reproduction is of

common occurrence in filaments growing in quiet water.

Mainx (1931) stated that it takes place when the

filaments have developed a certain sexual tonus, after a

certain period of active vegetative growth. The external

conditions which favour the process are light, sufficient

supply of CO2, hydrogen-ion concentration on the

alkaline side and nitrogen deficiency.

Distribution of sex organs: Several patterns of

distribution of sex organs occur in Oedogonium.

Depending on the distribution of sex organs, the species

of Oedogonium are grouped into two major categories,

macrandrous and nannandrous.

Macrandrous : In the macrandrous species, antheridia

occur on filaments of normal size. These may be (a)

monoecius or (b) dioecious.

(a) Macrandrous monoecius The antheridia and

oogonia in monoecious macrandrous species occur on

the same filament which is, thus, bisexual (Fig. 6A), (0.

nodulosum, O. fragile and O. himii).

(b) Macrandrous dioecious: The species in which the

two kinds of sex organs occur on separate filaments are

called macrandrous dioecious (Fig. 6BC), (0. crassum).

The filaments are thus unisexual. There are distinct male

and female filaments of normal size. They are similar

and indistinguishable in the vegetative state.

Physiologically of course they are different, one bears

antheridia (B), and the other oogonia (C). The common

examples of this category are O. aquaticum, O.

cardiacum and O. gracilius.

2. Nannandrous Some dioecious species of Oedogonium

exhibit dimorphism. The male and the female filaments

show distinct morphological differences. The antheridia

(a) are produced by special, much reduced male

filaments called the dwarf male plants or nannandria (O.

concatenatum). The latter grow epiphytically attached to

the female filaments. (Fig. 7).

Fig. 7 Oedogonium: distribution of sex organs in a nannandrous sp.

Fig. 6 (A-C), Oedogonium: Distribution of sex organs in macrandrous sp.- A. macrandrous monoecious with the antheridia (a) and oogonia (o) on the same filament; B-C. macrandrous dioecious, with antheredia on filament B and oogonia on filament C.

Antheridia (Fig. 8). The antheridia are flat, short,

cylindrical, disc-like cells or segments of the filament.

They lie in a row or series, consisting of a variable

number of 2 to 40 cells. The contents of each

antheridium commonly develop into two sperms, rarely

into one. The antheridia are either terminal or intercalary

in position. The antheridia in the macrandrous species

are developed by the rapid and repeated transverse

divisions of a vegetative cell, called the antheridial

mother cell. It is one of the cap cells (A). It divides into

two unequal cells (B), the upper much smaller

antheridium (a), and the lower

larger sister cell (b). The sister

cell divides again (C). The process

is repeated a number of times,

so that a row of antheridia are

formed. The protoplast within

each antheridium

commonly divides by a

transverse or vertical wall to form two sperms. They

escape in the same manner as the zoospores by the

transverse rupture of the cell wall, and are freed into a

thin vesicle (E), which soon dissolves. The liberated

sperms are pale-green, yellowish-green spherical bodies,

each with an apical ring of short flagella at the base of

the colourless, beak-like anterior end. They resemble the

zoospores in the type of flagellation, morphology and

method of liberation but are smaller in size and have

fewer flagella. They contain less chlorophyll. The

liberated sperms swim freely, and finally reach the

oogonia.

Fig. 8(A-F) Odogonium: stages in the development of antheridia and liberation of antherdiozooids Oogonia The oogonia are highly differentiated female

gametangia. (Fig. 9). Each oogonium develops from an

actively growing cap cell, called the oogonial mother cell.

It divides by a transverse wall into two daughter cells;

the upper or distal one is richer in cytoplasm. It contains a

larger nucleus than the lower and functions as an oogonium.

The oogonium gets distended to form a rounded or oval

structure. It always has one or more caps at the upper end

(A). The lower or sister daughter cell forms the supporting cell

or suffultory cell. It often remains undivided. In some species,

it again functions as an oogonium mother cell and undergoes

further segmentation to form a chain of two, three or four

oogonia. In the monoecious species the suffultory cell may

divide (C) to give rise to antberidia (O. nodulosum). Ohashi

(1930) reported that in O. americanum there is no supporting

cell.

The protoplast of the oogonium stores reserve food

materials and forms a single egg. The egg or oosphere has a

centrally located nucleus. Owing to the presence of chlorophyll,

the ovum is green. It is non-motile and retained within the

oogonium. In a mature ovum, the nucleus migrates to the

periphery. Prior to fertilization, the egg protoplast slightly

recedes from the oogonial wall to form a small clear patch, the

receptive spot, external to the nucleus.

In the macrandrous monoecious species, the antheridia

and oogonia are borne on the same filament. To ensure cross

fertilisation the antheridia usually develop one day after than

the oogonia. The macrandrous dioecious species have the

antheridia and oogonia developed on distinct filaments.

Fig. !9 (A-C). Oedogonium:

development of

oogonium (A and B) with the

upper one

in B ready for fertilization

The nannadrous species (Fig. 10) are

dioecious. They exihibit a curious dimorphism of sexual plants.

The sexual processes are more specialised and complicated.

The distribution of sex organs is peculiar. The oogonia are

produced on normal, large filaments (A). The antheridia (a)

are produced by special, very small filaments, the "dwarf male"

plants or nannandria (R C). The latter are produced by the

germination of peculiar type of motile spores called the

androsopres. The androspores are smaller than the zoospores

but larger than the sperms. In fact in shape and structure,

they are the small editions of the zoospores. The androspores

are produced within cells called androsporangia. The latter are

formed by the repeated transverse divisions of any vegetative

cell of a large filament (B). The androsporangia are flat,

discoid cells. They resemble antheridia of the

macrandrous species. They are formed in the same

manner, but are rather larger. The androspores are

formed singly within the androsporangia. The

androspores are motile and are provided with a subpolar

crown of flagella. When liberated, the androspore is

enveloped in a vesicle which soon vanishes. The

androspore swims in all directions till it reaches a female

filament and becomes affixed either on the wall of the

oogonium or the supporting cell. There it surrounds itself

with a cell wall. The attached androspore (L) then

germinates to form a minute dwarf male plant or

filament, called the nannandrium (RC). The nannandrium

consists of a rhizoid-like attaching or stalk cell. This one-

celled germling (rhizoidal holdfast or attaching cell) cuts

off one or more flat cells at its tip. These are antheridia

(a). The protoplast of the antheridium often divides to

form two sperms. In a few species the attaching cell

functions directly as an antheridium and produces two

sperms from its contents. Iyengar (1951), however,

reported that the antheridium of the dwarf male produces

a single sperm. The sperms are liberated either by the

disorganization of the antheridial cell or by the separation

of a lid at the top (D). The development of oogonia in the

nannandrous species is similar as in the macrandrous

dioecious species.

Fertilisation: It is accomplished both in the

macrandrous and nannandrous species by the sperm

swimming through a pore or transverse slit in the wall of

Fig. 10 (A -D), Oedgonium: sexual reproduction in nannandrous species.

the oogonium. Recent investigations have shown that the

mature egg secretes a chemical substance which diffuses

in the water and attracts the sperms. The sperms, thus

fascinated, reach the egg or the ovum. One of them,

probably the first to arrive, makes its entry and

penetrates the egg at the receptive spot. At the same

time, the protoplasmic membrane around the egg

changes in nature so as to prevent the entry of any more

sperms. The male ·and the female nuclei in the egg fuse

to form the diploid nucleus. The fertilized egg or zygote

soon secretes a heavy wall around it. Eventually the

oospore is liberated by the decay of the oogonial wall and

rests on the mud at the bottom of the pond, where it

enters upon a further period of rest.

Germination of oospore: Prior to germination, the

diploid oospore nucleus undergoes zygotic meiosis to

form four haploid nuclei (Hoffman, 1965). The protoplast

loses its red colour and turns green. The haploid nuclei

are organised into four uninucleate daughter protoplasts

by cleavage of the oospore protoplast. Soon after, each

of the 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. According to Smith (1950), the oospore

wall ruptures to liberate the mature, motile meiospores

or meiozoospore. They are, at first surrounded by a

delicate vesicle, which soon disappears. Fritsch (1935),

however, reported that in some species, the oospore wall

ruptures and the naked protoplast emerges. It is

liberated into a vesicle where it divides. The daughter

protoplasts soon develop flagella to become

meiozoospores.

Graphic life-cycle of Oedogonium