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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