Sexual Reproduction in Oedogonium

The sexual reproduction in Oedogonium is of advanced oogamous

type. Male and the female gametes differ morphologically as well as
physiologically. The male gametes are produced in antheridia and the
female gametes are produced in oogonia.

The factors influencing sexual reproduction are alkaline medium,

deficiency of nutrition, light and dark periods and increased
temperature.

Depending upon the nature of antheridia producing plants,

Oedogonium species are of two types:

(i) Macrandrous:
If antheridia are produced on normal size plant, Oedogonium forms
are called macrandrous. Macrandrous species may be monoecious or
dioecious. In monoecious macrandrous species antheridia and
oogonia are produced on the same plant e.g., O. fragile, O. hirnii, O.
nodulosum etc. In dioecious macrandrous species antheridia and
oogonia are produced on separate male and female plants of normal
size.

(ii) Nannandrous:
The female or oogonia bearing plants are normal. The antheridia are
produced on special type of small or dwarf plants, known as Dwarf
males or Nannandria. The dwarf males are formed by
androspores which are produced in androsporangia.

If androsporangia and oogonia are formed on same plant, the

Oedogonium forms are called gynandrosporous e.g., O.
concatinatum. If androsporangia and oogonia are formed on different
plants, Oedogonium forms are called idioandrosporous e.g., O.
confertum, O. iyengarii and O. setigerum. According to some
algologists, nannondrous species are more primitive.
Antheridia:
(i) In macrandrous forms:
The antheridia develop on normal filaments, terminal or intercalary in
position. The initial cell which gives rise to antheridia is called
antheridial mother cell. It is normally a cap cell. The antheridial
mother cell divides by transverse division to form an upper smaller
cell called antheridium and a lower larger cell called sister cell.

The sister cell divides repeatedly to form a row of 2-40 antheridia

(Fig. 6 A). The antheridia are broad, flat, short cylindrical, uninuleate
cells. The contents of an antheridial cells divide either longitudinally
or transversely into two and each metamorphoses into an antherozoid.

The two antherozoids are positioned side-by-side or one above the

other if divisions are longitudinal and transverse respectively. The
antherozoids are liberated in the same fashion as zoospores (Fig. 6 B).
The liberated antherozoids are pale green or yellow green, oval or
pear shaped.

Antherozoids are motile, about 30 sub-apical flagella present at the

base of beak or hyaline spot (Fig. 6 C). They swim freely in water
before they reach oogonia and take part in fertilization. The
antherozoids are similar to zoospores in structure but these are
smaller than zoospores.
(ii) In nannandrous forms:
The antheridia are formed on short or dwarf male plants called dwarf
males or nannandria (Fig. 7 G). The dwarf male filament is
produced by the germination of a special type of spore known as
androspore.

The androspore is produced singly within an androsporangium.

Androporangia are more or less similar looking to the antheridia of
macrandrous forms and are produced in a similar manner from a
mother cell (Fig. 7 A, B).

The androsporangia are flat, discoid cells slightly larger than

antheridia. Each androsporangium produces a single androspore just
as in the case of zoospore. Liberation of androspore is similar to that
of a zoospore. The androspores look similar to zoospore except for the
smaller size. The androspores are motile and have a subpolar ring of
flagella.

After swimming about for some time, the androspore settles on

oogonial wall e.g., O. ciliatum or on the supporting cell e.g., O.
concatenatum. The androspore germinates into a dwarf male or
nannandrium. Germlings at one celled stage may divide and produce
two antherozoids e.g., O. deplandrum, O. perspicuum (Fig. 7 C-G).
The nannandrium or dwarf male can be a few cells long. It has a
basal attaching cell the stipe and all other cells are antheridial cells.
Protoplasm of each antheridial cell divides to form two antherozoids
which are similar to antherozoids of macrandrous species.

According to Iyengar (1951) the antheridium of nannandrium

produces single antherozoid. The antherozoids are released by
disorganization of antheridial cell or through the opening. Pascher
considered the nanandrous forms as primitive and macrandrous as
specialized but a large number of phycologists consider that
nannandrous species have been evolved from macrandrous species.
Oogonia:
In Oedogonium the female sex organ oogonia are highly differentiated
female gametangia. These are mostly intercalary but sometimes can be
terminal e.g., O. palaiense.

The structure and development of oogonium is identical in

macrandrous and nannandrous species. Like antheridia any freely
divided or actively growing cap cell functions as the oogonial mother
cell. The oogonial mother cell divides by transverse division into two
unequal cells, the upper cell and the lower cell.

The upper larger cell forms oogonium and the lower smaller cell
function as supporting cell or suffultory cell. In some species the
oogonial mother cells directly forms the oogonium. Supporting cell is
absent is O. americanum.

In monoecious species the suffultory cell may divide to form

antheridia. The upper cell contains more cytoplasm, food and enlarges
into spherical or flask shaped oogonium. The oogonium also secretes
growth hormones which induce suffultory cell to increase in size (Fig.
8 A-C).
The protoplast in oogonium metamorphoses into a single egg or
ovum. The ovum is non-motile, green due to chlorophyll and has a
central nucleus. As the ovum matures, the nucleus moves to periphery,
the oosphere retracts slightly from the oogonial wall and develops a
hyaline or receptive spot just outside the nucleus. The receptive spot
receives antherozoids for fertilization.

At receptive spot a pore is formed by gelatinization of wall . In some

species a mucilage drop is extruded through opening to attract
antherozoids.
In macrandrous monoecious species, where antheridia and
oogonia develop on the same plant, the Oedogonium species are
protogynous i.e., the development of oogonia takes place before
development of antheridia to ensure cross-fertilization.

Fertilization:
The mature egg secretes chemical substance or mucilage to attract
antherozoids or the antherozoids may enter oogonium through the
slit. The antherozoids swim through the opening of oogonial wall and
enter the egg through hyaline receptive spot (Fig. 8 D-F). Only one
male antherozoid is able to fuse with ovum.

After plasmogamy and karyogamy the male nucleus and female

nucleus fuse to form a diploid zygote nucleus. The zygote secretes a
thick wall around itself and forms oospore. The colour of the oospore
changes from green to reddish brown. The oospore is liberated by the
disintegration of oogonial wall.

Germination of oospore:
Oospore is a resting spore but sometimes it can germinate directly.
The period of rest for oospore may be a year or more.

According to Mainx (1931) the zygote may require chilling before

germination. The diploid oospore divides meiotically to form four
haploid daughter protoplasts. Each daughter protoplast
metamorphosis into a zoospore also called as zoomeiospore.

The zoomeiospores are liberated in a vesicle (Fig. 9 A). Soon the

vesicle disappears and as in asexual reproduction the zoospores
develop to form Oedogonium plants.

In some cases out of four nuclei a few may degenerate forming less
than four zoomeiospores. In heterothallic forms e.g., O.
plagiostomum, out of four two give rise to male and the two give rise
to female plants.
In Oedogonium the thallus is haploid and the life cycle is haplontic
type. The diploid stage in life cycle is only zygote. It occurs for a short
period. The zygote or oospore undergoes meiosis to make four
meiozoospores which again form haploid Oedogonium thalli.