TEXT Systematic Position: Kingdom: Plantae Phylum:Chlorophyta

TEXT
Systematic Position:
Kingdom: Plantae
Phylum:Chlorophyta
Class:Chlorophyceae
Order:Volvocales
Family:Volvocaceae
Genus: Volvox L.
D:
INTRODUCTION,
IMPORTANCE:
OCCURRENCE
AND
Volvox is a Chlorophyte, or green alga. It exists
as a grand spherical colony. Each little alga within
the colony bears two flagella, whip-like hairs. The
individual alga are connected to each other by thin
strands of cytoplasm that enable the whole colony
to swim in a coordinated fashion. The individual
alga also have small red eye spots.
The colonies even have what we could call a
front and rear end. Or, since Volvox resembles a
little planet, a 'north and south pole'. In the
northern region the eyespots are more developed.
This helps the colony to swim towards the light.
This differentiation of cells make Volvox quite
unique. It is a colony that comes really close to
being a multi-celled organism.
Some Volvox colonies (A,B, & C) showing
clustered units in different stages of development:
“A”
“B”
“C”
Volvox can be found relatively easy. They need
quite clean nutrient rich water (Eutrophic) and it
has to be warm so summer is the time of the year
for a good Volvox catch. The best thing to do is to
use a plankton net but if you are lucky you can also
find them by squeezing pond scum. In a jar of pond
water they will swim towards the light so they are
often easy to find near the surface.
The Volvox has an interesting method of
reproduction which are both asexual as well as
sexual reproduction. One of the first things that
one notices on Volvox is that most colonies have
spheres inside. These are 'daughter' colonies,
called gonads. It is a means of asexual
reproduction. The gonads grow from cells around
the equator of the colony. These cells enlarge and
undergo a series of cell divisions until they form a
small sphere. There is only one complication, the
flagella will be on the inside of the new sphere. It
has to turn itself inside out so the flagella will be
situated towards the outside of the colony.
Volvox also has a sexual reproduction. As in the
asexual reproduction special cells are formed
around the equator. These cells develop into
special germ cells. Male and female colonies form
different germ cells. Sperm cells are formed by
division. The female germ cells do not divide. They
enlarge to form an ova.
Most species have male and female colonies. In
some species the colonies are hermaphroditic. They
are able to form sperm as well as ova. But not at
the same time so self fertilization is prevented.
This images “a” shows packages of sperm cells.
They will soon swarm out in search of ova in other
Volvox.
“a”
“b”
These “b” are fertilized egg cells, the zygotes,
of Volvox globator. After fertilization the egg cells
form a hard protective layer. Zygotes are able to
withstand harsh conditions and will survive a
winter.
Sometimes one can find inhabitants within the
colony. There is a small rotifer, called Proales
parasita that lives inside Volvox and feeds on the
cells.
The rotifer is able to detect when the colony
becomes immobile. When we observe Volvox under
the cover slip it may be trapped by the pressure of
the cover-slip. Proales will quickly make a hole and
escape.
Proales inside a Volvox colony
E. TYPICAL LIFE CYCLE OF VOLVOX:
Volvox is the most evolved genus of coenobial
form. It is normally found in fresh water pools and
ponds or in temporary water canals. Each colony is
usually spherical and sometimes appears as a small
pinhead on the surface of water bodies which are
predominantly stagnant. Its basic morphological
feature distinct from other members of the family
is the large number of cells per colony which is
visible even to a naked eye and depicts
phototactics.
Each colony is composed of a large number of
vegetative or somatic cells and relatively few
aflagellate reproductive cells known as gonidia. All
are arranged at the periphery of the coenobium.
The average number of cells per colony varies
from 500-50,000. Each colony irrespective of the
number of cells is spherical to ovoid and the
biflagellate cells lie in single layer within gelatinous
matrix.
Each cells is surrounded by its own gelatinous
sheath which may either be distinct from one
another or may be confluent with each other. If
distinct from each other the cells are angular due
to mutual compression and hexagonal in shape.
Electron micrograph have revealed a thickened
layer of unknown composition lying adjacent to the
plasma membrane. This is sometimes labeled as
cell wall. In some species cells are joined to one
another by conspicuous or delicate cytoplasmic
strands
Fig “a”
Fig “b”
Fig. “a” “b” Basic Morphological features of
Volvox colonies
Fig. “c” A close-up of individuals cells within the
colony, 2 flagella and red eyespot are visible
Individual cells of each colony reveal broad
intercellular connections which gives them stellate
appearance. The whole coenobium moves and
rotates by means of flagella of individual cells. Most
of the coenobial cells are vegetative not having a
capacity to give rise to progenial cells.
Each individual vegetative cell is biflagellate
with each flagella having a contractile vacuole at
its base or 2 to 5 contractile vacuoles irregularly
distributed at the anterior end of the cell.
The chloroplast of each cell is either laminate
or cup shaped. The nucleus is single pyrenoid and
centrally located. The eye spot is anteriorly
arranged. The eyespots also depict polarity wherein
those of anterior end are somewhat larger then
those of posterior.
Fig. “a” The individual alga are connected by thin
strands of cytoplasm.
“A”
“B”
“C”
“D”
“E”
“F”
Figs. A-F. Depicting various morphological characters
of Volvox of fresh waters
Complete
colonial attitude
Volvox
Body
depicting
Volvox colony 1) Chlamydomonas- like cell, 2)
Daughter colony, 3) Cytoplasmic bridges, 4)
Intercellular gel, 5) Reproductive cell, 6) Somatic
cell.
F. ASEXUAL REPRODUCTION:
Asexual reproduction in Volvox coenobium
starts with differentiation of asexual reproductive
cells known as Gonidia. The Gonidia formation
begins with the disappearance of flagella and
gradual thickening of the membrane. These
individuals producing daughter coenobia under go
division within their membranes to form the full
complement of cells of the future coenobium and,
after the latter’s liberation, there if no further celldivision until the new reproductive phase sets in.
“a”
“b”
“c”
“d”
Fig. a- Fig. d. Different stages of asexual
reproduction of Volvox
Usually a number of gonidia divide at the same
time,
many
new
colonies
are
formed
simultaneously. In each division three successive
longitudinal divisions result in formation of 8-celled
cruciate plate of plakea stage.
After the division ceases, the young
progenial colonies turns inside out by inversion
through a small pore
or opening known as
phialopore. The individual cells then develop
fagella. The new coenobium may or may not
immediately escape from the parental coenobium.
On an average each parental coenobium
can give rise to about 10-15 progenial colonies.
Fig “a” A daughter colony turns itself inside
Fig
“b” 'Birth' of daughter colonies after
out so the flagella will be orientated
the older colony disintegrates.
towards
the
outside
‘Grand daughter' colonies are
already developing!
of
the
cell.
Fig. A. The complete asexual reproduction
process of Volvox depicting complete formation of
progenial colonies within parental colonies (a-n)
G. SEXUAL REPRODUCTION:
The sexual reproduction in Volvox is strictly
oogamous. Depending upon various species and
strains coenobia may contain either eggs or sperms
(dioecious)
or
they
may
contain
both
(monoecious). Both kinds of gametes are produced
in small numbers and develop especially from cells
of posterior half of coenobium.
The egg cells are usually dense and darker
in
appearance
then
gonidia.
They
enlarge
considerably without undergoing any division and
lose their flagella during development.
Cells
forming
male
gametes
or
antherozoids divide sucessively giving rise to
numerous
spindle
shaped
biflagellate
antherozoides. Non motile female gametes are
much larger and are surrounded by gelatinous
sheath.
The group of sperm cells are liberated
from their parental colonies and presumably by
chemotactic stimulation swim to the surface of
female colonies.
The sperm cells finally dissociate into
individual sperms and penetrate female colonies
and fertilize the eggs. The resulting zygote divides
by meiosis and a new coenobium is formed through
the formation of single zoospore.
Fig
“a”
Packages
of
Fig “b” Fertilized egg cells.
sperm
cells
Fig: A-G Sexual reproduction of Volvox