Volvox is a genus of chlorophytes, a type of green algae. Volvox are one-celled algae that live
together in a colony.It
forms spherical colonies of up to 50,000 cells. They live in a
variety of freshwater habitats, and were first reported by Antonie van Leeuwenhoek
in 1700. Volvox developed its colonial lifestyle 200 million years ago.Volvox is a green alga that can be
considered as an aggregation of unicellular Chlamydomonas-like green algae: a
colony. The daughter cells of such unicellular algae doe not separate, but they are kept together by a gel mass. The colony which consists of merely 1 n cells, becomes larger by growth of individual cells and by -haploid- mitotic divisions. The daughter cells on their turn are kept in place by connecting arrays of plasma. This is how in Volvox hollow spheres of 500 up to 60 000 uniform cells can arise! The outwards oriented flagella generate a rotatory movement.
VEGETATIVE STRUCTURE
- Plant body is multicellular motile coenobium (colony with fixed number of cells arranged in a definite manner).
- The central hollow cavity is filled with mucilage.
- Each cell is biflagellate.
- Alga rolls on water surface by joined action of flagella.
- Each cell of coenobium performs its own function of nutrition, respiration & excretion.
- Individual cell of Volvox is spherical, elliptical or oval & biflagellate. The flagella are attached at anterior end.
- The cell has outer mucilaginous layer.
- A number of central vacuole are distributed near the surface of protoplast single cup shaped or curved plate like chloroplast.
- Each cell has single eye spot at anterior end.
- Single nucleus associated with nueromotor apparatus (blepharoplast + rhizoplast + centromere).
- Chloroplast is associated with 1-2 pyrenoids.
HABITATS
Volvox is a freshwater alga and is found in
ponds and ditches, even in shallow puddles."The most favorable place to
look for it is in the deeper ponds, lagoons,
and ditches which receive an abundance of rain water. It has been said that
where you find Lemna, you are likely to find Volvox;
and it is true that such water is favorable, but the shading is unfavorable.
Look where you find Sphagnum, Vaucheria, Alisma, Equisetum fluviatile,
Utricularia, Typha, and Chara. Dr. Nieuwland reports that Pandorina, Eudorina and Gonium are commonly found in summer as
constituents of the green scum on wallows in fields where pigs are kept. The
flagellate, Euglena, is often associated with these
forms."
DESCRIPTION
Volvox cells have
chlorophyll and make their own food by photosynthesis.Volvox is the most developed in a series of
genera that form spherical, subspherical, ellipsoidal or ovoid,colonies. Each
mature Volvox colony is composed of numerous flagellate cells similar to Chlamydomonas, up to 50,000 in total, and embedded
in the surface of a hollow sphere or coenobium containing an extracellular matrix
made of a gelatinous glycoprotein. The cells swim in a coordinated
fashion, with distinct anterior and posterior poles. The cells have eyespots, more developed near the anterior,
which enable the colony to swim towards light. The individual algae in some
species are interconnected by thin strands of cytoplasm, called protoplasmates. They are known
to demonstrate some individuality and working for the good of their colony,
acting like one multicellular organism. The flagellates on its outside resemble
Euglena.Two to 50 large reproductive cells
(gonidia) situated in posterior 1/2 to 2/3 of colony. Each cell enclosed by gelatinous
sheath which is distinct or confluent (species dependent). Somatic cells
spherical, ovoid or star-shaped, each with two equal flagella, a stigma, two
contractile vacuoles at base of flagella, and a cup-shaped chloroplast with
single pyrenoid. Cytoplasmic strands between cells are thick, thin or absent
and this is species dependent. Asexual reproduction by autocolony formation,
each reproductive cell dividing successively to form a hollow, spheroidal
plakea, which then inverts to form a daughter colony. Sexual reproduction
oogamous; in monoecious species the sexual colony with both sperm packets
(bundles of sperm) and eggs. In dioecious species, male colonies with
androgonidia which divide successively into sperm packets; such males may be
markedly reduced in size (dwarf male) or nearly as large as asexual colonies.
The female colony has eggs, whose number is nearly the same as that of gonidia
in asexual colonies (facultative female) or much larger (special female). After
fertilization zygotes develop a heavy cell wall that may be ornamented with
reticulation or spines. Upon germination, zygotes produce a single biflagellate
gone cell. Volvox is cosmopolitan in freshwater; V. aureus is
common. Volvox was established with V. globator and more than 12 additional
species have been described. The genera Besseyosphaera, Campbellosphaera,
Merrillosphaera, Copelandosphaera and Janetosphaera were
described by Shaw but are now included in Volvox. Each of these genera
was later considered to be a section of Volvox, with Euvolvox [=Volvox]
including species referable to Volvox by Shaw. Later, the sections Campbellosphaera
and Besseyosphaera were rejected and 18 species were recognized in four
sections based on differences in the gelatinous matrix, shape of somatic cells,
and cytoplasmic strands connecting cells in the colony. Embryogenesis of V.
carteri has been studied in detail, and chemical induction of sexual
colonies were described for V. aureus, V. carteri, V. rousseletii, V. gigas,
V. observus and V. capensis. Inducers are species-specific
glycoproteins of ca. 30,000 MW produced by sperm, except in V. capensis
where L-glutamic acid is reported. Ultrastructure of flagellar apparatuses in V.
rousseletii and V. carteri implied a remote relationship. The extracellular
matrix of the four sections of Volvox has also been studied
ultrastructurally, and a system of nomenclature was proposed in which the
matrix was divided into four main zones.
Volvox colony: 1) Chlamydomonas-like cell, 2) Daughter colony, 3)
Cytoplasmic bridges, 4) Intercellular gel, 5) Reproductive cell, 6) Somatic
cell.
LIFE CYCLE WITH ASEXUAL AND SEXUAL REPRODUCTION
In Volvox both
asexual and sexual propagation occurs. In large colonies some kind of
differentiation occurs between vegetative and not vegetative cells, although
the cells are all haploid.Asexual reproduction takes place during favourable
condition, but the sexual reproduction occurs during unfavourable condition
i.e., towards the end of summer months.
ASEXUAL REPRODUCTION
A few cells at
the posterior side of the coenobium enlarge about 10 times. The cells withdraw
theirs flagella and become more or less round. They are pushed inside the
colony during their development. These cells are called gonidia or
parthenogonidia or autocolonyinitials. The gonidium is separated from the
vegetative cells by its position and size.
DEVELOPMENT OF DAUGHTER COLONY
The gonidium
undergoes repeated division of about 15 or more times and can develop more than
3200 cells. Those cells ultimately form a colony.Initially the gonidium
undergoes longitudinal division with respect to colony and form to cells. The
second division is at right angle to first one and forms 4 celled stage. These cells again divide
longitudinally and form 8 celled stage. The cells are arranged in such a
pattern that their concave inner surface face towards the outer side of the
colony. This stage is called plakea stage or cruciate plate. The 4th
division forms 16 celled stage and at the time it becomes a hollow sphere eith
an opening towards the outer side, called phialopore. The division of cells
continues up to the number specific for a particular species. The cells now
face towards the centre. This group of cells then undergoes inversion through
the phialopore by which normal pattern of the colony is achieved.
INVERSION
During inversion
a constriction appears at point opposite to phialopore. This constricts region
becomes pushed gradually towards the phialopore. Simultaneously the phialopore
becomes enlarged, through which the lower part comes out and the edges of
phialopore hang backwards. With the help inversion the anterior side of the
cells changes their position from inner to the outer side and the position of
phialopore becomes reversed i.e. change its position from outer to inner side.
The phialopore gradually closes down and a complete hollow sphere is formed.
After completion of inversion the cells secrete their own gelatinous cell wall
and each develops two flagella. Thus the daughter colony is formed. Many such
colonies may develop in a coenobium and they swim freely inside the gelatinous
matrix of the mother coenobium. Later on the daughter coenobia come out by
rupture or disintegration of the mother colony. In same species like v. carteri
and v. Africans daughter colonies of 2-4 generations may remain within the
mother coenobium.
In preparation to sexual reproduction some
vegetative cells enlarge to form egg cells, while other divide to form
antherozoids. These sperm cells reach the egg cell by swim and fertilization
can occur. The zygote forms a thick and hard wall and remains in the cavity of
the colony until the later dies. After being liberated from its
"shell", the naked zygote goes into meiosis. The products of meiosis
can build a new colony.
SEXUAL REPRODUCTION
Sexual
In nature Volvox reproduces sexually at least once each year when
temporary ponds where the organism lives start to dry out in the heat of late
summer; the stimulus for switching from the asexual to the sexual mode of
reproduction is known to be a sex-inducing pheromone, a 32-kDa glycoprotein
triggers sexual development of gonidia at concentrations as low as 10(-16) M.
Sexual induction Gonidia that have been exposed to the sex-inducing pheromone
for at least 6–8 h before the initiation of embryonic cleavages modify their developmental
program and produce sexual progeny containing immotile eggs or motile sperm,
depending on the genetic sex of the individual; the sexual cycle is
initiated by a heat shock that causes the somatic cells of the asexual Volvox
spheroid to produce the sex-inducing pheromone; the level of pheromone is then
further amplified by the ability of sperm cells to produce more sex-inducing
pheromone. In volvox sexual reproduction is
oogamous. In monoecious species both types of gametes (male and female) are
borne by same colony while in dioecious species these are borne by separate
colonies.
The said gametes are borne by enlarged cells called gametangia (gamete bearing
cells) which lie in posterior side of the colony. Some of these cells are somatic cells (smaller spheres) and
androgonidia (larger spheres) arise in a 1:1 ratio, the protoplasm of which divides many
times and produces a cluster of minute biciliate male gametes called
antherizooids or sperms;
each containing 64 or 128 sperm; when the gametes are mature,
sperm packets are released into the surroundings,while other cells are oogonia or female reproductive organs,
the proplast of which forms a single large female gamete called egg or ovum.
Egg is large passive and non motile while sperms are very minute, active and
motile sperms may be in a plate like colony escaping from mother colony as a
unit or these may be arranged to form a hollow spore. In former case the unit
as it approaches an egg breaks up into individual sperms and in the latter case
sperms are liberated singly. Mode of fertilization is oogamous. Sperms swim and
enter by gelatinous sheath into organism lying in mother colony and one of the
fuses with egg. Thus the fertilization is affected.ZYGOTE:- After fertilization the zygote clothes itself with a thick spiny wall. It is set force from the mother colony only after the decay or disintegration of latter zygote sinks to bottom of pool of water and then after a period of rest is germinates with the approach of favourable reason. The protoplast of the zygote undergoes reduction division prior to germination. In some species the protoplast of the zygote divides and forms a new colony directly, whiten in others it forms a single bicliate zoospore which escapes by the rupture of the zygote wall and swims away. The free swimming zoospore then divides and forms new colony.
EVOLUTION
Ancestors of Volvox
transitioned from single cells to form multicellular colonies at least 200 million years ago, during the Triassic period.
An estimate using DNA
sequences from about 45
different species of Volvox and related species suggests that the
transition from single cells to undifferentiated multicellular colonies took
about 35 million years.
SIZE:- 350 to 500 μm (Two or three volvox cells
would fit in 1 mm.)
MOVEMENT :- Each
volvox cell has two flagella. The flagella beat together to roll the ball
through the water.
GROWING AND OBSERVING VOLVOX
Microscopists
who are interested in observing Volvox should try to investigate water samples
from ponds and puddles. It is also possible to grow Volvox at home. Volvox
likes to grow in nutrient-rich water. Dilute some plant fertilizer in water and
add some pond water containing Volvox (or other green algae that you want to
grow). Place the container on the window sill for several days but prevent
direct sunlight as this may cause overheating, and drives out the CO2 for
photosynthesis from the water. Alternatively, you can also use a plankton net
to catch the colonies.
For making permanent
mounts, it’s probably best to use a water-based mounting medium such as
glycerin gelatin. Solvent based media may dissolve the chlorophyll of the
chloroplasts and may cause the cells to lose water and shrink.
PHYLOGENY
The
family Volvocaceae contains several genera of green flagellated algae
that are intermediate in size and complexity between unicellular Chlamydomonas
and Volvox. Molecular phylogenetic analysis indicates that the family is
monophyletic, and shares common ancestor with Chlamydomonas that existed
about 50-200 million years ago. Thus, these algae provide great opportunity to
analyze evolutionary pathway leading from unicellularity to multicellularity
with complete division of labor.
Genomes of Chlamydomonas
and Volvox are remarkably similar: both genomes contain ~14,500
protein-coding genes, and the Volvox genome is slightly larger, 138-118
megabases, mostly because of its greater transposon/repetitive DNA content.
0 comments:
Post a Comment