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MONOCYSTIS
Structure
Monocystis is an extracellular parasite in the seminal
vesicles of earthworm.
Shape and Size
The adult feeding and growing stage of Monocystis is known
as trophozoite stage. The young trophozoite is small and ellipsoidal. It remains embedded in a group of developing
spermatozoa. It appears ciliated as the tails of the dead sperms remain
attached to it.
The mature trophozoite is elongated,
spindle-shaped and dorsoventrally flattened worm-like creature. It is
visible to the naked eye as a white thread.
Pellicle
The body is bounded by a thick, firm but elastic pellicle. It is formed of nitrogenous matter.
It may be striated or thrown into ridges and furrows or with hair-like
processes. It contains contractile microtubules.
Cytoplasm
The cytoplasm is differentiated into two layers:
1. Cortex or ectoplasm:
It is the outer clear layer of cytoplasm which is further differentiated into
three layers:
(a) the outer layer-epimyocyte,
(b) the middle layer-sarcocyte
and
(c) the inner layer-myocyte
The myocyte is traversed with longitudinal and transverse
contractile fibrils, the myonemes.
These help in the metabolic contraction of body and wriggling or gliding movement.
2. Medulla or endoplasm:
It is the inner granular mass consisting of minute granules of paraglycogen (the reserve food), fat
globules and sometimes volutin
(protein).
3. Nucleus
The nucleus is large, vesicular and ovoid or spherical. It
lies towards the anterior end and is bounded by a distinct nuclear membrane. It
contains one or sometimes two nucleoli,
endosomes or karyosomes.
The parapodia, cilia, flagella, cell mouth etc. and food
vacuoles and contractile vacuoles are absent due to parasitic mode of life.
Life-Cycle
Life-cycle of Monocystis is monogenetic. It is completed within one host (earthworm) the
life-cycle can be distinguished into following phases:
A. Gamontogamy
It is sexual phase of life-cycle. It includes
(1) pairing of gamonts, syzygy
(ii) formation of gametes, gametogony
(iii) syngamy or fertilisation
1. Syzygy - After
leading a short extracellular existence in the seminal vesicles and attaining
maturity, the trophozoite changes
into a spherical or oval gamont or
gametocyte. These come together and closely oppose each other. These secrete a
common double-layered gameto-cyst. It is hard, chitinous and protective. Its
rigid and thick outer layer is known as ectocyst,
while the inner thin membrane as the endocyst.
The gametocysts remain closely oppressed together inside the gameto-cyst but
they never fuse and are separated by a distinct wall. This type of association
of the gametocytes is known as syzygy
2. Gametogony - The
nucleus in each gametocyte divides by repeated divisions. The last one is
reduction division while others are simple mitotic. As a result a large number
of haploid nuclei are formed in each gametocyte. These get arranged at the
periphery and protrude out. The gametocyte now assumes appearance of a berry.
Each nucleus gets surrounded by a little cytoplasm to form the gamete. These are liberated by the
rupture of intervening wall of gametocytes.
fig. Life- cycle of Monocystis
The gametes thus formed in the two gametocytes are similar
in their morphology but of two different sizes- small motile microgametes and large, less motile and
somewhat rounded macrogametes.
3. Syngamy - The gametes so formed move freely for some time and then pairing occurs between the gametes of two different gametocytes. Although the gametes are similar in appearance and structure but fusion always occurs between the gametes formed from the different gametocytes. The process of their fusion is known as syngamy. As a result of fusion zygotes are formed which contain diploid number of chromosomes as found in the trophozoite.
4. Sporogony- The
development of the zygote occurs within the gametocysts . Each developing
zygote is known as sporoblast. It becomes oval and secretes a tough,
single-layered chitinuous sporocyst
or spore case. Thus sporoblast change into spores. Later on, the sporocyst
becomes boat-like and is termed as pseudonavicella.
The nucleus inside the sporocyst undergoes three successive mitotic divisions.
Simultaneously, the cytoplasm also divides. As a result eight sporozoites are
formed inside each sporocyst. These are elongated and sickle-shaped.
The spores containing sporozoites are set free inside the
seminal vesicles by the rupture of gametocysts. The release of sporozoites and
their further development takes place only inside the alimentary canal of New Host.
Transmission of
Spores and Infection of New Host
The transmission of spores from the infected into the fresh
host occurs in following ways:
1. During copulation-The spores are transmitted
from one worm to the other during copulation, because these are seen in the
sperm ducts and cocoon.
2. Due to death of
host-As a result of death and decay of the host body, the spores are
liberated into the soil and they make their way into the alimentary canal of
other worms which feed on that mud.
3. By birds- When
an infected earthworm is eaten by a bird, the spores are liberated in its
alimentary canal and come out unaltered with its faeces. These along with mud
enter the alimentary canal of the worm.
Inside the alimentary canal of earthworm, the spore wall
gets dissolved by the digestive juices and sporozoites are liberated. These
penetrate the wall of alimentary canal and enter seminal vesicles. Each
sporozoite enters a sperm morula and starts trophozoite phase of life-cycle.
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