ZOLOOGY LESSION-4 MONOCYSTIS

                         

               MONOCYSTIS

      

            Monocytis (Gr., Mono=Single+kystis=bladder) is a gregarine parasite of sporozoa. Its life cycle is completed in one host only. There are two forms of gregarines are recognized, which exhibits sexual reproduction only i.e., acephaline and cephaline. In acephaline forms, the body is not divided in to chambers and anterior in does not bear and organ for attachment. Thus, the monocytis is an but the cephaline forms possesses the chambered body as well as the organ for attachment. Thus, the monocystis is an acephaline gregarine. The various species of Monocystis are M. agilis; M. pheretimi; M.lumbrici; M.lloidi; M.beddardi and M. bengalensis, which parasitize the common earthworms. Besides Monocystis, there various allied genera like Nematocytis. Enterocytis, Rhabdocydtis; Dendrocystis; Apolocystis; Zygocystis; Pleurocystis; Cephalocystis; Dirhynchocystis; etc are also the parasitize the common earthworms. In case of mixed infection of an earthworm, it becomes difficult to recognize the stages of the parasite of respective genera. However, the tropozoites of different genera have characteristic shapes by which they may be identified (Fig.4.1). A generalized account of the genus monocystis has been described here, with particular reference to M. agilis.

                           MONOCYSTIS     AGILIS

                     SYSTEMATIC         POSITIONS

                         Phylum                 :      Protozoa

                   Sub- phylum    :      Sporozoa 

                    Super-class     :     Telosporca

                     Class            :     Gregarinia   

                     Order          Eugregarinida                         

                   Genus       :      Monocystis      

                 Species       :       Agilis

                H A B I T  A N D  H A B I T A T

         Monocystis lives as an endoparasites of earthworms and occur in their coelom and seminal vesicles. Its infection is so wide that practically all mature earthworm parasitized by this parasite.

 

          E X T E R N A L F E A T U R

     The adult mature stage of monocystis is called trophozoite. It is the feedind stage of the parasite. The young lives in the sperm morula or soerm ball of the host. Sometimes, it is seen surrounded by a thin layer of degenerating sperm morula, to which the tail of dead spermatozoa are attached and producing ciliated appearance. But, soon the sperm tails are detached from its body, the tropozoite becomes free.

             Shape  and  Size------A full grown mature trophozoite is elongated, spindle-shaped, flattened and worm-like organism with tapering body ends.

            It measures upto 500 in length and about 65 in width at its broadest portion. It is visible to the naked eye but it structural details can only be observed with the help of microscope.

               

            Pellicle----- The body of trophozoite is externally covered by a definite thick, smooth and firm pellicle. They are modified in different ways, i.e., it may be striated or thrown in to rides and furrow. It longitudinally arranged contractile microtubules.

          The cytoplasm is sharply differentiated into an outer clear ectoplasm and an inner granular endoplasm___ (i) Outer epicyte, (ii) middle sacrocyte, and (iii) inner myocyte. The inner myocyte layer consists of longitudinal and transverse contractile fibrils, called myonemes. The myonemes perform the characteristic metabolic movement of trophozite. The endoplasm contains numerous granules of paraglycogen  (a special form of glycogen) as a reserve food, fat globules and sometimes Volutin granules (a protein rich in phosphorse and nucleic acid).

         Nucleus___  The nucleus is single, large vesicular with spherical or ellipsoidal in shape. It placed  more anterior in the upper half of the body the nucleus is surrounded by a delicate nuclear membrane, which bears pores. The nucleoplasm contains usually one nucleolus or karyosome which stains deeply with iron heamatoxylin; sometimes more nucleoli four chromosomes, represent the haploid number.

          Structure   of  Sporozoite------- The electron microscopic structure of sporozoite reveals that it possesses all typical structures seen in protozoans, i.e., golgibody, mitochondria and nuclear components. A pair of elongated reservoir like are seen which secretes some tissue lysing substances, that help the trophozoite in peneterating through the host tissues. The anterior and also has conoids and micronemes , whose function is still unknown. The pellicle shows longitudinally arranged contractile microtubules.

 LOCOMOTION  

       Monocystis does not posses special organs or the locomotion, because is parasitic life and living in a medium having enough nourishment. The animal wriggles about like a worm with the help of rhythmic contractions and relaxations of myoneme fibrils of the ectoplasm and elastic pellicle. Such a type of movement is the characteristic of monocystid gregarine, it is known as gregarine movement, which are like euglenoid movement.  

       NUTRITION

         In Monocystis, the nutrition is saprozoic. It absorbs its nourishment by osmotrophy through the general body surface from the fluid of the seminal vesicle. The monocystis secretes digestive enzymes from its body which digest cytoplasm and developing sperms of the seminal vesicles, and the digested products are absorbed through the body surface by osmotrophy. The excess food material is stored as a paraglycogen granules in the endoplasm.

        RESPIRATION

       In Monocystis, the actual mechanism of respiration is not known. It is believed that in Monocystis the respiration is performed by diffusion through its pellicle from the cell contents of sperm morula and fluid of seminal vesicle of the host. The mitochondria synthesize respiratory enzymes for oxidation reactions involved in aerobic conversion of pyruvic acid to CO₂ and water. The CO₂ is diffuses out from the body and are finally eliminated through the blood of the host.

         EXCRETION 

      The excretory nitrogenous waste products are produced as a result of metabolism. These waste products are produced are diffuse out of the body of Monocystis in to the body of the host and are finally eliminated by the excretory organs of the host.

              REPRODUCTION

      Monocystis reproduces sexually and the process is always followed by asexual reproduction. It is the fact that both the processes are interdependent.

        LIFE CYCLE

      Monocystis is a monogenetic, i.e., its life cycle is completed in a single host, earthworms. Only zygotes are diploid while all other stages like gamonts and gametes are haploid. The life cycle of Monocystis is characterized by the absence of asexual multiplication by schizogony. The life cycle of Monocystis proceeds as follows------

       Gamontogony-------- The term gamontogony was suggested by Marshall and Williams in 1972. It is the method of sexual reproduction which involves the pairing of gamonts, formation of gametes and fertilization.

       Syzygy----- The two adult trophozoites, after a leading brief period of feeding, growing and wandering, become shortened and rounded called the gamount or gametocyte. The gametocytes associate in pairs and they secrete a common protective two layered cyst wall around themselves called the gametocyst  or gamontocyst. The gametocytes never fuse or conjugate within the cyst wall. This type of pairing of gametocytes is called syzygy.

       Gametogony------ within the gametocyst, the nucleus of each gametocyte divides mitotically several times  giving  rise to numerous daughter nuclei. The daughter nuclei are haploid because the gametocytes themselves are haploid having four chromosomes. The nuclei of a gametocyte move to the periphery and each gate surrounded by a small amount of cytoplasm to form gametes, leaving a certain amount of residual cytoplasm in the centre of the gametocyst which contains vacuoles and paraglycogen. All gametes from one gametocytes are of same sex. According to Hahn, the gametes are of unequal size and shape i.e., the gametes anisogamous. The male gametes are small and pointed called microgametes, and the female gametes are large and rounded called macrogametes. According to the M.A. Sleigh, 1973, all the gametes produced by both gametocytes are identical morphologically are called isogametes.

       
       
                                    
Syngamy--------- The wall between two gametocytes break, the gametes mingle and fuse in pairs to produce diploid zygotes. It is presumed that the two gametes uniting in pairs are from different gametocytes. This type of union and fusion of two gametes of a single gametocyst is called syngamy. If both pairing gametes are isogametes, it is called isogamy and if anisogametes, called anisogamy.

Sporogony-------- Each spherical zygote transforms itself into a unicellular oval body called sporoblast. It soon secretes a thick cyst wall around itself called sporocyst or zygocyst. Within the sporocyst, the nucleus and cytoplasm of the spore divided three times, first being reduction division to form eight spindle-shaped sporozoites. The sporozoites have been formed by asexual fission and constitute an asexual generation. At this stage the original cyst wall ruptures and spores are liberated into the cavity of seminal vesicle of earthworm. Liberated spores wait for their transmission to a new earthworm host.

However, the life cycle of monocystis exhibits an alternation of a sexual generation of gametocytes with an asexual generation of sporozoite.

Mode  of  Transmission---------- The exact manner of transmission of the spores or sporocysts of monocystis from one earthworm to another is not known with certainity. It may be brought about in any of the following ways----

By  death  of  host-------- When an infected earthworm dies and decays, the spores get scattered in soil. When infected soil is eaten by another earthworm, the spores enters the alimentary canal.

During   Copulation--------- The sporocysts are transferred from one host to another during copulation along with the spermatozoa and seminal fluid.

By  Birds-------- When an infected earthworm is devoured by a bird, the spores would pass out unaltered with its excreta and mixes with soil. When the soil containing the sporocyst is eaten by other earthworms, the cyst wall is dissolved in the intestine of earthworm and the sporozoites make their way into the seminal vesicle.

Autotomization-------- In some species of Monocystis, the spores are found in coelomic cavity of posterior segments of earthworm. When the posterior segments body is autotomized, then the spores are liberated in soil. A fresh earthworm is also infected with the ingestion of contaminated soil.