5. THE CILIATES
The phylum Ciliophora includes some of the most complex protozoa. Characteristics of the phylum Ciliophora are:
1. Ciliates are widelydistributed in freshwater and marine environments.
2. A few ciliates are symbiotic.
3. They have cilia for locomotion. Cilia ;ire also used for the generation of feeding currents in water.
4. They have rigid pellicle. It keeps their shapes has fixed.
5. They have distinct cytostome (mouth) structure.
6. They have demographic nuclei: A larger macronucleus and one or more smaller micronuclei.
Cilia and Other Pellicular Structures
1. Cilia: Cilia are generally similar to flagella. But they are much shorter, and more numerous. They are widely distributed over the surface of the ciliate. The ciliary movement is coordinated movements. The ciliary waves pass over the surface of the ciliate.
Many ciliates can reverse the direction of ciliary beating and the direction of cell movement. Some ciliates have evolved specialized cilia. Cilia may cover the outer surface of the protozoan. They may join to form cirri.Cirri are used in movement. Cilia may be lost from large regions of a ciliate.
2. Basal bodies (kinetosomes): It is the root of cilia. Cilia arise
from the basal bodies. Basal bodies of adjacent cilia re interconnected. They form an elaborate network of fibers. These fibers anchor the cilia and give shape to the organism.
3. Trichocysts: Trichocysts are sac like pellicular structures. They are arranged perpendicular to the plasma membrane. Theyare rod like or oval organelles. They are primarily used for protection. In Paramecium, they have a “golf tree” appearance. Trichocysts can come out of pellicle. They then remain connected to the body by a sticky thread.
Mech nism of Feeding
1. Paramecium: Paramecium has a ciliated oral groove along one side of the body. Cilia are present in oral groove. These cilia push the small food particles towards the cytopharynx. A food vacuole is formed in the cytopharynx. The food vacuole he ‘times larger. It then breaks from the cytopharynx and become free. It freely circulates through the endoplasm and digestion take place.
2. Didinium: Some free-living ciliates eat other protists or small animals. The prey touches the organism and they capture it. The ciliate Didinium feeds oi Paramecium. Its prey is bigger than itself. Didinium forms a temporary opening. This opening is greatly arged and engulf’ its prey.
3. Suctorians: Their cilia are attached to the substrate. They possess tentacles. Their secretions paralyze the prey. The prey is ciliates or amoebae. The tentacles form an opening in the pellicle of the prey. It then sucks the cytoplasm of the prey through tiny channels in the tentacle. This mechanism involves tentacular microtubules.
Gen tic Control and Reproduction
Ciliates have two kinds of nuclei.
(a) Macronucleus: It is a large polvploid macronucleus. It regulates daily metabolic activities.
(b) Micronuclei: There are one or more smaller micronuclei. These are the genetic reserve of the cell.
Ciliates reproduce asexually by transverse binary fission. Some reproduce by budding. Budding occurs in suctorians. It results in the formation of ciliated, free-swimming organisms. This organism attaches to the substrate and take the form of the adult.
Fig: Conjugationin paramecium (a) Random contact between conjuigant, (b) Meiosis (c) Mutual exchange of pronuclei (d-f)further division
Sexual Reproduction: Conjugation
Ciliates reproduce sexually by conjugation. The partners are called conjugants. Many species of ciliates have numerous mating types. All of these mating types are not mutually compatible.
1. Initial contact between individuals is random. The pellicles two conjugants secrete sticky secretion. These secretions help in adhesion of two conjugants.
2.The plasma membranes ciliates then fuse. They remain fused for several hours.
3. The macronucleus does not participate in the genetic exchange. The macronucleus breaks up. It then reforms from micronuclei of the daughter ciliates.
4. Meiosis takes place in the conjugants and four haploid pronuclei are formed.
5. The three pronuclie in each conjugant degenerate. The remaining pronuclie divides by mitosis. Thus each conjugant has now two pronuclei.
6. The conjugants exchange pronuclei mutually. The pronuclei fuse with each other in each conjugant.
7. The conjuagants separates from each other and they are now cal ed exconjugants.
8. Each exconjugant undergo a series of three nuclear divisions. Thus eight daughter nuclei are formed in each exconjugant. Two are degenerated.
9. Four of the remaining six nuclei become macronuclei. Each exconjugant divides at thi. stage. Now each daughter paramecium has two macronuclei and one micronucleus.
10. Each micronucleus again divides. Cytoplasmic division occurs. Thus four paramecia are formed from each exconjugant.
Fig: Phylogeny of protozoan based on 80S rRNA Sequence comparison
Most ciliates are free living. However, some are commensalistic or mutualistic and parasitic.
1. Balantidium coli: It is an important parasitic ciliate. It lives in the large intestines of humans, pigs and other mammals. Sometimes, it is a ciliary feeder. Sometimes, it produces proteolytic enzymes for the digestion of host epithelium. It causes a flaskshapled ulcer. B. coli form cysts.
2.Large numbers of different species of ciliates also inhabit the rumen of many ungulates (hoofed animals). These ciliates help in the digestive processes of their hosts.