Life History of Paramecium | Zoology Optional Notes for UPSC PDF Download

Habitat and Habit

• Paramecium is found globally, thriving in freshwater environments like pools, lakes, ditches, ponds, and slow-flowing waters enriched with decaying organic substances.

Movement and Feeding

• The body is covered with hair-like structures called cilia, facilitating movement at a speed four times its body length per second, including forward motion and rotation around its axis, aiding in food intake.
• Reversing cilia motion allows for backward movement.
• Utilizes phagocytosis for feeding; cilia push food into the gullet, leading to the formation of food vacuoles.
• Digestion involves enzymes and hydrochloric acid; waste is expelled through the cytoproct (pellicles).
• Contractile vacuoles expel water absorbed via osmosis to maintain osmotic balance.
• P. bursaria, a species forming a symbiotic relationship with algae, offers habitat to algae in exchange for nutritional benefits during food scarcity.
• Consumes microorganisms like yeasts and bacteria, using cilia to draw them into the oral groove and into the gullet.
• Food accumulates, forming a vacuole that circulates within the cell for digestion, with enzymes from the cytoplasm breaking down the food.
• Post-digestion, nutrients are absorbed into the cytoplasm, and waste is expelled through the anal pore into the environment.

Symbiosis

• Symbiosis denotes a mutually beneficial relationship between two organisms.
• Paramecium species such as P. bursaria and P. chlorelligerum engage in symbiosis with green algae, gaining food, nutrients, and protection from predators like Didinium nasutum.
• Notable endosymbiotic relationships include Kappa particle bacteria within paramecium, enabling them to kill other strains lacking this bacteria.

Reproduction

• Paramecium possesses a dual nuclear apparatus: diploid micronuclei for genetic stability and passing genes to offspring, and a polyploid macronucleus for non-reproductive cell functions and gene expression.
• Asexually reproduces via binary fission, dividing into two cells, with rapid multiplication under favorable conditions, up to three times daily.
• Binary fission involves transverse cell division, mitotic division of the micronucleus, and amitotic division of the macronucleus.
• Sexual reproduction occurs under food scarcity, with methods including conjugation and autogamy.
• Conjugation
  • Involves the exchange of genetic material between two paramecia (syngen).
  • Requires asexual multiplication 50 times before proceeding.
  • A conjugation bridge forms; micronuclei undergo meiosis to produce four haploid nuclei, with three degenerating and the remaining haploid nuclei fusing to form new diploid micronuclei.
  • Results in identical yet genetically varied exconjugants, each dividing to form four daughter paramecia.
• Autogamy (self-fertilization)
  • Leads to the formation of a new macronucleus, rejuvenating the organism.
• Cytogamy
  • Involves contact between two paramecia without nuclear exchange, rejuvenating the organism with a new macronucleus formation.
  • Paramecia can age and die after 100-200 cycles of fission without conjugation due to DNA damage and the role of the macronucleus in clonal aging.
    

Aging

• Paramecium experiences a gradual loss of energy and aging due to clonal aging during the mitotic cell divisions in asexual reproduction.
• Specifically, P. tetraurelia shows that without sexual reproduction (conjugation or autogamy), a cell dies after approximately 200 fissions, indicating reliance on sexual reproduction for longevity.
• Clonal aging is associated with increased DNA damage in the macronucleus, contributing to the aging process in P. tetraurelia, aligning with the DNA damage theory of aging applicable to both single-celled protists and multicellular eukaryotes.

Genome

• The sequencing of P. tetraurelia's genome revealed evidence of three whole-genome duplications.
• In certain ciliates, including Stylonychia and Paramecium, UAA and UAG serve as sense codons, while UGA is a stop codon, differing from the more common genetic code usage.