Protozoa: Reproduction | Zoology Optional Notes for UPSC PDF Download

Introduction to Protozoan Reproduction

Reproduction Modes:
Asexual Reproduction:

• Occurs without the fusion of pronuclei.
• Commonly observed in protozoans during favorable conditions.

Conditions for Asexual Reproduction:

• Thrives in optimal conditions, including suitable temperatures, nutrient abundance, and ecologically favorable water conditions.

Schematic General diagram (Reproduction in Protozoa)

The variability in protozoan reproduction underscores the adaptability of these microorganisms to diverse environments and ecological niches. The balance between asexual and sexual reproduction strategies ensures the survival and perpetuation of protozoan populations under varying circumstances. In the subsequent sections, we delve into the specifics of asexual and sexual reproduction mechanisms in protozoa, shedding light on the intricate processes governing their reproductive biology.

Nuclear Division in Protozoa

• Definition: Direct division of the nucleus resulting in unequal chromosome distribution and the production of two daughter cells.
• Instances:
  • In ciliophora protozoans, macronucleus division is consistently amitotic, while micronucleus division follows mitosis.
  • Paramecium caudatum exhibits amitosis, where the macronucleus elongates without internal structural changes, forming two daughter nuclei.
  • Endomixis, observed in Paramecium aurelia, involves the disintegration of the macronucleus, leading to the formation of four daughter cells from a single individual.
  • Amitotic division is also found in vegetative nuclei of trophozoites like Myxosoma catostomi in Myxosporoidia.

2. Mitosis:

• Characteristics: Results in two daughter cells with the same chromosome number as the parent cell.
• Types:
  • Equational plate and metaphase (orthomitosis) observed in some protozoans.
  • Pleuromitosis, where spindle asymmetry occurs during mitosis.
  • In veritable mitosis, exemplified by Oxnerella maxitima, acentrically situated nucleus and centriole undergo division, followed by nuclear membrane disappearance and reconstruction of daughter nuclei.
• Additional Features:
  • Presence of Paradesm, an extra-nuclear filament connecting division centers in mitosis of flagellates.

3. Cytoplasmic Division:

• Definition: Division of the body through the middle, resulting in two nearly equal daughter individuals.
• Examples: Occurs in various planes such as longitudinal (Euglina), transverse (Paramecium), or irregular (Amoeba).

4. Multiple Division:

• Process: The body divides into numerous daughter individuals, often without residual cytoplasmic masses.
• Examples: Common in foraminifera, radiolaria, and sporozoa groups like plasmodium.

5. Budding:

• Description: Formation of one or more smaller individuals from the parent individual.
• Types:
  • Exogenous budding (e.g., Acanthocystis, Noctiluca, Myxosporida).
  • Endogenous budding (e.g., Arcellinida, Myxosporoidea, Suctoria).
• Unique Case: Tokophrya demonstrates a distinctive budding where the entire body transforms into a young bud.

6. Plasmotomy:

• Definition: Multinucleated protozoan bodies divide into two or more smaller, multinucleated individuals, with cytoplasmic division independent of nuclear division.
• Examples: Occurs in certain sarcodina like Pleomyxa; Doflein (1901) described a similar process in Olugea lophii, termed plasmotomy.

Sexual Reproduction in Protozoa

• Definition: Sexual reproduction in protozoa involves the union of two gametes to form a new individual. It includes the fusion of male and female gametes through cytoplasmic and nuclear interactions.
• Frequency: While many protozoans primarily reproduce asexually, some undergo sexual reproduction at irregular intervals.

Meiotic Division:

• Process: Sexual reproduction in protozoa involves meiotic division, reducing the chromosome number to haploid.
• Types:
  • Gametic Meiosis: Reduction division occurs shortly before syngamy, resulting in haploid gametes.
  • Zygotic Meiosis: Reduction division occurs after zygote formation, making the zygote diploid but the rest of the life cycle haploid.

Types of Sexual Reproduction

• Definition: Complete union of two specialized protozoan individuals or gametes, forming a zygote.
• Gamete Characteristics: Microgametes (motile, small, numerous) and macrogametes (non-motile, voluminous, few).

• Types of Syngamy:
  • Isogamy: Morphologically and physiologically identical gametes (reported in Foraminifera, Phytomonadina, Gregarinida).
  • Anisogamy: Gametes differ in size; microgametes are motile, while macrogametes are non-motile (observed in Plasmodium vivex).
• Life Cycle: The zygote develops into adults directly or through encystment and various fission types.

Life cycle of P. vivex

• Conjugation:
  • Definition: Temporary union of two mating types of individuals, facilitating the exchange of nuclear materials (DNA).
  • Participants: Ciliates and suctoria.
  • Events: Formation of micro and macroconjugants, attachment, disappearance of macronucleus, prezygotic nuclear division, amphimixis, and postzygotic nuclear divisions.
  • Purpose: Considered an episode in reproduction, serving to reorganize the meganucleus, rejuvenate lost vigor, and create new hereditary combinations.
• Automyxis:
  • Definition: A process in which a single individual undergoes meiosis, resulting in the production of gametes.
  • Significance: Represents a form of internal self-fertilization.
  • Example: Recognized in Paramecium sps. with different syngens, each including two mating types.

Visual Representation:

• Figure: Schematic representation of major nuclear stages during the conjugation of Paramecium caudatum, illustrating key events in nuclear division and differentiation.

Recognition of Syngens:

• Observation: Different syngens in Paramecium sps. identified, with each syngen comprising two mating types.
• Migration: Reciprocal migration of pronuclei observed during conjugation.
• Structural Changes: Development of a 'conical elevation' (Paroral cone) facilitating nuclear migration without cytoplasmic exchange.

Automixis in Protozoa

Introduction:

• Definition: Automixis refers to the fusion of two gametic nuclei originating from the division of a single nucleus within an individual protozoan.
• Comparison: Distinguished from amphimixis, where two nuclei originating in separate individuals fuse to form two new individuals.

Types of Automixis:

Autogamy:

• Process: In autogamy, nuclei from the same individual fuse. Similar to conjugation, but the union occurs between pronuclei of the same organism.
• Example: Observed in solitary Paramecium aurelia, involving micronuclear changes akin to conjugating individuals.
• Steps: Two micronuclei undergo two divisions, forming eight nuclei. Further divisions result in two functional nuclei, which fuse in the 'Paroral cone' to create the synkaryon, undergoing two additional divisions.

Paedogamy:

• Description: Fusion happens between nuclei from different cells of the same parent. The organism encysts, divides into gametocytes, and undergoes meiosis. Gametes unite in pairs, forming zygotes.
• Examples: Actinosphaerium, Actinophrys, myxosporidians.
• Process: Organism encysts, divides into uninucleate secondary cysts. Each cyst divides, and nuclei undergo meiosis. Paedogamy leads to zygote formation in the secondary cyst.

Cytogamy:

• Nature: Cytogamy represents a sexual process in conjugating Paramecium where there is no nuclear exchange between conjugants.
• Observation: Reported by Diller (1934) in Paramecium caudatum and termed cytogamy. Two individuals fuse on their oral surfaces. Three micronuclear divisions occur, similar to conjugation, but without nuclear exchange. Considered intermediate between conjugation and autogamy.

Hemimixis:

• Definition: Coined by Diller (1936 & 1938) for a process observed in Paramecium aurelia involving simple fragmentation of the macronucleus. This fragmentation, not correlated with specific micronuclear activity, is considered a form of 'Purification act.'

Meiosis in Protozoa:

• Occurrence: Protozoa undergoing sexual reproduction presumably experience meiosis to maintain chromosome number consistency.
• Observation: Prandtle (1906) noted a reduction in chromosome number during protozoan meiosis. For instance, in Didinium nasutum, 16 chromosomes reduced to eight in the second division.
• Example: In Monocystis rostrala, a gregarine parasite of earthworms, meiosis is involved in the complete fusion of gametes.

Other Reproductive Mechanisms:

• Regeneration: Protozoa exhibit regeneration, replacing lost parts, especially among free-living and some parasitic forms. Proper proportions of cytoplasm and nucleus can regenerate an entire individual.
• Parthenogenesis: Gametes failing to fertilize can develop parthenogenetically into adults. Parthenogenesis is the ability of gametes to develop without fertilization by opposite-sex gametes. It's typically associated with female gametes. Examples include Actinophrys and Chlamydomonas. In ciliates, endomixis is a parthenogenetic phenomenon.

Conclusion: Automixis in protozoa encompasses various processes, including autogamy, paedogamy, and cytogamy, demonstrating the diverse mechanisms by which these microorganisms reproduce sexually. Additionally, hemimixis, meiosis, regeneration, and parthenogenesis contribute to the repertoire of reproductive strategies exhibited by protozoans.