Model Answer
0 min readIntroduction
Alternation of generations is a reproductive life cycle in which a plant or animal alternates between two distinct multicellular forms: a haploid gametophyte and a diploid sporophyte. This phenomenon is particularly prominent in plants and certain algae, but is also observed in some invertebrates like cnidarians. The gametophyte produces gametes (sex cells) through mitosis, while the sporophyte produces spores through meiosis. These spores then develop into a new gametophyte, completing the cycle. Understanding this process is crucial for comprehending the evolutionary adaptations and reproductive strategies of diverse organisms. The life cycle of *Obelia*, a colonial hydrozoan, provides a classic example of this alternation, showcasing a relatively simple yet effective reproductive strategy.
Alternation of Generations: A Detailed Explanation
Alternation of generations is not simply a switch between haploid and diploid phases, but a cyclical process where each generation is structurally distinct and specialized for a particular function. The relative dominance of each generation varies among different groups of organisms. In some, the gametophyte is dominant (e.g., mosses), while in others, the sporophyte is dominant (e.g., ferns and flowering plants). In *Obelia*, the polyp stage represents the dominant sporophyte, and the medusa stage represents the gametophyte.
Life History of Obelia: Illustrating Alternation of Generations
Obelia, a marine hydrozoan, exhibits a distinct alternation of generations. Its life cycle involves both a sessile polyp stage (sporophyte) and a free-swimming medusa stage (gametophyte). Let's examine each stage in detail:
1. Sporophyte (Polyp Stage)
The sporophyte is the dominant phase in Obelia. It is a colonial organism consisting of a branched, stalk-like structure anchored to a substrate. This colony is composed of numerous polyps, each representing an individual zooid. Polyps are responsible for asexual reproduction through budding. These buds can remain attached, forming a branched colony, or detach to form new colonies. The polyps are diploid (2n).
- Feeding Polyps (Gastrozooids): These polyps are specialized for feeding and are equipped with tentacles containing nematocysts (stinging cells) to capture prey.
- Reproductive Polyps (Gonozooids): These polyps are responsible for producing medusae asexually. They arise as buds from the stem and mature into medusae.
2. Gametophyte (Medusa Stage)
The medusa is the sexual reproductive stage of Obelia. It is a bell-shaped, free-swimming organism produced asexually by the reproductive polyps of the sporophyte. Medusae are haploid (n) and possess gonads (reproductive organs) that produce gametes – eggs and sperm.
- Sexual Reproduction: Medusae release eggs and sperm into the water. Fertilization occurs externally, resulting in a zygote (2n).
- Planula Larva: The zygote develops into a ciliated, free-swimming larva called a planula.
3. Development of the Sporophyte
The planula larva settles on a suitable substrate and undergoes metamorphosis, transforming into a new polyp (sporophyte). This polyp then begins to bud, forming a new colony and restarting the cycle.
The following table summarizes the key features of each generation:
| Generation | Stage | Ploidy | Reproduction Mode | Dominance |
|---|---|---|---|---|
| Sporophyte | Polyp | Diploid (2n) | Asexual (budding) | Dominant |
| Gametophyte | Medusa | Haploid (n) | Sexual (gamete fusion) | Less Dominant |
Therefore, in Obelia, the sporophyte generation is dominant, being responsible for the majority of the organism’s life cycle and providing a means for rapid colonization. The gametophyte generation, while essential for sexual reproduction and genetic diversity, is relatively short-lived and less conspicuous.
Conclusion
In conclusion, alternation of generations is a fascinating reproductive strategy that allows organisms like *Obelia* to exploit both asexual and sexual reproduction. The life cycle of *Obelia* clearly demonstrates this phenomenon, with the dominant polyp stage (sporophyte) undergoing asexual reproduction and the medusa stage (gametophyte) facilitating sexual reproduction. This interplay between generations ensures both rapid propagation and genetic variation, contributing to the species’ survival and adaptability in its marine environment. Further research into the genetic mechanisms controlling this alternation continues to reveal the intricacies of evolutionary adaptation.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.