SSC
The phenomenon in which an asexually reproducing generation alternates with a sexually reproducing generation, as in Obelia, is called:
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Answer: Option A — Metagenesis
Detailed Solution
Metagenesis is the alternation of generations between an asexually reproducing form and a sexually reproducing form in the life cycle of an animal. It is found in many members of the phylum Cnidaria (formerly Coelenterata), such as Obelia (sea fur), and in some other invertebrates.
Cnidarians exist in two basic body forms:
Polyp: a cylindrical, sessile (attached) form, with the mouth and tentacles facing upward. Hydra and sea anemones are polyps.
Medusa: an umbrella-shaped, free-swimming form, with the mouth and tentacles facing downward. Jellyfish are medusae.
In Obelia, which lives as a branching colony attached to rocks and seaweeds in the sea, the life cycle shows metagenesis:
The polyps reproduce asexually by budding. Some buds develop into small, free-swimming medusae.
The medusae reproduce sexually, releasing sperm and eggs into the water. Fertilisation produces a zygote, which develops into a ciliated larva called a planula.
The planula settles on a surface and grows into a new polyp colony.
Thus polyps produce medusae asexually, and medusae form polyps sexually. Aurelia (a jellyfish) also shows metagenesis.
In plants, a similar alternation of generations occurs between the haploid gametophyte and the diploid sporophyte, as in mosses and ferns.
Related terms:
- Metamorphosis: the transformation of a larva into an adult through drastic changes in form, as in frogs (tadpole to frog) and butterflies (egg, caterpillar, pupa, adult).
- Parthenogenesis: the development of an egg into a new individual without fertilisation, as in honeybees (drones), aphids and some lizards.
- Regeneration: the ability to regrow lost body parts or form a new organism from a fragment, as in Hydra, Planaria and starfish.
- Budding: a form of asexual reproduction in which a new organism grows as an outgrowth of the parent, as in Hydra and yeast.
Why not the other options?
- ✗Metamorphosis is a dramatic change from larva to adult.
- ✗Parthenogenesis is development without fertilisation.
- ✗Regeneration is the regrowth of lost parts.
Quick tip
Remember: Metagenesis = alternation of asexual polyp and sexual medusa (Obelia, Aurelia); metamorphosis = larva to adult (frog, butterfly); parthenogenesis = drones of honeybees.
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Reproduction in Plants & Animals — Preparation
This question tests your understanding of Reproduction in Plants & Animals, a key topic in the General Knowledge section. The detailed solution above uses a step-by-step approach to show the reasoning, and where available, a shortcut that helps you solve similar questions in under 90 seconds during the actual exam.
This topic covers reproduction: asexual methods such as fission, budding, fragmentation and vegetative propagation, sexual reproduction in flowering plants including pollination and fertilisation, and the human reproductive system. Questions often ask which method of reproduction an organism uses (binary fission in amoeba and budding in yeast and hydra), which agent pollinates which flower, and what each part of a flower does. Know the difference between self-pollination and cross-pollination, and the role of the ovary and ovule in forming fruit and seed. Practise the reproduction questions below, written in the style of the general awareness sections of exams such as SSC, banking, railways, CUET GAT, MAH CET and CLAT. Each question comes with a detailed explanation that adds related facts, so every question helps you revise more than one point.