Model Answer
0 min readIntroduction
Metagenesis, often interchangeably but inaccurately used with "alternation of generations" in zoology, describes a life cycle phenomenon in certain animals, particularly cnidarians, where there is an alternation between an asexual reproductive phase and a sexual reproductive phase. Crucially, both phases in metagenesis are typically diploid, distinguishing it from the true alternation of generations seen in plants where haploid and diploid forms alternate. This biological strategy allows organisms to adapt to diverse environmental conditions and enhances species dispersal.
Understanding Metagenesis
Metagenesis is a complex life cycle pattern found in various invertebrate animals, most notably within the phylum Cnidaria. It involves the regular succession of morphologically distinct asexual and sexual forms within a single species' life cycle.- Asexual Phase (Polypoid Form): This stage is generally sessile (fixed) and reproduces asexually, primarily through budding or fragmentation. Polyps are typically specialized for feeding and colony formation.
- Sexual Phase (Medusoid Form): This stage is typically free-swimming, often bell-shaped (medusa), and responsible for sexual reproduction. Medusae produce gametes (sperm and ova) that, upon fertilization, give rise to a new larval stage that eventually develops into the asexual polyp.
Metagenesis in Obelia (Sea Fur)
Obelia, a colonial hydrozoan, serves as a classic example of metagenesis. Its life cycle vividly illustrates the alternation between a fixed, asexual polyp colony and free-swimming, sexual medusae.The life cycle of Obelia involves the following key stages:
- Hydroid Colony (Polyp Stage): The adult Obelia exists as a branched, sessile colony attached to a substratum. This colony consists of different types of zooids: nutritive polyps (hydranths or gastrozooids) and reproductive polyps (blastostyles or gonangia). The blastostyles reproduce asexually by budding to produce medusae.
- Medusa Stage: The medusae are free-swimming, bell-shaped individuals that develop from the blastostyles. These medusae are the sexual forms; they are dioecious (separate male and female individuals) and bear gonads that produce gametes.
- Fertilization and Larval Development: Male medusae release sperm and female medusae release ova into the water. Fertilization occurs externally, forming a diploid zygote. The zygote undergoes cleavage to form a ciliated, free-swimming planula larva.
- New Colony Formation: The planula larva settles on a suitable substratum, loses its cilia, and metamorphoses into a sessile hydrula. This hydrula then repeatedly buds, forming a new colonial polyp, thus completing the cycle.
The table below highlights the differences between true alternation of generations and metagenesis:
| Feature | Metagenesis | True Alternation of Generations (e.g., in Plants) |
|---|---|---|
| Ploidy Level | Both asexual (polyp) and sexual (medusa) stages are diploid (2n). | Alternates between a haploid (n) gametophyte and a diploid (2n) sporophyte. |
| Gamete Formation | Medusae produce gametes (sperm/ova) directly. | Gametophytes produce gametes by mitosis; sporophytes produce spores by meiosis. |
| Occurrence | Mainly in certain invertebrates (e.g., Cnidarians like Obelia, some flatworms). | Predominant in plants (mosses, ferns, algae) and some fungi. |
| Morphological Distinction | Distinct morphological forms (polyp and medusa) but both diploid. | Often distinct morphological forms, representing different ploidy levels. |
Conclusion
In essence, metagenesis is a sophisticated reproductive strategy, primarily observed in Cnidarians like Obelia, where asexual polyp stages alternate with sexual medusa stages, both being diploid. This ensures both sedentary colony expansion and wide dispersal of the species through free-swimming medusae and larvae. While often confused with the alternation of generations, its defining characteristic lies in the consistent diploidy of both alternating forms, offering significant evolutionary advantages in terms of survival and adaptation across diverse marine habitats.
Answer Length
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