What is metagenesis?discuss it with reference to the lifehistory of obelia.

What is metagenesis?discuss it with reference to the lifehistory of obelia.

Grade:12th pass

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4 years ago
In the life cycle of Obelia there is a regular alternation of hydroid and the medusoid phases. Medusae produce eggs which after fertilization develop into hydroids. The hyroids produce medusae by asexual budding. This phenomenon was formerly called as “alternation of generations”.
However, true alternation of generations occurs where a diploid asexually reproducing generation alternate with a sexual haploid generation. It is commonly found in ferns when the diploid plant (a gametophyte) produces spores having haploid number of chromosomes. The spores develop into haploid gametophytes. After fertilization the egg cell procudes a new diploid sporophyte.
In Obelia both hydroid and medusoid phases are diploid. Thus, infact, there is no true alternation of generations in Obelia. The medusa is only a modified zooid that swims freely. This free swimming medusa is very useful to Obeliabecause it causes wide dispersal of gametes. Secondly, the gametes carried in the gonads of the medusae do not actually originate there. These originate in the ectoderm of blastostyle, develop, and later migrate into medusae and enter the gonads. This, it is impossible to differentiate between sexual and asexual generations in Obelia. The alternation of fixed (hydroid) and free swimming (medusae) phases is only an example of polymorphism. The term metagenesis is used for such alternation of fixed and free phases. (Alternation between two diploid phases is known as metagenesis). There is no true alternation of generation in Obelia.

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