Alternation Of Generations

Definition & Examples

Oct 7, 2023 - 05:46
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fern: life cycle

alternation of generations, also called metagenesis or heterogenesis, in biology, the alternation of a sexual phase and an asexual phase in the life cycle of an organism. The two phases, or generations, are often morphologically, and sometimes chromosomally, distinct.

moss life cyclemoss life cycle

In algae, fungi, and plants, alternation of generations is common. It is not always easy to observe, however, since one or the other of the generations is often very small, even microscopic. The sexual phase, called the gametophyte generation, produces gametes, or sex cells, and the asexual phase, or sporophyte generation, produces spores asexually. In terms of chromosomes, the gametophyte is haploid (has a single set of chromosomes), and the sporophyte is diploid (has a double set). In bryophytes, such as mosses and liverworts, the gametophyte is the dominant life phase, whereas in angiosperms and gymnosperms the sporophyte is dominant. The haploid phase is also dominant among fungi. Although some algae have determinate life cycle stages, many species alternate between the sexual and asexual phases in response to environmental conditions.

Life cycle of the common jellyfish Aurelia. Eggs released by females pass out through the mouth and become lodged in pits on the tentacles. Sperm released from male jellyfish fertilize the eggs, which remain on the tentacles during early development. A fertilized egg develops into a ciliated larva, or planula, which settles on and attaches to a substrate (such as a rock) and develops into a polyp with a mouth and tentacles. The polyp reproduces asexually by budding off saucer-shaped immature medusae, which develop into mature sexually reproducing forms.

Among animals, many invertebrates have an alternation of sexual and asexual generations (e.g., protozoans, jellyfish, flatworms), but the alternation of haploid and diploid generations is unknown.

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