alternation of generations

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alternation

 [awl″ter-na´shun]
interrupted occurrence, being interspersed with different or opposite events.
alternation of generations metagenesis.

al·ter·na·tion of gen·er·a·tions

a succession of generations of individuals like and unlike the original parents, or an alternation of sexual and nonsexual generations.

alternation of generations

n.
The regular alternation of forms or of mode of reproduction in the life cycle of an organism, such as the alternation between diploid and haploid phases, or between sexual and asexual reproductive cycles.

alternation of generations

Alternation of generationsclick for a larger image
Fig. 26 Alternation of generations .(a) Life cycle of Obelia (Coelenterata). (b) Life cycle of fern (Pteridophyta). n = haploid number of chromosomes.

alternation of generations

the occurrence within the life cycle of an organism of a sexually reproducing generation followed by an asexual generation. In some COELENTERATES, such as Obelia, the sexual organs are carried on MEDUSAS, and subsequent fertilization gives rise to an asexually producing HYDROID stage; here both generations are DIPLOID. Amongst the plants, ferns have a diploid SPOROPHYTE generation (the plant itself). This undergoes MEIOSIS to form spores that give rise to a HAPLOID GAMETOPHYTE generation known as a PROTHALLUS, which reproduces sexually. Male and female GAMETES fuse to form a ZYGOTE, which then develops into a new fern plant (sporophyte). Some definitions of the term require a haploid generation alternating with a diploid generation, so excluding all animal examples.
References in periodicals archive ?
Current versions of this hypothesis do not address how an isomorphic alternation of generations was derived from the basically haploid life cycles of charophycean algae.
Celakovsky (1874) introduced a distinction between homologous and antithetic alternation of generations.
He borrowed Celakovsky's terminology and called this antithetic alternation of generations.
The discovery that the alternation of generations in land plants was accompanied by an alternation of chromosome number (Strasburger, 1894) weakened this version of the homologous theory because it accentuated the distinctiveness of gametophytes and sporophytes.
On antithetic as distinct from homologous alternation of generations in plants.
The origin of alternation of generations in land plants: a focus on matrotrophy and hexose transport.
Alternation of generations in land plants: new phylogenetic and palaeobotanical evidence.

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