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 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 ?
(Overton believed that the reduction of chromosome number occurred at synapsis not, as we now know, in the subsequent nuclear division.) He wrote, "It will be a matter of great morphological as well as physiological interest, to establish beyond the possibility of a doubt that the alternation of generations, which is so remarkable a feature in the life-history of plants, is dependent on a change in the configuration of the idioplasm; a change, the outward and visible sign of which is the difference in the number of the nuclear chromosomes in the two generations."
Debates about alternation of generations in the early twentieth century can be confusing because three ways of classifying 'generations' co-exist (sexual vs.
The green alga Ulva replaced the brown alga Dictyota as the favored algal exemplar of the homologous theory (Graham, 1985; Blackwell, 2003) after Foyn (1929) and Hartmann (1929) described isomorphic alternation of generations in Ulva and Enteromorpha.
Remy (1980), for example, argues that stoneworts evolved from an ancestor with isomorphic alternation of generations, but secondarily lost the diploid phase.
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. In Celakovsky's scheme, two generations were homologous if they obeyed the same growth-law but were antithetic if they obeyed different growth-laws.
He borrowed Celakovsky's terminology and called this antithetic alternation of generations. Scott (1895), by contrast, believed that the sporophyte had been derived from an asexual algal thallus.
The total paleobotancial evidence, though, as previously discussed, does not support either theory of alternation of generations conclusively; put another way, the existing evidence could be argued to support both theories more or less equally.

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