Alternation of Generations(redirected from xenogenesis)
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alternation of generations:see gametophytegametophyte
, phase of plant life cycles in which the gametes, i.e., egg and sperm, are produced. The gametophyte is haploid, that is, each cell contains a single complete set of chromosomes, and arises from the germination of a haploid spore.
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capacity of all living systems to give rise to new systems similar to themselves. The term reproduction may refer to this power of self-duplication of a single cell or a multicellular animal or plant organism.
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Alternation of Generations
in some invertebrates, for example, hydroids, the alternation of two or more generations differing in morphological characteristics, mode of life, and type of reproduction. The developmental cycle of most plants is marked by the alternation of two generations, or phases: one forms the organs of sexual reproduction, and the other has organs of asexual reproduction.
Alternation of Generations
a regular succession of generations differing in mode of reproduction.
Animals may have primary or secondary alternation of generations. A primary alternation of generations, which characterizes many protozoans, is the alternation of a sexual generation with a generation reproducing by asexual cells (agametes). In foraminifers, for example, the alternating generations consist of sexual and asexual individuals—gamonts and agamonts (schizonts), respectively. By repeated division of the nucleus, the gamonts form gametes, which copulate in pairs to form a zygote, which in turn develops into an agamont. The agamont divides into agametes—future gamonts—as a result of schizogony. Since reduction division, or meiosis, occurs before agametes form, the sexual generation, like the gametes, is haploid, whereas the zygote and agamonts are diploid. In sporozoans and flagellates only the zygote is diploid, because meiosis is effected during the first division. In heliozoans, some flagellates, and infusorians meiosis is associated with the formation of gametes, the only haploid stage in the life cycle. This pattern typifies all multicellular animals.
A secondary alternation of generations occurs in two forms in animals. The alternation of different forms of sexual reproduction, for example, the normal sexual process with parthogenesis, is called heterogony. The alternation of sexual and asexual reproduction by means of multicellular vegetative bodies or by transverse division is called metagenesis. Heterogony is characteristic of trematodes, some roundworms, rotifers, and some arthropods (including water fleas, aphids, gallflies, and some gall midges). Metagenesis is very characteristic of tunicates (salpae, Doliolidae, ascidians, and pyrosomes) and coelenterates (hydrozoans and scyphozoans) in which the sexual generation consists of single free-swimming medusae and the asexual generation consist of sessile polyps, which often form colonies. Metagenesis in the broad sense should also include polyembryony, since embryos that reproduce vegetatively more or less constitute an underdeveloped asexual generation.
REFERENCESMiasoedov, S. V. Iavleniia razmnozhenüa i pola v organicheskom mire. Tomsk, 1935.
Hartmann, M. Obshchaia biologiia, 2nd ed. Moscow-Leningrad, 1936. (Translated from German.)
Dogel’, V. A. Zoologiia bespozvonochnykh, 6th ed. Moscow, 1975.
The sporophytes, or sporogonia, of bryophytes develop on the gametophytes. The gametophytes of ferns exist independently, whereas those of seed plants develop on the sporophytes. The gametophytes of isosporous plants are monoecious; those of heterosporous plants are dioecious and more reduced (especially males) than those of isoporous plants. In angiosperms the male gametophyte is the pollen grain, and the female gametophyte the embryo sac.
REFERENCESTakhtadzhian, A. L. Vysshie rasteniia, vol. 1: Ot psilofitovykh do khvoinykh. Moscow-Leningrad, 1956.
Poddubnaia-Arnol’di, V. A. Tsitoembriologiia pokrytosemennykh rastenii. Moscow, 1976.
A. N. SLADKOV