Alternation of Generations

(redirected from xenogenesis)
Also found in: Dictionary, Thesaurus, Medical, Wikipedia.

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.
..... Click the link for more information.
; reproductionreproduction,
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.
..... Click the link for more information.
.
The Columbia Electronic Encyclopedia™ Copyright © 2013, Columbia University Press. Licensed from Columbia University Press. All rights reserved. www.cc.columbia.edu/cu/cup/
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

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.

REFERENCES

Miasoedov, 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.
A. V. IVANOV
The alternation of generations in plants usually refers to the alternation of diploid and haploid phases in the developmental cycles. It is characteristic of plants in which both the diploid phase (diplont) and the haploid phase (haplont) are multicellular. The diplont forms sporangia, in which spores result from meiosis (hence a diplont is also called a sporophyte); the haplont forms gametangia, whose gametes are formed without reduction division (a haplont is also called a gametophyte). A sporophyte develops from a zygote, and a gametophyte from a spore. In some plants, for example, the algae Ulva and Dictiota, the sporophyte and gametophyte develop equally. In others, however, there is dominance of either the gametophyte (some brown algae [for example, Cutleria] and all bryophytes) or the sporophyte (some brown algae [for example, Laminaria] and all ferns and seed plants). In many green and, possibly, in some red algae only the zygotes that divide by meiosis are diploid, whereas in Siphonales, diatoms, and some brown algae only the gametes are haploid, as in the great majority of animals. These plants do not actually have an alternation of generations, although there is a succession of nuclear phases.
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.

REFERENCES

Takhtadzhian, 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

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

alternation of generations

[‚ȯl·tər′nā·shən əv ‚jen·ə′rā·shənz]
(biology)
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
"Xenogenesis was a bit challenging because it's focused on just one thing.
'Contradiction' also connects Xenogenesis to concepts such as the Hegelian dialectic and Derridean deconstruction, despite Butler's ostensible allergy to critical theory.
Thomas Foster's "'We Get to Live, and So Do They': Octavia Butler's Contact Zones" is similarly noteworthy, offering sustained critical attention both to the Xenogenesis trilogy and to the short story "Amnesty." About the story, he argues that "'Amnesty' makes more explicit Butler's double-sided dialogue with both science fiction and African American traditions" (149) and "restores a dimension of complexity" to the Xenogenesis trilogy in its refusal of easy answers regarding reciprocity versus exploitation in human/alien encounters (162).
In her space fiction trilogy Xenogenesis, she has the three sexed alien culture (named the Oankali) treasure Lilith, her Eve figure, because they discover during a genetic 'reading' that she has cancerous cells in her body.
Savage's reading of "The Heterosexual Imperative in Octavia Butler's Xenogenesis Trilogy," and the previously mentioned pieces by Whitney, Cornelius, Parody, and Hersch--are worthwhile but relatively small in scope, I recommend this book most for college libraries.
In an interview with McCaffery, conducted just as she had completed Xenogenesis, Butler says that the seeds of the Xenogenesis story can be traced to Ronald Reagan's idea of a "winnable" nuclear war:
Among their topics are epistemology, cultural negotiation, the black female world of Octavia Butler's Xenogenesis Trilogy, ecology and evolution in Dan Simmons' Hyperion Cantos, and political myths and social reality in East German science fiction 1949-89.
Broad draws on romanticism and postcolonialism in "Body Speaks: Communication and the Limits of Nationalism in Octavia Butler's Xenogenesis Trilogy" to examine ideas of language, communication, and colonial discourses in Butler.
(2) The works by Octavia Butler that Michelle Erica Green examines include the Patternmaster novels, the Xenogenesis trilogy, Kindred, "Speech Sounds," "Bloodchild," and "The Evening and the Morning and the Night."
The fourth chapter focuses on Butler's Xenogenesis trilogy (Dawn [1987], Adulthood Rites [1988], and Imago [1989]), considers the black body's survival capacity in terms of race and gender, and re-envisions identity boundaries.
She acknowledged her realist understanding of human nature in a 2000 essay for NPR where she, echoing the Oankali (of the Xenogenesis trilogy), described humans as having two competing characteristics: intelligence, and a tendency to hierarchy.