Changes of generations
as changes of generations one designates a form of the reproduction in biology, with which the different possibilities of the reproduction (the sexual and the ungeschlechtliche) are distributed on different generations. Here there are two variants:
- With the Metagenese (= metagenetischer change of generations) follows after those sexual reproduction a ungeschlechtliche reproduction, which again a generation with sexual reproduction follows.
- With the Heterogonie (= heterogoner change of generations) changes itself a bisexuell reproducing generation with or for several itself university-sexually (is called: parthenogenetisch) reproducing generation off.
The change between a haploiden and a diploiden generation in the technical language of the biologists heterophasisch one calls (= heterophasischer change of generations); there is no such change of the Ploidie, one speaks of a homophasischen change of generations.
If the generations differ in their outside appearance, then this is called heteromorpher changes of generations- in contrast to the isomorphic change of generations, with which the generations look outwardly alike or each other at least to confound are similar.
The change of generations is with many Protisten, a phenomenon occurring with Moosen , ferns and seed plants, in addition, with Nesseltieren and Tunicaten. Itunites the advantages of both growth modes in itself: The ungeschlechtlich increasing generation can produce a large number of descendants, without needing for it a Sexualpartner. The sexually increasing generation provides for a mixing of the gene pool.
work on] examples of
- the Metagenese (frequently on the lower surface of the fern whisks) in ungeschlechtliche way billions of Sporen, which to only some millimeters grow up a large pro thallium. Only these of the laymanhardly generation ever noticed increases sexually and produces again each the well-known, “normal fern plant”. Ferns possess thus a heteromorphen and heterophasischen change of generations.
- Moose are characterized contrary to ferns by the fact that everyonedid not admit, “normal MOO plant” Sporen produced, but of Gameten (sex cells). The Sporen producing generation is usually only on also from laymen considered “MOO plant” increasing, low, haarförmiger stand with Sporenkapsel. Also the Moose possesses thus a heteromorphen and heterophasischen change of generations.
- Quallen produce sex cells, after of themFusion from the Zygote a larva develops. This larva settles at the soil and attains full growth to a polyp . From it again various Quallen pinches itself off. This change of generations is homophasisch heteromorph and diploid.
- Tapeworms are hermaphrodites (possibly even with self fertilization) in the Endwirt,occasionally can however the fairings by budding in the intermediate landlord increase.
- Plasmodien, the exciters of the malaria, cause the illness-typical fever thrusts due to their ability for asexuellen Vermehrung (division), possess in the change with this generation in addition, a sexually active generation.
examplesfor the Heterogonie
- sheet lice can increase in favorable weather in the spring extremely rapidly, because this happens with most kinds over several generations away by means of Parthenogenese. Only under more unfavorable environmental condition again a winged, sexually reproducing generation, forms those at the same time also the spreading formrepresents and able is, widens distances to new plant hosts flying to overcome.
- Dwarf thread worms of humans have a change of generations between parthenogenetischem female (as a parasite) and a free-living two-sexual generation.
the change of generationsand master history plants
master history plants, which transition of fresh water Grünalgen to Moosen and ferns as well as later to the Ginkgopflanzen, which resinous wood plants and the bloom plants becomes lighter particularly well comprehensibly, if this is regarded in the light of those consequences, some certainForm of the change of generations to the consequence has. As of closest, today still living person of the related ancestors all land plants is considered today the Grünalge Mesostigma viride.
early single-celled organisms of the earliest forms of single-cell organisms to today no secured fossil excessive quantities. Conclusions on theirrather only comparing investigations permit presumed shape with today still occurring single-celled organisms. One of these model organisms is the Cyanobakterium with the loud-pictorial name Chroococcus and the conceivablly simplest shape, the spherical shape (illustration see: ). One subordinates an equivalence of all cells, which to the early single-celled organismsalso it means at the same time that the only possible reproduction form exists in the division in two parts of the cells, whereby the daughter cells separate sooner or later from each other.
the invention of the Sexualität
the bare cell division as method of the Vermehrung has the large disadvantage that favourable changesthe hereditary property (is called: favourable mutations) only to the own descendants can be passed on not be able to meet, independently of each other developed mutations however in an individual; even simplest bacteria order therefore over conjugation a mechanism specified for the change of hereditary factors (see also for this: Parasexualität). One can represent a developing of genuine Sexualität to model detention as development of the conjugation to a complete cell fusion: Sexualität is therefore first the smooth opposite of Vermehrung.
As model organism one can state single-cell Grünalgen, for example Chlamydomonas (illustration see: ). Many single-cell Grünalgen carry outregularly and frequently a ungeschlechtliche lengthening hurrying; two individuals can merge however also into a Zygote with one another. With Chlamydomonas directly a Meiose and afterwards a Mitose follow after the fusion of the cell cores, so that the Sexualakt of this Grünalge developing four new, single-cell and haploiden individuals to the consequence has. From two fusing single-celled organisms thus in the long run four “new” single-celled organisms follow.
simple and double Chromosomensatz
as model organism the Grünalge Chlamydomonas selected for an early form of the Sexualität is a haploider single-celled organism, that itself as a rule by lengthening hurryingincreased and only occasionally also sexually, whereby the phase of the Diploidie connected compellingly with the cell nuclear fusion is with it only from short duration. It requires however no large fantasy, in order from such a way of life us admitted higher animals to derive: The stage of the Diploidiemust become prevailing finally extended and (speak: the ability to the Parthenogenese by simple division is lost), furthermore the organism must become more cellular and have specialized fabrics the production of haploiden sex cells.
The master history of the plant followed however a first less dramatic way; as model organismfor the next stage of the development some brown algae offer themselves , for example Ectocarpus and Dictyota, which exhibit both an isomorphic and heterophasischen change of generations (illustration see:  and ): There are thus individuals (“alga plants”), the Sporen produces and as far as possible equal looking individuals, thoseSex cells produce; the one are diploid, the others haploid. From comparable isomorphic algae easy all heteromorphen variants of the change of generations can be derived, with which either or the other generation (at least for the human eye) prevails, up to the complex change of generations of the red algae (detailsfor this see under Tetrasporophyt).
problems of the land settling
the evolution researchers assume removed used the today living Grünalgen the transition from a purely aquatic to an predominantly landsettling way of life carry out could. Here it applied among other things, the urgent problem tooovercome: the lack of drip-cash-liquid water. Developing genuine roots was only the most obvious answer to the problem of the drynesses. In the long run many more seriously were the consequences, which the change of generations existing with the forerunners of the land plants produced: What do with the sex cells, if Wassermangel prevails?Landinhabiting male animals knew - due to their ability for free local movement - which females follow purposefully and more or less long, ductile appendices of body develop, with whose assistance its sex cells will partner inside dismiss directly in the damp environment of the inside of their could. The caked-on plants struck in the long runa similar way.
Der Generationswechsel bei den Moosen
Auch die heute lebenden Moose produzieren in ihren männlichen Geschlechtsorganen (den Antheridien) begeißelte Geschlechtszellen (Spermatozoide). They must move in drip-cash-liquid water to the Eizellen, those in the female sex organs(the Archegonien) stick. The Moose descends however from forerunner plants, with which the Gametophyt (the sex cells producing generation) became prevailing regarding the size; also each layman the well-known “MOO plant” is thus also that generation, which produces sex cells - she is thus on at least occasionalfreely available water dependent. This development to a dominating Gametophyten obviously cut so far each way to really grosswüchsigen kinds to the Moosen off, since their Spermatozoiden can succeed only at ground level the way from a plant to the neighbour plant. Moose are (probably even primarilybecause of her special change of generations) specialist for damp locations become or remained.
the change of generations with the ferns
also begeisselte sex cells ( Spermatozoide) produce ferns, which must put their way back to the female sex organs in a liquid. Differently than with the Moosen is but with the ferns of the Sporophyt (the Sporen producing generation) regarding the size prevailing become. This opened the ferns all possibilities for size growth, up to 30 meters high tree ferns. However the Gametophyt (the sex cells producing generation) remained with the ferns - similarly howwith the Moosen - earthconnected and actually resembles with many kinds of fern a Lebermoos (details for this see under pro thallium).
the Sexualakt becomes independently of free water
1896 discovered a Japanese researcher that Ginkgo - trees begeisselte sex cells (Spermatozoide) develop; a little laterthe same phenomenon was proven also with Cycas revoluta. Both plants can be classified therefore as living fossil and as voucher for the biological evolution, for the transition from sperm cells fertilization to seeds - the education.
Ginkgo is one two-strike-triumphs plant, it givesthus male and female individuals. Regarding still the highly rising Ginkgo tree corresponds to existences of the change of generations to the Sporophyt of the ferns and Moose. However the Sporen of the female plants no more are not set free into air, them rather as it were in the Sporangium established became and relativelargely (called megapores; There are megapores also with various algae). Besides the megapores keep assigned to so much food from the Ginkgo plants, which form them, that they can later develop sex cells without independent nutrition. The Sporen of the male plants (called micro pores or Pollen) experience stillin the Mikrosporangium ( the Anthere) their first mitotic divisions and are spread in such a way to the micro gametophyten and as such over the wind. The Ginkgo tree (thus the Sporophyt) separates the water at the Mikropyle the Samenanlage, necessary for the progressive movement of the male sex cells, out (Pollinations droplet). The polling hose only growing after arrival with the female plant (with Ginkgo it serves particularly as holdfast) is nothing else as the vestige of a once independently growing Gametophyten. Thus the sex generation ( the Gametophyt) is with Ginkgo (and with all other seed plants)extremely reduces.
Formulated one can say Phylogenetisch thus: Inside the megapore (Empryosackmutterzelle) an extremely strongly reduced Megagametophyt ( fertilizable Embryosack ) develops, for which over its Megagametangium (Synergiden) produces a Megagameten (Eizelle). The micro pore becomes as strongly reduced Mikrogametophyt (more-cellular polling grain) blown from the wind to the Megagametophyten, where they out-germinate, Mikrogameten (Spermatozoide) dismiss and thus the Megagameten fertilize.
the constant reduction of the Gametophyten and finally the incorporation of the female Gametophyten into its “nut/mother” - Sporophyten led in the long run to complete independencethe Sexualaktes of drip-cash-liquid water and so the adjustment of the plants perfects to the land life: This can be regarded as the most important reason for the fact that seed plants are today the prevailing way of life under the land plants.
With some Nacktsamern (with Eiben and Zypressen) still Pollinations droplets exist,in addition, with them the polling hose grows the inside the megapore. So a male cell core (a Spermium as it were reduced on the substantial) can penetrate directly to the Eizelle. With the Bedecktsamern the Samenanlagen are finally into a Fruchtblatt - housings enclosed, the nutrition of the out-germinating seeds(thus the juvenile Sporophyten) likewise optimizes like their spreading by animals. The Pollen lands here on a special structure of the Fruchtblattes, the scar.
- walter Carpenter: History of the plants. An overview. Munich (2. Edition 1969), dtv volume 4043
- Klaus UlrichLeistikow, Franziska Kockel:To the history of the development of the plants - a didactical model. In: Palmarum Hortus Francofortensis (scientific reports Frankfurt palm garden), number 2, Frankfurt/Main, June 1990 (list of contents)
Web on the left of
- Sexualität with Ginkgo (on English)