The recognition of and the union between two cells or individuals are preconditions of a sexual union, but they are not sufficient to explain the co-operation and recombination of two genomes.
In a strictly technical sense, the fusion of two nuclei of the same species leads to a doubling of the genetic information. Subsequent fusions could thus during the course of several cell generations lead to an exponential increase in the genetic information of the cells.
This is not the case, since the amount of information that was doubled by fusion is subsequently reduced to half its size again. This means, that the formation of a zygote and meiosis are directly linked processes. Three different stages of increasing development can be distinguished:
The formation of a zygote with an immediately occurring subsequent meiosis. Compared to vegetative reproduction, the genetic information of the products of meiosis is recombined and these new combinations could be of more advantage than the original parental combination.
Some time passes in between the formation of the zygote and meiosis. The life cycle of the species consists of two phases, a diploid and a haploid phase. In this case, it is also spoken of an alternation of the nuclear phases. This alternation is often, though by far not always paralleled by an alternation of generations. In contrast to an alternation of the nuclear phase, an alternation of generations means not only the zygote or the gametes alone, but a certain vegetative development of the zygote or the spores, too.
During the diploid stage, the genomes of both parents may supplement each other. The presence of at least one fully functional allele is enough to reach a 100 percent performance of the respective gene. The perfection of diploidy required the evolution of a more fine-tuned adjustability of gene expression.
- The diploid phase prevails, while only the highly specialized gametes are haploid.
The stages 1., 2., and 4. display gradual transitions. The mode of the alternation of generations is an important taxonomic feature of cryptogams and phanerogams.
Alternation of generations: haplophase predominates. The reduction division takes place immediately after the development of the zygote.
Alternation of generations: the diploid phase predominates. Reduction division takes place immediately after the development of the zygote.
In most single-celled flagellates, the female and the male or the + and the – gametes, respectively have the same size. This state is called isogamy. Primitive multicellular organisms in contrast display a tendency towards gametes of different size. The female gamete is larger than the male gamete, a state called heterogamy or anisogamy.
The tendency progresses further towards oogamy. In oogamy, the female gamete, also called egg or oogonium has no flagellum. Its nucleus is surrounded by a voluminous plasma containing a lot of nutriments. The male gamete is small, mobile, and contains only a small amount of plasma, just enough to provide the energy required for the movements. In oogamy, always far more male than female gametes are produced.
The evolution towards oogamy is accompanied by the evolution of gametophytes that again protect the development of the gametes. While the male gametes leave the gametophyte after ripening, the egg cell becomes the embryo after fertilization that remains in the protection of the gametophyte.
The progression from isogamy to oogamy occurred several times in the course of evolution. It exists in the plant kingdom, the animal kingdom, and with fungi.
A haploid gametophyte develops from a haploid cell, a spore, that again is the product of a reduction division. In algae and most pteridophytes, always the same type of spore is produced, a state called homospory. In contrast, the more advanced groups of pteridophytes produce spores of different size. This is termed heterospory. Male and female gametophytes, respectively, develop from these spores. It is assumed that the heterosporic ferns are the precursors of seed plants.
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