Are gametes considered to be diploids or haploids
Recognition and union of two cells (individuals) are indeed prerequisites for a sexual union, but they are not sufficient to explain a cooperation and a new combination of two genomes.
Merging two nuclei of the same type leads, purely formally, to a duplication of genetic information. Subsequent fusions would ultimately (over several cell generations) lead to an exponential increase in the genetic information of the cells.
As is well known, this is not the case, because meiosis reduces the amount of information doubled by amalgamation by half. It follows from this that sexuality (zygote formation) and meiosis must be processes that are directly linked to one another. One can distinguish between three phases of increasing development:
Formation of zygotes followed by meiosis. The advantage over vegetative reproduction can be seen in a new combination of genetic information in the products of meiosis (gons), which can be more advantageous than that of the parents.
A certain period of time elapses between zygote formation and meiosis. The life cycle of the species is divided into two phases, a diploid and a haploid. One also speaks of a core phase change; often, but not always, this is linked to a generation change. In contrast to the nuclear phase change, a generation change involves a certain vegetative development of the zygote, or spores, or, viewed differently, the zygote alone or the gametes alone are not yet a generation.
During the diploid phase, the genomes of both parents can complement each other. The presence of a functional allele is sufficient for the corresponding gene to perform to its full potential. The perfection of diploidy required the evolution of a difficult regulation of gene expression.
- The diploid phase predominates, only the highly specialized sex cells are haploid.
There are gradual transitions between (1.), (2.) and (4.). The mode of generational change is an important taxonomic feature of the systematic groups of cryptogams and phanerogams.
The reduction division takes place immediately after the zygote formation.
The zygote formation takes place immediately after the reduction division.
In most unicellular flagellates, both gametes (female, male or +, -) are usually of the same size: isogamy. In primitive multicellular cells, there is a tendency to produce sex cells of different sizes (mobile gametes). The female gametes are larger than the male ones: heterogamy or anisogamy.
The progression series continues in the direction of oogamy: the female gamete (egg cell, oogonium) is flagellant. The core is surrounded by a voluminous, nutrient-rich plasma. The male gamete is small, mobile, and the cell contains only a small amount of plasma (sufficient to supply energy for movement). Much more male than female gametes are always produced.
The development of gametophytes goes hand in hand with the development of oogamy, in whose protection the gametes develop. While the male gametes leave the gametophyte after they have matured, the egg cell continues to develop into an embryo under the protection of the gametophyte after fertilization.
The progression series from isogamy to oogamy has been "invented" several times in the course of evolution. They are found in the plant kingdom, in the animal kingdom and in mushrooms.
A (haploid) gametophyte develops from a haploid cell (spore) created by reduction division. Algae and most pteridophytes always produce the same type of spores: homosporia. In the more highly developed groups of pteridophytes, spores of unequal size develop: heterosporia from which male or female gametophytes arise. The heterospore ferns are considered to be the precursors of the seed plants.
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