Meiosis: reduction division, stages, and role in sexual reproduction

After Édouard van Beneden had described in 1883 that the number of chromosomes is doubled during fertilization of the egg cell of the roundworm (Ascaris), Eduard Strasburger and August Weismann postulated that a reduction division must take place to compensate for this during the formation of the gametes. This was described for the first time in 1890 by Oscar Hertwig completely and in a way that is still valid today, also in the roundworm. At that time, chromosomes were known as structures that occur during nuclear division, but nothing was known about their function. It was not until 1900, when the hitherto unnoticed rules of heredity, which had been elucidated by the Augustinian monk Gregor Mendel and described as early as 1866, were rediscovered and confirmed by several scientists, that Walter Sutton noticed in 1902 that the behaviour of the chromosomes corresponded to Mendel's rules and therefore suspected a connection. Then, in 1904, Theodor Boveri postulated that chromosomes were the material carriers of hereditary traits (chromosome theory of heredity).

The term meiosis was coined by Farmer and Moore in 1905.

The two stages of meiosis have been called different names by different authors:

• First section: 1st meiotic division, 1st mature division, meiosis I or reduction division
• Second section: 2nd meiotic division, 2nd mature division, meiosis II or equation division.

The term "reduction division" is also used for meiosis as a whole.

The change between a haploid and a diploid phase in the course of sexual reproduction is called nuclear phase change. This can occur in several variants. In humans, as in all multicellular animals, the diploid phase is predominant; only the gametes are haploid. Such organisms are called diplonts. The reverse case is represented by many fungi, many algae, and some protozoa (flagellates), which are normally haploid and whose diploid phase is confined to the zygote (haplonts). Third, there are diplohaplounts, in which haploid and diploid generations alternate, as in all plants and most algae. In organisms with higher degrees of ploidy, halving also occurs during meiosis, for example from tetraploid (four sets of chromosomes) to diploid.

In asexual reproduction, there is no nuclear phase change and thus no meiosis. It occurs in numerous forms in plants, algae, fungi and lower animals. To be distinguished from this is unisexual reproduction, in which female individuals produce offspring without fertilization. In animals, this is known as parthenogenesis or virgin reproduction. In this process, meiosis may be omitted altogether or reversed by subsequent karyogamy. Parthenogenesis is widespread in the animal kingdom (with the exception of mammals). It usually occurs in alternation with sexual reproduction; but the latter may be omitted altogether. One group of animals in which this has apparently been the case for millions of years is the Bdelloida, which belong to the rotifers. Many flowering plants can form seeds without fertilization (agamospermia). This can occur both unisexually, in which meiosis is omitted (as in various composite plants such as the dandelion), and asexually, in which the embryo emerges from vegetative tissue (as in the citrus plants).