Non-random segregation of chromosomes is a deviation from the usual distribution of chromosomes during meiosis, that is, during segregation of the genome among gametes. While usually according to the 2nd Mendelian rule (“Law of Segregation of genes“) homologous chromosomes are randomly distributed among daughter nuclei, there are various modes deviating from this in numerous organisms that are "normal" in the relevant taxa. They may involve single chromosome pairs (bivalents) or single chromosomes without mating partners (univalents), or even whole sets of chromosomes, in that these are separated according to their parental origin and, as a rule, only those of maternal origin are passed on to the offspring. It also happens that non-homologous chromosomes segregate in a coordinated manner. As a result, this is a form of Non-Mendelian inheritance. This article describes cases where non-random segregation is the normal case for the particular organisms or occurs very frequently. A related phenomenon is called meiotic drive or segregation distortion. This is a higher than average transmission of a single chromosome relative to the homologous chromosome in inheritance. This can be due to non-random segregation during meiosis, but also to processes after meiosis that reduce the transmission of the homologous chromosome. In addition, there are pathological cases that result in aneuploidy and are almost always lethal.
Background and early history of research
According to the chromosome theory of inheritance formulated by Theodor Boveri in 1904, homologous chromosomes were expected to be randomly distributed among the daughter nuclei during meiosis. The first studies on this question appeared in 1908 and 1909. These papers dealt with spermatogenesis in aphids, i.e. meiosis in the male sex. In aphids, sex determination is mostly done according to the XX/X0 type: females have two X chromosomes, males only one. However, males only appear in one generation towards the end of the year, while otherwise there are only females, which reproduce by parthenogenesis. The question now was how it is achieved that all offspring in sexual reproduction are females. It turned out that meiosis I is inequal, i.e. results in two unequal-sized cells, and the X chromosome always ends up in the larger daughter cell. Only from this cell do two sperm cells emerge after meiosis II, while the smaller cell degenerates. Thus, each sperm - like the egg - contains an X chromosome, and only female offspring (XX) are produced. Also in 1909, a paper was published on the spermatogenesis of Coreus marginatus. There are two different X chromosomes and no Y chromosome (X1X20), and in meiosis I both X chromosomes are assigned to the same daughter nucleus. The same is apparently generally true in spiders, many species of which have been studied in subsequent years, as well as in various nematodess and in some aphids. The situation is somewhat more complicated in the American mole cricket Neocurtilla hexadactyla, which Fernandus Payne described in 1916: Here, three sex chromosomes are present (X1X2Y), two of which mate, while X1 is present as a univalent (unpaired). Although, as recent studies have confirmed, there is no mechanical linkage, the univalent X chromosome enters the same daughter nucleus that receives the other X chromosome.
It was only after all these counter-examples that a study by Eleanor Carothers on locusts appeared in 1917 - in the same journal as Payne's paper (Journal of Morphology) - which was seen as clear evidence for the expected random distribution. While earlier studies had been limited to sex chromosomes because homologous autosomes could not be distinguished, Carothers had found experimental animals in which homologous autosomes could also be partially distinguished. Payne's divergent findings were subsequently ignored, especially as they could not be confirmed in the European mole cricket. Thomas Hunt Morgan, who decisively contributed to the establishment of the chromosome theory of heredity, which was not yet generally accepted at that time, even explicitly wrote in his book The Physical Basis of Heredity (1919) that there was no contradictory evidence against the random segregation of maternal and paternal chromosomes (there is not a single cytological fact opposed to the free assortment of maternal and paternal chromosomes), although he was undoubtedly aware of the work of his former collaborator Payne. It was not until 1951 that Michael J. D. White rediscovered it and confirmed it through his own investigations. The third basic variant of non-random segregation, in which the complete sets of chromosomes of maternal and paternal origin are separated from each other, was studied - among some other peculiarities - in the 1920s and 30s by Charles W. Metz and co-workers in fungus gnats. Since then, numerous other counterexamples to random segregation have been described in very different creatures. It was not until 2001, however, that a first review paper appeared that was devoted precisely to this topic and was not limited to specific cases. The authors stated that most geneticists are unaware of non-random segregations or consider them rare exceptions. Due to the wide taxonomic distribution of the known cases, they argue that the importance of these phenomena has been underestimated so far.
Single chromosomes or chromosome pairs We first consider cases where only a single chromosome pair or a single unpaired chromosome (univalent) is affected, in the order of first description in the respective taxon.
Aphids
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