Non-random segregation of chromosomes
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.
[5] 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.
Under certain conditions, mostly due to the decreasing day length towards the end of the vegetation period of the host plants, one generation occurs in which males are also present.
After fertilisation, eggs are laid, which survive until the beginning of the next growing season and then only produce females (XX), which again reproduce parthenogenetically.
In this species, J. Seiler (1920), an associate of Richard Goldschmidt, studied the inheritance of sex and the behaviour of the univalent Z chromosome during oogenesis.
Similarly, more males were produced when mating occurred a few days after hatching and thus towards the end of the short life of the female Imago.
[6] The first case of non-random segregation of single chromosomes in a plant was described by Marcus Morton Rhoades in 1942 in maize.
[14] Since Catcheside only studied male meiosis, which usually gives rise to four fertile daughter cells, it cannot be concluded that non-random segregation contributes to the accumulation in inheritance that is characteristic of B chromosomes in general.
In 1957, Hiroshi Kayano described the behaviour of a B chromosome in female meiosis in the Japanese lily species Lilium callosum, which is mostly present only in singular and therefore exists as a univalent.
[15] This work by Kayano seems to be the only one so far to demonstrate the accumulation of a B chromosome as a result of non-random segregation during meiosis in the embryo sac mother cell.
[16][17] In contrast, an accumulation of B chromosomes in plants by a directional nondisjunction in mitoses before or after meiosis has been observed in many cases, as for the first time in 1960 by Sune Fröst in the Crepis pannonica.
If two Bs are present, then they mate during reduction division (which is meiosis II here, as it is generally in mealybugs, scale insects and aphids) and segregate in the normal way.
[30] In contrast, Zipora Lucov and Uzi Nur found an example of non-random segregation at oogenesis in the North American species Melanoplus femurrubrum in 1973.
The reduced transmission of the additional segment is most likely due to non-random segregation during oogenesis, because the alternative possibility of differential mortality of zygotes could be excluded.
In female meiosis I of this species, unpaired B chromosomes are preferentially assigned to the future egg nucleus and thus accumulate in the inheritance.
How this happens was unclear for a long time, but according to recent studies it is apparently due to the fact that the univalent X chromosome is preferentially allocated to the future egg nucleus during meiosis I.
[39] Such cosegregation of mechanically coupled sex chromosomes has also been described in spiders, nematodess, stoneflies, ostracods, in a scale insect, and in beetles.
This is part of the normal course of meiosis in the spermatogenesis of various Neuroptera, some Alticini, the cricket Eneoptera surinamensis, and the Mesostoma ehrenbergii (Turbellaria).
[54] However, an electron microscopic examination revealed some microtubules, which also make up the spindle fibers, and which here appear to form a fine connection between X1 and Y.
[4] Targeted irradiation of this microtubule junction with UV microbeams often (in about one-third of cases) resulted in X1 moving to the other half of the spindle.
Dwayne Wise et al. concluded that these four microtubule bundles form an "interacting network" that enables the coordinated segregation of sex chromosomes, i.e., the correct allocation of the X1.
Their segregation starts right after the nuclear envelope dissolution, the metaphase is omitted, and the paternal chromosomes enter a small daughter cell which, like the polar bodies, passes away during oogenesis.
[56] This doubling of their number is compensated for at an early stage of embryonic development by eliminating excess L chromosomes from the nucleus, so that exactly two always remain.
In the female sex, i.e., in the embryo sac mother cell, all the univalents migrate undivided at meiosis I to the spindle pole that lies in the direction of the micropyle.
In the embryo sac mother cell, on the other hand, they all migrate towards the micropyle with a greatly increased probability and thus preferentially enter the oocyte.
From the widespread occurrence of such phenomena (in plants, insects, and vertebrates) and the diversity of the respective sequence, they conclude that a functional asymmetry of spindle poles - one of the prerequisites of non-random segregation - is probably present in principle and not only exceptionally.
This is also true for humans, where non-random segregation occurs when structurally abnormal chromosomes are present as a result of Robertsonian translocations.
[7] Elsewhere, the two authors argue for a significance of non-random segregation of structurally different homologous chromosomes (as in Robertson translocations) in the emergence of new species in evolution (speciation).
They reproduce in two manners, firstly in a way that their progeny will differentiate, and thus contribute functionally to the tissue, secondly remaining uncommitted and replenishing the stem cell pool.
[66] H3T3P separates sister chromatids enriched with diverse pools of H3 in order to coordinate asymmetric segregation of "old" H3 into germ stem cells and that male germline activity requires tight regulation of H3T3 phosphorylation.
