Non-random segregation of chromosomes

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The non-random segregation of chromosomes is a deviation from the usual distribution of chromosomes in meiosis , i.e. in the segregation of the genetic material onto the germ cells . While homologous chromosomes are usually distributed randomly to the daughter nuclei according to Mendel's 2nd rule (splitting rule) , there are various modes that deviate from this in numerous living beings that are "normal" in the taxa concerned . They can affect individual chromosome pairs ( bivalents ) or individual chromosomes without mating partners (univalents) or even entire sets of chromosomes by separating them according to their parental origin and usually only those of maternal origin are passed on to the offspring. In addition, it happens that non-homologous chromosomes segregate in a coordinated manner. The result is a form of non-Mendelian inheritance .

This article describes cases in which non-random segregation is the normal case or occurs very frequently for the respective living beings. A related phenomenon is known as meiotic drive or segregation distortion. This is an above-average transmission of a single chromosome compared to the homologous chromosome in inheritance. This can be based on a non-random segregation during meiosis, but also on processes after meiosis, which reduce the transmission of the homologous chromosome.

There are also pathological cases that result in aneuploidy and are almost always fatal .

Background and early research history

Theodor Boveri

According to the chromosome theory of inheritance formulated by Theodor Boveri in 1904, it was to be expected that homologous chromosomes would be randomly distributed to the daughter nuclei during meiosis. The first studies on this question appeared in 1908 and 1909. This work dealt with the spermatogenesis in aphids , ie meiosis in males. In the case of aphids, sex is usually determined 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 that reproduce parthenogenetically . The question now was how it is achieved that all offspring are females during sexual reproduction . It turned out that meiosis I is inequitable, i.e. results in two cells of different sizes, and that the X chromosome always ends up in the larger daughter cell. Only from this do two sperm emerge after meiosis II, while the smaller cell degenerates. Every sperm - like the egg cell - contains an X chromosome, and only female offspring (XX) arise.

Also in 1909 there was a work on the spermatogenesis of the leather bug . There are two different X chromosomes and no Y chromosome (X 1 X 2 0), and in meiosis I both X chromosomes are assigned to the same daughter nucleus. The same is evidently the case with spiders , of which many species were examined in the following years, as well as with various nematodes and with some aphids. The relationships with the American mole cricket Neocurtilla hexadactyla , which Fernandus Payne described in 1916, are somewhat more complicated : Here there are three sex chromosomes (X 1 X 2 Y), two of which are paired, while X 1 is univalent (unpaired). Although, as recent studies have confirmed, there is no mechanical connection, the univalent X chromosome ends up in the same daughter nucleus that also contains the other X chromosome.

Thomas Hunt Morgan

Only after all these counterexamples did a study by Eleanor Carothers on locusts appear in 1917 - in the same journal as Payne's work ( Journal of Morphology ) - which was regarded as clear evidence of the expected random distribution. While earlier investigations were limited to sex chromosomes because homologous autosomes could not be distinguished, Carothers had found experimental animals in which homologous autosomes could also be partly differentiated. Payne's deviating findings were subsequently ignored, especially since they could not be confirmed in the case of the European mole cricket . Thomas Hunt Morgan , who made a decisive contribution to the establishment of the then not yet generally recognized chromosome theory of inheritance, even wrote in his book The Physical Basis of Heredity (1919) that there were no contradicting evidence against the accidental 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 familiar with the work of his former colleague Payne. Michael JD White only rediscovered this in 1951 and confirmed it through his own research.

The third fundamental variant of non-random segregation, in which the complete sets of chromosomes of maternal and paternal origin are separated from one another, was investigated in the 1920s and 30s by Charles W. Metz et al . Since then, numerous other counterexamples of random segregation in very different living things have been described. However, it was not until 2001 that a first review was published that was dedicated to this very topic and was not limited to specific cases. The authors stated that most geneticists are unaware of non-random segregations or consider them to be rare exceptions. Due to the wide taxonomic distribution of the known cases, they argue that the importance of these phenomena has so far been underestimated.

Single chromosomes or pairs of chromosomes

We will first consider cases in which only a single chromosome pair or a single unpaired chromosome (univalent) is affected, in the order in which they were first described in the respective taxon.

Aphids

Parthenogenetic birth of an aphid

As mentioned, the behavior of the X chromosome in the spermatogenesis of aphids was described as the first example of non-random segregation as early as 1908. These insects only exist as females for most of the year and reproduce parthenogenetically, i.e. without the participation of males. There is no fertilization and no meiosis, and the successive generations are genetically identical. Under certain conditions, mostly due to the decreasing day length towards the end of the host plants' growing season, a generation occurs in which males are also present. This is achieved by the fact that the two X chromosomes present in females pair like in a meiosis and their number is reduced to 1, so that males (X0) are formed.

The fact that after this one bisexual generation only females emerge is based, as shown above, on the fact that the X chromosome in spermatogenesis is always assigned to the daughter cell from which sperm arise. Hans Ris described the exact course of meiosis in 1942: According to this, the X chromosome in the anaphase does not take part in the movement towards the poles of the nuclear division spindle , but is stretched between the diverging poles. The chromosome also remains in this position during the subsequent cleavage (cell division). Only in a late stage of the furrow does the furrow groove shift to one side and the X chromosome is assigned to the opposite, larger daughter cell. Since only two sperm arise from this, all sperm as well as the egg cells contain an X chromosome. After fertilization, eggs are laid which last until the beginning of the next vegetation period and from which only females (XX) emerge, which reproduce again parthenogenetically.

Butterflies

The eponymous tube with which the caterpillar of the tube bag carrier surrounds itself

In butterflies , the sex of the offspring is not determined by whether or what kind of sex chromosome the sperm contributes, as is the most common case among animals, as is the case with humans, but rather by the features of the egg cell. With them, the female sex is heterogametic , the male homogametic. In such cases one does not speak of X and Y chromosomes, but of Z and W chromosomes. Males have two Z chromosomes (ZZ), females either one Z and one W chromosome (ZW) or only one Z chromosome (Z0). An example of the ZZ / Z0 type is the tubular sack carrier . In this species, J. Seiler (1920), a colleague of Richard Goldschmidt , examined the inheritance of sex and the behavior of the univalent Z chromosome in oogenesis . He found that the gender ratio among the offspring depends on the temperature and age of the mother. At cool temperatures ("room temperature of about 12-16 °") the Z chromosome got into the polar body in 57% of the investigated cases in meiosis I and in the future egg nucleus in only 43%. Accordingly, Seiler found an excess of females in the offspring. Conversely, the chromosome in the incubator at 30–37 ° was assigned preferentially to the egg cell, and there was an excess of 62% male offspring. Likewise, more males emerged if mating took place a few days after hatching and thus towards the end of the short life of the female imago . (As in most invertebrates , meiosis pauses here in metaphase I and is not completed until after fertilization. See standstill of female meiosis .)

Indications of non-random segregation in female meiosis were also found in butterflies of the ZZ / ZW type. In some species of the genera Danaus and Acraea , females occur that only produce female offspring (ZW). This is apparently based on the fact that the W chromosome always gets into the egg cell and not into the polar body. This modification of the meiotic chromosome distribution is hereditary and is linked to the W chromosome.

Sciarid gnats

The fungus gnats, the spermatogenesis of which has some peculiarities (summary for sciarid gnats # genetics ), were already mentioned. In meiosis II, a peculiarity occurs with the X chromosome. Normally in meiosis II (as in mitosis ) all chromosomes are divided into the two chromatids that make them up and these are assigned to the two daughter nuclei. In the case of sciarid gnats, however, the X chromosome goes to one of the spindle poles prematurely and only divides there or on the way there. Since a sperm emerges only from the cell that develops there, it then contains two X chromosomes, and the zygote after fertilization three accordingly. One of these X chromosomes is eliminated at an early embryonic stage, restoring the normal female chromosome makeup (XX).

Flowering plants

The first case of a non-random segregation of individual chromosomes in a plant was described by Marcus M. Rhoades in 1942 in maize . This non-randomness occurs when there is an abnormal form of chromosome number 10 that contains an extra segment. Since this additional segment can be recognized as a knot-like structure in the pachytan of the meiotic prophase (English knobbed ), the chromosome is referred to as K10. It occurs particularly with some old varieties of corn from the North American Indians. If there is only one K10 and one normal chromosome 10 and in female meiosis I the crossing-over takes place in such a way that the chromatids are of different lengths, then in meiosis II the chromatid containing the nodular additional segment reaches about 70 percent probability in the embryo sac and thus in the egg cell. The segment is thus accumulated to a large extent in the inheritance; it has a meiotic drive . This also applies if other chromosomes also carry the segment, but only if at least one K10 is present.

A corresponding accumulation of additional chromosome segments has also been described in some other plant species, but has not been investigated further. Investigations on additional chromosomes, the B chromosomes , which are not homologous with regular chromosomes and only occur in some of the individuals in a population, i.e. have no essential functions, are much more numerous . A non-random segregation of B chromosomes was first described by Catcheside in 1950 in the Guayule . This shrub-like daisy family , the B-chromosomes pair, if they are present in majority in meiosis I or only fleetingly, are therefore usually in the form univalents. Nevertheless, there is a high probability that they migrate to the same pole in anaphase I.

Since Catcheside only examined male meiosis, from which four fertile daughter cells usually arise, it cannot be concluded from this that the non-random segregation contributes to the accumulation in inheritance that is generally characteristic of B chromosomes. The situation is different with female meiosis, in which three of the four daughter nuclei degenerate. In 1957, Hiroshi Kayano described the behavior of a B chromosome in female meiosis in the Japanese lily species Lilium callosum , which is mostly only present in the singular and therefore present as a univalent. He found that about 80% of the chromosome is allocated to the future egg cell and that it is passed on to 80% of the offspring.

This work by Kayano seems to be the only one to date in which the accumulation of a B chromosome as a result of non-random segregation in meiosis in the embryo sac mother cell has been demonstrated. In contrast, an accumulation of B chromosomes in plants due to a directed nondisjunction in mitoses before or after meiosis was observed, for the first time in 1960 by Sune Fröst in the Pannonian Pippau . Often both chromatids end up in the same daughter cell (nondisjunction), and this is directed in such a way that an accumulation results in the inheritance. A non-accidental segregation in meiosis can therefore only be concluded if a directed nondisjunction in mitoses can be ruled out. This is largely assured for the Mediterranean saw-blade plantain Plantago serraria and for the spotted piglet . Another case is probably the tuberous timothy grass ( Phleum nodosum ).

To fly

Similar to maize, the fruit fly Drosophila melanogaster also exhibits non-random segregation in female meiosis if homologous chromosomes are of different lengths and, as a result of the crossing-over in meiosis II, chromosomes with chromatids of different lengths are present. Then there is a 70% probability that the shorter chromatid will get into the nucleus. This was discovered in 1951 by E. Novitski. Later it was also found in gold flies ( Lucilia ) and onion flies ( Hylemya ), so it is apparently a widespread phenomenon in flies.

In D. melanogaster , non-random segregation can also occur in male meiosis. This is the case when the sex chromosomes (X and Y) do not pair in meiosis I. Then the unpaired chromosomes mostly end up in the same daughter cell. Accordingly, there are many males of the X0 type among the offspring, but surprisingly only a few of the XXY type. The latter is due to the fact that the daughter cells with the XY constitution are disturbed in their development. On the other hand, the X0 males are sterile. In the end, the X chromosome involved is enriched in the inheritance (meiotic drive).

The mealybug Pseudococcus affinis

A female of Pseudococcus affinis

B chromosomes are also common in the animal kingdom. In 1962, Uzi Nur described a non-random segregation in both sexes of the mealybug Pseudococcus affinis . In oogenesis, the segregation behavior of the B chromosome depends on the number of it. If there are two Bs, then they mate in the reduction division (which is meiosis II here, as is generally the case with grease insects and scale insects, as well as with aphids) and segregate in the normal way. However, if only one is present, then in two thirds of the cases it gets into the polar body and only the remaining third into the nucleus. Likewise, the unpaired surplus B chromosome behaves when there are 3 or 5 Bs, while the paired segregate normally. Overall, there is a tendency in women to exclude B chromosomes from inheritance through non-random segregation, which is particularly important when only one is present. However, in the male sex there is a strong tendency towards the accumulation of B chromosomes. This is possible because half of the meiosis products of this species (like many other mealybugs and scale insects) regularly degenerate. With the reduction division (also here meiosis II) all B-chromosomes are assigned to the future sperm nucleus with about 90% probability.

Grasshoppers

Female of the spotted cadaver

The transmission of B chromosomes was also examined in various locusts . As with plants, it turned out that the number of B chromosomes can increase even before meiosis due to mitotic nondisjunction. In contrast, Zipora Lucov and Uzi Nur 1973 found an example of non-random segregation in oogenesis in the North American species Melanoplus femurrubrum . Since there was never more than one B chromosome, accumulation before meiosis was ruled out in this case. Nevertheless, this chromosome was passed on to about 80% of the offspring. Hewitt's (1976) study of the spotted cadaver was even more revealing . Hewitt found that when the eggs were fixed in metaphase I (time of oviposition), the B chromosomes were usually found in the inward half of the dividing spindle, i.e. near the future egg nucleus. This corresponded to the transmission rate of about 75%. How common such non-random segregation of B chromosomes is otherwise in grasshoppers cannot yet be estimated. It is true that many species of locusts are known to have B chromosomes. However, their transmission has only been investigated in a few cases, and the non-random segregation in meiosis is only one of several ways in which a non-Mendelian transmission can come about.

Another chromosomal abnormality that is often found in grasshoppers is additional segments on individual chromosomes. Such additional segments can segregate quite randomly, and it was actually locusts with homologous chromosomes of unequal length that Carothers first succeeded in detecting random segregation in 1917. In contrast, López-León et al. (1991, 1992) evidence of non-random segregation in two grasshopper species: In Eyprepocnemis plorans , an additional segment in the female sex is less likely to be transmitted than the normal homologous chromosome if a B chromosome is also present. The B chromosome thus influences the transmission of a regular chromosome pair, while even in this case it follows Mendel's rules. The reduced transmission of the additional segment is very likely due to a non-random segregation in the oogenesis, because the alternative possibility of a differential mortality of the zygotes could be excluded. In Chorthippus jacobsi , López-León et al. the transmission of various additional segments on three different chromosomes. While all additional segments are transmitted normally on chromosomes M 5 and M 6 , there is always an accumulation in both sexes when an additional segment is located on the small chromosome S 8 . Even if both S 8 chromosomes have additional segments of different sizes, these do not follow Mendel's rules, but the shorter segment is preferably passed on. Here, too, there is a high probability of non-random segregation in oogenesis. How the non-Mendelian transmission takes place through the male sex is unclear.

Rodents

The first description of non-random segregation in a mammal appeared in 1977 and dealt with the forest lemming . In some populations of this species up to 80% of the animals are female. Some of the females have the “male” chromosome constitution XY. The fact that these animals develop into females even though they have a Y chromosome is due to a mutation on the X chromosome. In meiosis, this mutated chromosome (X *) reaches the egg nucleus more frequently than the Y chromosome and is therefore more likely to be transmitted to the offspring. A second example concerns a B chromosome in the Siberian collar lemming Dicrostonyx torquatus . In female meiosis I of this type, unpaired B chromosomes are preferably assigned to the future nucleus and thus accumulated in the inheritance.

In Siberian populations of the house mouse there is a different form of chromosome 1 with two insertions . This extended variant is passed on by heterozygous females with a much higher probability than normal chromosome 1. As it turned out, this happens through non-random segregation of the homologous chromosomes or chromatids in both meiotic divisions. This allows up to 85% of the offspring of a heterozygous female to receive the insertions. The latter is only the case, however, if the males used in the crossing experiments are not also carriers of these insertions. If homozygous carriers of these insertions were taken instead, in which each sperm received the insertions, then the non-randomness was reversed with female meiosis: in this case only about 1/3 of the offspring of a heterozygous mother received the insertions from this mother. This surprising influence of the sperm on the meiosis in the egg cell is possible because in mice, as in general in vertebrates, female meiosis pauses in metaphase II until fertilization takes place (see standstill of female meiosis ).

It has been known since 1962 that female mice with only one X chromosome (XO) are fertile, but that their daughters predominantly have two X chromosomes. How this happens has long been unclear, but according to recent studies it is apparently based on the fact that the univalent X chromosome in meiosis I is preferably assigned to the future nucleus.

Coordinated segregation of non-homologous chromosomes

Mechanically coupled univalents

The fact that two non-homologous chromosomes segregate in a coordinated manner in meiosis was first described in 1909 for the leather bug . In her, males have two different X chromosomes (X 1 X 2 0), and in meiosis I these are both assigned to the same daughter nucleus. Later research on other bedbugs revealed that the X chromosomes are linked and that their co-segregation was apparently based on this. Up to five different X chromosomes can be present, and most species also have a Y chromosome that migrates to the opposite spindle pole. Such a co-segregation of mechanically coupled sex chromosomes has also been described in spiders, nematodes , stone flies , mussel crabs, scale insects and beetles .

Free univalents

In some species of aphids, the males have two different X chromosomes (X 1 X 2 0) that are not mechanically connected and still reach the same spindle pole in meiosis I. This is consistent with the mode of directed segregation of a single X chromosome described above. In other aphid species, four different chromosomes appear to cosegregate in this way. A cosegregation of free univalents has also been described in the giant crab spider Delena cancerides . There are three different X chromosomes in males that are not mechanically connected as in other spiders and are still assigned to the same spindle pole.

The vortex worm Mesostoma ehrenbergii

More interesting are those cases in which free univalents of different kinds segregate in a regulated manner to opposite spindle poles. This belongs in spermatogenesis various lacewings , some flea beetle , the cricket Eneoptera surinamensis and whirl worm Mesostoma ehrenbergii the normal course of meiosis. Netwings usually have an X and a Y chromosome. which do not mate in meiosis. However, some species have multiple univalent sex chromosomes, and univalent B chromosomes can be added. They all segregate in an orderly manner to the spindle poles. This is known as distance segregation . Similar relationships with multiple sex univalents have also been described in some flea beetles. The cricket Eneoptera surinamensis has three free univalent sex chromosomes (X 1 X 2 Y) that migrate to the spindle poles while the autosomes congregate at the spindle equator.

In the vortex worm Mesostoma ehrenbergii, only three of the five pairs of chromosomes pair in meiosis. So there are three bivalents and four univalents, and here too the univalents segregate before the bivalents. The univalents are often not correctly distributed in fixed specimens. Hilary A. Oakley found the reason for this when she observed the process on a living object. According to this, the univalents move back and forth between the poles in metaphase I, i.e. when the bivalents are at the equator. Usually only one univalent moves, and after a longer break (five to ten minutes) another one starts moving. This continues until all four are correctly distributed. This is followed by the anaphase, i.e. the segregation of the paired chromosomes.

The mole cricket Neocurtilla hexadactyla

The American mole cricket Neocurtilla hexadactyla

In the case of the American mole cricket Neocurtilla hexadactyla mentioned above , live observations of meiosis were also very informative. As with Eneoptera, there are three sex chromosomes (X 1 X 2 Y), but only X 1 is univalent. In this case, too, the segregation of the sex chromosomes takes place before that of the autososomes, in that the X 2 Y bivalent is already shifted in metaphase I from the metaphase plate to a spindle pole in such a way that the Y chromosome is close to it, while the univalent X 1 is at the other pole. Through micromanipulation experiments in which they shifted the bivalent or the univalent in the spindle, René Camenzind and R. Bruce Nicklas (1968) found that X 1 is the active element and is based on the orientation of the bivalent. The authors also found that there was no mechanical connection between the two. However, an electron microscope examination revealed some microtubules , which also make up the spindle fibers and which here apparently form a fine connection between X 1 and Y. Targeted irradiation of this microtubule connection with UV micro-rays often (in about a third of cases) resulted in X 1 migrating to the other half of the spindle. Surprisingly, irradiation of one of the three spindle fibers on which the sex chromosomes were attached had the same effect, while irradiation of autosomal spindle fibers had no effects. Dwayne Wise et al. concluded that these four microtubule bundles form an “interacting network” that enables the coordinated segregation of the sex chromosomes, i.e. the correct allocation of the X 1 .

Complete sets of chromosomes

Sciarid gnats

A fungus gnat of the genus Sciara

The behavior of the chromosomes in the spermatogenesis of sciarid gnats is very unusual in several ways. A detail of Meiosis II has already been discussed above; Meiosis I, however, is far more remarkable. Since the otherwise obligatory pairing of homologous chromosomes is completely omitted, and these are separated from one another according to their origin - maternal or paternal. Their segregation begins immediately after the nuclear envelope has dissolved, the metaphase is omitted, and the paternal chromosomes get into a small daughter cell, which, like the polar bodies, disappears during oogenesis. All sperm receive only the maternal chromosomes, and the males only act as intermediaries between purely female lines of inheritance. The construction of the spindle apparatus with this division is also unusual. It is not a bipolar spindle, just a half spindle with only one pole. The maternal chromosomes move towards this pole, the paternal away from it.

Some fungus gnats have germline limited in addition to the regular chromosomes or L-chromosomes (of English. Limited , Limited), which only in cells of the germ line are present and from somatic cells are eliminated. During spermatogenesis, these segregate with the maternal regular chromosomes, so they enter the sperm without being reduced. This doubling of their number is compensated for at an early stage of embryonic development by excreting excess L chromosomes from the cell nucleus, so that exactly two always remain.

Gall mosquitoes

In gall mosquitoes , too , the sperm contain only the chromosome set of maternal origin, while the paternal chromosomes are eliminated in meiosis I. Here, too, the pairing of homologous chromosomes does not take place, the cell division is inequitable, and only the maternal chromosomes move to a spindle pole, whereby they get into the daughter cell from which two sperm cells emerge after meiosis II, while the other daughter cell perishes. In addition, there are numerous germline-limited chromosomes which, like those in sciarid gnats, remain with the paternal regular chromosomes and are thus eliminated.

Scale insects

Female (shield-shaped) and male (winged) of a scale insect

For most scale insects the males are parahaploid : Although they have two sets of chromosomes, only the chromosomes of maternal origin are active, and only they will be passed on to offspring. The inactivation of the paternal chromosomes takes place at an early embryonic stage ( blastula ) when the chromosomes are strongly compressed ( heterochromatized ). (This also occurs in humans, where one of the two X chromosomes in the female sex becomes heterochromatic.) Elimination from inheritance can take place in different ways; only one of them occurs in meiosis. This is known as the lecanoid chromosome system. Meiosis is inverse in scale insects, as in the aphids discussed above. H. the actual reduction division is meiosis II. In the lecanoid mode, the chromosomes form a “double metaphase plate” in which all maternal chromosomes are on one side and all paternal chromosomes on the other. (Normally, this is random.) In the anaphase, the two complete sentences then separate and each form their own daughter kernel. Since meiosis II is not connected with cell division here and the two daughter structures of the first division also reunite with each other, a quadricuclear cell ultimately results (as is generally the case with spermatogenesis of scale insects). Of the 4 nuclei, however, only the two with the maternal chromosomes become sperm nuclei; the other two condense more and more and finally perish.

plants

Blossom of a dog rose

Polyploidy is very common in the plant kingdom . Mostly they are allopolyploid species in which each chromosome finds a homologous partner during meiosis. But there are also species with an odd number of chromosome sets. As a rule, these can only be apomictic , i.e. H. bypassing meiosis and fertilization, reproduce because univalents are randomly distributed to the daughter nuclei during meiosis. However, some plants are known in which the univalents are distributed non-randomly and which can therefore reproduce sexually. The oldest example are the dog roses , in which this was discovered as early as 1922. They are pentaploid, i.e. H. they have five sets of chromosomes. Only two of these pair in meiosis in both sexes, so that there are 7 bivalents and 21 univalents. In the female sex, i.e. in the embryo sac mother cell, all univalents in meiosis I migrate undivided to the spindle pole, which lies in the direction of the micropyle . Since the embryo sac with the egg cell is then formed there, it receives 4 complete sets of chromosomes. In pollen meiosis, on the other hand, many univalents remain in anaphase I or II (so-called lagging) and are thus lost. This chromosome loss is so high that more than 1/10 of the pollen grains only contain a haploid set of those chromosomes that were paired during meiosis. And since only these haploid pollen grains are functional, the complete pentaploid chromosome stock is restored during fertilization. In this way, 3 of the 5 sets of chromosomes are transmitted exclusively through the female line, while the other two behave normally.

The Australheide family Leucopogon juniperinus is triploid, and of its 3 sets of chromosomes, only two pair in meiosis I. The univalents of the third movement are distributed in a directed manner, in contrast to the dog roses in both sexes. As in related species ( tribe Stypheleae), pollen meiosis is associated with inequitable cell division: three of the four daughter nuclei gather at one end of the initially undivided pollen mother cell and form three small cells there that do not develop any further. Thus, a pollen grain emerges from only one of the meiosis products, and as a result of the directed segregation of the univalents in meiosis I this is mostly haploid, i.e. H. the univalents are not eliminated from the pollen core by lagging, but by directed distribution. In the embryo sac mother cell, on the other hand, they all migrate in the direction of the micropyle with a much higher probability and thus preferentially reach the egg cell. Although the directional distribution in this species is by no means 100% in both sexes and therefore many aneuploid sex cells are formed, it is effective enough to enable high fertility.

The South American sweet grass Andropogon ternatus is also triploid, and in meiosis one set of chromosomes remains unpaired. In anaphase I, the univalents in both sexes remain between the segregating half-bivalents and form their own third nucleus, which is taken up in one of the two daughter cells. In female meiosis, this is the daughter cell facing the micropyle. In accordance with the two previously discussed plant species, the univalents are thus assigned to the micropylar side in a directed manner. However, since the embryo sac arises here at the other end of the tetrad facing the chalaza , this results in the elimination of the univalents from the inheritance. This is compensated for by the pollen, in that only those pollen grains which arise from the binuclear meiocytes and are therefore diploid develop normally and become fertile.

meaning

Fernando Pardo-Manuel de Villena and Carmen Sapienza discussed the meaning of these non-randomnesses in a 2001 review that was limited to a non-random segregation of individual chromosomes or chromosome pairs. From the wide spread of such phenomena (in plants, insects and vertebrates) and the diversity of the respective processes, they deduce that a functional asymmetry of the spindle poles - one of the prerequisites for a non-random segregation - is probably fundamental and not just exceptional. This also applies to humans, who experience non-random segregation when there are structurally abnormal chromosomes as a result of Robertson translocations . Elsewhere, the two authors argue for the importance of non-random segregation of structurally different homologous chromosomes (as in the Robertson translocations) in the emergence of new species in evolution ( speciation ).

literature

  • Bernard John: Meiosis . Cambridge University Press, Cambridge u. a. 1990. Chapter Preferential segregation , pp. 238-247.
  • Fernando Pardo-Manuel de Villena, Carmen Sapienza: Nonrandom segregation during meiosis: the unfairness of females. In: Mammalian Genome 12 , pp. 331-339 (2001). PMID 11331939 , doi : 10.1007 / s003350040003

Individual evidence

  1. ↑ a b Hermann Schwartz: The chromosome cycle of Tetraneura ulmi DE GEER . In: Journal for Cell Research and Microscopic Anatomy 15 , pp. 645-687 (1932).
  2. ↑ a b c d Michael JD White : The origin and evolution of multiple sex-chromosome mechanisms . In: Journal of Genetics 40 , pp. 303-336 (1940).
  3. ^ A b Thomas Hunt Morgan : The predetermination of sex in phylloxerans and aphids . In: Journal of Experimental Zoology 19 , pp. 285-321 (1915).
  4. ↑ a b Donna F. Kubai, Dwayne Wise: Nonrandom chromosome segregation in Neocurtilla (Gryllotalpa) hexadactyla: an ultrastructural study . Journal of Cell Biology 88 , pp. 281-293 (1981).
  5. ↑ Arthur Forer: Do chromosomes segregate randomly during meiosis ?: Key articles by Fernandus Payne were ignored, and perhaps "supressed". In: Proceedings of the American Philosophical Society 140 , pp. 148-163 (1996).
  6. ^ A b c Charles W. Metz: Chromosome behavior, inheritance and sex determination in Sciara . In: American Naturalist 72 , pp. 485-520 (1938).
  7. ^ A b Fernando Pardo-Manuel de Villena, Carmen Sapienza: Nonrandom segregation during meiosis: the unfairness of females. In: Mammalian Genome 12 , pp. 331-339 (2001).
  8. ↑ Hans Ris: A cytological and experimental analysis of the meiotic behavior of the univalent X chromosome in the bearberry aphid Tamalia (= Phyllaphis) coweni (Ckll.) . In: Journal of Experimental Zoology 90 , pp. 267-330 (1942).
  9. ↑ Adolf Remane , Volker Storch , Ulrich Welsch: Kurzes Lehrbuch der Zoologie . 5th edition, Fischer, Stuttgart 1985, p. 291.
  10. ↑ J. Seiler: Sex Chromosome Investigations on Psychiden. I. Experimental influence on the sex-determining mole division in Talaeporia tubulosa Retz. In: Archiv für Zellforschung 15 , pp. 249-268 (1920).
  11. ↑ a b Terrence W. Lyttle: Segregation distorters . In: Annual Review of Genetics 25 , pp. 511-557 (1991).
  12. ^ MM Rhoades, Ellen Dempsey: The effect of abnormal chromosome 10 on preferential segregation and crossing over in maize . In: Genetics 53 , pp. 989-1020 (1966).
  13. ↑ Gary Y. Kikudome: Studies on the phenomenon of preferential segregation in maize . In: Genetics 44 , pp. 815-831 (1959).
  14. ↑ DG Catcheside: The B-chromosomes of Parthenium arge tatum . In: Genetica Iberica 2 , pp. 139-149 (1950).
  15. ↑ Hiroshi Kayano: Cytogenetic studies in Lilium callosum. III. Preferential segregation of a supernumerary chromosome in EMCs . In: Proceedings of the Japan Academy 33 , pp. 553-558 (1957).
  16. ^ R. Neil Jones: Tansley Review No. 85. B chromosomes in plants. In: New Phytologist 131 , pp. 411-434 (1995).
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