Mitosis with undivided chromosomes

Summary Cases of cell division with single chromatids are discussed in connection with a study on mitosis with undivided chromosomes made on living material of the endosperm of Haemanthus katharinae. Such divisions are known from certain abnormal mitoses in the microspores of a few plant species, and also from the second meiotic division, in which it is possible in numerous materials to study the behaviour of daughter univalents, and, in a few cases, also daughter chromosomes derived from chromosomes that were paired during the first division.The various cases of mitosis with single chromatids show a great variation with respect to the degree of scattering of the chromosomes over the spindle at metaphase. In a few cases there is practically no tendency to form a metaphase plate. In other cases the tendency to form such a plate is more or less pronounced, but also in these cases it is difficult for the chromosomes to form this arrangement. Some of them remain scattered over the spindle. After the metaphase a kind of anaphase usually follows in which the single chromatids, without division, move to the poles, often with other chromosomes lagging in intermediate positions.An approach of chromosomes to the poles may be caused by two different mechanisms in mitoses of this kind and only in a few cases is the information sufficient to show that active centromere movements occur during these anaphases.In many aspects of their behaviour on the spindle, single chromatids are similar to ordinary univalents of the first meiotic division. For this reason the movement mechanics of the chromosomes of the first meiotic division is briefly reviewed.The interpretation is expressed that the structure of the centromere region of a single chromatid shows some similarity to that of a univalent of the first meiotic division and that this may be the reason for their similar behaviour. The chromatid centromere would have a structural multiplicity with respect to its kinetic elements, corresponding to its subdivision in half-chromatids and also to the presence of two or three consecutive chromomeres in its longitudinal direction. As these kinetic elements are arranged close to one another on one side of the narrow cylinder of the centromere constriction, it is difficult for them to orient, towards both poles simultaneously. A single chromatid having a centromere of this kind will show orientation instability and change its orientation between the two unipolar orientations and various more or less bipolar orientations. The movements following these different orientations would cause the scattering of these single chromatids over the spindle. The orientation of ordinary mitotic metaphase chromosomes, consisting of two such chromatids, could often be the consequence of a process of co-orientation similar to that in meiotic bivalents.The anaphase movement of undivided chromosomes, which by active centromere movements are shifted in the polar directions without a separation of daughter components, is discussed with reference to a similar behaviour observed by Dietz in multivalents in Ostracods. These multivalents are stabilized in the equator during metaphase, in spite of the fact that they have two or three centromeres directed towards one pole and a single one towards the other. During anaphase their chromosomes do not separate but the whole configurations are shifted towards that pole towards which the majority of the centromeres are directed (this is followed by another type of movement which does not concern us in this connection). Undivided chromosomes that are oriented with more of their kinetic material towards one of the poles and less towards the other should by the same mechanisms as moved the multivalents be shifted in the equatorial direction during metaphase and in the polar direction during anaphase. The mechanism of these events is obscure. A change in the interpretation given by Dietz is suggested.This paper is dedicated to Professor Franz Schrader on the occasion of his seventieth birthday.     

The odd-even effect in mitotically unstable B chromosomes in grasshoppers

The odd-even effect, by which B chromosomes are more detrimental in odd numbers, has been reported in plants and animals. In grasshoppers, there are only a few reports of this effect and all were referred to as traits related to the formation of aberrant meiotic products (AMPs). Here we review the existing information about B chromosome effects on AMPs, chiasma frequency and the number of active nucleolus organizer regions (NORs) per cell. Polysomy for A chromosomes and B chromosomes are two kinds of chromosome polymorphism frequently found in grasshoppers. In some aspects, e.g. meiotic behaviour and mitotic instability leading to individual mosaicism (in the case of mitotically unstable Bs), polysomic As show similar characteristics to B chromosomes. In fact, polysomy is regarded as one of the main mechanisms for B chromosome origin. Here we review some features of meiotic behaviour in known cases of polysomy and mitotically unstable Bs in grasshoppers, in looking for possible causes for the odd-even effect. In all these traits, the odd-even effect was apparent, although its appearance was not universal in any case, with variation among species or populations within the same species. The equational division and lagging of the extra chromosomes, when univalents, could favour the appearance of abnormal meiotic products, and the formation of bivalents, when there are two or more extra chromosomes, inhibits this process. Therefore, the odd-even effect might be a consequence of the concomitant operation of both aspects of extra chromosome meiotic behaviour. The possibility that the odd-even effect might result from an increase in cell stress generated by odd numbers is suggested.     

A measure of the within-chromosome synergistic epistasis for Drosophila viability

In order to detect possible synergistic epistasis for viability in Drosophila melanogaster we assayed the relative viability of chromosomes II in: (i) panmixia, (ii) forced total homozygosity, and (iii) homozygosity for, on the average, half of their loci. As these genotypes were constructed using exactly the same set of chromosomes in the three cases, the design allows us to estimate the inbreeding depression rate at two different inbreeding levels in the absence of purging natural selection. Overall, no consistent synergistic epistasis was found. However, there was a small fraction of chromosomes whose severely deleterious effect when homozygous was almost significantly larger than expected from their viability when homozygous for half of their loci. This suggests occasional but important synergistic epistasis, which might confer evolutionary advantage to recombination in tightly linked genomes. Nevertheless, such epistasis is unlikely to be an evolutionary advantage driving the evolution of sexual anisogamous reproduction, as its contribution to overall viability is small when compared with the two-fold cost of anisogamy.     

Double satellites: Autoradiographic study of a chromosomal marker observed in two generations

Summary 2 cases of a familial balanced translocation of satellites and part of the short arm between two D-group chromosomes are described.One of these chromosomes shows double satellites on its short arm, and the other appears deprived of visible satellites and part of its short arm. These chromosomes have been identified by means of autoradiography as a number 15 and a number 14, respectively.The possible implications of this type of translocation are discussed in the text.

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Zusammenfassung Es wird bei Tochter und Mutter eine balancierte Translokation von Satelliten und einem Teil des kurzen Arms zwischen zwei D-Chromosomen beschrieben.Eines dieser Chromosomen zeigt doppelte Satelliten an seinem kurzen Arm, bei dem anderen ist ein Satellit nicht zu erkennen, und es scheint ein Teil des kurzen Arms zu fehlen. Mit Hilfe der Autoradiographie ist jenes als Nr. 15, dieses als Nr. 14 charakterisiert.Die mögliche Bedeutung dieses Typs von Translokation wird besprochen.

     

Retained heterodisomy for chromosome 12 in atypical lipomatous tumors: implications for ring chromosome formation

Atypical lipomatous tumor (ALT) is an intermediate malignant mesenchymal tumor that is characterized by supernumerary ring chromosomes and/or giant rod-shaped marker chromosomes (RGMC). Fluorescence in situ hybridization (FISH) and molecular genetic analyses have disclosed that the RGMCs always contain amplified sequences from the long arm of chromosome 12. Typically, RGMCs are the sole clonal changes and so far no deletions or other morphologic aberrations of the two normal-appearing chromosomes 12 that invariably are present have been detected. The mechanisms behind the formation of the RGMCs are unknown, but it could be hypothesized that RGMC formation is preceded by trisomy 12 or, alternatively, that ring formation of one chromosome 12 is followed by duplication of the remaining homolog. The latter scenario would always result in isodisomy for the two normal-appearing chromosomes 12, whereas the former would yield isodisomy in one-third of the cases. In order to investigate these possible mechanisms behind ring formation, we studied polymorphic loci on chromosome 12 in 14 cases of ALT showing one or more supernumerary ring chromosomes and few or no other clonal aberrations at cytogenetic analysis. The molecular genetic analyses showed that the tumor cells always retained both parental copies of chromosome 12, thus refuting the trisomy 12 and duplication hypotheses.     

A major evolutionary transition to more than two sexes?

Two recently discovered cases of genetic caste determination in social insects might provide the first example of a major evolutionary transition from two to more than two sexes. I argue here that the system can be interpreted as comprising primarily individuals requiring gametes from three parental types and having four sexes from the perspective of demographic extinction. Additionally, I show how this mating system can be seen as a major evolutionary transition. For these populations, it is apparent that the mechanism for a three- or four-sex system does not lie within the myriad of possible arrangements of chromosomes within individuals, but at the next level of evolutionary complexity, with the arrangement of chromosomes among individuals within a social system.     

Differential C-heterochromatin distribution in two species of freshwater fish, Anabas testudineus (Bloch.) and Puntius sarana (Hamilton.)

Anabas testudineus (2n = 46) had the more conserved pattern of its C-heterochromatin distributed mainly in the centromeric region, whereas Puntius sarana (2n = 50) exhibited a rather unorthodox pattern, many chromosomes showing interstitial, some telomeric and a few chromosomes showing centromeric C-band localization. Further, lateral asymmetry in distribution of heterochromatin was also noted in two pairs of chromosomes in P. sarana. The possible implications of the differential distribution noted in these two species has been discussed.     

Integration of a B chromosome into the A genome of a wasp

B chromosomes are genome symbionts, the presence of which in many eukaryote species is explained, in most cases, by their violation of Mendelian rules, usually based on meiotic or mitotic instability, leading to their accumulation in the germ line (drive). However, B chromosome integration into the genome as a regular member of the chromosome set should imply the loss of drive. A possible way of bypassing this difficulty is to regularize meiosis when the B chromosome is frequent in the population, in order to yield gametes with one B chromosome. In diploid organisms, this task needs to be achieved in the two sexes, but in haplodiploids the problem simplifies to only the diploid sex. We have found, to the authors' knowledge, the first evidence of a B chromosome that is regularizing its meiotic behaviour and limiting its number to one B chromosome per haploid genome, the same dosage as the standard (A) chromosomes, in the solitary wasp Trypoxylon albitarse. It suggests a possible mechanism for B chromosome integration as a regular member of the chromosome complement.     

The influence of fixation on the morphology of mitotic chromosomes

It is well known that there is a strong influence of fixation, i.e., acetic methanol versus formaldehyde, on the chromosome morphology at stages of the first meiotic division. In this study the influence of both these types of fixation on the morphology of mitotic chromosomes was examined in human lymphocytes. After methanol-acetic acid (3:1) fixation, the chromosomes show the "classical" condensed shape in which it is not always possible to recognize the two sister chromatids. These chromosomes are accessible to the conventional G-, R-, and C-banding techniques. After formaldehyde fixation at a relatively high pH, the chromosomes are thinner and longer (two to six times) when compared with chromosomes following methanol-acetic acid fixation. They show a scaffold-like morphology, sometimes with a halo of thin material around it. In all cases the two sister chromatids could be recognized. This chromosome structure could be easily stained with silver, Giemsa, 4,6-diamino-2-phenyl-indole (DAPI), and fluorescein isocyanate isomere 1 (FITC). The results obtained following these stainings gave no indication to any specific chemical composition of a probable central scaffold. The scaffold-like structures were not accessible to G-, R-, or C-banding techniques. The only effect observed following these banding techniques was the disappearance of the halo of thin material around the central scaffold-like structure.     

Raciation and speciation in house mice from the Alps: the role of chromosomes

There are at least 24 different karyotypic races of house mouse in the central Alps, each characterized by a different complement of ancestral acrocentric and derived metacentric chromosomes; altogether 55 different metacentric chromosomes have been described from the region. We argue that this chromosome variation largely arose in situ. If these races were to make contact, in most cases they would produce F1 hybrids with substantial infertility (sometimes complete sterility), due to nondisjunction and germ cell death associated with the formation of long-chain and/or ring configurations at meiosis. We present fertility estimates to confirm this for two particular hybrid types, one of which demonstrates male-limited sterility (in accordance with Haldane's Rule). As well as a model for speciation in allopatry, the Alpine mouse populations are of interest with regards speciation in parapatry: we discuss a possible reinforcement event. Raciation of house mice appears to have happened on numerous occasions within the central Alps. To investigate one possible source of new karyotypic races, we use a two-dimensional stepping stone model to examine the generation of recombinant races within chromosomal hybrid zones. Using field-derived ecological data and laboratory-derived fertility estimates, we show that hybrid karyotypic races can be generated at a reasonable frequency in simulations. Our model complements others developed for flowering plants that also emphasize the potential of chromosomal hybrid zones in generating new stable karyotypic forms.     

Sexually antagonistic chromosomal cuckoos

The two kinds of sex chromosomes in the heterogametic parent are transmitted to offspring with different sexes, causing opposite-sex siblings to be completely unrelated for genes located on these chromosomes. Just as the nest-parasitic cuckoo chick is selected to harm its unrelated nest-mates in order to garner more shared resources, sibling competition causes the sex chromosomes to be selected to harm siblings that do not carry them. Here we quantify and contrast this selection on the X and Y, or Z and W, sex chromosomes. We also develop a hypothesis for how this selection can contribute to the decay of the non-recombining sex chromosome.     

Centromeric genotyping and direct analysis of nondisjunction in humans: Down syndrome

In species with chiasmate meioses, alterations in genetic recombination are an important correlate of nondisjunction. In general, these alterations fall into one of two categories: either homologous chromosomes fail to pair and/or recombine at meiosis I, or they are united by chiasmata that are suboptimally positioned. Recent studies of human nondisjunction suggest that these relationships apply to our species as well. However, methodological limitations in human genetic mapping have made it difficult to determine whether the important determinant(s) in human nondisjunction is absent recombination, altered recombination, or both. In the present report, we describe somatic cell hybrid studies of chromosome 21 nondisjunction aimed at overcoming this limitation. By using hybrids to “capture” individual chromosomes 21 of the proband and parent of origin of trisomy, it is possible to identify complementary recombinant meiotic products, and thereby to uncover crossovers that cannot be detected by conventional mapping methods. In the present report, we summarize studies of 23 cases. Our results indicate that recombination in proximal 21q is infrequent in trisomy-generating meioses and that, in a proportion of the meioses, recombination does not occur anywhere on 21q. Thus, our observations indicate that failure to recombine is responsible for a proportion of trisomy 21 cases. Received: 12 January 1997; in revised form: 16 February 1998 / Accepted: 19 February 1998     

Widespread Recombination Suppression Facilitates Plant Sex Chromosome Evolution

Classical models suggest that recombination rates on sex chromosomes evolve in a stepwise manner to localize sexually antagonistic variants in the sex in which they are beneficial, thereby lowering rates of recombination between X and Y chromosomes. However, it is also possible that sex chromosome formation occurs in regions with preexisting recombination suppression. To evaluate these possibilities, we constructed linkage maps and a chromosome-scale genome assembly for the dioecious plant Rumex hastatulus. This species has a polymorphic karyotype with a young neo-sex chromosome, resulting from a Robertsonian fusion between the X chromosome and an autosome, in part of its geographic range. We identified the shared and neo-sex chromosomes using comparative genetic maps of the two cytotypes. We found that sex-linked regions of both the ancestral and the neo-sex chromosomes are embedded in large regions of low recombination. Furthermore, our comparison of the recombination landscape of the neo-sex chromosome to its autosomal homolog indicates that low recombination rates mainly preceded sex linkage. These patterns are not unique to the sex chromosomes; all chromosomes were characterized by massive regions of suppressed recombination spanning most of each chromosome. This represents an extreme case of the periphery-biased recombination seen in other systems with large chromosomes. Across all chromosomes, gene and repetitive sequence density correlated with recombination rate, with patterns of variation differing by repetitive element type. Our findings suggest that ancestrally low rates of recombination may facilitate the formation and subsequent evolution of heteromorphic sex chromosomes.     

Euchromatic domains in plant chromosomes as revealed by H4 histone acetylation and early DNA replication.

Using specific polyclonal antisera raised against acetylated isoforms of histone H4, we have analyzed their distribution in the dioecious plant Silene latifolia (syn. Melandrium album) possessing heteromorphic sex chromosomes. Our previous studies on this species have shown that one of the two X chromosomes in homogametic female cells is heavily methylated and late replicating, as a possible consequence of dosage compensation. Here we report that there are no detectable differences in intensity and distribution of H4 acetylation between these two X chromosomes. In S. latifolia only distal-subtelomeric chromosome regions, on both the sex chromosomes and autosomes, display strong signals of H4 acetylation at N-terminal lysines 5, 8, and 12. These acetylated domains correspond to the very early replicating distal chromosome regions as revealed by 5-bromodeoxyuridine pulses followed by the indirect immunofluorescence microscopy. The distribution of H4 acetylated at lysine 16 was uniform along the chromosomes. The unique distal-subtelomeric H4 acetylation signals were also observed in three other Silene species (S. vulgaris, S. pendula, and S. chalcedonica), but not in two non-related plant species tested (Allium cepa and Nicotiana tobacum). The presented data as well as our recent studies on the structure of S. latifolia chromosome ends indicate that Silene species possess the specific distal-subtelomeric location of euchromatin, gene-rich regions on chromosomes.     

The development of XO gynogenetic mouse embryos

Diploid gynogenetic embryos, which have two sets of maternal and no paternal chromosomes, die at or soon after implantation. Since normal female embryos preferentially inactivate the paternally derived X chromosome in certain extraembryonic membranes, the inviability of diploid gynogenetic embryos might be due to difficulties in achieving an equivalent inactivation of one of their two maternally derived X chromosomes. In order to investigate this possibility, we constructed XO gynogenetic embryos by nuclear transplantation at the 1-cell stage. These XO gynogenones showed the same mortality around the time of implantation as did their XX gynogenetic counterparts. This shows that the lack of a paternally derived autosome set is sufficient to cause gynogenetic inviability at this stage. Autosomal imprinting and its possible relation to X-chromosome imprinting is discussed.     

Characterisation of the double genome structure of modern sugarcane cultivars (Saccharum spp.) by molecular cytogenetics

Cultivated sugarcane clones (Saccharum spp., 2n=100 to 130) are derived from complex interspecific hybridizations between the speciesS. officinarum andS. spontaneum. Using comparative genomic DNA in situ hybridization, we demonstrated that it is possible to distinguish the chromosomes contributed by these two species in an interspecific F1 hybrid and a cultivated clone, R570. In the interspecific F1 studied, we observed n+n transmission of the parental chromosomes instead of the peculiar 2n+n transmission usually described in such crosses. Among the chromosomes of cultivar R570 (2n=107–115) about 10% were identified as originating fromS. spontaneum and about 10% were identified as recombinant chromosomes between the two speciesS. officinarum andS. spontaneum. This demonstrated for the first time the occurrence of recombination between the chromosomes of these two species. The rDNA sites were located by in situ hybridization in these two species and the cultivar R570. This supported different basic chromosome numbers and chromosome structural differences between the two species and provided a first bridge between physical and genetical mapping in sugarcane.     

Characterisation of the double genome structure of modern sugarcane cultivars (Saccharum spp.) by molecular cytogenetics

Cultivated sugarcane clones (Saccharum spp., 2n=100 to 130) are derived from complex interspecific hybridizations between the speciesS. officinarum andS. spontaneum. Using comparative genomic DNA in situ hybridization, we demonstrated that it is possible to distinguish the chromosomes contributed by these two species in an interspecific F1 hybrid and a cultivated clone, R570. In the interspecific F1 studied, we observed n+n transmission of the parental chromosomes instead of the peculiar 2n+n transmission usually described in such crosses. Among the chromosomes of cultivar R570 (2n=107–115) about 10% were identified as originating fromS. spontaneum and about 10% were identified as recombinant chromosomes between the two speciesS. officinarum andS. spontaneum. This demonstrated for the first time the occurrence of recombination between the chromosomes of these two species. The rDNA sites were located by in situ hybridization in these two species and the cultivar R570. This supported different basic chromosome numbers and chromosome structural differences between the two species and provided a first bridge between physical and genetical mapping in sugarcane.     

Steps in the evolution of heteromorphic sex chromosomes

We review some recently published results on sex chromosomes in a diversity of species. We focus on several fish and some plants whose sex chromosomes appear to be 'young', as only parts of the chromosome are nonrecombining, while the rest is pseudoautosomal. However, the age of these systems is not yet very clear. Even without knowing what proportions of their genes are genetically degenerate, these cases are of great interest, as they may offer opportunities to study in detail how sex chromosomes evolve. In particular, we review evidence that recombination suppression occurs progressively in evolutionarily independent cases, suggesting that selection drives loss of recombination over increasingly large regions. We discuss how selection during the period when a chromosome is adapting to its role as a Y chromosome might drive such a process.     

A new test for detecting ongoing selection

Much effort has been made to search for signatures of past natural selection in DNA sequences. However, currently acting selection is rarely detected in natural populations because of its rarity, low detection power of available methods, or both. Here, we develop a new test to detect viability selection over a single generation. In this test, one specific type of chromosomes is chosen as a reference, while all other chromosomes are designated as "focal". The test compares measures of variation between two groups of "focal" chromosomes: those found in reference/focal heterozygous individuals and those found in focal/focal homozygous individuals. In the absence of selection, we do not expect differences between these two groups as long as mating is random. On the other hand, currently acting selection can cause differences in some measures of variation. We applied this test to typing data for In(2L)t inversion polymorphism in a Drosophila melanogaster population, using "standard" (non-inverted) chromosomes as the focal class. Although the frequencies of In(2L)t and standard chromosomes did not deviate from the Hardy-Weinberg equilibrium, we found differences in allele frequency and the number of haplotypes between the two groups of standard chromosomes. This new test, in conjunction with the Hardy-Weinberg test, may shed light on how often strong selection is operating in extant populations.