Nonhomologous Chromosome Pairing Research Articles

Non-homologous chromosome pairing during meiosis in haploid Brassica rapa.

Cytological observations of chromosome pairing showed that evolutionarily genome duplication might reshape non-homologous pairing during meiosis in haploid B. rapa. A vast number of flowering plants have evolutionarily undergone whole genome duplication (WGD) event. Typically, Brassica rapa is currently considered as an evolutionary mesohexaploid, which has more complicated genomic constitution among flowering plants. In this study, we demonstrated chromosome behaviors in haploid B. rapa to understand how meiosis proceeds in presence of a single homolog. The findings showed that a diploid-like chromosome pairing was generally adapted during meiosis in haploid B. rapa. Non-homologous chromosomes in haploid cells paired at a high-frequency at metaphase I, over 50% of examined meiocytes showed at least three pairs of bivalents then equally segregated at anaphase I during meiosis. The fluorescence immunostaining showed that the cytoskeletal configurations were mostly well-organized during meiosis. Moreover, the expressed genes identified at meiosis in floral development was rather similar between haploid and diploid B. rapa, especially the expression of known hallmark genes pivotal to chromosome synapsis and homologous recombination were mostly in haploid B. rapa. Whole-genome duplication evolutionarily homology of genomic segments might be an important reason for this phenomenon, which would reshape the first division course of meiosis and influence pollen development in plants.

Hybrids between Brassica napus and B. nigra show frequent pairing between the B and A/C genomes and resistance to blackleg.

High frequencies of homoeologous and even non-homologous chromosome recombination in Brassica hybrids can transfer useful traits between genomes, but also destabilise synthetic allopolyploids. We produced triploid hybrids (2n = 3x = ABC) from the cross B. napus (rapeseed, 2n = 4x = AACC) × B. nigra (black mustard, 2n = 2x = BB) by embryo rescue and allohexaploid hybrids (2n = 6x = AABBCC = 54) by chromosome doubling of the triploids. These hybrids demonstrated resistance to blackleg disease (causal agent: Leptosphaeria maculans) inherited from their B. nigra parent. In order to assess the possibility of transfer of this resistance between the B genome and the A and C subgenomes of B. napus, as well as to assess the genomic stability of allohexaploids from the cross B. napus × B. nigra, frequencies of non-homologous chromosome pairing in these hybrids were assessed using classical cytogenetics and genomic in-situ hybridization. Meiosis was highly irregular, and non-homologous chromosome pairing between the B genome and the A/C genomes was common in both triploid hybrids (observed in 38% of pollen mother cells) and allohexaploid hybrids (observed in 15% of pollen mother cells). Our results suggest that introgression of blackleg resistance from the B genome into the A or C genomes should be possible, but that allohexaploids from this genome combination are likely unstable.

Essential role of the Cdk2 activator RingoA in meiotic telomere tethering to the nuclear envelope.

Cyclin-dependent kinases (CDKs) play key roles in cell cycle regulation. Genetic analysis in mice has revealed an essential role for Cdk2 in meiosis, which renders Cdk2 knockout (KO) mice sterile. Here we show that mice deficient in RingoA, an atypical activator of Cdk1 and Cdk2 that has no amino acid sequence homology to cyclins, are sterile and display meiotic defects virtually identical to those observed in Cdk2 KO mice including non-homologous chromosome pairing, unrepaired double-strand breaks, undetectable sex-body and pachytene arrest. Interestingly, RingoA is required for Cdk2 targeting to telomeres and RingoA KO spermatocytes display severely affected telomere tethering as well as impaired distribution of Sun1, a protein essential for the attachment of telomeres to the nuclear envelope. Our results identify RingoA as an important activator of Cdk2 at meiotic telomeres, and provide genetic evidence for a physiological function of mammalian Cdk2 that is not dependent on cyclins.

The Fate of Chromosomes and Alleles in an AllohexaploidBrassicaPopulation

Production of allohexaploid Brassica (2n = AABBCC) is a promising goal for plant breeders due to the potential for hybrid heterosis and useful allelic contributions from all three of the Brassica genomes present in the cultivated diploid (2n = AA, 2n = BB, 2n = CC) and allotetraploid (2n = AABB, 2n = AACC, and 2n = BBCC) crop species (canola, cabbages, mustards). We used high-throughput SNP molecular marker assays, flow cytometry, and fluorescent in situ hybridization (FISH) to characterize a population of putative allohexaploids derived from self-pollination of a hybrid from the novel cross (B. napus × B. carinata) × B. juncea to investigate whether fertile, stable allohexaploid Brassica can be produced. Allelic segregation in the A and C genomes generally followed Mendelian expectations for an F2 population, with minimal nonhomologous chromosome pairing. However, we detected no strong selection for complete 2n = AABBCC chromosome complements, with weak correlations between DNA content and fertility (r(2) = 0.11) and no correlation between missing chromosomes or chromosome segments and fertility. Investigation of next-generation progeny resulting from one highly fertile F2 plant using FISH revealed general maintenance of high chromosome numbers but severe distortions in karyotype, as evidenced by recombinant chromosomes and putative loss/duplication of A- and C-genome chromosome pairs. Our results show promise for the development of meiotically stable allohexaploid lines, but highlight the necessity of selection for 2n = AABBCC karyotypes.

Non-homologous chromosome pairing and crossover formation in haploid rice meiosis

While many studies have provided significant insight into homolog pairing during meiosis, information on non-homologous pairing is much less abundant. In the present study, fluorescence in situ hybridization (FISH) was used to investigate non-homologous pairing in haploid rice during meiosis. At pachytene, non-homologous chromosomes paired and formed synaptonemal complexes. FISH analysis data indicated that chromosome pairing could be grouped into three major types: (1) single chromosome paired fold-back as the univalent structure, (2) two non-homologous chromosomes paired as the bivalent structure, and (3) three or more non-homologous chromosomes paired as the multivalent structure. In the survey of 70 cells, 65 contained univalents, 45 contained bivalents, and 49 contained multivalent. Moreover, chromosomes 9 and 10 as well as chromosomes 11 and 12 formed non-homologous bivalents at a higher frequency than the other chromosomes. However, chiasma was always detected in the bivalent only between chromosomes 11 and 12 at diakinesis or metaphase I, indicating the pairing between these two chromosomes leads non-homologous recombination during meiosis. The synaptonemal complex formation between non-homologs was further proved by immunodetection of RCE8, PAIR2, and ZEP1. Especially, ZEP1 only loaded onto the paired chromosomes other than the un-paired chromosomes at pachytene in haploid.

Nonhomologous chromosome pairing in female Drosophila. Before or after exchange?1

Reanalysis of certain data published earlier showed that the meiotic loss of the Y chromosome in the female meiosis of Drosophila melanogaster was slightly but significantly lower in those meioses where the nonhomologous pairing partner of the Y chromosome had undertaken an exchange, thus preventing nonrandom heterologous segregation, than in those meioses where exchange had not taken place, and nonrandom heterologous segregation of Y and an autosome occurred. This observation is inconsistent with the distributive pairing hypothesis, i.e. the view that heterologous pairing occurs as a de novo phenomenon after exchange. The result supports the alternative hypothesis, i.e. that heterologous pairing is initiated before exchange but can involve the segregation of noncrossover chromosomes only. Thus, heterologous pairing becomes segregationally effective after exchange.

Position-effect variegation and z1 mediated white repression in the In(1)wis system in Drosophila melanogaster.

We have characterized a new X-chromosomal inversion in Drosophila melanogaster, extending from just distal of white to just proximal of the bb locus. The inversion places the w-isoxanthopterinless (wis) allele close to heterochromatin and under the influence of position-effect variegation (PEV). The wis gene activity is also regulated by chromosome pairing-dependent z1-mediated repression. By changing the environment, using specific second site modifiers, altering the amount of heterochromatin, and disturbing the chromosome pairing, we have been able to separately affect the two regulatory phenomena and analyse their respective impact on the wis regulation. We provide evidence that under normal conditions PEV and z1 mediated white repression are additive. However, at extreme levels of wis repression by PEV, changes in the z1-mediated interactions are not observable. This indicates that PEV is epistatic to z1-mediated regulation of wis. We also show that deficiencies in the short arm of Y act as suppressors of the z1-mediated white repression. This suppression does not influence PEV and is thus not due to the lower amount of heterochromatin. We propose that nonhomologous chromosome pairing between X and Y is important for the synapsis-dependent z1-mediated repression of white transcription activity in this system.

Chromosome behaviour in the male and female sex mother cells of wheat(Triticum aestivumL.), oat (AvenasativaL.) and pearl millet(Pennisetum americanum(L.) Leeke)

Abstract Chromosome behaviour in respect of chiasma distribution and frequency was studied in the pollen mother cells and embryosac mother cells of two populations each of wheat (Triticum aestivum) and oat (Avena sativa) and three populations of pearl millet (Pennisetum americanum). While regular bivalent formation was a normal feature in both the sex mother cells of oat with no variation observed in the chiasma frequency and distribution in male and female sex cells, in wheat one population showed regular bivalent formation in the two sex cells whereas the other population carried a reciprocal interchange involving two pairs of nonhomologous chromosomes in 80 and 71.42 per cent male and female sex cells, respectively. However, the presence of interchange did not effect the overall chiasma frequency which was almost similar in all the cells studied with or without a quadrivalent. In pearl millet, unlike oat and wheat, the chiasma frequency was higher in female sex cells than the male cells. Moreover, the distribution pattern of chiasmata varied drastically in the two sex cells at diakinesis and metaphase-I stages.It was predominantly terminal in male cells whereas in female cells interstitial chiasmata were mostly observed. It is concluded that in oat and wheat the contribution of the two sex cells towards generating variability through recombination are identical. In pearl millet it seems likely that the effective recombination is brought about chiefly by female meiotic bivalents and the male side provide for the retention of certain linkage groups. The importance of differential behaviour of chromosomes in the two sex cells is discussed.

Karyotypes and C-banding patterns in species of Cyphomandra Mart. ex Sendtner (Solanaceae)

The orcein and C-banded karyotypes of 11 species of Cyphomandra (Solanaceae) were described. All species were diploid with 2 n = 2 x = 24. The chromosomes were large, ranging from 4 to 10 μm in length, and in each complement were largely metacentric or submetacentric with few subtelocentrics. There was a significant negative correlation between chromosome length and arm ratio within a complement as well as between taxa. In general, chromosomes of the larger complements were more symmetrical in terms of both relative chromosome length and arm ratio, implying that similar amounts of DNA had been added to or taken away from every chromosome are of each complement during evolutionary divergence. Two pairs of non-homologous chromosomes were seen to contain subterminal secondary constrictions in most species. The two Brazilian species studied differed from those of Andean origin in the location of one of these secondary constrictions, suggesting a major evolutionary divergence between these two groups of species. Non-homologous chromosomes were difficult to distinguish from one another without the aid of C-banding, due to a continuum in the distribution of chromosome lengths and arm ratios. Telomeric and interstitial bands were shown in all species but not all chromosomes in each complement were banded. There were no centromeric bands. Nuclear DNA amount and the length, but not proportional length, of C-bands were correlated in each species. One species ( C. luteoalba (Pcrs.) Child, section Cyphomandropsis ) was unique in its banding pattern, providing further evidence for the delimitatation of this species and perhaps section from other Cyphomandra taxa.

Karyotypes and C-banding patterns in species of Cyphomandra Mart. ex Sendtner (Solanaceae)

The orcein and C-banded karyotypes of 11 species of Cyphomandra (Solanaceae) were described. All species were diploid with 2n = 2x = 24. The chromosomes were large, ranging from 4 to 10 μm in length, and in each complement were largely metacentric or submetacentric with few subtelocentrics. There was a significant negative correlation between chromosome length and arm ratio within a complement as well as between taxa. In general, chromosomes of the larger complements were more symmetrical in terms of both relative chromosome length and arm ratio, implying that similar amounts of DNA had been added to or taken away from every chromosome are of each complement during evolutionary divergence. Two pairs of non-homologous chromosomes were seen to contain subterminal secondary constrictions in most species. The two Brazilian species studied differed from those of Andean origin in the location of one of these secondary constrictions, suggesting a major evolutionary divergence between these two groups of species. Non-homologous chromosomes were difficult to distinguish from one another without the aid of C-banding, due to a continuum in the distribution of chromosome lengths and arm ratios. Telomeric and interstitial bands were shown in all species but not all chromosomes in each complement were banded. There were no centromeric bands. Nuclear DNA amount and the length, but not proportional length, of C-bands were correlated in each species. One species (C. luteoalba (Pcrs.) Child, section Cyphomandropsis) was unique in its banding pattern, providing further evidence for the delimitatation of this species and perhaps section from other Cyphomandra taxa.

GENE DUPLICATION AND PHYLOGENY IN CLARKIA.

Direct evidence for the descent of a group of species from a single common ancestor is rarely possible in the absence of fossils. Consequently, phylogenetic taxonomy relies on comparative analysis of morphological, chromosomal, and biochemical characters for evidence of relationship. However, even when many characters are concordant in their variation patterns, phylogenetic inferences can be confused by character convergence, functional correlation, unequal rates of evolution in different lineages, and ignorance of primitive and derived character states. In principle, a monophyletic relationship for a large number of diploid species could be inferred if they possessed in common a character that had a single and unique origin. Such a character appears to have been identified in the annual plant genus Clarkia (Onagraceae). It is shared by more than a dozen diploid. species which are morphologically and karyotypically diverse, and assigned to four sections in the current taxonomy (Lewis and Lewis, 1955). The character is an apparent duplication of the structural gene which codes the subunits of the cytoplasmic isozyme of phosphoglucoisomerase (PGI) (EC 5.3. 1.9). PGIs are dimeric enzymes that catalyze the reversible isomerization of glucose-6phosphate and fructose-6-phosphate. The duplication was discovered during the course of electrophoretic studies of enzyme variation in Clarkia (Gottlieb, 1973, 1974, 1977a). Its electrophoretic variability in several species, mode of inheritance, linkage, and several biochemical properties were recently described (Gottlieb, 1977b). The duplication is considered unique because its presumed mode of origin (Gottlieb, 1977b) required the sequential occurrence of a number of independent and rare events: (1) two reciprocal translocations that involve the same two pairs of nonhomologous chromosomes; (2) each translocation must overlap the chromosomal segment that bears the ancestral PGI locus; (3) a chromosomal heterozygote for the translocations must be formed presumably from a cross between the single translocations; and (4) the duplicated progeny class must become homozygous and be viable and fertile. These steps have been demonstrated experimentally in maize (Burnham, 1962). The major genetic prediction of origin by translocation is that the ancestral and duplicate loci not be linked. This appears to be the case in Clarkia since the two loci assort independently in C. xantiana (Gottlieb, 1977b), and we report here similar results in C. biloba, C. dudleyana, and C. unguiculata. This paper describes the electrophoretic variability of PGI and the number of coding gene loci in 30 species of Clarkia (about 75% of those currently recognized). In addition, we provide both genetic and biochemical evidence of overlaps on starch gels of the electrophoretic mobilities of isozymes coded by the ancestral and duplicate genes. The taxonomic distribution of the duplication in Clarkia requires a new phylogenetic alignment of the sections of the genus.

TELOMERIC ASSOCIATIONS OF GAMETIC AND SOMATIC CHROMOSOMES IN DIPLOID AND AUTOTETRAPLOIDORNITHOGALUM VIRENS

In acetocarmine root-tip squashes, diploid cells of Ornithogalum virens in prophase exhibit configurations resulting from end-to-end associations of the six chromosomes. Homologues lie opposite one another in a ring. Prophase chromosomes of the autotretraploid cells likewise associate end-to-end; however, four homogues instead of two generally lie adjacent to one another and four (or eight) ends are often connected instead of the two (or four) found in diploid cells. Prophase chromosomes in a haploid pollen grain of a diploid form an open chain of three chromosomes, whereas in pollen from the autotetraploid balanced gametes form a configuration in which homologous pairs lie adjacent to one another and are attached end-to-end to other homologous pairs of nonhomologous chromosomes to form a chain. These observations are discussed in terms of the role telomeric associations may play in recognition and pairing of homologous chromosomes during meiotic prophase. This recognition of homologues may occur as early as syngamy.

NON-HOMOLOGOUS ASSOCIATIONS OF HAPLOID BARLEY CHROMOSOMES IN THE CYTOPLASM OF HORDEUM BULBOSUM L.

Haploids obtained from interspecific crosses between H. bulbosum (2n=14) and H. vulgare (2n=14) resembled the male parent (H. vulgare) with regard to vegetative and spike characteristics. The somatic chromosomes of these plants resembled those of the haploid complement (x=7) of the male parent H. vulgare. Thus, the haploids produced were considered androgenetic possessing the cytoplasm of H. bulbosum and the nucleus of H. vulgare. Detailed meiotic studies from pachytene to metaphase I in these haploids revealed non-homologous chromosome pairing, identical to that of haploids produced in the cytoplasm of H. vulgare. It was concluded that the influence, if any, of bulbosum cytoplasm on meiotic behavior of vulgare chromosomes is not different from that of vulgare cytoplasm.

The initiation of nonhomologous chromosome pairing before exchange in female Drosophila melanogaster.

The initiation of nonhomologous chromosome pairing before exchange in female Drosophila melanogaster.

MODIFICATION OF ABNORMAL GAMETIC RATIOS IN DROSOPHILA. I. EVIDENCE FOR AN INFLUENCE OF Y CHROMOSOMES AND MAJOR AUTOSOMES ON GAMETIC RATIOS FROM BAR-STONE TRANSLOCATION MALES

IF one considers the meiotic behavior of two pairs of non-homologous chromosomes, designating one pair A and A’ and the other pair B and B’, then it is expected that A will separate from A’ and B from B’ and that, following a random assortment of the two pairs of chromosomes, four gametic types (AB, A’B’, AB’ and A’B) will be produced with, under ordinary circumstances, equal frequency. NOVITSKI and I. SANDLER (1957) reported a case in Drosophila melanogaster in which they showed that, despite the absence of evidence for departures from regular separation and random assortment in their experiments, certain gametic types were, nonetheless, recovered appreciably more frequently than others. They studied the kinds of gametes produced by Drosophila males carrying the Bar translocation of STONE, T ( 1 ;4) Bs, where A or XD represents the large distal piece of a broken X chromosome attached to the centric piece of a broken fourth chromosome, and A’ or IV a normal fourth chromosome, and where B or Xp represents the proximal centric piece of the broken X chromosome, marked with the mutant Bar, and having the remaining acentric piece of the broken fourth chromosome attached to its free end, and B’ or Y a normal Y chromosome (Figure 1 ) . The relative frequencies of the four gametic types, XD + Xp, IV -F Y, XD + Y, and IV 4- Xp, observed in their experiments are given below in row I, and the relative rates of recovery of the members of each homologous pair of chromosomes are given in row 111.