Pairing In Meiosis Research Articles

A cyclical switch of gametogenic pathways in hybrids depends on the ploidy level

The cellular and molecular mechanisms governing sexual reproduction are conserved across eukaryotes. Nevertheless, hybridization can disrupt these mechanisms, leading to asexual reproduction, often accompanied by polyploidy. In this study, we investigate how ploidy level and ratio of parental genomes in hybrids affect their reproductive mode. We analyze the gametogenesis of sexual species and their diploid and triploid hybrids from the freshwater fish family Cobitidae, using newly developed cytogenetic markers. We find that diploid hybrid females possess oogonia and oocytes with original (diploid) and duplicated (tetraploid) ploidy. Diploid oocytes cannot progress beyond pachytene due to aberrant pairing. However, tetraploid oocytes, which emerge after premeiotic genome endoreplication, exhibit normal pairing and result in diploid gametes. Triploid hybrid females possess diploid, triploid, and haploid oogonia and oocytes. Triploid and haploid oocytes cannot progress beyond pachytene checkpoint due to aberrant chromosome pairing, while diploid oocytes have normal pairing in meiosis, resulting in haploid gametes. Diploid oocytes emerge after premeiotic elimination of a single-copied genome. Triploid hybrid males are sterile due to aberrant pairing and the failure of chromosomal segregation during meiotic divisions. Thus, changes in ploidy and genome dosage may lead to cyclical alteration of gametogenic pathways in hybrids.

Cytogenetic Analysis of F1Hybrids Derived From Crossings of Triticum aestivum L. Lines With Aegilops L. Species

As a result of the studies seed settings of hybrid combinations obtained from the crossings between Triticum aestivum L. lines and Aegilops L. species, also the cytogenetic analysis of the F1 hybrids belonging to these combinations have been established that possession joint subgenomes of parental forms is not guarantee a relatively high percentage of fertility at crosses, and a high level of chromosome pairing in meiosis. At the same time, no significant correlations were found between the level of chromosome conjugation and fertility in the studied intergeneric F1 hybrids.

The alternative transcription and expression characterization of Dmc1 in autotetraploid Carassius auratus.

Established autotetraploids often have a highly stable meiosis with high fertility compared with neo-autotetraploids. The autotetraploid Carassius auratus (4n = 200, RRRR) (4nRR), which stemmed from whole-genome duplication of Carassius auratus red var. (2n = 100, RR) (RCC), produces diploid gametes with an adopted diploid-like chromosome pairing in meiosis and maintains the formation of autotetraploid lineages. In this study, we focused on Dmc1, a meiosis-specific recombinase during the prophase of meiosis I, and elaborated on the genetic variation, alternative transcription, expression characterization, and epigenetic modification of Dmc1 in RCC and 4nRR. Two original Dmc1 from RCC were identified in 4nRR, and two duplicated Dmc1 differences in genetic composition were observed in 4nRR. Furthermore, we only noticed that one original and one duplicated Dmc1 were expressed in RCC and 4nRR, respectively. However, both possessed identical gene expression profiles, differential expression of sexual dimorphism, and hypomethylation levels. These results indicated that the specific expression of duplicated Dmc1 may be involve in the progression of meiosis of the diploid-like chromosome pairing in autotetraploid Carassius auratus. Herein, the findings significantly increase knowledge of meiosis of autopolyploid fish and provide meaningful insights into genetic breeding in polyploidy fish.

Autoploid origin and rapid diploidization of the tetraploid Thinopyrum elongatum revealed by genome differentiation and chromosome pairing in meiosis.

Polyploidy is a common mode of evolution in flowering plants. Both the natural tetraploid Thinopyrum elongatum and the diploid one from the same population show a diploid-like pairing in meiosis. However, debate on the chromosome composition and origin of the tetraploid Th. elongatum is ongoing. In the present study, we obtained the induced tetraploid Th. elongatum and found that the induced and natural tetraploids are morphologically close, except for slower development and lower seed setting. Using probes developed from single chromosome microdissection and a Fosmid library, obvious differentiations were discovered between two chromosome sets (E1 and E2 ) of the natural tetraploid Th. elongatum but not the induced one. Interestingly, hybrid F1 derived from the two different wheat-tetraploid Th. elongatum amphiploids 8802 and 8803 produced seeds well. More importantly, analysis of meiosis in F2 individuals revealed that chromosomes from E1 and E2 could pair well on the durum wheat background with the presence of Ph1. No chromosome set differentiation on the FISH level was discovered from the S1 to S4 generations in the induced one. In metaphase of the meiosis first division in the natural tetraploid, more pairings were bivalents and fewer quadrivalents with ratio of 13.94 II + 0.03 IV (n=31). Chromosome pairing configuration in the induced tetraploid is 13.05 II + 0.47 IV (n=19), with the quadrivalent ratio being only slightly higher than the ratio in the natural tetraploid. Therefore, the natural tetraploid Th. elongatum is of autoploid origin and the induced tetraploid Th. elongatum evolutionarily underwent rapid diploidization in the low generation.

On the features of evolutionary process in plants: diploidization of the genomes and karyotypes

As estimated by karyosystematists, from 30 to 50 % of land plant species exhibit polyploid karyotypes, respectively, from 50 to 70 % of plants are diploids. One of the mechanisms of diploidization of a neopolyploid karyotype is chromosomal rearrangements leading to a change in the chromosome number in genomes - so-called dysploidy. In parallel with the processes of karyotype diploidization, contributing to it and supplementing it, there are processes of structural and epigenetic diploidization of the genome, the loss of part of the duplicated genes. It may be that the role of genome polyploidy in plant evolution lies primarily in the fact that this ephemeral state is primarily an effective way to destabilize the genome, a source of many new combinations of alleles that pass through a rigid selection sieve and are realized later at the secondary diploid stage. The transition from a polyploid to a diploid state is justified by the fact that neopolyploids cannot provide a high percentage of gametes carring balanced chromosome sets due to problems with chromosome pairing in meiosis I. The mechanisms of transition to strict pairwise chromosome pairing are such that automatically leads to diploidization of both the genome and the karyotype. It is also important that the diploid karyotype provides more stringent, faster, more efficient selection of adaptively important new combinations of alleles, thereby contributing to the accumulation of taxonomically significant traits, speciation, and, as a result, to progressive evolution.

Transfer of the ph1b Deletion Chromosome 5B From Chinese Spring Wheat Into a Winter Wheat Line and Induction of Chromosome Rearrangements in Wheat-Aegilops biuncialis Hybrids.

Effective utilization of genetic diversity in wild relatives to improve wheat requires recombination between wheat and alien chromosomes. However, this is suppressed by the Pairing homoeologous gene, Ph1, on the long arm of wheat chromosome 5B. A deletion mutant of the Ph1 locus (ph1b) has been used widely to induce homoeologous recombination in wheat × alien hybrids. However, the original ph1b mutation, developed in Chinese Spring (CS) background has poor agronomic performance. Hence, alien introgression lines are first backcrossed with adapted wheat genotypes and after this step, alien chromosome segments are introduced into breeding lines. In this work, the ph1b mutation was transferred from two CSph1b mutants into winter wheat line Mv9kr1. Homozygous genotypes Mv9kr1 ph1b/ph1b exhibited improved plant and spike morphology compared to Chinese Spring. Flow cytometric chromosome analysis confirmed reduced DNA content of the mutant 5B chromosome in both wheat genotype relative to the wild type chromosome. The ph1b mutation in the Mv9kr1 genotype allowed wheat-alien chromosome pairing in meiosis of Mv9kr1ph1b_K × Aegilops biuncialis F1 hybrids, predominantly with the Mb-genome chromosomes of Aegilops relative to those of the Ub genome. High frequency of wheat-Aegilops chromosome interactions resulted in rearranged chromosomes identified in the new Mv9kr1ph1b × Ae. Biuncialis amphiploids, making these lines valuable sources for alien introgressions. The new Mv9kr1ph1b mutant genotype is a unique resource to support alien introgression breeding of hexaploid wheat.

Evaluation of Crossability between Nicotiana benthamiana and Nicotiana excelsior

Wild tobacco species in the Nicotiana section Suaveolentes are promising genetic resources to introduce their disease resistance to cultivated tobacco, Nicotiana tabacum. However, hybrid lethality is observed in hybrid seedlings from crosses between most Suaveolentes species and N. tabacum. In particular, N. benthamiana belonging to the section Suaveolentes produces only viable hybrids after crossing with N. tabacum. In the present study, crossability between N. benthamiana and N. excelsior (section Suaveolentes) was investigated to test the possible usefulness of N. benthamiana as the bridge parent to transfer desirable genes of N. excelsior to N. tabacum via bridge crossing. After reciprocal crosses using three accessions of N. benthamiana and N. excelsior each, several crossing barriers such as cross-incompatibility, seed abortion, and male and female hybrid sterility were observed. Although reciprocal hybrids between N. benthamiana and N. excelsior showed a high degree of chromosome pairing in meiosis, univalents and multivalents, as well as chromosome bridges and lagging chromosomes, were observed. These meiotic abnormalities were thought to cause hybrid sterility. The possible usefulness of reciprocal hybrids between N. benthamiana and N. excelsior is discussed.

Phase separation drives pairing of homologous chromosomes.

Pairing of homologous chromosomes is crucial for ensuring accurate segregation of chromosomes during meiosis. Molecular mechanisms of homologous chromosome pairing in meiosis have been extensively studied in the fission yeast Schizosaccharomyces pombe. In this organism, meiosis-specific noncoding RNA transcribed from specific genes accumulates at the respective gene loci, and chromosome-associated RNA-protein complexes mediate meiotic pairing of homologous loci through phase separation. Pairing of homologous chromosomes also occurs in somatic diploid cells in certain situations. For example, somatic pairing of homologous chromosomes occurs during the early embryogenesis in diptera, and relies on the transcription-associated chromatin architecture. Earlier models also suggest that transcription factories along the chromosome mediate pairing of homologous chromosomes in plants. These studies suggest that RNA bodies formed on chromosomes mediate the pairing of homologous chromosomes. This review summarizes lessons from S. pombe to provide general insights into mechanisms of homologous chromosome pairing mediated by phase separation of chromosome-associated RNA-protein complexes.

Генетический репродуктивный риск у носителей перицентрических инверсий

Перицентрические инверсии (ПИ) - это сбалансированные структурные хромосомные перестройки, частота которых в популяции варьирует от 0,12% до 0,7% [1]. Особенностью синапсиса и рекомбинации ПИ в мейозе является формирование инверсионной петли, единичный кроссинговер в которой может приводить к образованию рекомбинантных хромосом c дупликацией/делецией дистального по отношению к инверсии сегмента короткого плеча и делецией/дупликацией дистального сегмента длинного плеча соответственно. Клиническая значимость ПИ определяется риском формирования у носителя гамет (зигот) с хромосомным дисбалансом (ХД), приводящим к раннему спонтанному прерыванию беременности, мертворождению или рождению детей с пороками и/или аномалиями развития. Проведена оценка эмпирической частоты формирования сперматозоидов с рекомбинантными хромосомами, а также риска формирования гамет и выживаемости потенциальных зигот с ХД у четырех мужчин-носителей ПИ. Pericentric inversion are intrachromosomal balanced structural abnormalities. The frequency in the general population is ranged from about 0.12% to 0.7% [1]. To provide complete synapsis and recombination during meiosis pairing of the normal and inverted chromosomes requires the formation of inversion loop. One crossover within the inversion loop leads to the production of two complementary recombinant chromosomes with both duplicated and deleted chromosome segments including the regions distal to the inversion. The particular clinical relevance of inversion chromosomes is that they can set the stage for the generation of recombinant gametes that may lead to early miscarriages, stillbirth or congenital abnormalities. The estimation of empiric frequencies of recombinant spermatozoa, risk of abnormal gamete formation and potential zygote viability in four pericentric inversion carriers was performed.

Chromosome-associated RNA\u2013protein complexes promote pairing of homologous chromosomes during meiosis in Schizosaccharomyces pombe

Pairing of homologous chromosomes in meiosis is essential for sexual reproduction. We have previously demonstrated that the fission yeast sme2 RNA, a meiosis-specific long noncoding RNA (lncRNA), accumulates at the sme2 chromosomal loci and mediates their robust pairing in meiosis. However, the mechanisms underlying lncRNA-mediated homologous pairing have remained elusive. In this study, we identify conserved RNA-binding proteins that are required for robust pairing of homologous chromosomes. These proteins accumulate mainly at the sme2 and two other chromosomal loci together with meiosis-specific lncRNAs transcribed from these loci. Remarkably, the chromosomal accumulation of these lncRNA–protein complexes is required for robust pairing. Moreover, the lncRNA–protein complexes exhibit phase separation properties, since 1,6-hexanediol treatment reversibly disassembled these complexes and disrupted the pairing of associated loci. We propose that lncRNA–protein complexes assembled at specific chromosomal loci mediate recognition and subsequent pairing of homologous chromosomes.

A network of nuclear envelope proteins and cytoskeletal force generators mediates movements of and within nuclei throughout Caenorhabditis elegans development.

Nuclear migration and anchorage, together referred to as nuclear positioning, are central to many cellular and developmental events. Nuclear positioning is mediated by a conserved network of nuclear envelope proteins that interacts with force generators in the cytoskeleton. At the heart of this network are linker of nucleoskeleton and cytoskeleton (LINC) complexes made of Sad1 and UNC-84 (SUN) proteins at the inner nuclear membrane and Klarsicht, ANC-1, and Syne homology (KASH) proteins in the outer nuclear membrane. LINC complexes span the nuclear envelope, maintain nuclear envelope architecture, designate the surface of nuclei distinctly from the contiguous endoplasmic reticulum, and were instrumental in the early evolution of eukaryotes. LINC complexes interact with lamins in the nucleus and with various cytoplasmic KASH effectors from the surface of nuclei. These effectors regulate the cytoskeleton, leading to a variety of cellular outputs including pronuclear migration, nuclear migration through constricted spaces, nuclear anchorage, centrosome attachment to nuclei, meiotic chromosome movements, and DNA damage repair. How LINC complexes are regulated and how they function are reviewed here. The focus is on recent studies elucidating the best-understood network of LINC complexes, those used throughout Caenorhabditis elegans development.Impact statementDefects in nuclear positioning disrupt development in many mammalian tissues. In human development, LINC complexes play important cellular functions including nuclear positioning, homolog pairing in meiosis, DNA damage repair, wound healing, and gonadogenesis. The topics reviewed here are relevant to public health because defects in nuclear positioning and mutations in LINC components are associated with a wide variety of human diseases including muscular dystrophies, neurological disorders, progeria, aneurysms, hearing loss, blindness, sterility, and multiple cancers. Although this review focuses on findings in the model nematode Caenorhabditis elegans, the studies are relevant because almost all the findings originally made in C. elegans are conserved to humans. Furthermore, C. elegans remains the best described network for how LINC complexes are regulated and function.

Evidence of dramatic sterility in F1 male hybrid catfish [male Clarias gariepinus (Burchell, 1822) × female C. macrocephalus (Günther, 1864)] resulting from the failure of homologous chromosome pairing in meiosis I

Sterile hybrid animals exhibit spermatogenic disruptions with a decreased number and/or malformation of mature sperm. F1 hybrid catfish (2n = 55) is an important cultured fish derived from male North African catfish [Clarias gariepinus (Burchell, 1822); 2n = 56] and female bighead catfish [C. macrocephalus (Günther, 1864); 2n = 54]; they are sterile with gametogenic failure in males. Despite the generality of this phenomenon, the spermatogenic phenotype has not been well described and a comprehensive understanding of the genetic basis of the disruption remains elusive. Our observations in the F1 male hybrid showed abnormal morphology of testes with small size, while meiotic configuration indicated that meiosis succeeded in the early pachytene stage but failed to progress beyond the diplotene-diakinesis stage in primary spermatocytes. This suggests the presence of a number of degenerated spermatocytes. Histological examination recorded no postmeiotic cells and spermatozoa in F1 male hybrid testes. A high frequency of apoptotic testicular cells was also present in the F1 male hybrid, as shown by histochemical results for activated Caspase-3 and TUNEL assays. Low expression levels of Caspase-3, p53 and MCL1 (isoform 2), which activate pro-apoptotic signals in the development process, were also observed in both parental species, but significantly high expression levels were observed in the F1 male hybrid. By contrast, the level of BCL2 expression was very low in the F1 male hybrid, supporting the state of apoptosis as BCL2 inhibits the actions of pro-apoptotic proteins. These results collectively suggest that sterility in male hybrids is caused by spermatogenic disruptions in the pachytene stage, resulting from the failure of homologous chromosome pairing due to chromosomal incompatibility between parental genomes, and subsequent elimination by apoptosis leading to spermatogenic breakdown in the F1 male hybrid.

Out-of-position telomeres in meiotic leptotene appear responsible for chiasmate pairing in an inversion heterozygote in wheat (Triticum aestivum L.).

Chromosome pairing in meiosis usually starts in the vicinity of the telomere attachment to the nuclear membrane and congregation of telomeres in the leptotene bouquet is believed responsible for bringing homologue pairs together. In a heterozygote for an inversion of a rye (Secale cereale L.) chromosome arm in wheat, a distal segment of the normal homologue is capable of chiasmate pairing with its counterpart in the inverted arm, located near the centromere. Using 3D imaging confocal microscopy, we observed that some telomeres failed to be incorporated into the bouquet and occupied various positions throughout the entire volume of the nucleus, including the centromere pole. Rye telomeres appeared ca. 21 times more likely to fail to be included in the telomere bouquet than wheat telomeres. The frequency of the out-of-bouquet rye telomere position in leptotene was virtually identical to the frequency of telomeres deviating from Rabl's orientation in the nuclei of somatic cells, and was similar to the frequency of synapsis of the normal and inverted chromosome arms, but lower than the MI pairing frequency of segments of these two arms normally positioned across the volume of the nucleus. Out-of-position placement of the rye telomeres may be responsible for reduced MI pairing of rye chromosomes in hybrids with wheat and their disproportionate contribution to aneuploidy, but appears responsible for initiating chiasmate pairing of distantly positioned segments of homology in an inversion heterozygote.

Chromosome painting in meiosis reveals pairing of specific chromosomes in polyploid Solanum species.

Analysis of chromosome pairing has been an important tool to assess the genetic similarity of homologous and homoeologous chromosomes in polyploids. However, it is technically challenging to monitor the pairing of specific chromosomes in polyploid species, especially for plant species with a large number of small chromosomes. We developed oligonucleotide-based painting probes for four different potato chromosomes. We demonstrate that these probes are robust enough to monitor a single chromosome throughout the prophase I of meiosis in polyploid Solanum species. Cultivated potato (Solanum tuberosum, 2n = 4x = 48) is an autotetraploid. We demonstrate that the four copies of each potato chromosome pair as a quadrivalent in 66-78% of the meiotic cells at the pachytene stage. Solanum demissum (2n = 6x = 72) is a hexaploid and has been controversial regarding its nature as an autopolyploid or allopolyploid. Interestingly, no hexavalent pairing was observed in meiosis. Instead, we observed three independent bivalents in 83-98% of the meiotic cells at late diakinesis and early metaphase I for the four chromosomes. These results suggest that S. demissum has evolved into a cytologically stable state with predominantly bivalent pairing in meiosis.

45S rDNA sites in meiosis of Lolium multiflorum Lam.: variability, non-homologous associations and lack of fragility

In this study, we evaluated the behavior of 45S ribosomal DNA (rDNA) sites during the meiosis of Lolium multiflorum. The reason to study it in this species is that 45S rDNA sites are usually visualized as gaps in mitotic metaphase chromosomes and were initially denominated fragile sites (FSs). In different species, FSs were related to rearrangements that alter the karyotype and affect the chromosome pairing in meiosis. However, our findings show that the chromosome pairing in L. multiflorum is regular and, as in mitosis, the number of sites is variable. In diakinesis with five sites, one of the bivalents was in hemizygous state while, in diakinesis with seven sites, one of the bivalents had three conspicuous signals, two in syntheny in one of the homologous. Only four cells had gaps in the region of the 45S rDNA. Owing to the lower number of signals observed at the initial stages of meiosis, it is assumed that they are involved both in homologous and non-homologous associations and that they might assist the chromosome pairing. Regarding segregation, only meiocytes with five and six 45S rDNA signals were observed, and they were characterized by the segregation of 2/3 signals in the poles of anaphases I up to metaphases II; 2/2 and 3/3 in anaphases II and telophases II; and also 2/2 and 4/4 in the nuclei of tetrads, unlike the number of 45S signals expected. The numerical non-equivalence of sites among nuclei at later stages of meiosis is explained by the presence of chromosomes with hemizygous sites.

Telomeric TERB1-TRF1 interaction is crucial for male meiosis.

During meiotic prophase, the meiosis-specific telomere-binding protein TERB1 regulates chromosome movement required for homologous pairing and recombination by interacting with the telomeric shelterin subunit TRF1. Here, we report the crystal structure of the TRF1-binding motif of human TERB1 in complex with the TRFH domain of TRF1. Notably, specific disruption of the TERB1-TRF1 interaction by a point mutation in the mouse Terb1 gene results in infertility only in males. We find that this mutation causes an arrest in the zygotene-early pachytene stage and mild telomere abnormalities of autosomes but unpaired X and Y chromosomes in pachytene, leading to massive spermatocyte apoptosis. We propose that the loss of telomere structure mediated by the TERB1-TRF1 interaction significantly affects homologous pairing of the telomere-adjacent pseudoautosomal region (PAR) of the X and Y chromosomes in mouse spermatocytes. Our findings uncover a specific mechanism of telomeres that surmounts the unique challenges of mammalian X-Y pairing in meiosis.

Cell scientist to watch – Kevin Corbett

ABSTRACT Kevin Corbett graduated in biology and biochemistry from the University of Virginia. He then went to the University of California, Berkeley, to work on the structure and function of DNA topoisomerases in bacteria and archaea for his PhD with James Berger. In 2005, he moved to the laboratory of Stephen Harrison at Harvard Medical School for his postdoctoral work on kinetochore structure and function, particularly the yeast monopolin complex, which promotes proper chromosome segregation in the first meiotic division. Kevin started his own research group at UC San Diego and the Ludwig Institute for Cancer Research in 2011. He received a Sidney Kimmel Scholar Award in 2012. His current research interests include the molecular mechanisms of homologous chromosome pairing in meiosis I, spindle assembly checkpoint regulation in mitosis and meiosis, and how misregulation of meiotic genes can contribute to carcinogenesis.

A cohesin-based structural platform supporting homologous chromosome pairing in meiosis.

The pairing and recombination of homologous chromosomes during the meiotic prophase is necessary for the accurate segregation of chromosomes in meiosis. However, the mechanism by which homologous chromosomes achieve this pairing has remained an open question. Meiotic cohesins have been shown to affect chromatin compaction; however, the impact of meiotic cohesins on homologous pairing and the fine structures of cohesion-based chromatin remain to be determined. A recent report using live-cell imaging and super-resolution microscopy demonstrated that the lack of meiotic cohesins alters the chromosome axis structures and impairs the pairing of homologous chromosomes. These results suggest that meiotic cohesin-based chromosome axis structures are crucial for the pairing of homologous chromosomes.

The effect of different genome and cytoplasm on meiotic pairing in maize newly synthetic polyploids

Abstract Allopolyploidization plays the special role in the evolution of many crops. Moreover, the evolution in early stage of some allopolyploidization events is predicted to be effected by nuclear-cytoplasmic interactions. Maize and teosintes are well model system for study of genetic recombination in allopolyploidization. In order to investigate the effects of genome organization and cytoplasm on genome evolution in newly synthesized allopolyploids (neoallopolyploids), a series of neoallopolyploids were produced by reciprocal crosses of maize and Zea perennis. By using dual-color genomic in situ hybridization, intra- and intergenomic meiosis pairing of these polyploids were quantified and compared with regard to its genome organization and cytoplasm background. In the four neoallopolyploids, the stability of maize genome is consistently lower than that of Z. perennis genome. Additional, the stability of maize genome is affected by genome ploidy. The cytoplasm, genome composition and their interaction do have the special role in chromosome paring and the meiosis behaviors in Zea allopolyploids vary significantly and showed non-diploidization. Z. perennis cytoplasm may give a relatively relaxed environment for maize genome.

FISH Karyotype and GISH Meiotic Pairing Analyses of a Stable Intergeneric Hybrid xBrassicoraphanus Line BB#5

xBrassicoraphanus line BB#5, a new synthetic intergeneric hybrid between Brassica rapa L. ssp. pekinensis and Raphanus sativus L. var. rafiphera induced by N-methyl-N-nitroso-urethane mutagenesis in microspore culture, shows high seed fertility and morphological uniformity. Dual-color fluorescence in situ hybridization (FISH) using 5S and 45S rDNA probes and genomic in situ hybridization (GISH) using B. rapa genomic DNA probe were carried out to analyze the chromosome composition and the meiosis pairing pattern compared to its parental lines. The somatic chromosome complement is 2n = 38, which consists of 17 metacentric and two submetacentric chromosomes with lengths of 2.18 to <TEX>$5.01{\mu}m$</TEX>. FISH karyotype analysis showed five and eight pairs of 5S and 45S rDNA loci. GISH meiosis pairing analysis showed that 19 complete bivalents were most frequent and accounted for 42% of the 100 pollen mother cells examined. Based on chromosome number, size, morphology, rDNA distribution, and meiosis pairing pattern, both parental genomes of B. rapa and R. sativus appear to exist in xBrassicoraphanus line BB#5, demonstrating its genome integrity. Such stable chromosome constitutions and meiotic pairing patterns in somatic and meiotic cells are very rare in natural and synthetic intergeneric hybrids. Chromosomal studies and genetic and phenotypic changes in allopolyploids a re discussed. The results p resented h erein will b e usef ul f or f urther g enomic s tudy o f xBrassicoraphanus lines and their improvement as promising new breeding varieties.