Characterization of meiotic chromosome behavior in the autopolyploid Saccharum spontaneum reveals preferential chromosome pairing without distinct DNA sequence variation

Autopolyploidy and allopolyploidy may represent an evolutionary advantage and are more common in plants than assumed. However, less attention has been paid to autopolyploidy than to allopolyploidy,and its evolutionary consequences are largely unclear, especially for plants with high ploidy levels. In this study, we developed oligonucleotide(oligo)-based chromosome painting probes to identify individual chromosomes in S. spontaneum. Using fluorescence in situ hybridization(FISH), we investigated chromosome behavior during pachytene, metaphase, anaphase, and telophase of meiosis I(MI) in autotetraploid,autooctoploid, and autodecaploid S. spontaneum clones. All autopolyploid clones showed stable diploidized chromosome behavior;so that homologous chromosomes formed almost exclusively bivalents during MI. Two copies of homologous chromosome 8 with similar sizes in the autotetraploid clone showed preferential pairing with each other with respect to the other copies. However, sequence variation analysis showed no apparent differences among homologs of chromosome 8 and all other chromosomes. We suggest that either the stable diploidized pairing or the preferential pairing between homologous copies of chromosome 8 in the studied autopolyploid sugarcane are accounted for by unknown mechanisms other than DNA sequence similarity. Our results reveal evolutionary consequences of stable meiotic behavior in autopolyploid plants.

Chromosome 5P of Agropyron cristatum induces chromosomal translocation by disturbing homologous chromosome pairing in a common wheat background

Wide hybridization is a strategy for broadening the genetic basis of wheat. Because an efficient method for inducing wheat–alien chromosome translocations will allow producing useful germplasm, it is desirable to discover new genes that induce chromosomal variation. In this study, chromosome 5P from A.cristatum was shown to induce many types of chromosomal structural variation in a common wheat background, including nonhomoeologous chromosome translocations, as revealed by genomic in situ hybridization, fluorescence in situ hybridization, and DNA marker analysis. Aberrant meiosis was associated with chromosomal structural variation, and aberrant meiotic behavior was observed in wheat–A.cristatum 5P monosomic and disomic addition lines, suggesting that the effect of chromosome 5P was independent of the number of chromosome 5P copies. Chromosome 5P disturbed homologous chromosome pairing at pachytene stage in a common wheat background, resulting in a high frequency of univalent formation and reduced crossing over. Thirteen genes involved in DNA repair or chromatin remodeling, including RAD52-like and MSH6 genes, were differentially expressed(upregulated) in wheat–A. cristatum 5P addition lines according to transcriptome analysis, implicating chromosome 5P in the process of meiotic double-strand break repair. These findings provide a new, efficient tool for inducing wheat–alien chromosome translocations and producing new germplasm.

Molecular Mechanisms of Homologous Chromosome Pairing and Segregation in Plants

In most eukaryotic species, three basic steps of pairing, recombination and synapsis occur during prophase of meiosis I. Homologous chromosomal pairing and recombination are essential for accurate segregation of chromosomes. In contrast to the well-studied processes such as recombination and synapsis, many aspects of chromosome pairing are still obscure. Recent progress in several species indicates that the telomere bouquet formation can facilitate homologous chromosome pairing by bringing chromosome ends into close proximity, but the sole presence of telomere clustering is not sufficient for recognizing homologous pairs. On the other hand, accurate segregation of the genetic material from parent to offspring during meiosis is dependent on the segregation of homologs in the reductional meiotic division （MI） with sister kinetochores exhibiting mono-orientation from the same pole, and the segregation of sister chromatids during the equational meiotic division （MII） with kinetochores showing bi-orientation from the two poles. The underlying mechanism of orientation and segregation is still unclear. Here we focus on recent studies in plants and other species that provide insight into how chromosomes find their partners and mechanisms mediating chromosomal segregation.

Centromere pairing precedes meiotic chromosome pairing in plants

Meiosis is a specialized eukaryotic cell division, in which diploid cells undergo a single round of DNA replication and two rounds of nuclear division to produce haploid gametes. In most eukaryotes, the core events of meiotic prophase I are chromosomal pairing,synapsis and recombination. To ensure accurate chromosomal segregation, homologs have to identify and align along each other at the onset of meiosis. Although much progress has been made in elucidating meiotic processes, information on the mechanisms underlying chromosome pairing is limited in contrast to the meiotic recombination and synapsis events. Recent research in many organisms indicated that centromere interactions during early meiotic prophase facilitate homologous chromosome pairing, and functional centromere is a prerequisite for centromere pairing such as in maize. Here, we summarize the recent achievements of chromosome pairing research on plants and other organisms, and outline centromere interactions, nuclear chromosome orientation,and meiotic cohesin, as main determinants of chromosome pairing in early meiotic prophase.

Cytogenetic Analysis of the Primary Amphidiploid Derived from Interspecific Hybridization in Cucumis and Its Selfed Progenies

Studies on the reproduction and cytogenetic characterization of a primary amphidiploid Cucumis species C. hytivus Chen and Kirkbride (2n = 4x = 38) indicated that a more comprehensive cytogenetic analysis of this species and its first selfed progeny would increase its potential utility in cucumber improvement. With tendrils used as source materials for mitotic analysis, chromosome numbers in all selfed progenies were 2n = 38, confirming chromosomal stability in this synthetic amphidiploid species. Detailed meiotic processes were described by comparing the primary and the selfed amphidiploids. Meiotic abnormalities, such as chromosome lagging, unequal separation, chromosome multi-polarization and polyads were observed frequently in all amphidiploid plants except for the selfed no.8, in which meiosis was arrested prior to metaphase Ⅰ. Generally, the frequency of multivalents was higher and the configurations were more complex in the selfed progenies, demonstrating a more extensive genetic exchange between cucumber and C. hystrix Chakr. Genome separation between cucumber and C. hystrix was observed through prophase Ⅰ to anaphase Ⅰ in both generations of the amphidiploids. Consequently, in addition to n = 19, a new gamete with n = 7 was produced, which was confirmed by the chromosome counts 2n = 14 in the backcrossing progenies from cucumber × amphidiploid mating. Fertility varied among the selfed amphidiploid plants. The selfed plant no.1 was found to have an improved fertility (e.g., pollen staining ability 40.8% and 25.6 seeds per fruit) and then was used as source germplasm in further introgression and gene exchange experiments.

Molecular cytogenetic study on the plants of Elymus nutans with varying fertility on the Qinghai-Tibet Plateau

A molecular cytogenetic investigation was conducted on plants of the allohexaploid species Elymus nutans with varying fertility on the Qinghai-Tibet Plateau.Molecular karyotyping revealed that chromosome variants were distributed unevenly among genomes and among different homologue chromosomes in each genome.The plants with varying fertility exhibited significantly higher numbers of chromosome variants than did the normal fertility samples,although both kinds of plants showed the same pattern of high-to-low polymorphism from the Y to St and H genomes.Heterozygosis and karyotype heterozygosity in the plants with varying fertility were 3-and 13-fold higher than those in normal samples,respectively.Significant negative correlations were found not only between seed setting rates and total genome heterozygosity but also between seed setting rates and heterozygosity of each genome in the plants of varying fertility.Chromosome pairing analysis was performed using genomic in situ hybridization in selected plants of different fertility levels.The pairing of chromosomes at meiotic metaphase I was mostly bivalent,although univalent,trivalent,quadrivalent,and other polyvalents also occurred;in addition,chromosome configuration forms and frequencies varied among the studied samples.ANOVA results showed that the average number of ring bivalents in the Y genome was significantly higher than those in the St and H genomes.Significant positive correlations between pollen grain fertility and ring bivalent number were found in the St and H genomes but not in the Y genome.Furthermore,chromosome configuration parameters(total bivalents,numbers of ring and rod bivalents)were found to be significantly correlated with heterozygosity and seed setting rates in the St and H genomes,respectively,but not in the Y genome.It was inferred that the seed setting rate and pollen grain fertility in E.nutans are strongly influenced by the heterozygosity of each genome,but the Y genome differs from the St and H genomes due to chromosome pair alterations.The St and H genomes may contain more chromosome structural variations than the Y genome in E.nutans.

The predictive value of chromosome 8p deletion for metastasis of hepatocellular carcinoma:a summary of works in 10 years

Hepatocellular carcinoma(HCC)represents an extremely poor prognostic cancer,which is mainly due to the high frequency of metastasis/recurrence after surgical operation.Exploring the molecular mechanisms involved in HCC metastasis could be helpful in the pre-diction and early diagnosis of HCC recurrence and could also provide new therapeutic targets for HCC metastasis.In the recent decade,we analyzed the genomic aberrations of the clinical specimens,as well as the metastatic models and cell lines of human HCC to identify the genetic mar-kers related to HCC metastasis and to verify their clinical values in the prediction and control of metastasis of HCC.Using the comparative genomic hybridization(CGH)technique,we compared the differences of chromosomal aberrations between primary HCC tumors and their matched metastatic lesions,and found that chromosome 8p deletions might contribute to HCC metastasis.This novel finding was further confirmed by comparison between nude mice models of HCC with different meta-static potentials.By the more sensitive genome-wide microsatellite analysis,8p deletion was defined to 8p23.3 and 8p11.2,which are two likely regions harboring meta-stasis-related genes of HCC.Using‘8p-specific’microar-rays,two novel metastatic suppressors(HTPAP and MRSA)were identified,and were proven to suppress in vitro invasion and in vivo metastasis of HCC.Clinical studies indicate that 8p deletion detected in HCC or cir-culating plasma DNA of patients is a useful predictor for metastatic recurrence and prognosis,even for patients with early stage HCC.These novel findings are regarded as important advances in the study of the molecular mechanisms of HCC metastasis,which provide not only a holistic view on the molecular cytogenetic bases of HCC metastasis,but also candidate regions for further study to identify metastatic suppressor genes.

Ten Years of Gene Discovery for Meiotic Event Control in Rice

Meiosis is the crucial process by which sexually propagating eukaryotes give rise to haploid gametes from diploid cells. Several key processes, like homologous chromosomes pairing, synapsis, recombination, and segregation, sequentially take place in meiosis. Although these widely conserved events are under both genetic and epigenetic control, the accurate details of molecular mechanisms are continuing to investigate. Rice is a good model organism for exploring the molecular mechanisms of meiosis in higher plants. So far, 28 rice meiotic genes have been characterized. In this review, we give an overview of the discovery of rice meiotic genes in the last ten years, with a particular focus on their functions in meiosis.