Control Of Chromosome Pairing Research Articles

The novel function of the Ph1 gene to differentiate homologs from homoeologs evolved in Triticum turgidum ssp. dicoccoides via a dramatic meiosis-specific increase in the expression of the 5B copy of the C-Ph1 gene.

The Ph1 gene is the principal regulator of homoeologous chromosome pairing control (HECP) that ensures the diploid-like meiotic chromosome pairing behavior of polyploid wheat. The HECP control was speculated to have evolved after the first event of polyploidization. With the objective to accurately understand the evolution of the HECP control, wild emmer wheat accessions previously known to differ for HECP control were characterized for the structure and expression of the candidatePh1 gene, C-Ph1. The C-TdPh1-5A and 5B gene copies of emmer wheat showed 98 and 99% DNA sequence similarity respectively with the corresponding hexaploid wheat copies. Further, the C-TdPh1-5B carried the C-Ph1-5B specific structural changes and transcribed three splice variants as observed in the hexaploid wheat. Further, single nucleotide changes differentiating accessions varying for HECP control were identified. Analyzed by quantitative expression analysis, the wild emmer accessions with HECP control showed ~ 10,000-fold higher transcript abundance of the C-TdPh1-5B copy during prophase-I compared to accessions lacking the control. Differential transcriptional regulation of C-TdPh1-5B splice variants further revealed that C-Ph1-5Balt1 variant is mainly responsible for differential accumulation of C-Ph1-5B copy in accessions with HECP control. Taken together, these results showed that the HECP control evolved via transcriptional regulation of splice variants during meiosis.

The Ph1 Locus from Wheat Controls Meiotic Chromosome Pairing in Autotetraploid Rye (Secale cereale L.)

The Ph1 locus on chromosome 5B enforces strictly bivalent pairing in polyploid wheat, but the exact mechanism of its action remains unknown. Pairing restriction involves not only wheat homoeologues and all alien introgressions but also differentiated homologues. In this study we show that chromosome 5B with its Ph1 locus also controls chromosome pairing in autotetraploid rye by apparently restricting chiasma formation between dissimilar homologues. Unlike in wheat, the effect appears to be dosage-dependent, which may be a reflection of an interaction between Ph1 and the rye chromosome pairing control system. With 2 doses of Ph1 present, chiasmate pairing was severely restricted resulting in a significantly higher number of univalents and bivalents per cell than in the controls. The restrictions imposed by Ph1 virtually eliminated MI pairing of chromosome arms polymorphic for their C-band patterns and did not appear to affect arms with similar patterns. If the polymorphism for C-bands is taken as a measure of the overall chromosome similarity/divergence, such differences were recognized and acted upon by the Ph1 locus. The fact that Ph1 operates in rye in the same fashion as in polyploid wheats suggests that it controls some basic mechanism of chromosome recognition.

Chromosome pairing of individual genomes in tall fescue (Festuca arundinacea Schreb.), its progenitors, and hybrids with Italian ryegrass (Lolium multiflorum Lam.)

A diploid-like pairing system prevents meiotic irregularities and improves the efficiency of gamete production in allopolyploid species. While the nature of the system is known in some polyploid crops including wheat, little is known about the control of chromosome pairing in polyploid fescues (Festuca spp.). In this work we studied chromosome pairing in allohexaploid F. arundinacea, its progenitors F. pratensis and F. glaucescens, and two intergeneric hybrids Lolium multiflorum (2x) ×F. arundinacea (6x) and L. multiflorum (4x) ×F. glaucescens(4x). The use of genomic in situ hybridization (GISH) permitted the analysis of homoeologous chromosome pairing and recombination of different genomes involved. We detected a diploid-like pairing system in polyploid fescues F. arundinacea and F. glaucescens, the latter being one of the progenitors of F. arundinacea. The pairing control system was absent in the second progenitor F. pratensis. Detailed analysis of intergeneric hybrids confirmed the presumed haploinsufficiency of the fescue system, which resulted in homoeologous pairing between all component genomes. This indicates that introgression of any specific chromosome segment from one genome to another is possible in all genome combinations. Our results not only contribute to the quest to discover the nature of the system controlling chromosome pairing in polyploid fescues, but may also have serious implications for design of hybrid breeding schemes in forage grasses.

Haploidy in Cultivated Wheats: Induction and Utility in Basic and Applied Research

The usefulness of haploid plants in basic research in cytogenetics, genetics, evolution, and practical plant breeding is well known. Haploid plants provide an efficient research tool for studies on induced mutagenesis and genetic transformation. They also help elucidate the genetic control of chromosome pairing inherently present in allopolyploids such as bread wheat, durum wheat, and oats. Genetic control of chromosome pairing in haploid nuclei has helped in assessing intergenomic relationships. By analyzing the degree and specificity of chromosome pairing in the Ph1‐ and ph1b‐euhaploids (2n = 3x = 21; ABD), we demonstrated that the A and D genomes of wheat are more closely related to each other than either one is to the B genome. It is significant that the totipotent nature of a haploid cell is being exploited in several facets of biological research. In addition to its numerous applications in basic research, the haploidy approach provides an efficient means of producing truly homozygous lines, thereby accelerating the breeding process. Wheat cultivars developed from doubled haploids (DHs) have been released for cultivation in Canada, China, Europe, and Brazil. General characteristics and classification of haploids derived from diploid and polyploid species are provided in this article. Methods of extracting haploids of polyploid wheats are described, and applications of haploidy in basic and applied research are discussed.

Molecular biogeographic study of recently described B- and A-genomeArachisspecies, also providing new insights into the origins of cultivated peanut

The cultivated peanut Arachis hypogaea is a tetraploid, likely derived from A- and B-genome species. Reproductive isolation of the cultigen has resulted in limited genetic variability for important traits. Artificial hybridizations using selected diploid parents have introduced alleles from wild species, but improved understanding of recently classified B-genome accessions would aid future introgression work. To this end, 154 cDNA probes were used to produce 1887 RFLP bands scored on 18 recently classified or potential B-genome accessions and 16 previously identified species. One group of B-genome species consisted of Arachis batizocoi, Arachis cruziana, Arachis krapovickasii, and one potential additional species; a second consisted of Arachis ipaënsis, Arachis magna, and Arachis gregoryi. Twelve uncharacterized accessions grouped with A-genome species. Many RFLP markers diagnostic of A. batizocoi group specificity mapped to linkage group pair 2/12, suggesting selection or genetic control of chromosome pairing. The combination of Arachis duranensis and A. ipaënsis most closely reconstituted the marker haplotype of A. hypogaea, but differences allow for other progenitors or genetic rearrangements after polyploidization. From 2 to 30 alleles per locus were present, demonstrating section Arachis wild species variation of potential use for expanding the cultigen's genetic basis.

Meiotic behaviour of individual chromosomes of Festuca pratensis in tetraploid Lolium multiflorum

Intergeneric hybrids of fescues (Festuca spp.) and ryegrasses (Lolium spp.) are unique for the ability of their chromosomes to pair essentially freely in meiotic metaphase I (MI). At the same time, their chromosomes can be readily recognized by genomic in-situ hybridization (GISH). Past genome-wide observations suggested that this homoeologous pairing was not completely random. In this study we extend the analysis to all seven individual chromosomes of F. pratensis introgressed into autotetraploid L. multiflorum and show that for any F. pratensis chromosome the choice of an MI pairing partner depends on the identity of the remaining chromosomes present in the quadruplet. In monosomic introgressions, the choice of a homologous or homoeologous partner was completely random; in disomics there was a slight preference for homologous pairing. Pairing preference was similar for each chromosome, suggesting that pairing affinity of all chromosomes is essentially the same and no structural rearrangements differentiate the two genera. Homoeologous crossover rates for individual chromosomes were similar and they were consistently lower than expected on the basis of the MI pairing. High homoeologous MI pairing in these hybrids may be due to a very permissive system of chromosome pairing control that overlooks differences between the parental chromosomes. Given the ease of genome discrimination by GISH in the Lolium-Festuca hybrids, the differences in repetitive DNA sequences must be substantial. On the other hand, it appears just as likely that while the DNA repeats diverged markedly during evolution, the sequences involved in chromosome pairing have been conserved enough to facilitate regular pairing partner recognition and crossing-over.

Condensation of rye chromatin in somatic interphase nuclei of Ph1 and ph1b wheat

The Ph1 locus in hexaploid wheat (Triticum aestivum L.) enforces diploid-like behavior in the first metaphase of meiosis. To test the hypothesis that this chromosome pairing control is exercised by affecting the degree of chromatin condensation, the dispersion of rye chromatin in interphase nuclei in somatic tissues of wheat-rye chromosome translocations 1RS.1BL, 2RS.2BL, 2BS.2RL, 3RS.3DL and 5RS.5BL was compared in Ph1 and ph1b isogenic backgrounds. No significant differences in rye chromatin condensation that could be attributed to the Ph1 locus were detected. Regardless of the Ph1 status, each rye chromosome arm tested conformed to the general Rabl’s orientation and occupied portions of the nuclei proportional to their length. Earlier observations that indicated the involvement of Ph1 locus in rye chromatin condensation in wheat could have been due either to specific loci on the studied 5RL rye arm that control the chromosome condensation process or to damage to the genetic system controlling chromatin condensation in the existing ph1b stocks of wheat. That damage might have been caused by homoeologous recombination and uneven disjunction of chromosomes from multivalents.

Microarray expression analysis of meiosis and microsporogenesis in hexaploid bread wheat

BackgroundOur understanding of the mechanisms that govern the cellular process of meiosis is limited in higher plants with polyploid genomes. Bread wheat is an allohexaploid that behaves as a diploid during meiosis. Chromosome pairing is restricted to homologous chromosomes despite the presence of homoeologues in the nucleus. The importance of wheat as a crop and the extensive use of wild wheat relatives in breeding programs has prompted many years of cytogenetic and genetic research to develop an understanding of the control of chromosome pairing and recombination. The rapid advance of biochemical and molecular information on meiosis in model organisms such as yeast provides new opportunities to investigate the molecular basis of chromosome pairing control in wheat. However, building the link between the model and wheat requires points of data contact.ResultsWe report here a large-scale transcriptomics study using the Affymetrix wheat GeneChip® aimed at providing this link between wheat and model systems and at identifying early meiotic genes. Analysis of the microarray data identified 1,350 transcripts temporally-regulated during the early stages of meiosis. Expression profiles with annotated transcript functions including chromatin condensation, synaptonemal complex formation, recombination and fertility were identified. From the 1,350 transcripts, 30 displayed at least an eight-fold expression change between and including pre-meiosis and telophase II, with more than 50% of these having no similarities to known sequences in NCBI and TIGR databases.ConclusionThis resource is now available to support research into the molecular basis of pairing and recombination control in the complex polyploid, wheat.

From Diploids to Allopolyploids: The Emergence of Efficient Pairing Control Genes in Plants

Polyploidy has played a major role in the evolution of higher plants. Precise control of chromosome pairing is vital for conferring meiotic regularity, and hence reproductive stability in allopolyploids. In this review, we examine whether strong evidence has accumulated for the presence and activity of pairing control genes in different allopolyploid species that are entirely bivalent forming and that display a strict disomic inheritance. We show that very good evidence has been adduced in Triticum species, Avena sativa, Festuca arundinacea, Brassica napus, Gossypium hirsutum, and G. barbadense, and in amphidiploids related to the diploid species Lolium perenne, L. multiflorum, and L. rigidum. More circumstantial evidence has been obtained for polyploids in the genera Aegilops, Hordeum, Nicotiana, and Coffea, which have received far less attention than the other species. Although these pairing regulators seem to control different processes operating throughout the premeiotic interphase and the meiotic prophase, little is known about their precise mode of action. We present three hypotheses that have been proposed to explain the origin and evolution of pairing control genes; none of them has been supported by direct evidence, and the origin of most pairing suppressors is still unknown. Accordingly, the study of pairing control genes is still an important task for understanding the stabilization and establishment of allopolyploid species.

PrBn, a major gene controlling homeologous pairing in oilseed rape (Brassica napus) haploids.

Precise control of chromosome pairing is vital for conferring meiotic, and hence reproductive, stability in sexually reproducing polyploids. Apart from the Ph1 locus of wheat that suppresses homeologous pairing, little is known about the activity of genes that contribute to the cytological diploidization of allopolyploids. In oilseed rape (Brassica napus) haploids, the amount of chromosome pairing at metaphase I (MI) of meiosis varies depending on the varieties the haploids originate from. In this study, we combined a segregation analysis with a maximum-likelihood approach to demonstrate that this variation is genetically based and controlled mainly by a gene with a major effect. A total of 244 haploids were produced from F(1) hybrids between a high- and a low-pairing variety (at the haploid stage) and their meiotic behavior at MI was characterized. Likelihood-ratio statistics were used to demonstrate that the distribution of the number of univalents among these haploids was consistent with the segregation of a diallelic major gene, presumably in a background of polygenic variation. Our observations suggest that this gene, named PrBn, is different from Ph1 and could thus provide complementary information on the meiotic stabilization of chromosome pairing in allopolyploid species.

Formation of 2n gametes in durum wheat haploids: Sexual polyploidization

Allopolyploidy, resulting from interspecific and intergeneric hybridization accompanied by sexual doubling of chromosomes, has played a major role in the evolution of crop plants that sustain humankind. The allopolyploid species, including durum wheat, bread wheat, and oat, have developed a genetic control of chromosome pairing that confers on them meiotic regularity (diploid-like chromosome pairing), and hence reproductive stability, and disomic inheritance. Being natural hybrids, they enjoy the benefits of hybridity as well as polyploidy that make them highly adaptable to diverse environments. Despite the complexities of sexual reproduction, it is widespread among plants and animals. Sexual polyploids are highly successful in nature. Sexual polyploidization is far more efficient than somatic chromosome doubling. Sexual polyploidization effected by functioning of unreduced (2n) gametes in the parental species or in their hybrids has been instrumental in producing our grain, fiber, and oilseed crops. Evidence is presented for the occurrence of sexual polyploidization in durum haploids. ThePh1-induced failure of homoeologous pairing is an important factor in the formation of first division restitution(FDR) nuclei and 2n gametes. The evolutionary and breeding significance of sexual polyploidization is discussed. It is emphasized that three factors, viz.,sexual reproduction, allopolyploidy, and genetic control of chromosome pairing,jointly constitute a perfect recipe for cataclysmic evolution in nature.

A strict diploid‐like pairing regime is associated with tetrasomic segregation in induced autotetraploids of kale

AbstractFertility and reproductive stability in polyploids depend on the precise control of chromosome pairing at meiosis. In this study, meiotic pairing configurations are related to isozyme segregation ratios in F1 hybrids between colchicine‐induced autotetraploids of kale. An immediate diploid‐like pairing regime is associated with tetrasomic segregation. Although the mechanisms driving such a behaviour have yet to be identified, pairing preferences are not responsible for the cytological diploidization of autotetraploid kales.

The cytogenetics and evolution of forage legumes from Rio Grande do Sul: a review

The work developed by the Cytogenetics Group of the Department of Forage Plants and Agrometeorology (Departamento de Plantas Forrageiras e Agrometerologia - DPFA), Agronomy Faculty, Federal University of Rio Grande do Sul (UFRGS), are reviewed in the present study. Topics discussed include: the chromosome numbers and meiotic behavior of Desmodium and Vigna; the application of cytogenetic methods (e.g., polyploidy induction in Trifolium riograndense) to plant breeding; the genetic control of chromosome pairing in autopolyploids of T. riograndense; karyotypes of the Vicia sativa aggregate in Southern Brazil as an example of a founder effect leading to a reduction in karyotype but not to morphological variability; data on the karyotypes of four Lathyrus species which show that the evolution of these species has been accompanied by a decrease in chromosome size, and the results of an investigation of variability in chromosome number in a complete genus, Leucaena. The main objectives of the group for the near future are also outlined.

Interspecific hybrids between a transgenic rapeseed (Brassica napus) and related species: cytogenetical characterization and detection of the transgene

In interspecific hybrids produced between a transgenic rapeseed, an allotetraploid species, resistant to herbicide, phosphinotricin, and five diploid related species, the risk for gene introgression in weed genomes was explored through cytogenetic and bar gene characterizations. Among the 75 hybrids studied, most had the expected triploid structure, with the exception of B. napus - B. oleracea amphidiploid plants and one B. napus - S. arvensis amphidiploid plant. In triploid hybrid plants, the reciprocal hybrids did not exhibit any difference in their meiotic behavior. The comparison of the percentage of chromosome pairing in the hybrids with that of haploid rapeseed permit to conclude that allosyndesis between AC genomes and related species genomes took place. This possibility of recombination was confirmed by the presence of multivalent associations in all the interspecific hybrids. Nevertheless, in B. napus - B. adpressa hybrids a control of chromosome pairing seemed to exist. The possibility of amphidiploid plant production directly obtained in the F1 generation increased the risk of gene dispersal. The B. napus - B. oleracea amphidiploid plant presented a meiotic behavior more regular than that of the B. napus - S. arvensis amphidiploid plant. Concerning the herbicide bar gene characterization, the presence of the gene detected by DNA amplification was correlated with herbicide resistance, except for two plants. Different hypotheses were proposed to explain these results. A classification of the diploid species was established regarding their gene dispersal risk based on the rate of allosyndesis between chromosomes of AC genomes of rapeseed and the genomes of the related species.

Progress and potential of interspecific hybrids of Lolium and Festuca

SUMMARYThe Lolium-Festuca complex of related species provides a wide range of variation for the development of versatile grasses adapted to meet the changing requirements of livestock farming. Various approaches have been used in order to harness the full potential of this variation. Tetraploid hybrids between the two ryegrass species L. perenne and L. multiflorum have resulted in stable and successful commercial varieties. Hybrids between more distantly related ryegrass and fescue species have demonstrated useful combinations of traits from both parents. Chromosome doubling is necessary in these hybrids to restore fertility in synthetic amphiploids. However, genetic instability can still be a problem and research is in progress to obtain tighter control of chromosome pairing. An alternative approach is to introduce a few specific traits from fescues into ryegrasses, or vice versa, using introgressive breeding procedures. This review outlines the progress already achieved in exploiting the ryegrass–fescue complex by using these various approaches and assesses the potential of hybrid derivatives.

Genotypic control of chromosome pairing in amphiploids involving the cultivated oat Avena sativa L.

A genotype of the diploid species Avena longiglumis (Cw 57) has been shown to modify the genetic control of diploid-like chromosome pairing in the cultivated oat, A. sativa (2n=6x=42) leading to increased homoeologous chromosome pairing in 4x hybrids between the two species (Rajhathy & Thomas, 1974). The Cw 57 genotype has a similar effect in increasing homoeologous chromosome pairing in amphiploids combining diploid and hexaploid genomes including associations between alien chromosomes and their corresponding pairs in hexaploid species. The effect of the Cw 57 genotype is probably in altering the specificity of chromosome pairing in the early stages of meiosis. The use of the Cw 57 genotype to induce homoeologous chromosome pairing as a technique for the transfer of desirable alien variation into the cultivated oat is discussed.

Induction of polyploidy and cytological characterization of autotetraploids of Trifolium riograndense Burkart (Leguminosae)

Nine polyploids (including two mixaploids) were obtained in Trifolium riograndense with three methods of colchicine application to seedlings: immersion, dropping and cotton. The best method showed to be immersion for 6 h in 0.30% colchicine. Leaves with more than 3 folioles were frequently observed in some (but not all) polyploids. The number of pollen grain pores increased from 3 to 4 in tetraploids. Stomatal cell size was not a reliable criterium to distinguish ploidy levels. Two percent of aneuploidy occurred in the C1 generation. Meiotic behavior was characterized by high bivalent and low quadrivalent numbers with suggestions of genetic control of chromosome pairing. Meiotic index ranged from 70% to 98% and pollen viability from 66 to 90%. An apparent tendency to decrease the number of quadrivalents and to increase mean meiotic index and pollen viability from C0 to C1 was detected. The use of these autopolyploids in basic and applied research as well as in genetic breeding is promising.

GENOME ANALYSIS OF HORDEUMJUBATUMANDH.COMPRESSUM

On the basis of the chromosome behavior of a backcross between a colchicine-induced amphiploid (6x) from a cross between Hordeum jubatum L. and H. compressum Griseb. and H. jubatum (4x), the following genome relationship between H. jubatum, H. compressum and closely related species is suggested: H. jubatum, AAA′A′, H. brachyantherum, A A A′A′, H. californicum, A A, and H. compressum, A′A′. Also, an argument is proposed for genetic control of chromosome pairing.

Genetic control of chromosome pairing in hexaploid oats.

THE common oat, Arena sativa, is a hexaploid (2n=6x=42) whose chromosomes form exclusively bivalents during meiosis. The bivalents are formed by the pairing of exact homologues resulting in disomic inheritance. Gene duplications and triplications were revealed by early inheritance studies1 and homoeologous relationships of the three genomes were later supported by the good tolerance of deficiency for single and for pairs of chromosomes2.

A Cytological Study of Certain Autotetraploid Avena Hybrids1

Autotetraploid hybrids of Avena brevis ✕ A. hirtula, A. hirtula ✕ A. strigosa, and reciprocal were studied for cytological behavior, self‐fertility, and cross‐fertility in the C1 and C2 generations. There was no statistical difference between hybrids or generations for these characters. Cells with 10 bivalents and 2 quadrivalents were the most common type of chromosome pairing at metaphase I of meiosis. The overall means were .52 univalents, 9.86 bivalents, .33 trivalents, and 1.69 quadrivalents. Complete bivalent pairing was occasionally observed. When compared to data for autotetraploids of A. strigosa, the hybrids showed an increase of 4 to 5 bivalents, a decrease in multivalents, and about 20% higher self‐fertility.It was suggested that the increased meiotic stability of the autotetraploid hybrids was caused either by genic control of chromosome pairing or by cryptic structural differences between homologues from the As genomes of the parental species.