Chromosome Arm Length Research Articles

GROWTH AND GENETIC CHARACTERISTICS OF CUCUMBER (CUCUMIS SATIVUS L.) CULTIVAR MERCY F1 HYBRID AND MUTANT POPULATIONS

Phenotypic and genotypic diversity is essential and needs more enhancement in different ways to achieve higher productivity and better quality vegetables. The presented research aimed to study the phenotypic characteristics and determine the genetic diversity of F1 populations compared with the colchicine mutant variants in the Mercy cultivar of cucumber (Cucumis sativus L.), held from March to May 2019 at the experimental field of the Department of Agrotechnology, Islamic University of Riau, Pekanbaru, Indonesia. A total number of 144 studied plants comprised 72 from each of the two plant groups. The data recorded on various parameters attained statistical analysis using paired T-test at a significant P < 0.05. Comparisons between the cucumber plants of both groups included karyotypes and chromosome shapes, idiograms, and sequencing. The results revealed that, on average, the taller plants came from F1 populations (93.49 cm), followed by mutants of the same cultivar (67.83 cm). The F1 hybrids showed early flowering (29.00 days) compared with colchicine-treated mutants (33.31 days). However, the number of fruits and fruit weight were higher in the mutant variants (9.39 fruits and 1055.39 g), followed by F1 populations. The karyotypes of cucumber cultivar Mercy F1 hybrids and mutants have different chromosomes, especially with the arm size. An idiogram also exhibited differences in chromosome length between the variants of both plant groups, while the primers of trnL-F and trnL-R target sequences were the same with the DNA sequence length. Using mutant cucumber seeds demonstrates a change in phenotypic character to increase fruit production. However, polyploidy did not occur, with genetic alterations measured by the length of chromosome arms. As a result, a future study with increased concentrations of colchicine is imperative to obtain significant chromosomal mutations.

Draft de novo Genome Assembly of the Elusive Jaguarundi, Puma yagouaroundi.

The Puma lineage within the family Felidae consists of 3 species that last shared a common ancestor around 4.9 million years ago. Whole-genome sequences of 2 species from the lineage were previously reported: the cheetah (Acinonyx jubatus) and the mountain lion (Puma concolor). The present report describes a whole-genome assembly of the remaining species, the jaguarundi (Puma yagouaroundi). We sequenced the genome of a male jaguarundi with 10X Genomics linked reads and assembled the whole-genome sequence. The assembled genome contains a series of scaffolds that reach the length of chromosome arms and is similar in scaffold contiguity to the genome assemblies of cheetah and puma, with a contig N50 = 100.2 kbp and a scaffold N50 = 49.27 Mbp. We assessed the assembled sequence of the jaguarundi genome using BUSCO, aligned reads of the sequenced individual and another published female jaguarundi to the assembled genome, annotated protein-coding genes, repeats, genomic variants and their effects with respect to the protein-coding genes, and analyzed differences of the 2 jaguarundis from the reference mitochondrial genome. The jaguarundi genome assembly and its annotation were compared in quality, variants, and features to the previously reported genome assemblies of puma and cheetah. Computational analyzes used in the study were implemented in transparent and reproducible way to allow their further reuse and modification.

Determination of Karyological Properties of Different Buckwheat (Fagopyrum esculentum Moench) Varieties

In this study, the karyological characteristics of two different varieties (Aktaş and Güneş) of buckwheat (Fagopyrum esculentum) were determined. The study was carried out in Kahramanmaraş Sutcu Imam University, Faculty of Agriculture, Department of Field Crops in 2021. The chromosome number of both varieties was determined as 2n=2x=16 and haploid karyotype formulas as 6 m + 2 sm. Chromosome length ranges from 3.35 µm to 4.78 µm in the Aktaş variety examined, while it varied between 2.89 µm and 3.33 µm in the Güneş variety. While the arm ratio varied between 1.30 and 1.75 in Aktaş variety, it was observed that it varied between 1.22 and 1.86 in Güneş variety. Chromosome length, arm lengths and ratio, relative length, centromere index, centromere state and asymmetric index values of these two varieties were determined. Also, pictures of the metaphase chromosomes of the varieties were taken, measurements were made and idiograms were drawn. Since both types of Fagopyrum esculentum used are bred, they are similar in terms of karyology.

Direct evidence for crossover and chromatid interference in meiosis of two plant hybrids (Lolium multiflorum×Festuca pratensis and Allium cepa×A. roylei).

Crossing over, in addition to its strictly genetic role, also performs a critical mechanical function, by bonding homologues in meiosis. Hence, it is responsible for an orderly reduction of the chromosome number. As such, it is strictly controlled in frequency and distribution. The well-known crossover control is positive crossover interference which reduces the probability of a crossover in the vicinity of an already formed crossover. A poorly studied aspect of the control is chromatid interference. Such analyses are possible in very few organisms as they require observation of all four products of a single meiosis. Here, we provide direct evidence of chromatid interference. Using in situ probing in two interspecific plant hybrids (Lolium multiflorum×Festuca pratensis and Allium cepa×A. roylei) during anaphase I, we demonstrate that the involvement of four chromatids in double crossovers is significantly more frequent than expected (64% versus 25%). We also provide a physical measure of the crossover interference distance, covering ~30-40% of the relative chromosome arm length, and show that the centromere acts as a barrier for crossover interference. The two arms of a chromosome appear to act as independent units in the process of crossing over. Chromatid interference has to be seriously addressed in genetic mapping approaches and further studies.

Aberrant Function of the C-Terminal Tail of HIST1H1E Accelerates Cellular Senescence and Causes Premature Aging

Histones mediate dynamic packaging of nuclear DNA in chromatin, a process that is precisely controlled to guarantee efficient compaction of the genome and proper chromosomal segregation during cell division and to accomplish DNA replication, transcription, and repair. Due to the important structural and regulatory roles played by histones, it is not surprising that histone functional dysregulation or aberrant levels of histones can have severe consequences for multiple cellular processes and ultimately might affect development or contribute to cell transformation. Recently, germline frameshift mutations involving the C-terminal tail of HIST1H1E, which is a widely expressed member of the linker histone family and facilitates higher-order chromatin folding, have been causally linked to an as-yet poorly defined syndrome that includes intellectual disability. We report that these mutations result in stable proteins that reside in the nucleus, bind to chromatin, disrupt proper compaction of DNA, and are associated with a specific methylation pattern. Cells expressing these mutant proteins have a dramatically reduced proliferation rate and competence, hardly enter into the S phase, and undergo accelerated senescence. Remarkably, clinical assessment of a relatively large cohort of subjects sharing these mutations revealed a premature aging phenotype as a previously unrecognized feature of the disorder. Our findings identify a direct link between aberrant chromatin remodeling, cellular senescence, and accelerated aging.

Immunocytological analysis of meiotic recombination in two anole lizards (Squamata, Dactyloidae).

Although the evolutionary importance of meiotic recombination is not disputed, the significance of interspecies differences in the recombination rates and recombination landscapes remains under-appreciated. Recombination rates and distribution of chiasmata have been examined cytologically in many mammalian species, whereas data on other vertebrates are scarce. Immunolocalization of the protein of the synaptonemal complex (SYCP3), centromere proteins and the mismatch-repair protein MLH1 was used, which is associated with the most common type of recombination nodules, to analyze the pattern of meiotic recombination in the male of two species of iguanian lizards, Anolis carolinensis Voigt, 1832 and Deiroptyx coelestinus (Cope, 1862). These species are separated by a relatively long evolutionary history although they retain the ancestral iguanian karyotype. In both species similar and extremely uneven distributions of MLH1 foci along the macrochromosome bivalents were detected: approximately 90% of crossovers were located at the distal 20% of the chromosome arm length. Almost total suppression of recombination in the intermediate and proximal regions of the chromosome arms contradicts the hypothesis that “homogenous recombination” is responsible for the low variation in GC content across the anole genome. It also leads to strong linkage disequilibrium between the genes located in these regions, which may benefit conservation of co-adaptive gene arrays responsible for the ecological adaptations of the anoles.

The Conformation of Yeast Chromosome III Is Mating Type Dependent and Controlled by the Recombination Enhancer.

Mating-type switching in yeast occurs through gene conversion between the MAT locus and one of two silent loci (HML or HMR) on opposite ends of the chromosome. MATa cells choose HML as template, whereas MATα cells use HMR. The recombination enhancer (RE) located on the left arm regulates this process. One long-standing hypothesis is that switching is guided by mating-type-specific and possibly RE-dependent chromosome folding. Here, we use Hi-C, 5C, and live-cell imaging to characterize the conformation of chromosome III in both mating types. We discovered a mating-type-specific conformational difference in the left arm. Deletion of a 1-kb subregion within the RE, which is not necessary during switching, abolished mating-type-dependent chromosome folding. The RE is therefore a composite element with one subregion essential for donor selection during switching and a separate region involved in modulating chromosome conformation.

Synaptonemal complex extension from clustered telomeres mediates full-length chromosome pairing in Schmidtea mediterranea.

In the 1920s, József Gelei proposed that chromosome pairing in flatworms resulted from the formation of a telomere bouquet followed by the extension of synapsis from telomeres at the base of the bouquet, thus facilitating homolog pairing in a processive manner. A modern interpretation of Gelei's model postulates that the synaptonemal complex (SC) is nucleated close to the telomeres and then extends progressively along the full length of chromosome arms. We used the easily visible meiotic chromosomes, a well-characterized genome, and RNAi in the sexual biotype of the planarian Schmidtea mediterranea to test that hypothesis. By identifying and characterizing S. mediterranea homologs of genes encoding synaptonemal complex protein 1 (SYCP1), the topoisomerase-like protein SPO11, and RAD51, a key player in homologous recombination, we confirmed that SC formation begins near the telomeres and progresses along chromosome arms during zygotene. Although distal regions pair at the time of bouquet formation, pairing of a unique interstitial locus is not observed until the formation of full-length SC at pachytene. Moreover, neither full extension of the SC nor homologous pairing is dependent on the formation of double-strand breaks. These findings validate Gelei's speculation that full-length pairing of homologous chromosomes is mediated by the extension of the SC formed near the telomeres. S. mediterranea thus becomes the first organism described (to our knowledge) that forms a canonical telomere bouquet but does not require double-strand breaks for synapsis between homologous chromosomes. However, the initiation of SC formation at the base of the telomere bouquet, which then is followed by full-length homologous pairing in planarian spermatocytes, is not observed in other species and may not be conserved.

Telomere Shortening and Chromosomal Instability in Chronic Lymphocytic Leukemia

Abstract 1761Chronic lymphocytic leukemia (CLL) is a neoplastic disorder of B-lymphocytes, typically with a high number of peripheral B-lymphocytes and small mature lymphocytes (M Hallek et al, Ann Oncol. 16, Suppl 1:i50-1 (2005). Clinically, some patients have a mild, largely asymptomatic course of the disease and a normal life expectancy, while others suffer from fulminant progression and have a very short survival. An important predictive factor is the presence of typical chromosome aberrations (H Doehner et al, N Engl J Med343, 1910–1916 (2000)). Due to a low proliferative rate of the cells, the gold standard for cytogenetic diagnostics in CLL is fluorescence in situ hybridization (FISH). Therefore, not much is known about the incidence of complex karyotypes, although they are strong predictors of a very poor prognosis in CLL (C Mayr et al, Blood107, 742–751 (2007)).By stimulating the cells with different interleukins and CpG-oligodeoxynucleotides, we were able to detect complex karyotypes in about 10% of investigated cases of CLL by classical banding analysis. In this study, we characterized 24 patients with CLL and complex karyotype by performing multicolor fluorescence in situ hybridization (mFISH). Hereby, we could identify cryptic aberrations and describe the karyotype in greater detail. In addition to typical aberrations involving 6q, 11q, 13q and 17p and trisomy 12, (iso)dicentric chromosomes and whole-arm translocations of chromosomes Y, 1, 3, 4, 5, 13, 15, 17, 18, 21 and 22 were detected. These chromosome aberrations were mostly generated by breaks in heterochromatic and telomeric regions indicating an increased breakage of these regions. This may indicate that epigenetic alterations and critically short telomeres predispose for the generation of chromosome aberrations in CLL. Telomere shortening and chromosomal instability are believed to play an important role in the development of neoplasia. Recently, it was shown that short telomeres in CLL are associated with a poor survival and increased genetic complexity (G Roos et al, Blood111, 2246–2252 (2008)). So far, published data are only available on the average telomere length in CLL, but not on the telomere length of individual chromosomes. We used a new technique, telomere/centromere-fluorescence in situ hybridization (T/C-FISH), which combines fluorescence R-banding and FISH using a probe against the telomere repeats to measure the telomere length of each chromosome arm. In line with previous results, patients with CLL showed significantly shorter telomeres than those of healthy controls. Comparing the telomere lengths of distinct chromosome arms with specific aberrations, there was no significant association. In addition, we could compare the telomere lengths of cells with aberrations and cells without aberrations within one patient. Aberrant metaphases of the same patient showed significantly shorter telomeres than metaphases with a normal karyotype (p<0.05).Thus, telomere shortening is not a basic mechanism affecting all hematopoietic cells in CLL patients, e.g. due to aging, but affects only the malignant cells, indicating that telomere attrition is involved in the pathogenesis of CLL with complex karyotypes. Disclosures:No relevant conflicts of interest to declare.

Ongoing GC-Biased Evolution Is Widespread in the Human Genome and Enriched Near Recombination Hot Spots

Fast evolving regions of many metazoan genomes show a bias toward substitutions that change weak (A,T) into strong (G,C) base pairs. Single-nucleotide polymorphisms (SNPs) do not share this pattern, suggesting that it results from biased fixation rather than biased mutation. Supporting this hypothesis, analyses of polymorphism in specific regions of the human genome have identified a positive correlation between weak to strong (W→S) SNPs and derived allele frequency (DAF), suggesting that SNPs become increasingly GC biased over time, especially in regions of high recombination. Using polymorphism data generated by the 1000 Genomes Project from 179 individuals from 4 human populations, we evaluated the extent and distribution of ongoing GC-biased evolution in the human genome. We quantified GC fixation bias by comparing the DAFs of W→S mutations and S→W mutations using a Mann–Whitney U test. Genome-wide, W→S SNPs have significantly higher DAFs than S→W SNPs. This pattern is widespread across the human genome but varies in magnitude along the chromosomes. We found extreme GC-biased evolution in neighborhoods of recombination hot spots, a significant correlation between GC bias and recombination rate, and an inverse correlation between GC bias and chromosome arm length. These findings demonstrate the presence of ongoing fixation bias favoring G and C alleles throughout the human genome and suggest that the bias is caused by a recombination-associated process, such as GC-biased gene conversion.

Karyotype characterization of some Tabanidae (Diptera) species

Karyotypic features of Tabanidae (Diptera: Insecta) are largely unexplored. In this study, karyotypes of some horse fly species, Tabanus quatuornotatus Meigen, 1820, T. sudeticus Zeller, 1842, Dasyrhamphis umbrinus (Meigen, 1820), Atylotus loewianus (Villeneuve, 1920) and Haematopota italica Meigen, 1804 are described for the first time, and also, karyotypes of T. autumnalis Linnaeus, 1761, T. bromius (Linnaeus, 1761), T. bifarius Loew, 1858, T. unifasciatus (Loew, 1858), A. fulvus (Meigen, 1820) are confirmed. Karyotypic differences between six genera in Tabanidae will be discussed. Adult and larval specimens were collected, injected cholchicine and brought to the laboratory. After 24-48 hours tissues were dissected, fixed and dyed, respectively. After dying, preparations were made by squashing tissues and investigated with light microscope and photographed. Based on the results of the present study, the chromosome number in the following reads as follows; T. autumnalis 2n=14, T. bromius 2n=10, T. bifarius 2n=16, T. quatuornotatus 2n=16, T. sudeticus 2n=14, T. unifasciatus 2n=12, D. umbrinus 2n=10, A. fulvus 2n=18, A. loewianus 2n=18 and H. italica 2n=14 chromosome number. Chromosome number, chromosome arm length and arm ratio,

Karyotype analysis ofAstragalus effususBunge (Fabaceae)

Abstract In this study, the karyotype analysis of Astragalus effusus Bunge species was studied using Image Analysis System. The protocol assayed consisted of the use of α-bromonaphthalene for pretreatment, Lewitsky solution for fixation, 1N NaOH for hydrolysis and finally, aceto iron-hematoxylin for staining stage. The species’ chromosome number was 2n = 4× = 32, chromosome lengths range from 1.57 to 2.99 μm and the length of the whole genome was 75.8 μm; also, the chromosome length and arm lengths showed this species was most likely natural autotetraploid and observed one pericentric inversion in chromosomes of the VIII group. Regarding the karyotypic symmetry, this species belongs to the 2A karyotypic symmetry class with a 7m + 2msat + 7sm karyotype formula. Asymmetrical parameters emphasize on rather symmetry karyotype in this species. In this paper, we assess the karyotype of this species for the first time.

Chromosome arm length and nuclear constraints determine the dynamic relationship of yeast subtelomeres

Physical interactions between distinct chromosomal genomic loci are important for genomic functions including recombination and gene expression, but the mechanisms by which these interactions occur remain obscure. Using telomeric association as a model system, we analyzed here the in vivo organization of chromosome ends of haploid yeast cells during interphase. We separately labeled most of the 32 subtelomeres and analyzed their positions both in nuclear space and relative to three representative reference subtelomeres by high-throughput 3D microscopy and image processing. We show that subtelomeres are positioned nonrandomly at the nuclear periphery, depending on the genomic size of their chromosome arm, centromere attachment to the microtubule organizing center (spindle pole body, SPB), and the volume of the nucleolus. The distance of subtelomeres to the SPB increases consistently with chromosome arm length up to approximately 300 kb; for larger arms the influence of chromosome arm length is weaker, but the effect of the nucleolar volume is stronger. Distances between pairs of subtelomeres also exhibit arm-length dependence and suggest, together with dynamic tracking experiments, that potential associations between subtelomeres are unexpectedly infrequent and transient. Our results suggest that interactions between subtelomeres are nonspecific and instead governed by physical constraints, including chromosome structure, attachment to the SPB, and nuclear crowding.

Chromosomal variation in Agromyzidae (Diptera)

The polytene chromosomes of nine species were investigated and chromosome maps are presented. Together with four earlier investigated species, two of which consist of two semispecies each, they are compared as to chromosomal relationships. The basic chromosome number 2n = 12 has been reduced by one centric fusion in one semispecies and by three centric fusions in three species each. The chromosome number has increased through polyploidy in the parthenogenetic species Phytomyza crassiseta to 2n = 18. In species with 2n = 12, five rods and one dot occur in five species, six rods occur in four species (or semispecies) and one V and five rods occur in one species. In polytene chromosome cells there are always five euchromatic major arms and (as a rule) one very small one. The relative lengths of chromosome arms are similar within genera. Intra-arm-rearrangements have resulted in a great dissimilarity among species as to band sequences. In two groups of species it has been possible to homologize the chromosomes and to estimate the number of breaks lying behind species differentiation. One of these groups included three and the other two species. Chromosomal polymorphism occurred in five species. Structural rearrangements, fixed in heterozygous condition, occurred in the parthenogenetic species. Inversions, fixed in homozygous condition, (incipient species differentiation) occurred in two species. Chromosomal evolution seems to have been due to paracentric inversions, changes in single bands and increase in the amount of constitutive heterochromatin.

Reference karyotype and cytomolecular map for loblolly pine (Pinus taeda L.)

A reference karyotype is presented for loblolly pine (Pinus taeda L., subgenus Pinus, section Pinus, subsection Australes), based on fluorescent in situ hybridization (FISH), using 18S-28S rDNA, 5S rDNA, and an Arabidopsis-type telomere repeat sequence (A-type TRS). Well separated somatic chromosomes were prepared from colchicine-treated root meristems, using an enzymatic digestion technique. Statistical analyses performed on chromosome-arm lengths, centromeric indices, and interstitial rDNA and telomeric positions were based on observations from 6 well-separated metaphase cells from each of 3 unrelated trees. Statistically, 7 of the 12 loblolly pine chromosomes could be distinguished by their relative lengths. Centromeric indices were unable to distinguish additional chromosomes. However, the position and relative strength of the rDNA and telomeric sites made it possible to uniquely identify all of the chromosomes, providing a reference karyotype for use in comparative genome analyses. A dichotomous key was developed to aid in the identification of loblolly pine chromosomes and their comparison to chromosomes of other Pinus spp. A cytomolecular map was developed using the interstitial 18S-28S rDNA and A-type TRS signals. A total of 54 bins were assigned, ranging from 3 to 5 bins per chromosome. This is the first report of a chromosome-anchored physical map for a conifer that includes a dichotomous key for accurate and consistent identification of the P. taeda chromosomes.

Incomplete sister chromatid separation of long chromosome arms

Chromosome segregation ensures the equal partitioning of chromosomes at mitosis. However, long chromosome arms may pose a problem for complete sister chromatid separation. In this paper we report on the analysis of cell division in primary cells from field vole Microtus agrestis, a species with 52 chromosomes including two giant sex chromosomes. Dual chromosome painting with probes specific for the X and the Y chromosomes showed that these long chromosomes are prone to mis-segregate, producing DNA bridges between daughter nuclei and micronuclei. Analysis of mitotic cells with incomplete chromatid separation showed that reassembly of the nuclear membrane, deposition of INner CENtromere Protein (INCENP)/Aurora B to the spindle midzone and furrow formation occur while the two groups of daughter chromosomes are still connected by sex chromosome arms. Late cytokinetic processes are not efficiently inhibited by the incomplete segregation as in a significant number of cell divisions cytoplasmic abscission proceeds while Aurora B is at the midbody. Live-cell imaging during late mitotic stages also revealed abnormal cell division with persistent sister chromatid connections. We conclude that late mitotic regulatory events do not monitor incomplete sister chromatid separation of the large X and Y chromosomes of Microtus agrestis, leading to defective segregation of these chromosomes. These findings suggest a limit in chromosome arm length for efficient chromosome transmission through mitosis.

Legume genome evolution viewed through the Medicago truncatula and Lotus japonicus genomes

Genome sequencing of the model legumes, Medicago truncatula and Lotus japonicus, provides an opportunity for large-scale sequence-based comparison of two genomes in the same plant family. Here we report synteny comparisons between these species, including details about chromosome relationships, large-scale synteny blocks, microsynteny within blocks, and genome regions lacking clear correspondence. The Lotus and Medicago genomes share a minimum of 10 large-scale synteny blocks, each with substantial collinearity and frequently extending the length of whole chromosome arms. The proportion of genes syntenic and collinear within each synteny block is relatively homogeneous. Medicago-Lotus comparisons also indicate similar and largely homogeneous gene densities, although gene-containing regions in Mt occupy 20-30% more space than Lj counterparts, primarily because of larger numbers of Mt retrotransposons. Because the interpretation of genome comparisons is complicated by large-scale genome duplications, we describe synteny, synonymous substitutions and phylogenetic analyses to identify and date a probable whole-genome duplication event. There is no direct evidence for any recent large-scale genome duplication in either Medicago or Lotus but instead a duplication predating speciation. Phylogenetic comparisons place this duplication within the Rosid I clade, clearly after the split between legumes and Salicaceae (poplar).

Sex-specific telomere length profiles and age-dependent erosion dynamics of individual chromosome arms in humans

During aging, telomeres are gradually shortened, eventually leading to cellular senescence. By T/C-FISH (telomere/centromere-FISH), we investigated human telomere length differences on single chromosome arms of 205 individuals in different age groups and sexes. For all chromosome arms, we found a linear correlation between telomere length and donor age. Generally, males had shorter telomeres and higher attrition rates. Every chromosome arm had its individual age-specific telomere length and erosion pattern, resulting in an unexpected heterogeneity in chromosome- specific regression lines. This differential erosion pattern, however, does not seem to be accidental, since we found a correlation between average telomere length of single chromosome arms in newborns and their annual attrition rate. Apart from the above-mentioned sex-specific discrepancies, chromosome arm-specific telomere lengths were strikingly similar in men and women. This implies a mechanism that arm specifically regulates the telomere length independent of gender, thus leading to interchromosomal telomere variations.

Limitations of in situ hybridization with total genomic DNA in routine screening for alien introgressions in wheat

In situ hybridization with total genomic DNA (GISH) has become a powerful tool in characterization of alien introgressions in wheat. With recent simplification it can now be used in large scale screening for new chromosome constructs. Its level of resolution in routine applications was tested on sets of recombined wheat-rye chromosomes with genetically determined positions of the translocation breakpoints. The resolution level of GISH visualized by an enzymatic color reaction was much lower than that of GISH with fluorescent probes but both techniques failed to reveal the presence of some distally located breakpoints. The limits of resolution for the two methods were at least 9.8 and 3.5 cM of the relative genetic lengths of chromosome arms, respectively, in configurations with proximal rye and terminal wheat segments when rye DNA was used as a probe. When wheat DNA was used as a probe, a terminal wheat segment estimated to be ca. 1.6 cM in length could not be visualized. An example of induced recombination between a chromosome of Agropyron elongatum and wheat illustrates that these resolution limits of GISH may hamper isolation of critical translocation breakpoints in a chromosome engineering effort.

C‐Banding Analyses of Bromus inermis Genomes

Smooth bromegrass (Bromus inermis Leyss.) has both tetraploid (2n = 28) and octaploid (2n = 56) ploidy levels that have been difficult to characterize cytogenetically because of similar chromosome morphology. Objectives of this study were to identify individual chromosomes of tetraploid and octaploid B inermis with C‐banding procedures along with chromosome length and arm length ratios, develop more detailed karyotypes than those previously available, and use the karyotypes to examine the genomic relationship of tetraploid and octaploid B inermis Root tips of the plants from four tetraploid and three octaploid accessions were used to produce chromosome squash preparations for cytogenetic analysis. The tetraploid B inermis genome consisted of 12 chromosomes with a telomeric band on each arm and sixteen chromosomes with only one telomeric band on one arm. All of the chromosomes of the tetraploid form, except for four chromosomes, were identified by C‐banding patterns, chromosome length, and arm length ratio. The octaploid B inermis genome consisted of four chromosomes with no C‐bands, ≈14 chromosomes with two telomeric bands, and ≈38 chromosomes with only one telomeric band on either the short or long arm. The combined use of C‐banding, chromosome size, and arm length ratio only enabled groups of 2, 4, 6, or 8 similar chromosomes to be identified because of similarities in chromosome morphology of the octaploids. Results indicate that tetraploid B inermis is an allotetraploid since all chromosomes except four could be separated into identifiable pairs. Because of differences between expected and actual numbers of satellite chromosomes and chromosomes with specific C‐banding patterns, octaploid B inermis is probably not a doubled form of the tetraploid B inermis