Alien chromosome segment from Aegilops speltoides and Dasypyrum villosum increases drought tolerance in wheat via profuse and deep root system.

BACKGROUND: Recurrent drought associated with climate change is a major constraint to wheat (Triticum aestivum L.) productivity. This study aimed to (i) quantify the effects of addition/substitution/translocation of chromosome segments from wild relatives of wheat on the root, physiological and yield traits of hexaploid wheat under drought, and (ii) understand the mechanism(s) associated with drought tolerance or susceptibility in wheat-alien chromosome lines. METHODS: A set of 48 wheat-alien chromosome lines (addition/substitution/translocation lines) with Chinese Spring background were used. Seedling root traits were studied on solid agar medium. To understand the influence of drought on the root system of adult plants, these 48 lines were grown in 150-cm columns for 65 d under full irrigation or withholding water for 58 d. To quantify the effect of drought on physiological and yield traits, the 48 lines were grown in pots under full irrigation until anthesis; after that, half of the plants were drought stressed by withholding water for 16 d before recording physiological and yield-associated traits. RESULTS: The alien chromosome lines exhibited altered root architecture and decreased photochemical efficiency and seed yield and its components under drought. The wheat-alien chromosome lines T5DS·5S#3L (TA5088) with a chromosome segment from Aegilops speltoides (5S) and T5DL.5 V#3S (TA5638) with a chromosome segment from Dasypyrum villosum (5 V) were identified as drought tolerant, and the drought tolerance mechanism was associated with a deep, thin and profuse root system. CONCLUSIONS: The two germplasm lines (TA5088 and TA5638) could be used in wheat breeding programs to improve drought tolerance in wheat and understand the underlying molecular genetic mechanisms of root architecture and drought tolerance.

[Prostate gland - what would urologists like to know from radiologists?] / Die Vorsteherdrüse ­ was möchte der Urologe vom Radiologen wissen?

A more than 100-year period, where the prostate was only seen and treated as a whole is coming to an end right now. Finally, high resolution imaging is providing deep insights and detailed information so that new therapeutic procedures can aim for the smallest targets within the gland. The long-standing wish of patients for individual noninvasive diagnostics and treatment of prostate diseases can now be fulfilled by providing new tailored concepts; however, in order to transfer the enormous amount of new information into the specific clinical patient situation, a closely knit interdisciplinary approach is required. In this setting, the traditional outpatient consultation service is overstretched in every aspect. It is now the time for new innovative constructs. The current one-sided service concept for urologists, radiologists and radiation therapists is therefore behind the times and the development of a "prostate management team" with equally cooperating partners from each specialty is the task for the future.

Sequence organization and evolutionary dynamics of Brachypodium-specific centromere retrotransposons.

Brachypodium distachyon is a wild annual grass belonging to the Pooideae, more closely related to wheat, barley, and forage grasses than rice and maize. As an experimental model, the completed genome sequence of B. distachyon provides a unique opportunity to study centromere evolution during the speciation of grasses. Centromeric satellite sequences have been identified in B. distachyon, but little is known about centromeric retrotransposons in this species. In the present study, bacterial artificial chromosome (BAC)-fluorescence in situ hybridization was conducted in maize, rice, barley, wheat, and rye using B. distachyon (Bd) centromere-specific BAC clones. Eight Bd centromeric BAC clones gave no detectable fluorescence in situ hybridization (FISH) signals on the chromosomes of rice and maize, and three of them also did not yield any FISH signals in barley, wheat, and rye. In addition, four of five Triticeae centromeric BAC clones did not hybridize to the B. distachyon centromeres, implying certain unique features of Brachypodium centromeres. Analysis of Brachypodium centromeric BAC sequences identified a long terminal repeat (LTR)-centromere retrotransposon of B. distachyon (CRBd1). This element was found in high copy number accounting for 1.6 % of the B. distachyon genome, and is enriched in Brachypodium centromeric regions. CRBd1 accumulated in active centromeres, but was lost from inactive ones. The LTR of CRBd1 appears to be specific to B. distachyon centromeres. These results reveal different evolutionary events of this retrotransposon family across grass species.

Genetic compensation abilities of Aegilops speltoides chromosomes for homoeologous B-genome chromosomes of polyploid wheat in disomic S(B) chromosome substitution lines.

The S genome of Aegilops speltoides is closely related to the B and G genomes of polyploid wheats. However, little work has been reported on the genetic relationships between the S-genome and B-genome chromosomes of polyploid wheat. Here, we report the isolation of a set of disomic substitutions (DS) of S-genome chromosomes for the B-genome chromosomes and their effects on gametophytic and sporophytic development. Ae. speltoides chromosomes were identified by their distinct C-banding and fluorescence in situ hybridization patterns with the Ae. speltoides-derived clone pGc1R-1. Although no large structural differences between S-genome and B-genome chromosomes exist, significant differences in gametophytic compensation were observed for chromosomes 1S, 3S, 5S and 6S. Similarly, chromosomes 1S, 2S, 4S, 5S and 6S affected certain aspects of sporophytic development in relation to spike morphology, fertility and meiotic pairing. The DS5S(5B) had disturbed meiosis with univalents/multivalents and suffered chromosome elimination in the germ tissues leading to haploid spikes in 50% of the plants. The effect of the Ph1 gene on meiosis is well known, and these results provide evidence for the role of Ph1 in the maintenance of polyploid genome integrity. These and other data are discussed in relation to the structural and functional differentiation of S- and B-genome chromosomes and the practical utility of the stocks in wheat improvement.

Fate of Aegilops speltoides-derived, repetitive DNA sequences in diploid Aegilops species, wheat-Aegilops amphiploids and derived chromosome addition lines.

The present study reports the cloning and characterization of an Aegilops speltoides-derived subtelomeric repeat, designated as pSp1B16. Clone pSp1B16 has 98% sequence homology with the previously isolated Ae. speltoides repeat Spelt1. The distribution of pSp1B16 and another Ae. speltoides repeat, pGc1R1, was analyzed in diploid Aegilops species, tetra- and hexaploid wheats, wheat-Aegilops amphiploids and derived chromosome addition lines by fluorescence in situ hybridization (FISH). Clones pSp1B16 and pGc1R1 revealed FISH sites in Ae. speltoides, Ae. sharonensis and Triticum timopheevii, whereas additional pGc1R1 FISH sites were observed in Ae. longissima and Ae. caudata. The pSp1B16 and pGc1R1 FISH patterns of the Aegilops chromosomes in the wheat-Aegilops amphiploids and chromosome addition lines are similar to those present in the Aegilops parent accession. We did not observe any evidence of pSp1B16 and pGc1R1 sequence elimination, which is in contrast to previous studies using similar hybrids and repeats. The presented data suggest that the genomic changes in synthetic amphiploids observed in previous studies might be caused by homoeologous recombination, which was suppressed in the amphiploid analyzed in this study.

Upper-Urinary-Tract Effects After Irreversible Electroporation (IRE) of Human Localised Renal-Cell Carcinoma (RCC) in the IRENE Pilot Phase 2a Ablate-and-Resect Study.

PURPOSE: Irreversible electroporation (IRE) is a new potential ablation modality for small renal masses. Animal experiments have shown preservation of the urine-collecting system (UCS). The purpose of this clinical study was to perform the first evaluation and comparison of IRE's effects on the renal UCS by using urinary cytology, magnetic-resonance imaging, and resection histology in men after IRE of pT1a renal-cell carcinoma (RCC). METHODS: Seven patients with biopsy-proven RCC pT1a cN0cM0 underwent IRE in a phase 2a pilot ablate-and-resect study (IRENE trial). A contrast-enhanced, diffusion-weighted MRI and urinary cytology was performed 1 day before and 2, 7, and 27 days after IRE. Twenty-eight days after IRE the tumour region was completely resected surgically. RESULTS: Technical feasibility was demonstrated in all patients. In all cases, MRI revealed complete coverage of the tumour area by the ablation zone with degenerative change. The urographic late venous MRI phase (urogram scans) demonstrated normal morphological appearances. Urine cytology showed a temporary vacuolisation of the cyto- and caryoplasmas after IRE. Whereas the urothelium showed signs of regeneration 28 days after IRE-ablation, the tumour and parenchyma below it showed necrosis and permanent tissue destruction. CONCLUSIONS: Renal percutaneous IRE appears to be a safe treatment for pT1a RCC. The preservation of the UCS with unaltered normal morphology as well as urothelial regeneration and a phenomenon (new in urinary cytology) of temporary degeneration with vacuolisation of detached transitional epithelium cells were demonstrated in this clinical pilot study.

Plant cytogenetics at the dawn of the 21st century.

The years 1996-1997 saw advances in plant chromosome handling, structure, behaviour and manipulation. Improved protocols were developed for flow sorting, microdissection and microcloning. Fibre FISH was used to map a range of DNA sequences at a resolution of a few kilobases. Over 400 wheat deletion stocks were reported and healing of broken chromosomes by de novo addition of telomeric sequences was demonstrated. Centromeric DNA sequences were identified. The role of telomeric ends in pairing was demonstrated. Apparently unusually long chromosome arms can interfere with mitosis. Novel phenomena and potential of wide hybrids for genome analysis were noteworthy.

[Focal therapy for small renal masses : Observation, ablation or surgery]. / Fokale Therapie von kleinen Nierentumoren : Beobachtung, Ablation oder Operation.

BACKGROUND: The rising incidence of renal cell carcinoma, its more frequent early detection (stage T1a) and the increasing prevalence of chronic renal failure with higher morbidity and shorter life expectancy underscore the need for multimodal focal nephron-sparing therapy. DISCUSSION: During the past decade, the gold standard shifted from radical to partial nephrectomy. Depending on the surgeon's experience, the patient's constitution and the tumor's location, the intervention can be performed laparoscopically with the corresponding advantages of lower invasiveness. A treatment alternative can be advantageous for selected patients with high morbidity and/or an increased risk of complications associated with anesthesia or surgery. Corresponding risk stratification necessitates previous confirmation of the small renal mass (cT1a) by histological examination of biopsy samples. Active surveillance represents a controlled delay in the initiation of treatment. RESULTS: Percutaneous radiofrequency ablation (RFA) and laparoscopic cryoablation are currently the most common treatment alternatives, although there are limitations particularly for renal tumors located centrally near the hilum. More recent ablation procedures such as high intensity focused ultrasound (HIFU), irreversible electroporation, microwave ablation, percutaneous stereotactic ablative radiotherapy and high-dose brachytherapy have high potential in some cases but are currently regarded as experimental for the treatment of renal cell carcinoma.

Robertsonian translocations in wheat arise by centric misdivision of univalents at anaphase I and rejoining of broken centromeres during interkinesis of meiosis II.

The mechanism of origin of Robertsonian translocations was investigated in plants monosomic for chromosome 1A of wheat and 1H(t) of Elymus trachycaulus by GISH. Chromosomes 1A and 1H(t) stayed univalent in all metaphase I cells analyzed, suggesting that Robertsonian translocations do not originate from meiotic recombination in centromeric regions with shared DNA sequence homology. At ana-/telophase I, the 1H(t) and 1A univalents underwent either chromosome or chromatid segregation and misdivided in 6-7% of the pollen mother cells. None of the ana-/telophases I analyzed had Robertsonian translocations, which were only observed in 2% of the "half tetrads" at ana-/telophase II. The frequency of Robertsonian translocations observed at ana-/telophase II corresponds well with the number of Robertsonian translocations (1-4%) detected in progenies derived from plants monosomic for group-1 chromosomes of wheat (1A, 1B, and 1D) and 1H(t) of E. trachycaulus. Our data suggest that Robertsonian translocations arise from centric misdivision of univalents at ana-/telophase I, followed by segregation of the derived telocentric chromosomes to the same nucleus, and fusion of the broken ends during the ensuing interkinesis.

Gametocidal genes induce chromosome breakage in the interphase prior to the first mitotic cell division of the male gametophyte in wheat.

Male gametogenesis was cytologically analyzed in wheat lines homozygous or hemizygous for gametocidal (Gc) factors with different modes of action. The first and second meiotic divisions in all lines were cytologically normal. The postmeiotic mitoses were normal in the homozygous lines; however, chromosome fragments and bridges were observed in the mitoses of the hemizygous lines. The morphology of the chromosome fragments suggests that the Gc genes induce chromosome breaks in the G1 phase prior to DNA synthesis of the first postmeiotic mitosis. The age of an anther was correlated with the frequency of aberrant second mitosis. Younger anthers contained a higher number of pollen undergoing normal second mitosis. This observation suggests that the arresting of the cell cycle occurs as the result of chromosome breaks during the first mitosis. Because chromosome bridges were more frequent than fragments in the second mitosis, breakage-fusion-bridge cycles possibly occurred during gametogenesis, which led to further chromosomal rearrangements. The Gc factors located on chromosomes 2S of Aegilops speltoides and 4Ssh of Ae. sharonensis induce severe chromosome breakage in pollen lacking them. However, the Gc factor on telosome 2CcL of Ae. cylindrica only induced chromosome breaks at a low frequency. The observed partial fertility of Gc lines is presumably due to cell cycle arrest and the competition among gametes with and without chromosome breakage.

Induction of sister-chromatid exchanges and exchange aberrations by UV light and quinacrine mustard in relation to chiasma formation in a standard line and two oligochiasmatic mutants of Vicia faba L.

The frequencies of sister-chromatid exchanges (SCE) and exchange aberrations (EA) in root tip cells were compared to chiasma formation in pollen mother cells of a standard line and two oligochiasmatic lines of Vicia faba L. SCE and EA were induced by UV light and quinacrine mustard. Between the lines SCE frequencies were not different. The background level of SCE was doubled after UV irradiation and 4 times higher after exposure to quinacrine mustard in all Vicia lines analysed. However, the induced frequencies of EA were found to be different under the same treatment conditions for the standard line and the oligochiasmatic mutants. Between the frequencies of induced EA and the frequencies of chiasmata a correlation could be shown. The relationship between the formation of SCE and EA due to the reduced ability of meiotic recombination in the mutants of Vicia faba is discussed.

Radiographic techniques for the localization and treatment of gastrointestinal bleeding of obscure origin.

PURPOSE/BACKGROUND: Acute gastrointestinal bleeding (GIB) is an emergency with high mortality rates, which requires a quick localization and treatment of the bleeding site. In this article, we give a summary of the diagnostic and interventional treatment of acute GIB with an emphasis on radiological methods. METHODS: The MEDLINE database identified relevant studies up until January 2011. Furthermore, experiences drawn from a highly experienced department of diagnostic radiology (~60 transarterial embolizations [TAEs]/year) were taken into account. RESULTS: Clinical risk scores are useful tools to triage patients for appropriate treatment. High-risk patients should undergo emergency endoscopy within 24 h. If endoscopical control of the bleeding cannot be achieved, a computed tomography angiography (CTA) should be done. If active bleeding (or if an active bleeding site) is found, in most cases, TAE should be performed prior to surgery because of the equal mortality rates with lower complication rates. If the site of bleeding is not identified and the patient is stable, a "watch-and-wait" strategy can be pursued. Especially for intermittent bleeding, scintigraphy with tagged red blood cells can be useful. CONCLUSION: Managing obscure acute GIB remains a challenge. The best patient care is achieved with a multidisciplinary team of endoscopists, experienced surgeons, and interventional radiologists. If emergency endoscopy fails, a CTA has to be done, which is more sensitive than conventional angiography. Based on CTA findings, a decision must be made between TAE and surgical intervention.

[Differential Giemsa staining of heterochromatic regions in the chromosomes of Vicia faba, Allium cepa and Paeonia tenuifolia]. / Spezifische Giemsa-Färbung von heterochromatischen Chromosomensegmenten bei Vicia faba, Allium cepa und Paeonia tenuifolia.

Hypotheses for the mechanism of differential Giemsa staining of euchromatin and heterochromatin are discussed. The banding-patterns of Vicia faba, Allium cepa and Paeonia tenuifolia are described. Heterochromatic chromosome segments have been identified in metaphase chromosomes to correspond with those to be found in interphase nuclei. The fact that the number of chromocenters is usually smaller than the number of Giemsa bands in the metaphase chromosomes is supposed to be due to fusion of heterochromatic sections during interphase. Within each species non-homologous chromosomes exhibit similar patterns. The significance of this phenomenon, which has been called "aequilocal heterochromacy" by Heitz, is so far unknown.

C-banding pattern and powdery mildew resistance of Triticum ovatum and four T. aestivum-T. ovatum chromosome addition lines.

C-banding patterns of T. ovatum (Ae. ovata) and four T. aestivum cv 'Poros'-T. ovatum chromosome addition lines are presented, and the added chromosomes of T. ovatum have been identified. Furthermore, nucleolar activity and powdery mildew resistance were analyzed in the 'Poros'-ovatum addition lines and compared to that of T. ovatum and T. aestivum cv 'Poros'. The addition lines II, III and IV and 'Poros' were highly susceptible to powdery mildew isolates nos. 8 and 9, whereas the addition lines VI1 and VI2 showed high resistance. Even for an Ml-k virulent isolate, these two lines were highly resistant. By combining the cytological results and those of the powdery mildew analysis, the added chromosomes of T. ovatum can be excluded from responsibility for the high powdery mildew resistance of the addition lines VI1 and VI2. The same is true for a modified chromosome 6B, which is present in the 'Poros'-ovataum addition lines II, III and VI. The high variation in C-banding pattern observed in the A-, B- and D-genome complement of the addition lines is believed to be the result of crossing different lines of T. aestivum instead of 'Poros' alone. Thus, we cannot trace the powdery mildew resistance back to a specific chromosome.

Control of Extracellular beta-1,3-glucanase activity in a basidiomycete species.

The basidiomycete QM 806 excreted large amounts of beta-1,3-glucanase into the culture medium. Synthesis and excretion of the enzyme were triggered by a critically low concentration of carbon source. The extracellular beta-1,3-glucanase exhibited a remarkable stability. Addition of glucose or other carbon sources to a culture after consumption of the initial carbon source led to an inactivation of the extracellular beta-1,3-glucanase by an inactivating system, which could be separated from the cells. The inactivation of beta-1,3-glucanse was prevented by cycloheximide. This indicates the necessity of active protein synthesis for the inactivation process but does not prove that the inactivating system itself is a protein. Marked changes in the electrophoretic mobility and immunological properties of beta-1,3-glucanase indicate rather profound alterations of the enzyme protein in the course of inactivation.

Identification of a complete set of isogenic wheat/rye D-genome substitution lines by means of Giemsa C-banding.

A complete set of isogenic wheat/rye D-genome substitutions were produced by crossing an inbred line of spring rye Secale cereale L. cv. "Prolific" to a tetraploid wheat, the A-and B-genomes of which had previously been extracted from hexaploid wheat, Triticum aestivum L. em Thell. cv. "Thatcher". After chromosome doubling, the derived hexaploid triticale (x Triticosecale Wittmack) was backcrossed to 6x "Thatcher" and selection for wheat/rye substitution lines was carried out in BCF3 to BCF6 families by using Giemsa C-banding. Five fertile disomic wheat/rye D-genome substitution lines were obtained and their chromosomal constitution was determined to be 1D/1R, 2D/2R, 7D/4R, 6D/6R, 7D/7R. The two remaining 3R and 5R substitutions are at the moment in a monosomic condition. Another 1D/7R substitution was detected but this plant was very weak and sterile, indicating that only substitutions between homoeologous chromosomes result in fertile, vigorous plants. Furthermore, many rye telocentrics as well as rye-rye and rye-wheat translocations were selected. Since all lines selected in this program share the same genetic background of "Thatcher" wheat, genetic heterogeneity is excluded. The material is very useful, therefore, for analyzing the effects of different rye chromosomes or chromosome segments in an otherwise homozygous background.

Cytological characterization, powdery mildew resistance and storage protein composition of tetraploid and hexaploid 1BL/1RS wheat-rye translocation lines.

Progenies of a tetraploid 1BL/1RS wheat-rye translocation line, CV 256, selected from the cross 'Cando' x 'Veery', were analyzed by means of Giemsa C-banding. CV 256 is cytologically stable for the presence of the 1BL/1RS translocation but still segregating for A- and B-genome chromosomes of 'Cando' and 'Veery'. In CV 256, nucleolar activity of the 1RS NOR locus is suppressed, as judged by the absence of a secondary constriction in that rye segment and the capability of organizing nucleoli. PAGE analysis of prolamins confirmed the presence of two 1RS secalins in all single seeds analyzed. SDS-PAGE analysis of reduced glutenins of single seeds indicated that some seeds contained the 'Cando' Glu-B1 locus (subunits 6+8), some contained the 'Veery' Glu-B1 locus (subunits 7+9) while others contained all four subunits, indicating that the material was heterozygous. Pm8 resistance is expressed in the tetraploid 1BL/1RS translocation line based on the reactions of six well-defined powdery mildew isolates. However, Pm8 resistance is not expressed in the hexaploid wheat cultivars 'Olymp', 'Heinrich' and 'Florida', which also contain the 1BL/1RS translocation. Obviously, the existence of the 1BL/1RS translocation is not a proof for the expression of the associated genes. PAGE results did not show a clear linkage between powdery mildew resistance and the presence of 1RS secalins.

Recombination in an isochromosome preferentially occurs between cis isochromatids.

An isochromosome has identical arms attached to the same centromere. At the pachytene stage of meiosis, it has four isochromatids and recombination can occur either between cis isochromatids (attached to the same half-centromere) or trans isochromatids (attached to different half-centromeres). Normally such recombination cannot be detected because all four chromatids are homogenetic (arose from misdivision of a centromere to which genetically identical sister chromatids were attached). We isolated an isochromosome of wheat that is heterogenetic for the distal 64% of the long arm. The heterogenetic isochromosome was recovered from the progeny of a cross between Triticum aestivum cv. Chinese Spring containing an isochromosome for the long arm of chromosome 5B (i5BL) and a disomic substitution line of Triticum turgidum ssp. dicoccoides chromosome 5B in Chinese Spring wheat. New recombinants were produced when the two arms of i5BLrec paired at metaphase I of meiosis. Only trans isochromatid exchanges led to some homozygous loci in i5BLrec, whereas exchanges between cis isochromatids resulted in heterozygosity at all loci similar to the parental type. There was an average frequency of 0.87 chiasmata per pollen mother cell for the heterogenetic i5BL, which will result in 0.44 cis and 0.44 trans isochromatid exchanges, assuming that both are occurring at the same frequency. The average crossover frequency based on recombination between trans isochromatid exchange detected by restriction fragment length polymorphism analysis in 98 plants was 0.29. This observed value is significantly lower (P<0.01) than the value of 0.44 as expected from chiasmata counts. Our study provides the first experimental evidence that crossovers preferentially occur between cis isochromatids rather than trans isochromatids.

Gametocidal factor-induced structural rearrangements in rye chromosomes added to common wheat.

The gametocidal factor on the Aegilops cylindrica chromosome 2Cc was used to induce and analyze the nature of chromosomal rearrangements in rye chromosomes added to wheat. For this purpose we isolated plants disomic for a given rye chromosome and monosomic for 2Cc and analyzed their progenies cytologically. Rearranged rye chromosomes were identified in 7% of the progenies and consisted of rye deficiencies (4.6%), wheat rye dicentric and rye ring chromosomes (1.8%), and terminal translocations (0.6%). The dicentric and ring chromosomes initiated breakage-fusion-bridge cycles (BFB) that ceased within a few weeks after germination as the result of chromosome healing. Of 56 rye deficiencies identified, after backcrossing and selfing, only 33 were recovered in either homozygous or heterozygous condition covering all rye chromosomes except 7R. The low recovery rate is probably caused by the presence of multiple rearrangements induced in the wheat genome that resulted in poor plant vigor and seed set, low transmission, and an underestimation of the frequency of wheat rye dicentric chromosomes. Genomic in-situ hybridization (GISH) analysis of the 33 recovered rye deficiencies revealed that 30 resulted from a single break in one chromosome arm followed by the loss of the segment distal to the breakpoint. Only three had a wheat segment attached distal to the breakpoint. Although some of the Gc-induced rye rearrangements were derived from BFB cycles, all of the recovered rye rearrangements were simple in structure. The healing of the broken chromosome ends was achieved either by the de-novo addition of telomeric repeats leading to deficiencies and telocentric chromosomes or by the fusion with other broken ends in the form of stable monocentric terminal translocation chromosomes.

Meiotic metaphase I pairing behavior of a 5BL recombinant isochromosome in wheat.

A recombinant isochromosome i5BLrec of wheat was developed with one arm and the proximal 36% of the other arm of Chinese Spring (CS) origin and the distal 64% of the recombined arm of Triticum turgiduM subsp. dicoccoides origin. The i5BLrec provides an unusual opportunity to analyze the role of the centromere or arm heterozygosity in chromosome prealignment and synapsis during meiosis. In monosomic condition, the i5BLrec formed a ring univalent in 86.8% of the pollen mother cells (PMCs) at meiotic metaphase I. In the disomic condition, the two i5BLrec preferentially paired as a normal bivalent in 74.8% of the PMCs, which differed significantly (p <0.01) from the normal bivalent pairing of 51% observed in diisosomic 5BL chromosomes of the CS (Di5BL(CS)) control plants. In plants with one i5BLrec and a normal 5B(CS), the long arm of 5B(CS) paired with the homologous arm of i5BLrec in 54.4% of the PMCs, and 40.4% of the PMCs had a 5B(CS) univalent and a i5BLrec ring univalent. The implications of the i5BLrec pairing data on the mechanism of Ph1 gene action are discussed.