GrainGenes

ITEMS FROM THE UNITED KINGDOM

JOHN INNES CENTRE

Norwich Research Park, Colney, Norwich NR4 7UH, United Kingdom.

Chromosome 5BS-7BS and its control of adult-plant resistance to yellow rust.

C.N. Law and A.J. Worland.

The near-centromeric reciprocal translocations, 5BL-7BL and 5BS-7BS, are present in many west European wheats. The absence of the 5BS arm, as in plants nullisomic for 5BS-7BS and ditelosomic for 7BS, in cultivars showing adult-plant resistance (APR) to races of yellow rust produces adult plants that are highly susceptible. Because this effect was observed in cultivars that had a history of durability, we thought that the gene or genes for APR on chromosome arm 5BS of these cultivars could be responsible for their durability.

To test whether this might be possible, six cultivars known to carry the translocation, but differing in their level of APR to yellow rust, were chosen for further investigation. Two of these, Hybride du Joncquois and Nord­Desprez, were both highly susceptible to yellow rust at the adult-plant stage, whereas Cappelle-Desprez, Caribo, Vilmorin 27, and Bersee were all highly resistant and, because of their history, could be classified as exhibiting durability. Removal of either the 5BS-7BS chromosome or the 5BS arm in these four resistant cultivars produced high levels of infection to yellow rust. The 5BS-7BS chromosome from each of the six cultivars, when substituted into each of the four resistant cultivars, produced high levels of infection to yellow rust. The 5BS-7BS chromosomes from each of the six cultivars were substituted into each of the four resistant cultivars and gave 24 different substitution lines. If durability and the resistance carried by the 5BS arm were related causally, we reasoned that the substituted 5BS-7BS chromosomes would correlate exactly with their donor varietal behavior. In this event, and following extensive field experimentation, all 5BS-7BS chromosomes were found to be identical, all giving the same levels of APR. Therefore, because the susceptible cultivars Hybride du Joncquois and Nord-Desprez carry the same 5BS-7BS chromosomes as the durably resistant cultivars, then this chromosome clearly cannot be solely responsible for the durability.

Substitution lines for 5BS and 7BS arms from three cultivars, Bezostaya 1, Poros, and Mara, without the translocation, also were produced using Cappelle-Desprez as the recipient. In these lines, the normal karyotype of wheat is re-instated in a Cappelle-Desprez background. The only difference from the recipient cultivars is that the 5BS and 7BS arms originate from the donor cultivar. These three lines also were tested in the field and all three were susceptible at the adult stage, with similar infection levels as the Cappelle-Desprez nullisomic 5BS-7BS line. Therefore, the gene (or genes) for APR on 5BS was either absent or inactive in these nontranslocation-carrying cultivars.

Of the nine cultivars surveyed, six with the translocation had APR on 5BS, whereas the three lacking the translocation were susceptible. Therefore, the presence of the translocation and resistance are correlated completely. The gene(s) on 5BS may be linked closely to the breakpoint, so the probability is high that the gene(s) is transmitted with the translocation and contribute to the high frequency of this translocation in west European wheats. An alternative but less likely explanation is that the translocation itself is responsible for APR.

Genes for inhibition of flowering and day-length sensitivity on the group 6 chromosomes.

M.N. Islam-Faridi (now at Texas A & M University), A.J. Worland, and C.N. Law.

The study of the tetrasomics, nullisomic-tetrasomics, and ditelosomics of group 6 chromosomes of `Chinese Spring', grown under vernalized/ unvernalized and short/long day conditions, has established a clear and large chromosome-dose effect on flowering or ear-emergence time.

An increased dosage of the group 6 chromosomes delays ear-emergence. Reducing the dosage of the same chromosomes has the reverse effect, accelerating ear-emergence. Both effects are removed by vernalization. We propose that genes on these chromosomes produce a flowering inhibitor that is sensitive to vernalization. This inhibitor possibly may be affected by the group 5 Vrn genes for vernalization requirement through their production of a flowering promoter, which either suppresses the group 6 genes or complexes with the inhibitor, reducing the vernalization requirement and accelerating ear-emergence.

A gene (or genes) for sensitivity to day length was detected on the long arm of chromosome 6B, which, in its absence, delayed ear-emergence under short days. The genes for inhibition of emergence time probably were located on the long arms of the group 6 chromosomes, but were separate from the gene(s) for day-length sensitivity.

The control of aspects of bread-making quality using the chromosome substitution lines of Bezostaya 1 into Cappelle-Desprez.

A.F. Krattiger, P.I. Payne (Plant Breeding International, Cambridge), and C.N. Law.

The group 1 and 6 intervarietal chromosome substitution lines of Bezostaya 1 in Cappelle-Desprez were intercrossed, along with the parental cultivars, to give 36 genetically different families. An analysis of the means of these families, using weighted least square procedures to estimate a range of parameters, showed that variation in SDS­sedimentation volume fit a predominantly additive model. No significance within or between-chromosome interactions occurred among the group 1 and 6 chromosomes and no evidence existed for interactions between these chromosomes and those of the background. However, significant dominance/within chromosome interactions were detected among the background chromosomes. Some of the positive effects on SDS-sedimentation volume were associated with increased grain hardness. Chromosome effects on percent grain protein were not correlated with SDS-sedimentation

Location of a gene for resistance to Septoria nodorum.

A.J. Worland, S. Lewis, and P. Nicholson.

Single-chromosome substitution lines to introduce individual chromosomes of the Septoria nodorum-resistant line `Synthetic 6x' into the susceptible cultivar Chinese Spring previously demonstrated that the resistance in Synthetic 6x is determined primarily by genes on chromosomes 2A, 3D, 5D, and 7D.

To determine the genetic control of resistance on chromosome 5D, single-chromosome recombinant lines between chromosomes 5D of Synthetic 6x and 5D of Chinese Spring were developed in a Chinese Spring background. Septoria nodorum testing of 49 recombinant lines at the seedling stage demonstrated that the resistance was determined by a single gene. To locate the gene on the 5D chromosome, the recombinant lines also were classified for spring habit, determined by Vrn3 from Chinese Spring, or winter habit, determined by the recessive vrn3 allele from Synthetic 6x, and classified for allelic variation at the isozyme loci Ibf-D1 (iodine binding factor) and Mdh-D3 (malate dehydrogenase) located on the long and short arms of chromosome 5D, respectively.

The results of the analysis show that the S. nodorum resistance gene maps on the long arm of chromosome 5D between Ibf-D1 and the centromere, about 17 cM from the centromere and 12.2 cM from Ibf-D1. Further analysis using RFLP markers is underway to more precisely map the resistance gene.

The enhancement of fluorescent in situ hybridization using pre-annealing of DNA probes.

S.M. Reader and K. Anamthawat-Jónsson (Agricultural Research Institute, Reykjavik, Iceland).

Fluorescent in situ hybridization has greatly increased the sensitivity of cytological analysis of both mitotic and meiotic plant chromosomes. The advent of rapid in situ hybridization allowed routine application of the technique and increased its value to the cytologist.

The recent advancement of pre-annealing, which utilizes this rapid protocol, has permitted an even more detailed analysis of plant chromosomes. The technique exploits the variation in homology present in interspecific hybrids to optimize simultaneous genomic in situ hybridization. Fluorescently labelled, genomic DNA probes of both parents involved are pre-annealed at a temperature lower than that of the hybridization. Sequences that are common to the two parental genomes, or are repetitive, hybridize during the pre-annealing stage. Repetitive DNA has rapid reassociation kinetics compared with unique sequences, thus pre-annealed probes are expected to have an increased specificity, because a proportionally higher concentration of species- or genome-specific sequences are available to the subsequent in situ hybridization. The temperature of both pre-annealing and in situ hybridization can be varied to attain optimum differentiation of genomes. Thus, the technique is especially useful to study meiotic chromosome pairing or to visualize translocated chromosomes.

Molecular tagging of Pm12, a powdery mildew resistance gene transferred from Aegilops speltoides to wheat.

J. Jia (now at Institute of Crop Germplasm Resources, CAAS, Beijing 100081, China), K.M. Devos, S. Chao (now at Institute of Botany, Academia Sinica, Nankang, Taipei, Taiwan), T.E. Miller, S.M. Reader, and M.D. Gale.

A dominant powdery mildew gene, Pm12 was introgressed from Ae. speltoides into line #31. Monosomic analysis indicated that Pm12 was located on one of the homoeologous group 6 chromosomes. Therefore, genetic maps of the homoeologous group 6 chromosomes of bread wheat, T. aestivum, spanning 103 cM on 6A, 90 cM on 6B, and 124 cM on 6D, were transferred to a `Chinese Spring x line #31' cross, but could not be mapped more precisely because of the lack of recombination between the 6S Ae. speltoides segment and chromosome 6B. The 6BS segment in the 6BS-6SS·6SL chromosome is less than 18 % of the short arm. Thus, the yield depression incurred with the Pm12 resistance gene is associated with a large segment of alien DNA. Work is underway to reduce the Ae. speltoides segment by induced recombination in the absence of the homoeologous pairing control gene, Ph1, followed by selection for both the target gene and flanking markers.

Structural evolution of wheat chromosomes 4A, 5A, and 7B and its impact on recombination.

K.M. Devos, J. Dubcovsky (now at IRB-INTA, Villa Udaondo, m 1712 Castelar, Buenos Aires, Argentina), J. Dvorak (Department of Agronomy and Range Science, University of California, Davis, CA, USA), C.N. Chinoy, and M.D. Gale.

The construction of comparative genetic maps of chromosomes 4Am and 5Am of T. monococcum and chromosomes of homoeologous groups 4, 5, and 7 of T. aestivum has provided insight into the evolution of these chromosomes. The structures of chromosomes 4A, 5A, and 7B of modern-day hexaploid bread wheat can be explained by a 4AL/5AL translocation that occurred at the diploid level and is present both in T. monococcum and T. aestivum. Three further rearrangements, a 4AL/7BS translocation, a pericentric inversion, and a paracentric inversion all took place in the tetraploid progenitor of hexaploid wheat. Translocations between the long arms of chromosomes 4 and 5 also were observed in a range of other Triticeae species such as T. urartu, Ae. umbellulata, Th. bessarabicum, and S. cereale. The presence of a 4L/5L translocation in several Triticeae genomes raises two questions: which state is the more primitive and is the origin of the translocation poly-phylogenetic?

The rearrangements that have occurred in chromosome 4A resulted in segments of both arms having different positions relative to the telomere, compared to 4Am and to 4B and 4D. Comparisons of map length in these regions indicate that genetic length is a function of distance from the telomere with the distal regions showing the highest recombination.