Pyramiding of Ryd2 and Ryd3 conferring tolerance to a German isolate of Barley yellow dwarf virus-PAV (BYDV-PAV-ASL-1) leads to quantitative resistance against this isolate
Barley yellow dwarf virus (BYDV) is an economically important pathogen of barley, which may become even more important due to global warming. In barley, several loci conferring tolerance to BYDV-PAV-ASL-1 are known, e.g. Ryd2, Ryd3 and a quantitative trait locus (QTL) on chromosome 2H. The aim of the present study was to get information whether the level of tolerance against this isolate of BYDV in barley can be improved by combining these loci. Therefore, a winter and a spring barley population of doubled haploid (DH) lines were genotyped by molecular markers for the presence of the susceptibility or the resistance encoding allele at respective loci (Ryd2, Ryd3, QTL on chromosome 2H) and were tested for their level of BYDV-tolerance after inoculation with viruliferous (BYDV-PAV-ASL-1) aphids in field trials. In DH-lines carrying the combination Ryd2 and Ryd3, a significant reduction of the virus titre was detected as compared to lines carrying only one of these genes. Furthermore, spring barley DH-lines with this allele combination also showed a significantly higher relative grain yield as compared to lines carrying only Ryd2 or Ryd3. The QTL on chromosome 2H had only a small effect on the level of tolerance in those lines carrying only Ryd2, or Ryd3 or a combination of both, but the effect in comparison to lines carrying no tolerance allele was significant. Overall, these results show that the combination of Ryd2 and Ryd3 leads to quantitative resistance against BYDV-PAV instead of tolerance.
Strategies for Pyramiding Resistance Genes Against the Barley Yellow Mosaic Virus Complex (BaMMV, BaYMV, BaYMV-2)
Barley Yellow Mosaic Virus disease caused by different strains of BaYMV and BaMMV is a major threat to winter barley cultivation in Europe. Pyramiding of resistance genes may be considered as a promising strategy to avoid the selection of new virus strains and to create more durable resistances. However, this goal cannot be achieved by phenotypic selection due to the lack of differentiating virus strains. For pyramiding of resistance genes rym4, rym5, rym9 and rym11, located on chromosomes 3H and 4H of barley two different strategies have been developed. These strategies are based on doubled haploid lines (DHs) and marker assisted selection procedures. On the one hand F1 derived DH-plants of single crosses were screened by molecular markers for genotypes being homozygous recessive for both resistance genes. These genotypes were crossed to lines carrying one resistance gene in common and an additional third gene, leading to a DH-population of which 25% carry three resistance genes, 50% have two resistance genes and 25% possess a single resistance gene homozygous recessively. Alternatively, F1 plants having one resistance gene in common were directly inter-crossed [e.g. (rym4 × rym9) × (rym4 × rym11)] and about 100 seeds were produced per combination. Within these complex cross progenies plants were identified by markers being homozygous at the common resistance locus and heterozygous at the others. From such plants, theoretically present at a frequency of 6.25%, DH-lines were produced, which were screened for the presence of genotypes carrying three or two recessive resistance genes in a homozygous state. Besides DH-plants carrying all possible two-gene combinations, 20 DH-plants out of 107 analysed carrying rym4, rym9, and rym11 and 27 out of 187 tested carrying rym5, rym9, and rym11 homozygously have been detected using the second strategy which is faster but needs co-dominant markers, because in contrast to the first strategy marker selection is carried out on heterozygous genotypes.
Pasteurella haemolytica serotype 1 colonization of the nasal passages of virus-infected calves.
Nasal passages of calves with a virus-induced respiratory tract disease became colonized by Pasteurella haemolytica serotype 1 after they were inoculated intranasally with P haemolytica. Inoculation with infectious bovine rhinotracheitis virus caused a more severe clinical illness and resulted in a greater degree of colonization with P haemolytica than developed after inoculation with parainfluenza-3 virus. Nasal passages of parainfluenza-3 virus-inoculated calves were colonized to a greater degree with P haemolytica than were those of healthy, nonstressed calves. Calves were susceptible to P haemolytica colonization during or shortly after virus-induced illness, even though they had been previously exposed to P haemolytica and had serum antibody and nasal secretion antibody to P haemolytica.