The genomic in situ hybridization (GISH) technique was used to discriminate between chromosomes of the C genome and those of the A and A/D genomes in allopolyploid oat species (genus Avena). Total biotinylated DNA from A. strigosa (2n = 2x = 14, AsAs genome) was mixed with sheared, unlabelled total DNA from A. eriantha (2n = 2x = 14, CpCp) at a ratio of 1:200 (labelled to unlabelled). The resulting hybridization pattern consisted of 28 mostly labelled and 14 mostly unlabelled chromosomes in the hexaploids. Attempts to discriminate between chromosomes of the A and D genomes in A. sativa (2n = 6x = 42, AACCDD) were unsuccessful using GISH. At least eight intergenomic translocation segments were detected in A. sativa 'Ogle', several of which were not observed in A. byzantina 'Kanota' (2n = 6x = 42, AACCDD) or in A. sterilis CW 439-2 (2n = 6x = 42, AACCDD). At least five intergenomic translocation segments were observed in A. maroccana CI 8330 'Magna' (2n = 4x = 28, AACC). In both 'Ogle' and 'Magna', positions of most of these translocations matched with C-banding patterns.

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Symbiotic relationships between wheat plants and mycorrhizal fungi may play an important role in the growth and productivity of wheat as a crop. Wheat cultivars differ in their ability to form such relationships, but little is known concerning the genetic basis of such differences. A set of intervarietal substitution lines having individual chromosomes from 'Cheyenne' (nonresponsive to mycorrhizae) substituted into 'Chinese Spring' (nonresponsive to mycorrhizae) were tested for mycorrhizal response in greenhouse experiments. Chromosomes 1A, 5B, 6B, 7B, 5D, and 7D of 'Cheyenne' had positive effects on the trait, with homologous groups 5 and 7 in the B and D genomes having the largest effects. Chromosome 5B of 'Hope', a nonresponsive cultivar, also had a positive effect in a 'Chinese Spring' background. In addition, the mycorrhizal responsiveness of a range of other cultivars and ancestors was tested to expand information on the trait in wheat and its relatives. Data on chromosome location of mycorrhizal-response genes and genotypic responsiveness will permit more effective genetic manipulation of this trait. Key words: pathogenesis, growth response, vesicular–arbuscular mycorrhizae.

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Species belonging to the genus Agropyron are an important source of disease resistance for bread wheat. Several resistance genes have been transferred. C‐banding and in situ hybridization are the methods of choice for analyzing introgressed alien chromatin. Using these methods, we characterized radiation‐induced wheat‐Agropyron elongatum chromosome translocation lines carrying the rust resistance genes Sr26 and Sr25/Lr19. Data show that Sr26 is located on the translocation chromosome T6AS. 6AL‐6Ae#1L. The Ag. elongatum segment in this translocation has a length of 2.48 μm. The Sr25/Lr19 genes in cultivar ‘Agatha’ are located on a 2.55 μm long Ag. elongatum segment present in the translocation chromosome T7DS. 7DL‐7Ae#1L. The wheat‐Ag. elongatum translocation chromosome present in the ethyl methane‐sulfonate (EMS) treated derivative ‘Agatha‐28’ is similar to that present in Agatha. The Ag. elongatum segment present in the EMS treated derivative ‘Agatha‐235’ has a length of 1.99 pm and is inserted intercalary in the long arm of wheat chromosome 7D, resulting in the translocation chromosome T7DS‐7DL‐7Ae#1L‐7DL. A compensation (CI) index was calculated as the ratio of the length of the missing wheat segment as percent of the corresponding wheat arm divided by the length of the transferred alien segment as percent of the corresponding alien chromosome arm. The CIs in T6AS‐6AL‐6Ae#1L and T7DS‐7DL‐7Ae#1L are 1.15 and 0.95, respectively. These are close to the desirable value of I and indicate good compensation ability. The CI in T7DS. 7DL‐7Ae#1L‐7DL is 0.60 and indicates poor compensation ability. This index may allow the prediction of the performance of any compensating wheat‐alien chromosome translocation.