Cytological and Molecular Characterization of Wheat Lines with Thinopyrum intermedium Chromosome Additions, Substitutions and Translocations Resistant to Barley Yellow Dwarf Virus

Molecular markers show a complex mosaic pattern of wheat-Thinopyrum intermedium translocations carrying resistance to YDV

Thinopyrum intermedium translocations derived from the wheat (Triticum aestivum L.) substitution line P-29 were previously characterized by RFLP. We have further analyzed these lines and additional related germplasm with publicly available STS and SSRs. Primers which showed a polymorphism between wheat and P-29, were tested in all recombinant and nulli-tetrasomic lines confirming their position on chromosome 7D. The resulting 7D/7E chromosome maps appeared as a mosaic of wheat and Th. intermedium chromatin sections. To verify the composition of the translocation lines suggested by the RFLP-PCR map, F(2) progeny of two crosses (CS/216-1 and CS/260-1) were analyzed with molecular markers. Both populations gave an unexpectedly diverse number of recombinant individuals, suggesting that interstitial translocations occur more frequently than previously thought. This analysis also showed that there is a wide range in the number and position of the interstitial translocations within a given line such as the mosaic chromosome in recombinant line 260-1/CS-26, which has four Th. intermedium chromosome segments. Phenotypic data of the two populations suggested the presence of one gene which we have called Bdv3 to differentiate it from the previously reported orthologous gene Bdv2. Using the PCR-based molecular markers identified in this study, 5 out of 12 elite lines that showed good yields and no YDV symptoms contained Th. intermedium chromatin. Due to the multiple components involved in the YDV disease complex, combining selection for YDV resistance with the molecular markers and maps identified in this study will increase the efficiency of introgressing Th. intermedium chromatin containing YDV resistance or other beneficial traits into elite wheat germplasm.

CAPS markers specific to Eb, Ee, and R genomes in the tribe Triticeae

Wild Triticeae grasses serve as important gene pools for forage and cereal crops. Understanding their genome compositions is pivotal for efficient use of this vast gene pool in germplasm-enhancement programs. Several cleaved amplified polymorphic sequence (CAPS) markers were developed to distinguish the Eb, Ee, and R genomes. With the aid of disomic addition lines of wheat, it was confirmed that all 7 chromosomes of Eb, Ee, and R genomes carry these genome-specific CAPS markers. Thus, the identified CAPS markers are useful in detecting and monitoring the chromosomes of these 3 genomes. This study also provides evidence suggesting that some Purdue and Chinese germplasm lines developed for barley yellow dwarf virus (BYDV) resistance are different from those developed in Australia. Furthermore, Thinopyrum intermedium and Thinopyrum ponticum were shown to have different genome constitutions. Sequence analyses of the 1272 bp sequences, containing Ty3/gypsy retrotransposons, from the Eb, Ee, and R genomes also shed light on the evolution of these 3 genomes.

Identification and characterization of wheat-wheatgrass translocation lines and localization of barley yellow dwarf virus resistance

Stable introgression of agronomically important traits into crop plants through wide crossing often requires the generation and identification of translocation lines. However, the low efficiency of identifying lines containing translocations is a significant limitation in utilizing valuable alien chromatin-derived traits. Selection of putative wheatgrass-wheat translocation lines based on segregation ratios of progeny from γ-irradiated seed using a standard phenotypic analysis resulted in a low 4% success rate of identifying barley yellow dwarf virus (BYDV) resistant and susceptible translocation lines. However, 58% of the susceptible progeny of this irradiated seed contained a Thinopyrum intermedium chromosome-specific repetitive sequence, which indicated that γ-irradiation-induced translocations occurred at high rate. Restriction fragment length polymorphism (RFLP) analysis of susceptible lines containing alien chromatin, their resistant sister lines and other resistant lines showed that more than one third of the progeny of γ-irradiated double monosomic seeds contained wheatgrass-wheat translocations. Genomic in situ hybridization (GISH) analysis of selected lines confirmed that these were wheatgrass-wheat translocation lines. This approach of initially identifying BYDV susceptible deletion lines using an alien chromosome-specific repetitive sequence followed by RFLP analysis of their resistant sister lines efficiently identified resistant translocation lines and localized the BYDV resistance to the distal end of the introgressed Th. intermedium chromosome.Key words: gene introgression, wide crosses, chromosome, repetitive elements, RFLP, Thinopyrum intermedium.