Molecular cytogenetics and genomics of novel wheat-Thinopyrum bessarabicum recombinant lines carrying intercalary translocations

The diploid wild grass Thinopyrum bessarabicum (2n = 2x = 14, JJ or EbEb) is a rich source of important genes for bread wheat (2n = 6x = 42) improvement because of its salinity tolerance and disease resistance. Development of wheat–Th. bessarabicum translocation lines by backcrossing amphiploids in the absence of the Ph1 gene (allowing intergenomic recombination) enables its practical utilization in wheat improvement. Using genomic in situ hybridization (GISH) and repetitive probes for fluorescent in situ hybridization (FISH), six novel wheat–Th. bessarabicum translocation lines involving different chromosome segments (T4BS.4BL-4JL, T6BS.6BL-6JL, T5AS.5AL-5JL, T5DL.5DS-5JS, T2BS.2BL-2JL, and the whole arm translocation T1AL.1JS) were identified and characterized in this study. No background translocations between wheat genomes were observed. The involvement of 5 of the 7 chromosomes, and small terminal segments of the Th. bessarabicum chromosome arm were important, contributing to both reduced linkage drag of the derived lines by minimizing agronomically deleterious genes from the alien species, and high stability including transmission of the alien segment. All three wheat genomes were involved in the translocations with the alien chromosome, and GISH showed the Th. bessarabicum genome was more closely related to the D genome in wheat. All the introgression lines were disomic, stable and with good morphological characters. The work also generated a high-resolution karyotype of two accessions of Th. bessarabicum using multiple repetitive DNA probes for chromosome identification. A complete CS-Th. bessarabicum amphiploid (2n=8x=56, AABBDDJJ) was used and each individual Jgenome unambiguously identified. The established karyotype will be useful for the rapid identification of potential donor chromosomes in wheat improvement programs, allowing appropriate alien-chromosome transfer. Genotyping-by-sequencing (GBS) data was collected from the wheat-Th. bessarabicum introgression lines, but the complexity of the wheat genome and need for further development of data analysis pathways limited interpretation.