Comparative cytogenomic characterization of 18 wheat core collection cultivars from six continents using triple-color...

Triple-color PLOP-FISH reveals agro-climate-associated variation in rDNA organization among globally diverse wheat cultivars, despite conserved chromosome number and 45S rDNA locus stability. Wheat (Triticum aestivum L.) is the world's most extensively cultivated crop, yet, its allohexaploid genome is highly complex and repetitive. Elucidating its chromosome-level genome organization is critical to understanding genome evolution and stability. In this study, fluorescence in situ hybridization (FISH) was used to analyze 18 wheat cultivars collected from six continents, comparing their chromosomal composition and rDNA distribution. Three universal pre-labeled oligonucleotide probes (PLOPs) targeting 5S rDNA, 45S rDNA, and the Arabidopsis-type telomeric repeat were used for FISH. All cultivars consistently exhibited chromosome numbers of 2n = 6x = 42. Interestingly, the number of 5S rDNA loci varied between six and eight, whereas the 45S rDNA loci remained conserved at twelve. Intensity patterns of both 5S and 45S rDNA varied among cultivars across the three agro-climatic zones. However, cultivars originating from arid/semi-arid climatic zones exhibited the highest proportion of major signals for both loci. Telomeric repeat signals were observed at both chromosome ends across all cultivars, with no obvious differences in intensity. Additionally, the number of chromosomes with adjacent 5S-45S rDNA was either four or six, depending on the number of 5S rDNA loci. These results demonstrate intra-specific variation among wheat cultivars from different continents associated with agro-climatic conditions. These findings provide valuable insights for future research into wheat evolution and breeding.