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Thymus, an aromatic medicinal plant, is a well-known, perennial and woody herb from Lamiaceae family. Thymus is taxonomically a very complex genus with a high frequency of hybridization and introgression among sympatric species, and some species of this herb are endemic to Iran. In the present study, in order to identification genetic variability in this medicinal plant seven Thymus spp. accessions collected from different regions of Iran along with London agricultural species were studied by karyotypic characteristics. The secondary basic numbers in all ecotypes was x = 15 that probably originate from a primary basic number x = 7. The ploidy levels of these ecotypes were diploid and tetraploid. The Thymus ecotypes under study occupied classes 1A and 1B of Stebbins’ karyotype classification, indicating the presence of a primitive symmetrical karyotype in these ecotypes. The mean chromosome length ranged from 1.03 to 1.53 µm. chromosome types were detected as metacentric “m”. Furthermore, the cluster analysis using chromosomal parameters and based on UPGMA assigned the ecotypes into four groups.

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Drought stress is one of the most important factors involved in reducing wheat production and identifying genetic structure and gene action type in controlling grain yield in water stress condition is essential for choosing appropriate breeding methods. In this study, 9×9 one way diallel crosses were used to study the genetic structure of wheat grain yield at stress and non- stress conditions. Combining ability analysis by the second Griffing method for both conditions resulted in significant GCA and SCA variances, representing grain yield may be controlled by additive and non- additive effects of genes. The results of applying combining ability analysis indicated that among the parents, genotypes Ghods and Bam had the highest GCA for grain yield and the best specific crosses were Arg × Ghods, Navid × Moghan, Bam × Alvand (for both irrigation regimes) and Bam × Ghods (in stress condition). Biplot analysis of diallel data was used to display GCA and SCA for parents and to determine heterotic groups and the best crosses. In general, according to the results, Bam, Ghods and Arg were tolerant cultivars and had the ability to maintain yield in drought stress condition as well as to transfer these properties to the hybrids. So these genotypes can be used to improve stress tolerance in breeding programs.