Karyological and Nuclear DNA Content Variation in Some Iranian Endemic Thymus Species (Lamiaceae)

Thymus is a medicinal plant which contains one of the world's top ten essential oils, exhibiting antibacterial, antioxidative, food preservative and mammalian age-delaying properties. This work was aimed at identifying between-species variations requiring for selecting appropriate parents for hybridization. Six Iranian endemic Thymus accessions belonging to Thymus daenensis, T. eriocalyx and T. migricus were studied. Root tips were examined for karyological studies and fresh young leaves of the standard reference (Parsley, Petroselinum crispum, 2C DNA= 4.45 pg) and the Thymus samples stained with propidium iodide (PI) for flow cytometric (FCM) measurements. Two ploidy levels (diploid and tetraploid) and 3 chromosome numbers (30, 56, 60) were recognized. The latter chromosome number is being reported for the first time on T. daenensis accession. FCM measurements showed that 2C DNA contents varied from 1.02 to 2.42 pg, verifying more than 2-fold variations and showing a genome size range of 499 to 1182 Mbp, correspondingly. The mean amount of 2C DNA/chromosome and mean of monoploid genome size were not proportional to ploidy. 2C-values were correlated with, and linearly regressed upon somatic metaphase, considering either total chromosome volume (TCV) or total chromatin length (X).