Cytogenetic Study of the Genus Thymus (Lamiacea) in Iran

 The Genus Thymus is one of the most important genera of the Lamiaceae family. According to Jalas (1971), Thymus is divided into eight sections: Micantes, Mastichina, Piperella, Teucrioides, Pseudothymbra, Thymus, Hyphodromi, and Serpyllum. About 18 species exist in some areas of Iran and most of them belong to Serpyllum Section and kotschiani Subsection. Chromosomal information is an important key for taxonomy, phylogeny, evolution, genetics and breeding in thyme. Therefore, this work was carried out to study the cytogenetic characteristics of genus Thymus belonging to the Subsection Kotschyani for breeding purposes and taxonomy.

Seeds were collected from 5 wild populations and germinated on wet filter paper at 20 oC. One cm long roots were pretreated with 8-hydroxyquinolin and then washed in distilled water and fixed in carnoy solution for 24 hours. Hydrochloric acid (1N) was applied for 7 min to hydrolyze the roots. Staining was done by orcein for 24 h at room temperature. Stained roots squashed in one drop of 45% acetic acid and examined by a ZEISS Axiophot compound microscope. Cells in metaphase stage were photographed with a D450; Canon Inc. Japan digital camera. Ten well prepared metaphasic cells were selected and some chromosomal characteristics such as total chromosomal length (TL), long arm length (L), short arm length (S), the arm ratio (AR) [LA/SA], and centromeric index (CI), were measured using Micro Measure ver. 3.3 software. The following karyological parameters were determined: total chromosome form (TF%), intrachromosomal asymmetry index (A1) and interchromosomal asymmetry index (A2). Karyotypic characteristics have been determined using the symmetry classes of Stebbins (SC). Karyotype formula was determined from chromosome morphology based on centromere position in accordance with the classification of Levan.

 This study reports the chromosome number and meiotic behavior of 31 populations belonging to 8 species of Thymus Subsect. kotschyani from Iran. In most species of this subsection, the base number of chromosome x = 15 and two diploid and tetraploid ploidy levels with chromosome number of 2n = 30 and 60 were observed. Only in T. migricus species, the base number of chromosome x = 14 was reported with two levels of diploid and tetraploid ploidy and chromosome numbers of 2n= 28 and 56. The chromosomes were mostly metacentric (m) or sub-metacentric (sm) in all species Among the various species with basic number of x= 15, the highest percentage of TF and the lowest amount of A1 were observed in populations belonging to T. eriocalyx (T7) indicating that this species has the most symmetric karyotypes. The lowest percentage of TF and the highest amount of A1 are in population of T. lancifolious (27) and T. fedtschenkoi (T28) species; this shows that the species has the most asymmetric karyotype among the species of this subsection. The highest amount of A2 was observed in T. eriocalyx (T7) species, indicated the asymmetry between chromosomes and chromosomal length. The difference in the number of chromosomes, ploidy levels and karyotype asymmetry in the populations and different species may be related to different environmental conditions in their habitat, pollination system (cross pollination) or polyploidy (Aneuploidy) in this genus.

 The results showed that basic chromosome number in most species is 15 (x= 15). Two ploidy levels (diploid and tetraploid) were observed among different species. Chromosomes in most species, were metacentric and sub-metacentric. Based on intra- chromosomal symmetry (A1 and TF) T. eriocalyx species (T7) has the most symmetric and most primitive karyotype and the species T. lancifolius (T27) and T. fedtschenkoi (T28) have the most complete and the most asymmetric karyotypes. Also based on inter- chromosomal symmetry, (A2) T. eriocalyx (T) species has the most asymmetric karyotype.