Karyomorphology and genome size of 15 St John's wort (Hypericum perforatum L.) populations are reported for the first time. Root tips and fresh young leaves were used for karyological studies and flow cytometric (FCM) measurements, respectively. The chromosome length varied from 0.81 µm to 1.16 µm, and chromosome types were determined as "m". Eight different somatic chromosome numbers were found (2n = 16, 22, 24, 26, 28, 30, 32, 38). Based on the observed basic (x) chromosome numbers of x = 8, 11, 13, 14, 15, 19, this may correspond to diploid (2x), triploid (3x), tetraploid (4x), respectively. Interestingly, we found mixoploidy (3x - 4x) in the root tips of one of the populations. Hybridization, polyploidy and dysploid variation may be the main factors associated with the chromosome number evolution of this species. FCM showed that 2C DNA contents vary from 0.87 to 2.02 pg, showing more than a 2-fold variation. The mean amount of 2C DNA/chromosome and the mean of monoploid genome size were not proportional to ploidy.
Keywords: Hypericum perforatum; dysploidy; genome size; karyotype variability; mixoploidy; polyploidy.