A comparative karyological study of Helianthemum (Cistaceae): karyotype size, karyotype symmetry and evolution of chromosome number

In this study we assessed karyotype size and symmetry for a comprehensive taxonomic and geographic representation of Helianthemum and reconstructed chromosome number evolution in the genus. Using root tips, we photographed mitotic metaphase spreads to obtain chromosome number, total haploid (monoploid) length of the chromosome set (THL), karyotype formula, Stebbins’ classification of karyotype asymmetry, interchromosomal coefficient of variation of chromosome length (CVCL) and intrachromosomal mean centromeric asymmetry (MCA) using MATO (Measurement and Analysis Tools). We found that shifts in chromosome number are not a major driver in the evolution of Helianthemum, whose chromosome number evolved at a constant rate of single chromosome gain or loss. Karyotype asymmetry is very low and little variable in all taxonomic categories studied, with a predominance of metacentric and submetacentric small to medium-sized chromosomes about 3 μm at the genus level. However, total karyotype length varies from 16.91 μm to 47.84 μm at the species level, with a cytogenetic signature that is not conserved within subgenera and most sections. Overall, H. subg. Plectolobum shows both the longest and the most symmetrical karyotypes. We hypothesize that the variation in karyotype size in Helianthemum is likely a consequence of chromosome rearrangements that have occurred under selective pressures.