Muzeum Vysočiny Jihlava

rDNA clusters are an important cytogenetic marker for studying karyotype evolution and chromosomal changes. The variability of this cytogenetic characteristic is, however, still almost unknown in the karyotypes of the entire class Arachnida (Arthropoda: Chelicerata). This situation is particularly evident in harvestmen (Arachnida: Opiliones), with 97 species studied cytogenetically, for which there is information on the number and position of rDNA clusters for only 13. Moreover, previous studies indicate that the number of rDNA loci is highly variable in the species analysed, ranging from one to five pairs of rDNA clusters. Based on this fragmentary information, which is for rare species mainly from the limits of the distribution of their families, it is still not possible to reconstruct the ancestral state for this important cytogenetic feature in this order. Building upon recent research in Central Europe, we analysed the number and position of 18S rDNA in 13 species belonging to the suborder Eupnoi. This revealed that their karyotypes were variable in terms of the diploid number (2n = 16-36) and number of 18S rDNA clusters (from one to seven pairs). For the first time, an 18S rDNA cluster was detected on B chromosomes in harvestmen. Our study sheds new light on the karyotype evolution and 18S rDNA distribution in harvestmen and provides an improved understanding of the ancestral state of karyotypes in the order Opiliones.