Comparative Chromosomal Mapping of the 18S rDNA Loci in True Bugs: The First Data for 13 Genera of the Infraorders Cimicomorpha and Pentatomomorpha (Hemiptera, Heteroptera)

<b>Background/Objectives:</b> Sites of ribosomal RNA genes are the most widely documented regions of chromosomes in various groups of eukaryotes, including insects. Data on the number and chromosomal location of 45S rDNAs (25S, 5.8S, and 18S rDNA) are actively used to study the diversity of karyotypes, the organization of individual chromosomes, and the evolution of entire genomes. In true bugs (suborder Heteroptera), the number and chromosomal distribution of 18S rDNA loci are currently known for less than 0.5% of described species. Although some patterns of rDNA distribution can already be identified both in individual taxa of true bugs and in the suborder as a whole, there are still negligible data. In order to expand our understanding of the diversity of rDNA distribution in Heteroptera, we studied for the first time the location of 18S rDNA in 13 species from 13 genera (seven families) of the infraorders Cimicomorpha and Pentatomomorpha (=Terheteroptera, the terminal group of Heteroptera). <b>Methods</b>: Fluorescence in situ hybridization (FISH) with an 18S rDNA probe was used in our study. <b>Results</b>: In total, we have identified three main types of rDNA arrangement: (1) on autosomes, (2) on the X chromosome, and (3) on autosomes and on the X chromosome simultaneously. In most of the studied species, 18S rDNA loci were detected in the terminal position on one pair of autosomes. <b>Conclusions</b>: This study contributed to the understanding of the chromosomal distribution of rDNA loci in the infraorders Cimicomorpha and Pentatomomorpha and confirmed the importance of rDNA in the reorganization of the genomes of Heteroptera as a whole.