Location and expression of ribosomal RNA genes in grasshoppers: Abundance of silent and cryptic loci

Abstract

We investigate regularities and restrictions in chromosome location of ribosomal RNA genes, analysed by fluorescent in situ hybridization (FISH), and their phenotypic expression assessed by nucleolus formation at first meiotic prophase cells, analysed by silver impregnation, in 49 grasshopper species. High variation was found for rDNA location between species within most genera analysed. The mean haploid number of rDNA loci detected by FISH was 2.47, but some species had up to 10 loci. Chromosome distribution of rDNA loci differed between the Gomphocerinae and Oedipodinae subfamilies, most loci being proximal to the centromere in the former and distal to it in the latter. Chromosomes 2, 3 and X frequently carried rDNA in Gomphocerinae species with 2n♂=17 chromosomes, whereas chromosomes 6 and 9 were the most frequent rDNA locations in the Oedipodinae. About 13% of the 126 rDNA loci detected by FISH were silent, although this figure might be even higher. The comparison of FISH and silver-impregnation results also suggested the existence of cryptic NORs, i.e. those forming small nucleoli with no apparent presence of rDNA revealed by FISH. This was especially clear after the same cells in two species were sequentially treated with both silver impregnation and FISH. The abundance of silent and cryptic loci might thus suggest that rDNA spreads through grasshopper genomes by the Dubcovsky and Dvorak mechanism—that is, the transposition of a few rRNA genes to new chromosome locations, their amplification giving rise to new NORs, and the elimination of the old NORs. The cryptic NORs might correspond to nascent NORs, i.e. a few rRNA gene copies moved to new locations, whereas the inactive rDNA loci might correspond to those being in the process of elimination.