Unusually high amount of inactive ribosomal DNA in the grasshopper Stauroderus scalaris

Fluorescence in-situ hybridization (FISH) was employed to determine the chr omosomal location of the ribosomal DNA cistrons in spermatocytes of two pop ulations of the grasshopper Stauroderus scalaris. The results showed that p aracentromeric C-bands, which in this species constitute about 50% of the t otal chromatin, contain substantial amounts of rDNA in all chromosomes. How ever, silver impregnation showed the presence of a single active nucleolus organizing region (NOR) in chromosome 3 of primary spermatocytes, indicatin g an extremely high amount of silent rDNA across the whole genome of this s pecies in the two geographically distant populations analysed. The signific ance of such an unusual phenomenon is discussed.