Population differences in the expression of nucleolus organizer regions inthe grasshopper Eyprepocnemis plorans

Fluorescence in situ hybridization revealed the presence of ribosomal RNA g enes in paracentromeric regions of all A chromosomes and in the distal half of B chromosomes in embryonic cells from Moroccan specimens of the grassho pper Eyprepocnemis plorans. The expression of these genes was monitored by the presence of nucleoli attached to each chromosome bivalent in diplotene cells from males collected from two different Moroccan populations and was compared to previous data of Spanish populations. Whereas only the nucleolu s organizer regions (NORs) on S-9-S-11 and X chromosomes were active in the Spanish specimens, Moroccan individuals showed NOR activity in all chromos omes. The rRNA genes on the B chromosome were inactive in both populations. The S-9 and S-10 NORs were less active in Moroccan specimens than in Spani sh specimen, which might be partly explained by the negative interdependenc e for expression of the S-10 NOR with respect to those on L-2 and X chromos omes. On the other hand, the X NOR was more active in Moroccan specimens th an in Spanish specimens, and this might be partly due to the positive effec t that the presence of B chromosomes has on the expression of this NOR. The implications of these observations on current models of NOR activity regul ation are discussed.