Effects of B chromosomes on the activity of nucleolar organizer regions in the grasshopper Eyprepocnemis plorans: activation of a latent nucleolar organizer region on a B chromosome fused to an autosome

Abstract

Four nucleolar organizer regions (NORs) are active in standard males of the grasshopper Eyprepocnemis plorans. They are located near the centromeric regions of the S9, S10, S11, and X chromosomes. Changes in the pattern of NOR activity have been observed in the presence of a B2 type supernumerary chromosome. Males with one B show a higher mean number of active NORs per cell than do zero B males owing to significant increases in the activity of the NORs on the S11 and the X. Zero B and one B embryos, however, showed similar patterns of activity. In a male carrying a centric fusion between a B and one of the L1 chromosomes, the activation of a latent NOR, present at the telomere of the long arm of the B, parallelled a significant decrease of NOR activity on the S9 and S10 bivalents stemming from a competition between different NORs in the presence of the B. Thus, while in zero B males the activity of the S10 NOR influences that of the NORs on the X and S9 in a negative way, in one B males it does not do so, although such an influence is observed in the B–L1 fusion male where the activity of the S10 NOR again decreases significantly. On the other hand, the activities of the NORs on the S9 and S11 show a significant positive interdependence in both zero B and one B males where S11 NOR activity is increased but do not do so in the B–L1 fusion male, which shows a significant decrease in the S9 NOR activity. Key words: Eyprepocnemis plorans, B chromosome, nucleolus.