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

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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.