Effects of supernumerary chromosome segments on the activity of nucleolar organiser regions in the grasshopper Chorthippus binotatus

Abstract

Eight different supernumerary chromosome segments have been detected in natural populations of the grasshopper Chorthippus binotatus. These differ in chromosomal location, size and their response to C-banding. Two different segments occur in each of the M₅, M₆, M₇ and S₈ chromosome pairs. Standard males which lack all eight extra segments show regular primary active nucleolar organiser regions (NORs) located interstitially in the L₂ and L₃ chromosomes plus occasional secondary active NORs located in paracentromeric regions of the M₄ and M₅ chromosomes which give rise to small nucleoli. However, males carrying extra distal segments on the M₅ chromosome show a significant increase in the activity of the secondary NORs in either or both of the M₄ and M₅ chromosomes. In contrast, males carrying extra segments in chromosomes other than the M₅ give a pattern of NOR activity similar to that of standard males. This increase in the activity of the secondary NORs is presumed to result from the presence of extra heterochromatin in the distal region of the M₅. We propose that this region contains genes which regulate NOR expression and that the presence of extra segments in their vicinity affects their normal functioning, resulting in an increased transcription of the ribosomal DNA contained in the secondary NORs.