SUPERNUMERARY HETEROCHROMATIC SEGMENTS ASSOCIATED WITH THE NUCLEOLAR CHROMOSOMES OF PYRGOMORPHA-CONICA (ORTHOPTERA) CONTAIN METHYLATED RDNASEQUENCES

The two nucleolus organizing chromosome pairs of the grasshopper Pyrgo morpha conica can carry a proximal supernumerary heterochromatic segme nt. We employed different cytological techniques to characterize and a nalyze the possible origin of this segment. The supernumerary segment and the nucleolus organizing regions (NORs) show similar responses aft er C-banding plus either Giemsa or acridine orange, and chromomycin A3 /distamycin A staining to detect GC-rich chromosome regions. Fluoresce nce in situ hybridization with a biotinylated rDNA probe demonstrated that the segment originated by amplification of the rDNA genes. Howeve r, as the silver staining indicates, the ribosomal genes present in th e segment are not active since no nucleolus is formed. The use of in s itu digestion with the isoschizomeric MspI and HpaII restriction endon ucleases and subsequent Giemsa, ethidium bromide or chromomycin A3/dis tamycin A staining, suggests that the segment has been inactivated by DNA methylation.