VARIATION IN SIZE OF AG-NORS AND FLUORESCENT RDNA IN-SITU HYBRIDIZATION SIGNALS IN 6 BREEDS OF DOMESTIC PIG

Variation of the size of silver-stained nucleolar organizer regions (N ORs) of chromosomes 10 and 8 was studied in pigs of six breeds (Sus sc rofa L.). The silver deposits were quantified by image analysis and th e results were normalized for each Ag-NOR chromosome. In general, norm alized values for chromosomes 10 were higher than those for chromosome s 8, suggesting that the NOR activity of chromosomes 10 is dominant as compared to that of chromosomes 8. However, high values for chromosom es 8 were found in the Meishan breed and in some Pietrain pigs, indica ting a high transcriptional activity of the rRNA genes on these chromo somes. In some pigs, the relative quantities of rDNA in chromosomes 10 and 8 were investigated by fluorescent in situ hybridization and the results were compared with those of the silver staining procedure. It is concluded that Ag-NOR sizes on chromosomes 10 are relatively well c orrelated to the number of rRNA genes, whereas the absence or the smal l size of Ag-NORs on chromosomes 8, often observed in pigs, is the res ult of low NOR activity rather than of absence of rDNA.