Variation in Size of Ag‐NORs and Fluorescent rDNA in Situ Hybridization Signals in Six Breeds of Domestic Pig

Variation of the size of silver‐stained nucleolar organizer regions (NORs) of chromosomes 10 and 8 was studied in pigs of six breeds (Sus scrofa L.). The silver deposits were quantified by image analysis and the results were normalized for each Ag‐NOR chromosome. In general, normalized values for chromosomes 10 were higher than those for chromosomes 8, suggesting that the NOR activity of chromosomes 10 is dominant as compared to that of chromosomes 8. However, high values for chromosomes 8 were found in the Meishan breed and in some Piétrain pigs, indicating a high transcriptional activity of the rRNA genes on these chromosomes. 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 correlated to the number of rRNA genes, whereas the absence or the small size of Ag‐NORs on chromosomes 8, often observed in pigs, is the result of low NOR activity rather than of absence of rDNA.