The DNAs of the A and B chromosomes of the mealy bug, Pseudococcus obscurus

When the DNA of mealy bugs carrying B chromosomes (+B:DNA( was compared to the DNA of individuals not possessing Bs (-B:DNA), no significant differences were found using isopycnic centrifugations in CsCl or thermal denaturation analyses. Both DNAs had buoyant densities of 1.693 g/cm3 in neutral CsCl gradients and 1.748 g/cm3 in alkaline CsCl gradients. Satellite DNAs were not detected. The average Tm of +B:DNA was 67.9 degrees C in 0.1 SSC while -B:DNA had an average Tm of 67.4 degrees C in the same solution. However, in situ molecular hybridizations with complementary RNAs (cRNAs) transcribed in vitro from each type of DNA showed considerable differences with regard to the amount of labeling of B chromosomes. Using cRNA to +B:DNA, the average number of silver grains over a B chromosome was 2.1 X the average number of silver grains over individual non-B chromosomes (A chromosomes). In contrast, the ratio (B/A) using cRNA to -B:DNA was less than 0.14. The results are interpreted as meaning that very little DNA is shared in common by both A and B chromosomes.