CYTOLOGICAL AND ELECTROPHORETIC ANALYSIS OF DNA METHYLATION IN THE HOLOCENTRIC CHROMOSOMES OF MEGOURA-VICIAE (HOMOPTERA, APHIDIDAE)

Chromosomal and purified DNA methylation patterns were determined in the holocentric chromosomes of Megoura viciae by treatment with MspI and HpaII. Both enzymes produced a clear C-like banding pattern but widely digested one telomere of the X chromosome, which appeared as heterochromatic after C-banding treatment and brightly fluorescent after chromomycin A(3) staining. Quantitative microfluorometric evaluations of DNA extraction performed on cytological preparations showed that both isoschizomers resulted in the same DNA extraction (about 30%). Contrary to what was found by in situ endonuclease treatment, the electrophoretic patterns of purified and digested DNA showed that digestion with MspI was slightly more extensive than that with HpaII in a zone of fragments ranging from 23 to 9 kb. This result indicates that aphid chromatin is not wholly unmethylated. The discrepancy between electrophoretic and cytological data has been explained by taking into consideration that DNA fragments with high molecular weights could be cleaved in situ by the enzymes but not extracted from the chromatin.