NUCLEAR-DNA CHANGES WITHIN HELIANTHUS-ANNUUS L - VARIATIONS IN THE AMOUNT AND METHYLATION OF REPETITIVE DNA WITHIN HOMOZYGOUS PROGENIES

Complex alterations in the redundancy and methylation of repeated DNA sequences were shown to differentiate the nuclear genome of individual s belonging to single progenies of homozygous plants of the sunflower. DNA was extracted from seedlings obtained from seeds collected at the periphery of flowering heads (P DNA) or from seedlings obtained from seeds collected in their middle (M DNA). Three fractions of repeated s equences were isolated from genomic DNA: a highly repetitive fraction (HR), which reassociates within an equivalent Cot of about 2 x 10(-1), and two medium repetitive fractions (MR1 and MR2) having Cot ranges o f about 2 x 10(-1)-2 and 2-10(2), respectively. Denaturation kinetics allowed different sequence families to be recognized within each fract ion of repetitive DNA, and showed significant differences in sequence redundancy. to occur between P and M DNA, particularly as far as the M R2 fraction is concerned. Most DNA sequence families are more represen ted in P DNA than in M DNA. However, the redundancy of certain sequenc es is greater in the latter than in the former. Each repetitive DNA fr action was hybridized to Southern blots of genomic P or M DNA which wa s digested to completion by three pairs of isoschizomeric restriction endonucleases which are either insensitive or sensitive to the methyla tion of a cytosine in the recognition site. The results obtained showe d that the repetitive DNA of H. annuus is highly methylated. Clear-cut differences in the degree of methylation of P and M DNA were found, a nd these differences were particularly apparent in the MR2 fraction. I t is suggested that alterations in the redundancy of given DNA sequenc es and changes in their methylation patterns are complementary ways to produce continuous genotypic variability within the species which can be exploited in environmental adaptation.