INTRAINDIVIDUAL HETEROGENEITY OF RDNA ALLOWS THE DISTINCTION BETWEEN 2 CLOSELY-RELATED SPECIES IN THE GENUS HELIANTHUS

The Ribosomal DNAs of Helianthus annuus and H. argophyllus were analys ed. Total DNA from single individuals of six cultivated lines, one wil d ecotype of H. annuus and three ecotypes of H. argophyllus, were dige sted with various restriction enzymes. Hybridisation of Southern blots with sunflower ribosomal probes containing most of the interspaces re gions (R3) or the 25 s coding region (R2) reveals different patterns f rom those expected: while no difference between H. annuus and H. argop hyllus had been observed in previous rDNA RFLP analysis, our study cle arly distinguished the two species on the basis of two different patte rns when using R3 and BamHI, BstYI, or EcoRI/BamHI. Furthermore, the s um of the fragment weights of the BamHI restriction patterns was much greater than that of the rDNA entire unit-weight space. The co-existen ce of different rDNA units within single individuals is proposed as a model to explain these results. Four rDNA units were distinguished, wh ich differed in their state of methylation and by the presence of muta tions at two BamHI restriction sites. H. annuus individuals displayed two types of rDNA units while H. argophyllus individuals displayed fou r types.