CHROMOSOMAL DIFFERENTIATION IN HELIANTHUS-ANNUUS VAR MACROCARPUS - HETEROCHROMATIN CHARACTERIZATION AND RDNA LOCATION

The 2n = 34 chromosomes of the inbred line HA89, and the Flamme and Mi rasol hybrids of Helianthus annuus var. macrocarpus possess centromeri c heterochromatin as established by Giemsa C-banding. This heterochrom atin can not be differentiated by fluorochromes such as DAPI or Chromo mycin A3, with selective affinity for specific DNA base pairs. This si tuation probably results from either a balanced AT/GC composition of t he involved repeat or the existence of alternating repetitive sequence s of opposite base pair composition in these heterochromatic areas. Ho wever, there is also heterochromatin associated with the secondary con strictions on three pairs of chromosomes. This heterochromatin appears to be GC-rich according to its response to the fluorochrome treatment s, thus indicating heterochromatin heterogeneity in H. annus. Silver s taining reveals the existence of active NORs associated with these sec ondary constrictions. In situ hybridization with an rDNA probe confirm s these results and makes the existence of other inactive rDNA sites u nlikely. These results are relevant to evolutionary and breeding studi es on sunflowers.