The genome of a Helianthus annuus (2n = 34) x Helianthus tuberosus (2n = 10
2) hybrid was studied at cytological, biochemical and molecular levels and
compared to those of the parental species. Cytophotometric analyses showed
that the hybrid has a 4C DNA content higher than expected and with a larger
variability than in the parents. This high variability is probably not rel
ated to chromosome-number variations since the hybrid always had 2n = 68 ch
romosomes. Moreover, hybrid interphase nuclei showed lower heterochromatin
condensation than the parental ones. Thermal denaturation of genomic DNAs i
ndicated that, quantitative variation of some DNA families occurred in the
hybrids compared to parents. Finally, molecular analyses of DNAs restricted
with different enzymes, after Southern blotting and hybridization with HR
probes, showed restriction patterns in the hybrid different from those obse
rved in parents. These results indicate that interspecific hybridization be
tween H. annuus and H. tuberosus may determine quantitative variation of so
me DNA families and differential DNA methylations that probably modify the
nuclear structure. These phenomena are probable responses to a "genomic sho
ck" following the interspecific cross.