Phylogenetic relationships between annual and perennial species of Helianthus: evolution of a tandem repeated DNA sequence and cytological hybridization experiments.

The amplification and chromosomal localization of tandem repeated DNA sequences from Helianthus annuus (clone HAG004N15) and the physical organization of ribosomal DNA were studied in annual and perennial species of Helianthus. HAG004N15-related sequences, which did not show amplification in other Asteraceae except for Viguiera multiflora, were redundant in all the Helianthus species tested, but their frequency was significantly higher in perennials than in annuals. These sequences were located at the ends and intercalary regions of all chromosome pairs of annual species. A similar pattern was found in the perennials, but a metacentric pair in their complement was not labelled. Ribosomal cistrons were carried on two chromosome pairs in perennials and on three pairs in annuals except for H. annuus, where rDNA loci were on four pairs. No difference was observed between cultivated H. annuus and its wild accessions in the hybridization pattern of the HAG004N15 and ribosomal probes. These findings support the hypothesis that the separation between annual and perennial Helianthus species occurred through interspecific hybridization involving at least one different parent. However, GISH in H. annuus using genomic DNA from the perennial Helianthus giganteus as blocking DNA failed to reveal different genomic assets in annual and perennial species.