Chromosomal characterization and physical mapping of the 5S and the 18S-5.8S-25S ribosomal DNA in Helianthus argophyllus, with new data from Helianthus annuus

The characterization of the mitotic chromosomes of Helianthus argophyllus b y means of Feulgen staining, Giemsa C-banding, and the usual DNA sequence-s pecific fluorochromes allows a chromosomal classification of this species, and shows that two kinds of heterochromatin co-exist equilocally in its chr omosomes. One type is confined to the pericentromeric areas of all the chro mosomes and the other is associated with the secondary constrictions of the satellite chromosomes. This species is evolutionarily very close to H. ann uus with which it is involved in introgression breeding programs. Since the se two species show no intra- or interspecific differences with the above t reatments, we have used C-banding followed by DAPI, chromomycin A(3) or Acr idine Orange, and the fluorescent in situ hybridization (FISH) with 5S and 18S-25S ribosomal DNA probes to characterize further the chromosomes of bot h species in an attempt to find practically applicable chromosomal markers. Our results confirm the heterogeneity of the heterochromatin in these spec ies and show that neither its distribution nor its response to distinct flu orochrome treatments allows better discrimination of the chromosomes within or between the species. On the other hand, the 18S-5.8S-25S rDNA arrays ar e located in the secondary constrictions of the satellited SM7, SM10, and S T13 pairs and the 5S-rDNA genes are interstitially placed on the short arm of the SM7 and SM11 chromosomes in both species. This permits these chromos omes to be distinguished and provides markers which may be helpful for furt her physical mapping of DNA sequences in these species.