Characterization of the chromosome complement of Helianthus annuus by in situ hybridization of a tandem repeated DNA sequence.

A tandemly repeated sequence isolated from a clone (HAG004N15) of a nebulized genomic DNA library of sunflower (Helianthus annuus L., 2n=34) was characterized and used to study the chromosome complement of sunflower. HAG004N15 repeat units (368 bp in length) were found to be highly methylated, and their copy number per haploid (1C) genome was estimated to be 7800. After in situ hybridization of HAG004N15 repeats onto chromosome spreads, signals were observed at the end of both chromosome arms in four pairs and at the end of only one arm in eight other pairs. Signals were also observed at the intercalary (mostly subtelomeric) regions in all pairs, in both arms in eight pairs, and in only one arm in the other nine pairs. The short arm of one pair was labelled entirely. The chromosomal location of ribosomal DNA was also studied by hybridizing the wheat ribosomal probe pTa71. Four chromosome pairs contained ribosomal cistrons at the end of their shorter arm, but a satellite was seen in only three pairs. These hybridization patterns were the same in the three sunflower lines studied (HA89, RA20031, and HOR). The chromosomal localization of HAG004N15-related sequences allowed all of the chromosomes pairs to be distinguished from each other, in spite of small size and similar morphology.