Characterisation of pericentromeric and sticky intercalary heterochromatinin Ornithogalum longibracteatum (Hyacinthaceae)

The hexaploid liliaccous plant Ornithogalum longibracteatum (2n=6x=54) has a heterochromatin-rich bimodal karyotype with large (L) and small (S) chrom osomes. The composition and subgenomic distribution of heterochromatin was studied using molecular and cytological methods. The major component of cen tromeric heterochromatin in all chromosomes is Sat1, an abundant satellite DNA with a basic repeat unit of 155 bp and an average A+T content (54%). Th e major component of the large blocks of intercalary heterochromatin in L c hromosomes is Sat2, an abundant satellite DNA with a basic repeat unit of 1 15 bp and a high A+T content (76%). Additionally, traces of Sat2 can be det ected at the centromeric regions of S chromosomes, while minor amounts of S at1 are discernible in intercalary heterochromatin of L chromosomes. The ch romosomal localisation pattern of Sat2 is consistent with the fluorescent s taining pattern obtained with the A+T-specific DNA ligand 4'-6-diamidino-2- phenylindole (DAPI). A+T-rich intercalary heterochromatin is sticky and ten ds to associate ectopically during mitosis. Sister chromatid exchange clust ering was found at the junctions between euchromatin and heterochromatin an d at the centromeres. The pattern of mitosis-specific phosphorylation of hi stone H3 was not uniform along the length of the chromosomes. In all L and S chromosomes, from early prophase to ana-/telophase, there is hyperphospho rylation of histone H3 in the pericentromeric chromatin and a slightly elev ated phosphorylated histone H3 level at the intercalary heterochromatin of L chromosomes. Consequently, the overall phosphorylated histone H3 metaphas e labelling resembles the distribution of Sat1 in the karyotype of O. longi bracteatum.