C-bands and chiasma distribution in Scilla siberica (Hyacinthaceae)

The distribution of chiasmata and their relationship to heterochromatic segments in Scilla siberica (Hyacinthaceae) have been analyzed in pollen mother cell meiosis by means of a Giemsa C-banding technique. All chromosomes could be identified in the meiotic cells. C-bands have a clear effect on chiasma distribution. Regions adjacent to intercalary homomorphic C-bands showed higher chiasma frequencies than other euchromatic regions. Chiasma distribution in bivalents of chromosome variants without these intercalary C-bands is different from those with C-bands. Strongly heterozygous C-blocks lead to a pronounced reduction of chiasma frequency in their vicinity. Mechanical clues for this are found in pachytene pairing behaviour of heterozygous C-bands, which also indicates that euchromatin is constant and C-bands are variable and additional segments. Within one anther, equational separations of C-band heteromorphisms were less than expected from corresponding metaphase I bonds, presumably because of a lower chiasma frequency in early meiotic cells. Heteromorphic intercalary C-bands show that chiasmata do not terminalize. Only at the onset of anaphase I does chiasma resolution result in transitory configurations reminiscent of terminalization.