Different replication patterns of chromocentres and C-bands in Lilium henryi

To determine if interphase chromocentres are fully equivalent to mitotic C-bands in plants, their times of replication have been compared in the large genome (1C=35 pg) of Lilium henryi. Nuclei of the root-tip cortex were pulse labelled with³H-thymidine and labelling patterns carefully followed in semi-thin sections during a 12 h chase period. Chromocentres decondense and replicate in the later stages of S-phase, after euchromatin has completed its replication. Late-replicating regions, reflecting a portion of the chromocentric material, were then mapped in mitotic chromosomes and found to be localized to the sub-distal and distal regions of all long chromosome arms. Most of the chromatin in these regions is non C-banded and, further, not all C-bands are located here. Some of the 11 inter-calary and 2 nucleolar C-bands are found in earlier replicating regions, as are the 12 centric bands. Therefore Lilium C-bands do not all replicate at the end of S-phase. Chromocentres occupy 17-18% of interphase nuclear volume while C-bands make up only 3.7% of the area of mitotic chromosomes. We conclude that Lilium chromocentres contain much other chromatin in addition to C-bands, and therefore that chromocentres and C-bands cannot be universally equated.