Differential Replication of the Chromosomes of Pallavicinia longispina, Liverwort

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Abstract
The estimated duration of the mitotic cell cycle of meristematic cells in the female gametophyte of P. longispina is presented. The 1st appearance of Ag grains over the mitotic prometaphase chromosomes was at 6 h after the beginning of 2 h pulse labeling. At this hour, Ag grains were only over the euchromatic regions, and not over the heterochromatic regions. This labeling pattern was also observed at 8-10 h. The heterochromatic regions having Ag grains were observed for the 1st time at 15 h when euchromatic regions were also labeled. At 20-25 h, the heterochromatic regions have more grains than the euchromatic regions. These results show that the duration of DNA replication in heterochromatin is shorter than that of euchromatin and that the heterochromatin completes its DNA replication earlier than euchromatin. Differential replication of DNA is found among chromosomes in a complement: X chromosome completes its DNA replication earlier than any other autosomes; individual autosomes vary in the distribution of early replicating segments.