Chromosome size variation during pollen grain development in Scilla sibirica.

The first pollen grain mitosis in Scilla sibirica takes place within three weeks after the completion of meiosis. Within one anther the duration of the first pollen grain mitotic cycle varies substantially. The duration of the mitotic cycle affects the length of chromosomes at metaphase of the first pollen grain mitosis. In grains which divide "early" the chromosomes at metaphase are longer, up to twice the length, of the chromosomes in grains dividing "late". The diminution in length with increase in the mitotic cycle is due to more intensive coiling which, in turn, is explained by a lengthening of G2 and of prophase. The relationship between the duration of the mitotic cycle and chromosome length at metaphase would account, at least largely, for the variation in chromosome length between different tissues within organisms. It explains also why the chromosome at metaphase of mitosis are shorter in polyploids than in their diploid ancestors.