Decreased DNA content of birch (Betula) chromosomes at high ploidy as determined by cytophotometry

Relative amounts of DNA were determined on telophase nuclei by Feulgen cytophotometry for euploid taxa of birch (Betula) with somatic chromosome numbers of 28, 42, 56, 70, and 84. A direct correlation was found between observed DNA absorbance and chromosome number except for plants of B. papyrifera with 84 somatic chromosomes. The DNA density value for nuclei of the 84-chromosome plants fitted a 12.25 ratio instead of the expected 13.0 ratio. The DNA density value for these plants was calculated to be approximately equivalent to plants which would possess 63 somatic chromosomes. The average DNA value per chromosome was 2.73 for the 84-chromosome plants in contrast to 3.50 per chromosome in each of the lower euploids. Nuclear diameters of the 84-chromosome plants were directly related to chromosome number and not to DNA density value. The genomic number of Betula was considered to be x=7, rather than x=14, since a DNA value equivalent to 63 chromosomes is a multiple of 7 and not 14. Diploid birch species (2n=2x=28), therefore, would actually be tetraploids (2n=4x=28). The reduction in DNA content may be an adaptation for the establishment of higher ploidy in birches.