Relation of nuclear volume and radiosensitivity to ploidy level (haploid to 22-ploid) in higher plants and a yeast

Abstract

The relation between ploidy level and radiosensitivity for lethality among plants was examined to see if any major determinant could be found for the relation observed, and if any generalization could be derived. Data are presented on nuclear volume (NV), interphase chromosome volume (ICV) and acute lethal dose for x% killing (LD x% ) for polyploid series of species within 11 genera, 117 species of varieties of herbaceous higher plants and one species of the yeast Saccharomyces. All were irradiated while in active, vegetative growth. The series embrace a 44-fold range of ploidy from haploid to 22-ploid. Radiosensitivity data were obtained for 6 genera, 66 species' or varieties. In general, within any one genus, NV increases proportional to ploidy level. The relative increase of NV with ploidy is about the same for all 12 genera; it is less rapid than linear in the range 2 x to 4 x but almost linear from 4 x to the higher ploidy levels. On the average, relative acute lethal dose (LD x% ) of haploids is half that of diploids, is about constant from diploid to 10-plploid, and then decreases with increasing ploidy. When we compare the dose absorved per average chromosome for x% killing ( “chromosome dose x%” , CD x% ), greater consistency is found both within and between genera, (CD x% is derived by multiplying the lethal dose required for x% killing by the volume of the average chromosome, its ICV). In general, the average haploid chromosome, and cell, is twice as sensitive as the diploid. In the range 2- to 10-ploid, chromosome sensitivity remains about constant and equal to the diploid. From 10- to 20-diploid, chromosome sensitivity increases rather smoothly up to about three to four times the diploid value. This is true for all six general of herbaceous plants. Sensitivity of yeast chromosomes increases with ploidy at a faster rate, and to a much greater extent (by 26 times from diploid to 6-ploid). In the range 2- to 10-ploid, it appears that the sensitizing effect of increased target (genome) size is about compensated for by the protective effect of the equivalent increase in genetic redundancy, leading to the net constancy in sensitivity. At the higher ploidy levels, genomic and chromosomal size do not account for the radiosensitivity increase. It may be that these later evolving, high ploidy species are less well adapted to their ploidy state than the lower levels.