Studies on some nuclear factors controlling radiation sensitivity and the induced mutation rate in eu- and para-sorghum species

Abstract

The interaction between the length and number of chromosomes, DNA content and nuclear volume in relation to radio-sensitivity was studied in S. nitidum and S. purpureosericeum, two species with 2n=10 belonging to the Para-Sorghum group and S. vulgare, a cultivated Eu-Sorghum species with 2n=20. The LD 50 dosages for Co 60 gamma- and 50 kV X-irradiation of seeds of S. nitidum were respectively 26,060 and 16,000 r and for S. vulgare were 65,160 and 63,680 r. With regard to radiosensitivity, S. vulgare behaves like a polyploid. The total length of the chromosome complement varied in the ratio 1 : ·16 : 1·33 in S. vulgare, S. nitidum and S. purpureo-sericeum, the ratio of nuclear DNA contents being 1 : 1·70 :1·62. The absence in these three species of any consistent relationship between chromosome length and DNA content may be due to the presence of variable amounts of heterochromatin. S. vulgare and S. purpureo-sericeum had a similar nuclear volume but no well defined proprtionality existed between total chromosome length and nuclear volume. Our results suggest that in these three Sorghum species (i) no direct relationship may exist between chromatin length and DNA content in species characterized by chromosomes having distinct eu- and heterochromatic segments, (ii) no well defined relationship may exist between nuclear volume, chromatin length and DNA content. Nuclear volume alone, therefore, may not not always be a reliable index of radiation sensitivity and (iii) ‘tetraploid’ Sorghum vulgare with smaller chromosomes and lesser DNA content is more resistant to radiation than the diploid Para-Sorghum species with much longer chromosomes and higher DNA value. Phenotypic mutations occurred in S. vulgare but were absent in the Para-Sorghum species. It is suggested that diplontic selection effects may be responsible for this.