Somatic crossing over in Glycine max (L.) Merrill: mutagenicity of sodium azide and lack of synergistic effect with caffeine and mitomycin C.

Glycine max (soybean) is one angiosperm which lends itself to the study of somatic crossing over. This is made possible because some varieties have gene combinations Y(11)Y(11), Y(11)y(11) and y(11)y(11) in the segregating populations from Y(11)y(11) plants. The gene in question is responsible for chlorophyll synthesis. The Y(11)Y(11) plants have dark green leaves, Y(11)y(11) are light green and y(11)y(11) plants are golden yellow. The heterozygous plants have dark green, yellow and dark green-yellow (double) spots on the leaves of the untreated control material, whereas the two homozygotes are almost always devoid of somatic sectoring. Application of caffeine, or mitomycin C, to the seeds increased the frequency of double, dark green and yellow spots on the Y(11)y(11) background. Possibly, some dark green or yellow spots originate by failure of one of the two components of what might start as a double spot due to somatic crossing over. The application of NaN(3) increases the frequency of dark green or yellow spots, almost exclusively. The two spots increase in equal frequency. The y(11)y(11) plants so treated do not have any light green sectors, but dark green, Y(11)Y(11), plants do develop a few light green or very dark green spots. The data indicate that NaN(3) is capable of inducing nondisjunction, but does not cause mutations (at this locus), chromosome fragmentations (segmental losses) or somatic crossing over to an appreciable degree. It has previously been shown that caffeine-induced chromosome rejoining in Vicia faba can be inhibited by treating the roots with NaN(3). In the present experiments NaN(3) did not affect the processes of somatic crossing over as induced by caffeine or mitomycin C. The effect was additive. This system offers advantages for studying chemical mutagens in that somatic crossing over, point mutations, segmental losses through chromosome breakage and nondisjunction can all be studied in a single treatment to the seeds.