Effect of a substitution of a short chromosome segment carrying a histone H1 locus on expression of the homeotic gene Tl in heterozygote in the garden pea Pisum sativum L.

Abstract

Mutation tlx was induced in pea by gamma-irradiation in the homeotic gene Tl, responsible for transformation of the upper leaflets into tendrils. Heterozygotes tlx/Tl+ possess ‘flat tendrils’ having narrow rudimentary leaf blades. By 15 generations of selfing the line Delta was established, in which closely linked loci Tl and His1 were maintained heterozygous, His1 encoding the most abundant histone H1 subtype. The heterozygous chromosome region was flanked by several recessive markers. The genotype of line Delta with respect to loci His1 and Tl was: tlxHis12/Tl+His11. There were derived two sublines, Delta-s and Delta-f, homozygous for each of the His1 alleles but heterozygous for the Tl and flanking markers. The three lines were compared for a number of quantitative traits, including parameters of flat tendrils reflecting the expression of the gene Tl in heterozygote, and for recombination rate within the heterozygous segment. The line Delta-s exceeded Delta-f in terms of the level of the first inflorescence, length of the internode 8–9, number of pods on the main stem and mean seed mass, but was inferior regarding the mean number of seeds per pod. The flat tendrils were wider in the Delta-s as compared with the Delta-f line ; the difference was highly significant and exceeded 15% at node 17. Recombination rate between the flanking markers also showed significant differences, being highest in the Delta-f and lowest in the Delta line. We do not exclude the possibility that the observed differences in the quantitative traits are caused by the substitution of alleles of the histone H1 gene. The effects of such substitution appeared to be especially strong for expression of the homeotic gene Tl.