Genetics of corolla colour in periwinkle: relationship between genes determining violet, orange-red and magenta corolla

Abstract: In periwinkle (Catharanthus roseus), pink, white and red-eyed (white corolla with red centre) are three common corolla colours, which are due to epistatic interaction between two genes R and W. Violet corolla, which is not found commonly in natural populations, is reported to be due to another gene B which blues the pigment in pink genotypes (R- W-). Recently, another gene O and its allele O^m have been reported to determine two uncommon corolla colours, orange-red and magenta corolla, respectively. Since, the relationship between genes determining violet, orange-red and magenta corolla was not known, a strain possessing violet corolla and white eye (VI) was crossed with strains possessing orange-red corolla and white eye (OR) and magenta corolla and white eye (MJ-1) to study: (i) the relationship between genes involved in the production of violet, orange-red and magenta corolla, (ii) to study the possibility of producing novel corolla colours and (iii) to determine the validity of the gene interaction models proposed earlier. The F₁ plants of both crosses, VI x OR and VI x MJ-1, had violet corolla. The F₂ generation of the cross VI x OR segregated into plants with (i) violet corolla, (ii) pink corolla, (iii) orange-red corolla, and (iv) white corolla in the ratio of 45:12:3:4, while the progeny of the backcross F₁ x OR segregated into three types of plants, (i) violet corolla, (ii) pink corolla, and (iii) orange-red corolla in the ratio of 2:1:1. The F₂ generation of the cross VI x MJ-1 segregated into five kinds of plants viz., (i) violet corolla, (ii) pink corolla, (iii) magenta corolla, (iv) rose corolla, and (v) white corolla in the ratio of 144:48:12:36:16, while the progeny of the backcross, F₁ x MJ-1 segregated into four types of plants viz., (i) violet corolla, (ii) magenta corolla, (iii) rose corolla and (iv) pink corolla in the ratio of 1:1:1:1. The results suggested that genes involved B, R, W, O/O^m and J were inherited independently and that the gene B blued the corolla pigment in B-RRwwO- genotypes but not in B-RRwwO^m-jj and B-RRwwO^m-JJ genotypes. No new corolla colours were observed in the studied crosses due to the interaction between genes governing violet, orange-red and magenta corolla. The observed segregation for different corolla colours in the studied crosses was same as that expected from independent segregation and known interactions between the genes involved, validating the earlier proposed models.