Complementary genes control biparental plastid inheritance in Pelargonium

Zonal pelargoniums exhibit biparental plastid inheritance. After G x W plastid crosses the progeny are a mixture of green, variegated and white embryos corresponding to a maternal, biparental or paternal inheritance of plastids, respectively. There are two patterns of segregation: type-I females have families in which the majority of embryos are green, variegated are of intermediate frequency and white are the least frequent. Type-II females have families in which green and white embryos are present at about the same frequency and variegated are the least common. The results of many selfs and crosses made within and between 8 type-I and 8 type-II plants led us to conclude that the type of female was determined by its genotype with respect to a pair of complementary genes. Plants giving rise to the type-II pattern contained one or two copies of the dominant alleles of both genes, whereas in the absence of either one or both dominant alleles the plants were type I. The genes were called Pr1/pr1 and Pr2/pr2, an adaptation of symbolism used previously. All 8 type IIs were double heterozygotes Pr1pr1, Pr2pr2, whereas we found 3 genotypes among the type Is, Pr1Pr1, pr2pr2; pr1pr1, Pr2Pr2 and pr1pr1, Pr2pr2. In unrelated experiments we found type IIs of which some were again double heterozygotes and others single heterozygotes Pr1pr1, Pr2Pr2 or Pr1Pr1, Pr2pr2. The model displaces an earlier model based on the proposed operation of a gametophytic lethal or incompatibility system.