The control of plastid inheritance in Pelargonium. III

Abstract

Selfing two heterozygous green plants derived from crosses between the Pelargonium cultivars Dolly Varden and Flower of Spring produced solely green F2 progeny in a ratio of 1 homozygote, Pr1Pr1 : 1 heterozygote, Pr1Pr2 instead of the expected mendelian 3:1 or 1:2:1 ratio. The fact that these F2 are all green rules out the possibility that the ratio could be biased through loss of white or variegated embryos as was argued in previous experiments; the fact that the mean fertilisation and embryo survival is the same for families of homozygotes and heterozygotes contradicts zygotic selection; and the fact that embryo death is insufficient to swing the ratio from 3 : 1 to 1 : 1 all point to gametophytic selection against the Pr2 allele on the female side. This implies that the conspicuous differences between the segregation patterns from heterozygous females after G × W plastid crosses occurs despite the fact that the female eggs are of uniform genotype Pr1. Hence the differences between the segregation patterns must be determined by the female parent before meiosis when still heterozygous and diploid.