Summary
Clones of the Pelargonium cultivars Dolly Varden and Flower of Spring when used as female parent in G × W plastid crosses, give quite distinct segregation patterns. With D. Varden most offspring are green, rather less variegated and a few are white—type I pattern. With F. of Spring green and white offspring are approximately equally frequent and variegated few—type II pattern. There is considerable phenotypic variation within each pattern but no overlap between them.
The differences between the segregation patterns are related to differences in the female nuclear genotype, D. Varden is homozygous for the type I pattern and F. of Spring is heterozygous. Differences in the male nuclear genotype may slightly modify the segregation pattern but are unable to switch it from one type to another. It is suggested that the two patterns are controlled by alternative alleles of a nuclear gene, named Pr1 and Pr2, with an effect on plastid replication. It is further suggested that the major control of plastid inheritance is by the female parent, because the female produces the eggs, and the nuclear genotype of the eggs determines the behaviour of the plastids. Thus the Pr1 allele favours the replication of green plastids, and when all the eggs have this genotype a type I pattern results, whereas the Pr2 allele favours the replication of white plastids, and when about half the eggs have the Pr1 genotype and half the Pr2 genotype a type II pattern results. Unexpected deviations from simple monohybrid and backcross ratios are explained on the assumption that the Pr2 allele is lethal or semilethal, particularly on the female side, and particularly in combination with mutant plastids. The general demonstration of a nuclear control of the extranuclear plastid inheritance pattern is thought to have important implications for all studies of organelle genetics.
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References
Tilney-Bassett, R A E. 1970a. Genetics and plastid physiology in Pelargonium. III. Effect of cultivar and plastids on fertilisation and embryo survival. Heredity, 25, 89–103.
Tilney-Bassett, R A E. 1970b. The control of plastid inheritance in Pelargonium. Genet Res Camb, 16, 49–61.
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Tilney-Bassett, R. The control of plastid inheritance in Pelargonium II. Heredity 30, 1–13 (1973). https://doi.org/10.1038/hdy.1973.1
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DOI: https://doi.org/10.1038/hdy.1973.1
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