Irregular segregation at the Pr locus controlling plastid inheritance in Pelargonium: gametophytic lethal or incompatibility system?

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Abstract
Two distinct segregation patterns are recognized after G X W plastid crosses in Pelargonium. Type I parents produce offspring in which maternal zygotes are frequent, biparental intermediate, and paternal zygotes rare (MZ>BPZ>PZ), as defined by the presence or absence of green or white plastids in the young embryos into which the zygotes develop. Type II parents produce offspring in which maternal and paternal zygotes are frequent with biparental zygotes the least frequent class (MZ>BPZPr 1 Pr 1. Type II plants, which do not breed true, are regarded as heterozygotes — Pr 1 Pr 2. The nuclear gene is symbolized as Pr as it is presumed to control alternative patterns of plastid segregation through an effect on plastid replication. Selfs and intercrosses of heterozygous plants segregate in an “unexpected” 1:1 ratio and not the expected 3:1 (1:2:1). The alternative homozygote — Pr 2 Pr 2 — could not be detected. Reciprocal crosses between heterozygotes (Pr 1 Pr 2) and homozygotes (Pr 1 Pr 1) give the expected 1:1 ratio when the Pr 2 allele is derived from the male, whereas there is often, but not always, a highly significant deviation from 1:1 when the Pr 2 allele is derived from the female. A simple explanation, which is not wholly satisfactory, is to assume that Pr 2 is a gametophytic lethal on the female side. An alternative, or additional, explanation is that an incompatibility mechanism is involved in which Pr 1 is a self-compatible allele, Pr 2 a self-incompatible allele, and Pr 1-Pr 2 cross-compatible alleles. Successful fertilization is then determined by sporophytic control on the male side and gametophytic control on the female side.