INHERITANCE OF THE CAPACITY TO TRANSFER PLASTIDS BY THE POLLEN PARENT IN PETUNIA-HYBRIDA HORT

In Petunia hybrida Hort., exceptional biparental inheritance of plastids was obtained reproducibly at a rate of about 2% within the progeny of the Tb 1-3 genotype when Tb 1-3 was used as the pollen parent. Variegated individuals that contained both yellow-virescent maternal plastids and green paternal plastids were also produced by hybrids between Tb 1-3 and nontransmitter inbreds as pollen donors. Analysis of one backcross generation supported the hypothesis that two major loci determined the differences between Tb 1-3 and Skr4 (a nontransmitter). Moreover, testing chimeral individuals as male parents suggested that genes favoring paternal transfer acted at the male gametophytic level. Deviations from additivity obtained among different F1 pollen donors, and a further test of a series of 22 inbreds, which detected five additional inbreds yielding low rates of chimera, suggested that loci responsible for transfer are polymorphic within the Petunia genus. Rare biparental inheritance can be explained tentatively by the following hypothesis: the likelihood of plastid genetic units remaining in a microspore fluctuates among genotypes despite the asymmetrical divisions that lead to differentiation of the spermatic cells.