Expression of nuclear-cytoplasmic genomic incompatibility in interspecific Petunia somatic hybrid plants

Somatic hybrid plants were regenerated following calcium-high pH fusion of the unidirectional, sexually incompatible cross of Petunia parodii wild-type leaf mesophyll protoplasts with protoplasts from a cytoplasmic determined chlorophyll-deficient mutant of P. inflata. Genic complementation to chlorophyll synthesis and sustained growth in the selective medium was used to visually identify hybrid calluses. Hybrid calluses were subsequently regenerated to shoots, rooted, and confirmed as somatic hybrids by their intermediate floral and leaf morphology based on comparison to the 2 n = 4 x = 28 sexual counterpart, dominant anthocyanin expression in the corolla, chromosome number, and peroxidase and maleic dehydrogenase isozyme patterns. Certain cytologically stable somatic hybrids displayed aberrant reproductive and floral morphologies including subtle to moderate corolla and leaf pigment variegation, floral dimension changes and reduced pollen viability. In contrast, cytologically unstable somatic hybrids showed various degrees of aneuploidy coupled with corolla splitting, and irregularities in reproductive organs such as double stigmas and styles in addition to reduced pollen viability. Postulated mechanisms to account for these phenotypic changes in stable and unstable somatic hybrids include nuclear-cytoplasmic genomic incompatibility, chromosome loss in a biparental cytoplasm, or a phenomenon similar to hybrid dysgenesis occurring as a result of somatic fusion.