Protoplast fusions were performed between two sexually produced alloplasmic male-sterile tobacco cultivars, with cytoplasms from Nicotiana bigelovii [Nta(big)S] and N. undulata [Nta(und)S], both of which exhibit homeotic-like phenotypes affecting the petal and stamen whorls. Among the fusion products obtained, both novel male-sterile and pollen-producing cybrid plants were identified. Of the pollen-producing cybrid plants, all of which were indehiscent, some had flowers with stamens that appeared normal when compared to male-fertile tobacco plants. Other hybrid plants were incompletely restored as they exhibited petaloid structures on the anther-bearing pollen-producing stamens. In this study, gel-blot analyses with mitochondrial gene-probes were conducted comparing the mitochondrial DNA of cybrids and male-sterile parents. It was found that the flower morphology typical of the Nta(big)S parental plants, as well as of the novel male-sterile cybrids, coincided with the presence of a chimeric atp A gene copy where an open reading frame of unknown origin was found to be linked in-frame to the 3'-end of a truncated atp A gene. RNA gel-blot hybridizations revealed the presence of atp A transcripts in the male-sterile parent Nta(big)S and novel male-sterile cybrids, but which were absent in cybrids capable of pollen production.
