Species of Nicotiana as the Sources of Cytoplasmic Male Sterility for Cultivated Tobacco

In tobacco, cytoplasmic male sterility (CMS) is the most extensively studied and practically exploited aspect of extrachromosomal inheritance. It finds utility in the economically viable production of intervarietal hybrids as an alternative to purebred cultivars. Formerly, little commercially exploitable heterosis per se was expected from intervarietal tobacco hybrids, but the recent studies have revealed that breeding for heterotic effects in several market types of tobacco can be a viable strategy. The major merits of a hybrid cultivar is the mitigation of deleterious linkages and the ease with which some important traits can be combined in one genotype. Protection against unauthorized propagation and other infringements of proprietary rights is another added value of cytoplasmic male sterility. Genetic principles of cytoplasmic male sterility are reviewed, disruption of complementarity between nuclear and mitochondrial DNA caused by the replacement of native cytoplasm with that of an alien species being the underlying cause of the phenomenon. Various floral modifications associated with cytoplasmic male sterility are tabulated on a species-by species basis. Subsequent sections examine the methods to develop cms lines of N. tabacum, including conventional sexual transfer of cytoplasmic factors and somatic hybridization that involves the development of highly asymmetric somatic hybrids. A list of 32 species, the reported donors of alien genetic material in cytoplasmically male sterile forms of N. tabacum is compiled. Of these species, four were also involved in the origin of cms forms as male parents, one was involved as a male parent and as a dubious cytoplasm donor, and one (N. tabacum) as a putative cytoplasmic mutant. Restoration of fertility to alloplasmic cms forms and the underlying mechanisms were discussed. A list of eight N. tabacum alloplasmics, to which male fertility was restored by introgressing genetic factors from cytoplasm donor species, was compiled. Controversial issues and discrepancies regarding identity and origin of some alloplasmics maintained in germplasm collections are examined. In particular, the provenance and identity of some of the alloplasmics, in the origin of which N. plumbaginifolia and N. glutinosa were involved, are put into question. In connection with ‘tabacum-mutant’, allegedly a lineage of cms glauca, the author hypothesizes that it might have been a product of uncontrolled cytoplasm transfer through pollen. The same hypothesis is tentatively advanced to account for the origin of the male sterile systems which had an alien species as the male parent in their pedigrees. The final parts of the review contain information on agronomic merits or flaws of the 32 cms alloplasmics. Separately, data on agronomic performance are compiled for four major cms systems known to be deployed in commercialized tobacco hybrid cultivars.