Experimental Introgression from Nicotiana Species to Cultivated Tobacco

A number of breeding goals and strategies involving the use of different Nicotiana species can be envisaged. Yet it is only breeding for resistance and the deployment of cytoplasmic male sterility that have unequivocally profited from resorting to tobacco’s wild relatives. Resistance to pests among the species of Nicotiana is limited to only a few insects. Resistance to diseases is widespread among the species of Nicotiana and the chapter provides relevant data in a table that includes 47 species and 17 diseases. Three major types of introgression in Nicotiana are briefly reviewed of which the the transfer o f simply inherited monogenic, dominant traits has been most important. The mechanics of interspecific gene transfer in tobacco is reviewed. The transfer of resistance to tobacco mosaic (TMV) from N. glutinosa to N. tabacum is expounded in detail as the classical study in the field. Instances of alien substitutions or additions are arranged on a species-by-species basis, quoting 19 species from which such transfers were accomplished. Different routes of introgression are described including the bridge-cross method. Eleven bridge-cross transfers are listed. Fiftyfour reported trigenomic and four tetra-genomic hybrids involving N. tabacum genome are listed as potential starting hybrids. Merits and disadvantages of tetraploid variants of N. tabacum in interspecific gene transfer are discussed and the once-proposed highly contested „egg transformation” is recalled. Various barriers, setbacks and stumbling blocks confronting the interspecific tobacco breeders are reviewed. They include lack of chromosomal homology between species involved in gene transfer, preferential chromosome pairing, disturbed gametogenesis and sterility issues in the allopolyploid hybrids resulting from asynaptic and desynaptic phenomena, chromosome loss in meiosis and mitosis of interspecific hybrids and their derivatives, low transmission rates of alien chromosomes, and unstable behaviour of incorporated alien chromosomes or chromosome fragments. Undesirable effects related to the introgressed trait per se include various modifications in expression of the transferred gene, linkage drag, epistatic and pleiotropic effects. Two issues received a detailed treatment. One was concerned with the epistatic interaction between the recessive PVY-resistance factor from N. raimondii and the symptomless host response to PVY from N. tabacum . If present in one genotype, the two factors cancelled each other’s effects resulting in strong susceptibility, a potentially useful system to help tracking the recessive factor from N. raimondii in future introgression projects. The other issue was a peculiar instance of linkage drag associated with resistance to TSWV from N. alata resulting in development of leaf deformations and tumorous outgrowths. The author of this review tries to account for this unusual manifestation of linkage drag by recalling genetic tumors reported in many interspecific hybrids of Nicotiana and pointing to similarities between the two phenomena. Instances of linkage drag in several cases of introgressing alien genes into the genome of N. tabacum are listed on a species-by-species basis, including eight source species and several manifestations of this undesirable effect. Several other introgression-related issues are discussed including the role of synthetic tobacco, molecular marker-assisted selection of introgressed genes, linkage drag in heterozygous condition and the concept of designer chromosome. The review sums up with listing reports on transfer of diverse traits, mostly resistance factors from 30 species to N. tabacum. Well documented accounts and mere mentions, completed or advanced but apparently discontinued projects were all included and treated on an equal basis.