Experimental Interspecific Hybrids in Nicotiana and Barriers to Hybridization

The history of interspecific hybridization in Nicotiana is reviewed, starting with the work of Koelreuter who in the eighteenth century produced a hybrid between N. paniculata and N. rustica followed by 22 hybrids, most of them synthetized in the nineteenth century, and 43 produced later. Throughout the twentieth century, the number of newly created hybrid combinations grew progressively and was estimated at ca. 350 in the late 1970s. The author of this review found records for ca. 450 different hybrid combinations, not including multi-specific hybrids and reciprocals. The data on the involvement of individual Nicotiana species in interspecific hybrids are compiled containing information on whether or not a particular species hybridized with N. tabacum, the number of hybrids, if any, that it produced with species other than N. tabacum and, if available, the chromosome pairing data for the hybrid with N. tabacum. The subsequent discussions are focused almost exclusively on hybrids involving N. tabacum. Pre-and post zygotic barriers to produce viable hybrids are examined and tabulated on a species-to-species basis for each particular hybrid combination. Lethality of hybrid seedlings is given a special attention. All known combinations showing seedling lethality are tabulated and morphological manifestations of seedling death, distinguished into five types, are discussed. The causes and phenomena associated with hybrid lethality, type II lethality in particular, are reviewed. The development of N. tabacum monosomics and the importance of monosomic analysis in assigning gene/s responsible for hybrid seedling lethality to particular N. tabacum chromosomes is discussed. The inconsistency of reports on the subgenomic, chromosomal, and linkage group assignment of the type II lethality factor is raised by the author and an attempt is made to account for and possibly to reconcile the discrepancy. Causes of cross incompatibility, embryo abortion, seed inviability, and seedling lethality are reviewed, and methods, both natural and laboratory-based, to manage these conditions are discussed. In-vitro methods to overcome different types of incongruity are tabulated species by species. A separate section is devoted to the appearance of maternal and paternal phenotypes, both haploid and diploid, in the offspring of interspecific crosses. A special treatment is given to diploid maternal plants produced by crossing female N. tabacum with male N. alata. Conversion of sterile amphihaploid hybrids to their fertile amphidiploid counterparts including natural spontaneous chromosome doubling, the use of chemical agents and culturing allohaploid explants in vitro is reviewed. Hybrids obtained by both spontaneous or induced methods are listed in several tables on a species-by-species basis, indicating for each case the method adopted and cytogenetic status of converted hybrid. Optional methods of interspecific hybridization are reviewed including somatic, gametosomatic, and graft hybrids. The review ends up with the lists of all interspecific combinations involving N. tabacum including, if applicable, their converted amphidiploid counterparts and somatic hybrids.