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Resistance to tobacco mosaic virus (TMV) is controlled by the single dominant gene N in Nicotiana glutinosa L. This gene has been transferred to cultivated tobacco (N. tabacum L.) by interspecific hybridization and backcrossing, but has historically been associated with reduced yields and/or quality in flue-cured tobacco breeding materials. Past researchers have suggested the role of pleiotropy and/or linkage drag effects in this unfavorable relationship. Introduction of the cloned N gene into a TMV-susceptible tobacco genotype (cultivar 'K326') via plant transformation permitted investigation of the relative importance of these possibilities. On average, yield and cash return ($ ha(-1)) of 14 transgenic NN lines of K326 were significantly higher relative to an isoline of K326 carrying N introduced via interspecific hybridization and backcrossing. The negative effects of tissue culture-induced genetic variation confounded comparisons with the TMV-susceptible cultivar, K326, however. Backcrossing the original transgenic lines to non-tissue cultured K326 removed many of these unfavorable effects, and significantly improved their performance for yield and cash return. Comparisons of the 14 corresponding transgenic NN backcross-derived lines with K326 indicated that linkage drag is the main factor contributing to reduced yields in TMV-resistant flue-cured tobacco germplasm. On average, these transgenic lines outyielded the conventionally-developed TMV-resistant K326 isoline by 427 kg ha(-1) (P < 0.05) and generated $1,365 ha(-1) more (P < 0.05). Although transgenic tobacco cultivars are currently not commercially acceptable, breeding strategies designed to reduce the amount of N. glutinosa chromatin linked to N may increase the likelihood of developing high-yielding TMV-resistant flue-cured tobacco cultivars.

The tobacco (Nicotiana tabacum L.) breeding line NC 152 is a doubled haploid that possesses an addition chromosome from N. africana [Merxm. and Buttler]. A gene on this chromosome confers potyvirus resistance (Poty(R)). Our objective was to use the addition chromosome as a base on which to construct a designer chromosome containing a foreign gene linkage package. A mutant dhfr gene conferring resistance to methotrexate (Mtx) was inserted into NC 152-haploid (n = 25) leaf tissue via Agrobacterium tumefaciens-mediated transformation. After chromosome doubling, 135 NC 152dhfr transformants (2n = 50) were pollinated with the potyvirus-susceptible (Poty(S)) cultivar 'McNair 944' (2n = 48). Linkage analysis was performed in the BC1 generation. Two transformants, NC 152dhfr-996 and NC 152dhfr-1517 exhibited complete linkage between Mtx resistance (Mtx(R)) and Poty(R). Segregants from these two transformants which were Mtx(R) and Poty(R) possessed 49 chromosomes, while Mtx sensitive (Mtx(S)) and Poty(S) progeny possessed 48 chromosomes. Eighty percent of the NC 152dhfr transformants transmitted the dhfr gene as one locus. Other foreign genes can be directed to the addition chromosome through transformation followed by selection for single loci with linkage to Poty(R) or Mtx(R). The integrity of both the foreign-gene linkage package and the rest of the genome will be maintained because recombination between the N. africiana and the N. tabacum chromosomes has not been observed.

Potato virus Y (PVY), susceptible tobacco (Nicotiana tabacum L.) cultivar, McNair 944, was subjected to in vitro anther culture to determine if genetic variability for virus resistance could be induced among resulting haploids. Five hundred and forty-five haploids were produced and inoculated with a highly necrotic strain (NN) of PVY. One haploid plant survived, even though it was infected with the virus. Selfed progenies of a chromosome-doubled plant of this variant, designated NC 602, proved to be highly resistant to the necrotic effects of the virus. An investigation into the genetic nature of this variant showed the resistance mechanism to be controlled by a single gene exhibiting incomplete dominance. Cytoplasmic and maternal effects were not involved in the disease resistance reaction. The variant was challenged with ten additional strains of PVY from an international collection, and it proved to be resistant to three (VAM-B, MM, and Spanish) strains. NC 602 was evaluated for five agronomic traits and concentrations of total alkaloids as nicotine and reducing sugars in cured leaf. The gametoclonal variant differed from McNair 944 only for cured leaf yield, where an 18.4% reduction was measured.

In tobacco (Nicotiana tabacum L.), anther-derived doubled haploid populations have been shown to exhibit large amounts of unexpected genetic variation and a severe depression in cured leaf yield when compared to conventionally inbred genotypes from comparable sources. A previous study had predicted that the yield depression observed in a doubled haploid population-derived from a near homozygous cultivar, NC95, might be overcome through a recurrent selection program. In the current study, progress from three cycles of full-sib family selection for improved yield in an anther-culture derived population of NC95 was measured, as well as the remaining genetic variation within the population. A design II experiment was conducted in the population following three cycles of selection. Results indicate that the NC95 yield level has been recovered in the third selection cycle population. Although most of the genetic variation in the population appears to be exhausted, the additive genetic variance among maternal half-sib families for yield is significant, and it appears that continued yield improvement can be made through recurrent selection. Significant additive-genetic variance for yield was found among maternal half-sib families but was essentially zero among the paternal half-sib families, suggesting that remaining genetic variation is not being transmitted through pollen. One possible explanation results from the phenomenon of DNA amplification that can occur during the anther culture process, and that may enable extraordinary recombinational events and reduce the viability of male gametes.