Abstract
In tobacco (Nicotiana tabacum L.), there is a very little scope for intervarietal gene transfer, hence interspecific hybridization utilising wild species of the genus Nicotiana possessing resistance to one or more pests, is the best solution. Eventhough N. benthamiana is resistant to tobacco caterpillar (Spodoptera litura F.), in view of its interspecific incompatibility with N. tabacum, a trispecific amphiploid (N. benthamiana×N. glutinosa×N. tabacum) was synthesised with N. glutinosa as a bridge species. Progressive elimination of N. benthamiana and N. glutinosa chromosomes was achieved through four successive back-crosses of the amphiploid with flue-cured N. tabacum varieties, resulting in plants having most of the tabacum characters and caterpillar resistance. All the plants were male-sterile and most of them possessed 48-50 mitotic chromosomes, 48 from N. tabacum and 1-2 from N. bentamiana thus indicating that the resistance factors are confined to one or two chromosomes. Based on the mortality of inoculated larvae and the extent of damage, the plants were grouped into highly resistant, moderately resistant and susceptible types.
The heavy mortality of the caterpillar within 24 hours indicate that certain toxic exudates from the leaf surface may be the primary cause of death. Starvation due to “non-preference” may be another factor responsible for death of most of the remaining larvae within the next 24-240 hours. The retarded growth of a few surviving larvae, their resistant feeding and the slow pace at which they proceed from one instar to another indicate that some “antibiosis” properties may also be associated with the factors for resistance.
