The nature of gene action in a Nicotiana rustica cross revealed by the recombinant inbred and second-cycle hybrid analysis

A unique set of data recorded on 60 randomly extracted single-seed-descent ([F.sub.infinity]) lines of a highly heterotic cross between two varieties of Nicotiana rustica and their 870 reciprocally produced pairwise crosses, the second-cycle hybrids (SCH), are analysed to investigate the true nature of genetical control in the cross and the results are compared with those in earlier publications. The analysis revealed that epistasis, genotype-by-micro-environmental interaction, maternal effects and linkage are significant for several characters and the additive and non-additive components of variation take large values for all of the traits. Epistasis is predominantly duplicate and not complementary. Dominance is high but partial, all estimates of dominance ratio lying between 0.5 and 0.9. Dominance is predominantly unidirectional for leaf length, leaf width and final height, while for the remaining traits, some genes show ambidirectional dominance, although the incidence of unidirectional dominance is much higher throughout. The direction of dominance is predominantly for the increased score, except for flowering time where alleles conferring earliness are up to five times more frequently dominant. The present study has also confirmed that the [F.sub.2] and [SCH.sub.i] distributions are very similar and that the former can be used to predict the transgression in the latter with confidence. The reduced range of the [SCH.sub.i] families compared to the recombinant inbreds, further indicated that heterosis among many of the [SCH.sub.i] is due to gene dispersion and there is little evidence for the presence of over-dominance.