THE NATURE OF GENE-ACTION IN A NICOTIANA-RUSTICA CROSS REVEALED BY THE RECOMBINANT INBRED AND 2ND-CYCLE HYBRID ANALYSIS

A unique set of data recorded on 60 randomly extracted single-seed-des cent (F-infinity) lines of a highly heterotic cross between two variet ies of Nicotiana rustica and their 870 reciprocally produced pairwise crosses, the second-cycle hybrids (SCH), are analysed to investigate t he true nature of genetical control in the cross and the results are c ompared with those in earlier publications. The analysis revealed that epistasis, genotype-by-micro-environmental interaction, maternal effe cts and linkage are significant for several characters and the additiv e and non-additive components of variation take large values for all o f the traits. Epistasis is predominantly duplicate and not complementa ry. Dominance is high but partial, all estimates of dominance ratio ly ing between 0.5 and 0.9. Dominance is predominantly unidirectional for leaf length, leaf width and final height, while for the remaining tra its, 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 flow ering time where alleles conferring earliness are up to five times mor e frequently dominant. The present study has also confirmed that the F -2 and SCHi distributions are very similar and that the former can be used to predict the transgression in the latter with confidence. The r educed range of the SCHi families compared to the recombinant inbreds, further indicated that heterosis among many of the SCHi is due to gen e dispersion and there is little evidence for the presence of over-dom inance.