Selection under negative linkage disequilibrium. Random mating versus inbreeding.

A population resulting from the diallel cross of a set of elite inbred lines was selected under two strategies: (i) inbred strategy (IS) under brother-sister mating; and (ii) random strategy (RIS) under random mating. The final lines in both cases were completely inbred. RIS populations clearly responded better than IS populations. Although most of the RIS advantage was achieved in the second generation, the average response under the RIS strategy increased with respect to that of the IS strategy from the beginning to the end of the experiment. In general terms and on theoretical grounds this RIS advantage is expected only if the initial disequilibrium is negative. One of the final inbred lines performed significantly better than the sum of the two inbred ancestors, strongly suggesting a heterotic epistatic combination fixed in homozygosity. Strong negative disequilibrium and partial epistatic control of quantitative characters are to be expected in breeding programmes of self-pollinating crops. In all likelihood, recurrent selection under forced random mating in such crops would result in better responses.