Recurrent Selection for Specific Combining Ability and Type of Gene Action Involved in Yield Heterosis in Corn 1

The extensive literature reviews of heterosis presented elsewhere will not be repeated here. Recurrent selection for specific combining ability was proposed by Hull (3) in 1945. Where yield is the attribute under consideration, the procedure involves the selfing and out-crossing of selected plants to a narrow-base t ster. Under optimum efficiency the tester ~vould be a long-time inbred line. After testcross evaluation, S, progeny of the plants having superior genotypes are intercrossed in a diallel manner and. the intercross populations provide source material for a repetition of the selfing, testcrossing, evaluation, and intercrossing cycle. This method of breeding was designed for efficiency if overdominance is of major importance in yield heterosis. It should be pointed out, however, that it also would be effective, for all loci for which the tester parent was recessive, if dominance and partial dominance were of importance. Sprague and Miller (6) outlined a procedure that should make it possible to distinguish xvhether dominance or overdominance is of major importance in yield heterosis in corn. The assumptions involved ~vere as follows: If the selection, as practiced, is effective in changing gene frequency then with successive repetitions gene frequencies will tend toward 1.0 as a limit if dominance and partial dominance are of major importance. If overdominance is of majo__r importance, and if thb gent frequency of the tester parent is q, gene frequencies in the populations undergoing selection will tend toward 1-q as a limit. At this point the genotype of the selected population would be the exact counterpart of the genotype of the tester parent. The test for the direction of change in gene frequency is provided by carrying through the selection operation simultaneously with two populations, A and B, using a common i bred tester. Intercrosses between Ao X Bo, A, X B1, A.~ X B~ should exhibit an increasing yield trend if dominance and partial dominance are of major importance in yield heterosis. However, if overdominance is of major importance the same series of crosses should exhibit a decreasing yield trend. This follows since each cycle of effective selection tends toward increasing genetic ’similarity between the two populations of A~ and B,,. If the initial heterozygo_sities and array of alleles were similar for A and B, then at the 1-q limit the two populations would be of identical genotype. Sprague and Russell (8) presented some evidence derived from the procedures just outlined and concluded that the results obtained were in agreement with expectations under the dominance hypothesis. It should, perhaps, be stressed that the procedures outlined do not provide clear-cue evidence that overdominance does or does not exist, iRather, it provides a measure of whether dominance (partial or complete) is of greater importance than overdominance in yield heterosis in corn.