HETEROSIS IN SOYBEAN AND ITS PREDICTION BY GENETIC SIMILARITY MEASURES

Coefficient of parentage (CP) and restriction fragment length polymorp hism-based genetic similarity estimates (RFLP-GS) have been proposed a s measures of genetic distance in crop species. If these measures are to have application in practical breeding, it is important to validate their utility in predicting genetic traits of interest such as hetero sis. The objectives of this paper were to (i) estimate heterosis for y ield in soybean [Glycine max (L.) Merr.] adapted to the southern USA, and (ii) predict heterosis by mi:ans of CP and RFLP-GS as genetic dist ance measures. Twenty-four F2 populations were developed by crossing t hree testers ('Young', 'Centennial', and 'Tracy') to eight contrasting parents, representing a wide range of CP and RFLP-GS. The experimenta l material was divided into three sets representing the testers above, and was evaluated in eight replications at Clayton and Plymouth, NC, in 1994. Midparent heterosis for yield was 7.9, 4.5, and 7.9% for Sets 1, 2, and 3, respectively. Heterosis was 3.5, 1.6, and 3.0% for 100-s eed weight, and 4.1, 5.4, and 13.2% for plant height. The CP and RFLP- GS were highly correlated (r = 0.80, 0.92 and 0.95 for Sets 1, 2, and 3, respectively, P = 0.01), but neither predicted heterosis well for y ield averaged across locations because of a large genotype x environme nt (G x E) interaction. In contrast, CP and RFLP-GS predicted heterosi s well for 100-seed weight and plant height in two of the three sets a veraged over locations. Our estimates of high parent heterosis for yie ld (as high as 11% over locations), may justify soybean hybrids as a b reeding objective. However, the limited predictive value of CP and RFL P-GS in our study indicates that the identification of favorable heter otic combinations may require extensive field testing.