Heterosis and Isozyme Divergence in Indica Rice

Earlier studies conducted in China indicated an association of heterosis in rice (Oryza sativa L.) crosses with certain esterase and peroxidase isozyme patterns. To search for methods that predict yield heterosis in hybrid rice breeding program at IRRI, we studied isozyme variation of 18 rice cultivars (seven maintainers and 11 restorers of ‘WA’ cytosterility system) and 75 F1, hybrids derived from these parents to determine the relationship between yield heterosis in the F1s and isozyme diversity among their parents. Isozyme variation among parental lines was found for six genes, viz., Est‐9, Est‐2, Amp‐3, Sdh‐1, Pgi‐2, and Pgd‐1. Midparent heterosis for yield ranged from −44.6 to 156.7%. Our studies did not show any association between magnitude of heterosis in F1s and isozyme variation among parents. The reason our results differed from those reported in China may be because most of the parental lines used in our studies were elite rice cultivars bred at IRRI from complex crosses involving parents of various geographic origin that allowed extensive recombination among genes. Consequently, linkage disequilibrium between isozyme markers and gene blocks involved in heterosis for yield might have disappeared.