Combining Ability, Genetic Distances, and Heterosis among Elite CIMMYT and IITA Tropical Maize Inbred Lines

Maize (Zea mays L.) breeding programs are increasingly using molecular tools to enhance the efficiency and speed of developing productive cultivars. As a preliminary step toward using each other's germplasm, 20 elite inbred lines, 10 each from CIMMYT and IITA, were crossed according to a modified North Carolina Design and were genotyped using 62 simple sequence repeat (SSR) markers. The design used six sets of 25 hybrids, including all combinations among groups of five lines each from CIMMYT and IITA's respective heterotic groups A and B. The 150 single-cross hybrids were evaluated in six environments in Zimbabwe and Nigeria during 2003 and 2004, with average grain yield ranging from 5.3 to 11.9 Mg ha(-1). Combining ability analysis indicated that CML444 and CML312 can contribute to the breeding program of IITA, and that TZMI702 should be useful to CIMMYT. The SSR markers identified 209 alleles with an average 3.4 alleles locus(-1) and revealed moderate genetic distances among the parent lines. Genetic distances were not significantly associated with hybrid performance, specific combining ability effects, or midparent heterosis for grain yield, but they effectively grouped the lines according to known pedigree relationships and approximately according to heterotic patterns used by CIMMYT and IITA. Although SSR-determined genetic distances were not useful to predict hybrid grain yield, they could be helpful for preliminary classification and use of exotic or poorly characterized germplasm.