Genetic diversity among CIMMYT maize inbred lines investigated with SSR markers. II. Subtropical, tropical midaltitude, and highland maize inbred lines and their relationships with elite U.S. and European maize

Characterization of genetic diversity of maize (Zea mays L.) germplasm is of great importance in hybrid maize breeding. The objectives of this study were to (i) investigate genetic diversity in CIMMYT subtropical, tropical midaltitude and highland inbreds with simple sequence repeat (SSR) markers, (ii) identify appropriate testers for the development of new inbred lines, (iii) compare this sample to U.S. and European elite maize lines and CIMMYT tropical lowland inbreds, and (iv) use the marker and pedigree information as a guide to understanding the heterotic relationship among the CIMMYT maize lines (CMLs) and their potential practical use in maize breeding programs worldwide. Inbreds included in the study were assayed with 79 SSR markers. The CIMMYT inbred lines originated from 35 mostly broad-based populations and pools with mixed origins. A total of 566 alleles were scored, (averaging 7.2 and ranging from 2 to 16 alleles per locus). The modified Roger's distance (MRD) between pairs of inbreds averaged 0.78, with a range of 0.45 to 0.93. Unweighted paired group method using arithmetic averages (UPGMA) cluster analysis revealed no clear clustering. This reflects a mixed composition of CIMMYT subtropical, tropical midaltitude and highland maize populations and pools, and indicates that large amounts of variation have been incorporated into CIMMYT germplasm. Temperate heterotic groups were separated based on the markers, and nontemperate CIMMYT maize was genetically distinct from temperate lines. Discrete clusters were difficult to identify within and often between megaenvironments. Specific recommendations for nontemperate hybrid maize breeding are made.