Determining heterotic orientation of South African maize inbred lines towards USA temperate and CIMMYT- tropical testers and genetic analyses under contrasting environments.

Abstract

Drought and low soil fertility are major abiotic stresses limiting maize productivity in South Africa. Developing drought and low nitrogen-tolerant varieties is part of a long-term solution to improved maize productivity under climate change. The employment of well-defined heterotic groups has been the prime cause of success of most hybrid breeding programs in the private sector. The public maize program in South Africa utilises seven heterotic groups. These require many different testers, and the resultant many cross combinations require a lot of resources for extensive field testing. Reducing the number of heterotic groups is essential for improving breeding efficiency. The objectives of this study were to classify the South Africa maize inbred lines into fewer heterotic groups based on their orientation towards temperate and tropical testers, and to identify superior genotypes under stress and non-stress environments. A sample of 42 lines drawn from the seven heterotic groups was genotyped with 56110 SNP DNA markers. The lines were also crossed to two inbred line testers representing the heterotic groups A and B for tropical CIMMYT and temperate USA Corn Belt. The resultant hybrids were evaluated in an (0, 1) α-lattice design under stress and non-stress conditions during the 2014/15 and 2015/16 summer seasons. Data were collected on grain yield and secondary traits. Using the specific combining ability and SNP-marker data, the seven heterotic groups could be reorganised into two major clusters. This information would be useful in designing superior hybrids. Correlation between genetic distance with grain yield and specific combining ability was negligible, making it prudent to perform multi-location trials to identify superior genotypes. The lines FO215W, I-42, I-16 and K64 displayed good general combining ability for grain yield. The most superior hybrids were FO215W x CML444 and I-42 x CML444, which combined high productivity with stability. However, performance of hybrids generally differed under stress and non-stress conditions. Overall, results showed success in simplifying the heterotic grouping of the public maize germplasm in South Africa and the possibility of improving heterosis and obtaining high yields under low input and water limited environments by exploiting temperate × tropical hybrid combinations.