ABSTRACT
In another chapter of this book, Franks examined the pre-breeding, development and testing of inbred through the level of topcross testing on F4/F5 inbreds. As he discussed, the development of A/B-lines and R-lines differ due to the need to develop a cytoplasmic male sterile through backcrossing. Topcross tests are often based on a single uniform tester, with the goal of selecting among siblings within a population and in identifying superior populations. The focus is generally on identifying yield potential, but the hybrids may also be evaluated for disease, insect or abiotic tolerance characteristics if the target
environment is limited by these issues. These topics are also discussed in a review by Rooney (2004). While the development of inbreds is central to the creation of unique hybrids, once the inbreds are homozygous enough for testing, continued advancement is based on hybrid performance if the goal is a commercial hybrid. Various schemes have been used to advance inbreds, some programmes name or code inbred lines directly at the topcross level, others may not name the inbred until after a second year of testing on multiple testers. The naming of an inbred merely simplifies the pedigree of the hybrid as it moves through the testing process. Depending on the resources available, an advancement cycle may follow a procedure similar to that outlined in Table 1. For commercial production, it helps to identify inbreds with good general combining ability as opposed to lines with high specific combining ability as a good general combining line may find use across a number of products. As more materials are genetically fingerprinted and phenotypically evaluated, it is also possible to use genomic prediction tools to plan new combinations for wider testing (Heffner et al., 2009).
