Genetic and Agronomic Aspects of Heterosis in Maize: QTL Analysis, Development and Evaluation of Near Isogenic Lines for Heterotic QTL

摘要

Heterosis indicates the hybrid superiority over its parental lines and is often found for traits of agronomic importance. Heterosis is sizable for allogamous species, especially maize (Zea mays L.), and is crucial in crop sustainability through the optimization of yield physiology. Hybrid superiority has been extensively exploited but, despite a century of investigations (Shull, 1908), its genetic basis is not completely understood, yet. On the other hand, there is today a renewal in the attempt to understand the mechanisms of heterosis, owing to the availability of new genetic tools. In this connection, we undertook a three-step research with the objective to gain information on agronomic and genetic aspects of heterosis in maize. We first conducted a study to detect Quantitative Trait Loci (QTL) affecting heterosis. A set of 142 Recombinant Inbred Lines (RILs), derived from the single cross B73 H99, was crossed to the three testers B73, H99 and B73 H99; then testcrosses were evaluated, together with the RILs, in three environments. Several QTL for agronomic traits were detected and most of them were characterized by dominant or overdominant gene action (Frascaroli et al. 2007). The most important QTL for grain yield overlapped with QTL for othertraits like plant height and yield components, suggesting that QTL affected first these traits and then grain yield, with a pleiotropic gene action. As it has been recently reviewed (Holland 2007), the ultimate objective of QTL mapping is to identify the causal genes that underlie them. For this purpose, as a second step, we conducted a Marker Assisted Selection (MAS) aimed at developing pairs of Near Isogenic Lines (NILs) for six QTL, i.e., lines sharing the same genetic background but differing for the parental alleles at one QTL.