The pure-line hybrid concept traces its roots back to experiments on heterosis and its complement in- breeding conducted by SHULL (1908, 1909) at Cold Spring Harbor Laboratories in New York, and EAST

Today the concept of heterotic groups and patterns is fundamental to hybrid breeding theory and practice. Our objectives were to (i) review various hypotheses on the causes of heterosis in maize and on the experimental evidence supporting or refuting them, (ii) examine advantages and disadvantages of the concept of heterotic groups and patterns, (iii) describe the status of heterotic patterns in different regions, and (iv) discuss strategies for the establishment of heterotic patterns. Various hypotheses explaining the phenomenon of heterosis were reviewed. Although many hypotheses have been suggested, the genetical, physiological, and biochemical bases of heterosis still remain largely unexplained. Theoretical and experimental arguments are given suggesting that grouping of germplasm into divergent heterotic groups is advantageous due to (i) a higher mean heterosis and hybrid performance and (ii) a reduced specific combining ability (SCA) variance and a lower ratio of SCA to general combining ability (GCA) variance. Thus early testing becomes more effective and superior hybrids can be identified and selected mainly based on their prediction from GCA effects. The status of heterotic patterns were described for the U.S. Corn Belt, Central Europe, Japan, China, and tropical maize growing regions. While in most regions heterotic patterns have been established, grouping failed in the tropical germplasm due to the complex population structure. Two strategies to establish heterotic groups were discussed. Further research is required incorporating recent advances on the genetic architecture of quantitative traits and on the genetic basis of heterosis to develop optimal procedures for establishing and maintaining heterotic patterns.