Mega-Environment vs. Which-Won-Where
Dividing the target environment into meaningful mega-environments and deploying different cultivars for different mega-environments is the only way to utilize positive GE and avoid negative GE and the sole purpose for genotype by environment interaction analysis (Yan et al., 2007, Crop Sci). A mega-environment is defined as a group of locations that consistently share the same best cultivar(s) (Yan and Rajcan, 2002). This definition involves several essential elements:
1) mega-environments are defined by different winning cultivars, noting that different genotypes can be equally adapted to the same mega-environment and that a mega-environment may need different types of genotypes to stablize the overall production;
2) mega-environment is a concept of geographical locations; and
3) the cultivar-location interaction pattern should be repeatable across years.
All three aspects are essential for the declaration of different mega-environments. The following biplot based on the multi-environment trials (MET) data of barley yield illustrates two points: 1) A mega-environment may have more than one winning genotype, and 2) even if there exists a universal winner (line "829"), it is still possible, and beneficial, to divide the target environments into meaningful mega-environments.
Each dot represents a genotype.