Relacion Entre Suelo Planta Y Animal

Identifying

Mega-Environments and Targeting Genotypes
Hugh G. Gauch, Jr.,* and Richard W. Zobel locations in the same mega-environment (but of course not outside the location’s own mega-environment). The term mega-environment appears to have been coined by researchers at CIMMYT. "Mega-environments are broad, not necessarily contiguousareas, usually international and frequentlytranscontinental, defined by similar biotic and abiotic stresses, cropping system requirements, consumer preferences, and, for convenience, by a volume of production of the relevant crop sufficient to justify ... attention," for example, "tropical lowland, late-maturing, white dent" corn with relevant disease resistances, which occupies 3.8 million hectares across 18 countries (CIMMYT, 1989, p. 58). Note thatthis definition encompasses environmental, genotypic, geographical, and even economic aspects of mega-environments. Mega-environments are used to allocate resources in a breeding or research program, to rationalize germplasm and information exchanges between breeding programs (allowing even small programs to progress by focusing on the most promising material), to increase heritabilities withinrelatively well-defined and predictable environments, to increase the efficiency of testing and breeding programs, and to target genotypes to appropriate production areas (Brown et al., 1983; Peterson and Pfeiffer, 1989; Abdalla et al., 1996). Manyother terms, however, have essentially the same meaning, such as agroclimatic or ecogeographic regions. Because the need for mega-environments arises fromgenotype-environment interactions and furthermore these interactions involve both genotypes and environments, there are also corresponding terms applied to genotypes for identifying adaptation traits, such as the maturity groups in corn and soybean [Glycine max (L.) Merr.] that are used for delineating adaptation zones. Virtually the entire literature on interaction, clustering environments, yieldstability and dependability, wide and narrow adaptation, and heritability is relevant for understanding and characterizing mega-environments, whether or not the word mega-environment appears in a given paper. Subdivision of a crop’s growing region into several mega-environments could be avoided if genotypes could be found with yield superiority throughout the region, meaning that cultivars bred infavorable environments wouldalso perform best in different or unfavorable environments. But this expectation assumes that "the same genetic system controls yield" in diverse environments, contrary to extensive evidence from genetics, physiology, and yield trials (Ceccarelli and Grando, 1993; Ceccarelli, 1989; Simmonds, 1991). Consequently, for many crops, acceptable yields have necessitatedtargeting narrowly adapted genotypes to several different, wellAbbreviations:AMMI, Additive MainEffects and Multiplicative Interaction; AOV,analysis of variance; CIMMYT, Centro Internacional de Mejoramientode Maizy Trigo; df, degrees of freedom; IPCA,interaction PCAaxis; PCA,principal components analysis; S/N, signal-to-noise; SS, sum of squares; G, genotype; E, environment; L, location; Y, year; R,replication.

ABSTRACT
To maximizeyield throughout a crop’s heterogeneous growing region, despite differences in cultivar rankings from place to place due to genotype-environment interactions, frequently it is necessary to subdivide a growingregion into several relatively homogeneous megaenvironments and to breed and target adapted genotypes for each mega-environment. objectives of this study areto identify relevant The criteria for evaluating mega-environment analyses and to apply the Additive Main Effects and Muitiplicative Interaction (AMMI) model to mega-environment analysis. The proposed analysis is illustrated using a Louisianacorn (Zea maysL.) trial. Statistical strategies for identifying mega-environments should meet four criteria: flexibility in handlingyield trials with...