Genetic analysis of adaptation differences between highland and lowland tropical maize using molecular markers

Molecular-marker loci were used to investigate the adaptation differences b etween highland and lowland tropical maize. An F-2 population from the cros s of two inbred lines independently derived from highland and lowland maize germplasm was developed, and extracted F-3:4 lines were phenotype in repli cated field trials at four thermally diverse tropical testing sites, rangin g from lowland to extreme highland (mean growing season temperature range 1 3.2-24.6 degrees C). Traits closely related with adaptation, such as biomas s and grain yield, yield components, days from sowing to male and female fl owering, total leaf number, plant height and number of primary tassel branc hes (TBN), were analyzed. A large line x environment interaction was observ ed for most traits. The genetic basis of this interaction was reflected by significant, but systematic, changes from lowland to high land sites in the correlation between the trait value and genomic composition (designated by the proportion of marker alleles with the same origin). Joint analysis of quantitative trait loci (QTLs) over sites detected 5-8 QTLs for each trait (except disease scores, with data only from one site). With the exception o f one QTL for TEN, none of these accounted for more than 15% of the total p henotypic variation. In total, detected QTLs accounted for 24-61% of the va riation at each site on average. For yield, yield components and disease sc ores, alleles generally favored the site of origin. Highland-derived allele s had little effect at lowland sites, while lowland-derived alleles showed relatively broader adaptation. Gradual changes in the estimated QTL effects with increasing mean site temperature were observed, and paralleled the ob served patterns of adaptation in highland and lowland germplasm. Several cl usters of QTLs for different traits reflected the relative importance in th e adaptation differences between the two germplasm types, and pleiotropy is suggested as the main cause for the clustering. Breeding for broad thermal adaptation should be possible by pooling genes showing adaptation to speci fic thermal regimes, though perhaps at the expense of reduced progress for adaptation to a specific site. Molecular marker-assisted selection would be an ideal tool for this task, since it could greatly reduce the linkage dra g caused by the unintentional transfer of undesirable traits.