Demonstration of local adaptation in maize landraces by reciprocal transplantation

Populations are locally adapted when they exhibit higher fitness than
foreign populations in their native habitat. Maize landrace adaptations to
highland and lowland conditions are of interest to researchers and
breeders. To determine the prevalence and strength of local adaptation in
maize landraces, we performed a reciprocal transplant experiment across an
elevational gradient in Mexico. We grew 120 landraces, grouped into four
populations (Mexican Highland, Mexican Lowland, South American Highland,
South American Lowland), in Mexican highland and lowland common gardens
and collected phenotypes relevant to fitness and known highland-adaptive
traits such as anthocyanin pigmentation and macrohair density. 67k DArTseq
markers were generated from field specimens to allow comparison between
phenotypic patterns and population genetic structure. We found phenotypic
patterns consistent with local adaptation, though these patterns differ
between the Mexican and South American populations. Quantitative trait
differentiation (QST) was greater than neutral allele frequency
differentiation (FST) for many traits, signaling directional selection
between pairs of populations. All populations exhibited higher fitness
metric values when grown at their native elevation, and Mexican landraces
had higher fitness than South American landraces when grown in these
Mexican sites. As environmental distance between landraces' native
collection sites and common garden sites increased, fitness values
dropped, suggesting landraces are adapted to environmental conditions at
their natal sites. Correlations between fitness and anthocyanin
pigmentation and macrohair traits are stronger in the highland site than
the lowland site, supporting their status as highland-adaptive. These
results give substance to the long-held presumption of local adaptation of
New World maize landraces to elevation and other environmental variables
across North and South America.