Asymmetrical local adaptation of maize landraces along an altitudinal gradient

Abstract

Crop landraces are managed populations that evolve in response to gene flow and selection. Cross-pollination among fields, seed sharing by farmers, and selection by management and environmental conditions play roles in shaping crop characteristics. We used common gardens to explore the local adaptation of maize (Zea mays ssp. mays) landrace populations from Chiapas, Mexico to altitude. We sowed seeds of 21 populations from three altitudinal ranges in two common gardens and measured two characteristics that estimate fitness: likelihood of producing good quality seed and the total mass of good quality seed per plant. The probability of lowland plants producing good quality seed was invariably high regardless of garden, while highland landraces were especially sensitive to altitude. Their likelihood of producing good seed quadrupled in the highland site. The mass of good quality seed showed a different pattern, with lowland landraces producing 25% less seed mass than the other types at high elevations. Combining these two measures of fitness revealed that the highland landraces were clearly adapted to highland sites, while lowland and midland landraces appear more adapted to the midland site. We discuss this asymmetry in local adaptation in light of climate change and in situ conservation of crop genetic resources.

Independent Molecular Basis of Convergent Highland Adaptation in Maize

“…It may be more likely that alleles adapted in the highlands are slightly deleterious at lower elevation, consistent with empirical findings in reciprocal transplant experiments in Mexico (Mercer et al 2008); in the Appendix we find the rate at which such an allele already present in the Mesoamerican highlands would transit the intervening lowlands and fix in the Andean highlands. The resulting values depend most strongly on the population density, the selection coefficient, and the rate at which seed is transported long distances and replanted.…”

“…Common garden experiments in Mexico reveal that highland maize has successfully adapted to high-elevation conditions (Mercer et al 2008), and phenotypic comparisons between Mesoamerican and South American populations are suggestive of convergent evolution. Maize landraces (open-pollinated traditional varieties) from both populations share a number of phenotypes not found in lowland populations, including dense macrohairs and stem pigmentation (Wellhausen et al 1957;Wilkes 1977), differences in tassel branch and ear husk number (Brewbaker 2015), and a changed biochemical response to UV radiation (Casati and Walbot 2005).…”

“…After domestication, maize spread rapidly across the Americas, reaching the lowlands of South America and the high elevations of the Mexican Central Plateau by $ 6000 YBP (Piperno 2006) and the Andean highlands by $ 4000 YBP (Perry et al 2006;Grobman et al 2012). The transition from lowland to highland habitats spanned similar environmental gradients in Mesoamerica and South America (Supporting Information, Figure S1) and presented a host of novel challenges that often accompany highland adaptation, including reduced temperature, increased ultraviolet radiation, and reduced partial pressure of atmospheric gases (Körner 2007).Common garden experiments in Mexico reveal that highland maize has successfully adapted to high-elevation conditions (Mercer et al 2008), and phenotypic comparisons between Mesoamerican and South American populations are suggestive of convergent evolution. Maize landraces (open-pollinated traditional varieties) from both populations share a number of phenotypes not found in lowland populations, including dense macrohairs and stem pigmentation (Wellhausen et al 1957;Wilkes 1977), differences in tassel branch and ear husk number (Brewbaker 2015), and a changed biochemical response to UV radiation (Casati and Walbot 2005).…”

Assessing the vulnerability of traditional maize seed systems in Mexico to climate change

“…How these processes may affect a cultivated species like maize, in which the tracking of environmental conditions depends on human decisions and which, from a population genetics perspective, shows a metapopulation structure (21), remains to be studied. Already, however, there is evidence of greater local adaptation of highland than of lowland landraces in southern Mexico, and the former do not seem to express the plasticity necessary to sustain productivity under warming conditions (22). Highland Mexican races, which show introgression with teosinte (Zea mays subspecies mexicana) (23) and have a high genetic diversity among New World maize races (24), may be the most threatened because of their strong local adaptation.…”

Back to the Origin: In Situ Studies Are Needed to Understand Selection during Crop Diversification

“…Reciprocal common garden studies with maize have found that highland landraces show higher fitness and seed quality in highland conditions, while lowland landraces have higher fitness in mid-altitude locations (Mercer et al, 2008). Traits such as pigmentation, stem hair, plant height, and flowering time have been shown to be adaptive to altitude, but completely different genes underlie local adaptation to highland conditions in Mesoamerica and South America (Mercer et al, 2008). Field sampling of Mexican teosinte populations helped to clarify that maize adaptation to the Mexican highlands resulted due to introgression from wild teosinte .…”