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Abstract:
People introduced maize (Zea mays ssp. mays) to the southwestern US by 4,000 years ago, quickly establishing maize agriculture in the lowland deserts while adoption was delayed in the temperate uplands for 2,000 years. Because reduced flowering time characterizes modern temperate maize, we test for early flowering in the earliest established upland agriculture. Turkey Pen Shelter, at 1830m in southeastern Utah, is one of the earliest established agricultural sites in the temperate Southwest. We sequenced fifteen 1,800-year old archaeological maize cobs from Turkey Pen Shelter with high endogenous content to 5-20X coverage, allowing genotyping on HapMap 3.21 variants. An additional 1018 GBS-genotyped teosinte and landrace individuals enriched for the Southwest US and North Mexico cover the parameter space of tropical and temperate adapted landraces, allowing us to genetically situate Turkey Pen maize within modern temperate Southwestern landraces. A subset of 110 Southwest US/N. Mexican individuals, representing 80 accessions, were crossed to a common tester and the progeny evaluated for DTA and DTS in 9 replicate/environments (H = 0.89 and 0.9, respectively). Using the diverse Ames inbred panel to train genomic prediction models, which had a prediction accuracy of 0.72 in the phenotyped Southwestern landraces, we confidently predicted that Turkey Pen maize flowered comparably early to modern temperate lines, and would have been marginally adapted in Utah. Our results suggested that temperate adaptation drove modern population differentiation and was selected in situ from ancient standing variation. Reliance on standing genetic variation highlights the importance of germplasm maintenance in light of unprecedented climate change.
