Investigating the evolutionary history of maize in South America

Maize (Zea mays ssp. mays), a crop of great importance worldwide, is characterised by its great adaptability and its extraordinary variability both at the molecular and the phenotypic level. Behind these remarkable features lies a complex evolutionary history. The main aim of this research is to contribute to a better understanding of it by answering questions about the origin, phylogenetic relationships, and initial dispersion of maize, using South America as a case study. To achieve this aim, archaeological specimens and primitive landraces were genotyped at 20 microsatellite loci. Sequences of Adh2-UM, a locus informative of the routes of initial dispersion of maize in South America, were also obtained. The data gathered were analysed using tree-building and network methods, and a Bayesian model-based clustering approach to infer population structure. Our results are consistent with a model of initial dispersion by which after its introduction, maize spread along the Andes and through eastern Bolivia and northwest Argentina towards the east of the continent to the area of Paraguay and southern Brazil, and gave rise to the maize distributed to the east of the Andean mountains. Analysis of population structure indicate that variation among the races analysed is more phenotypic than genetic, and that considerable amounts of gene flow has been occurring at a macro-regional level. We believe that cultural practices and cultural contacts played a fundamental role in making maize on these areas more genetically similar, while at the same time keeping maize phenotypically different. Further studies of ancient DNA, coupled with archaeological and ethnographical knowledge of cultural trajectories, have the huge potential of contributing to a better understanding of how and since when these macro-regional areas of gene flow have been in place.