Abstract
BackgroundMaize landraces from South America have traditionally been assigned to two main categories: Andean and Tropical Lowland germplasm. However, the genetic structure and affiliations of the lowland gene pools have been difficult to assess due to limited sampling and the lack of comparative analysis. Here, we examined SSR and Adh2 sequence variation in a diverse sample of maize landraces from lowland middle South America, and performed a comprehensive integrative analysis of population structure and diversity including already published data of archaeological and extant specimens from the Americas. Geographic distribution models were used to explore the relationship between environmental factors and the observed genetic structure.ResultsBayesian and multivariate analyses of population structure showed the existence of two previously overlooked lowland gene pools associated with Guaraní indigenous communities of middle South America. The singularity of this germplasm was also evidenced by the frequency distribution of microsatellite repeat motifs of the Adh2 locus and the distinct spatial pattern inferred from geographic distribution models.ConclusionOur results challenge the prevailing view that lowland middle South America is just a contact zone between Andean and Tropical Lowland germplasm and highlight the occurrence of a unique, locally adapted gene pool. This information is relevant for the conservation and utilization of maize genetic resources, as well as for a better understanding of environment-genotype associations.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-016-0874-5) contains supplementary material, which is available to authorized users.
Highlights
Maize landraces from South America have traditionally been assigned to two main categories: Andean and Tropical Lowland germplasm
We recovered Highland Mexico and US landraces forming a single cluster, and two of the main groups previously described for South America by Vigouroux et al [8], i.e. Andean and Tropical Lowland
alcohol dehydrogenase 2 gene (Adh2) sequence analysis To assess whether the marked differentiation detected for the Northeastern Argentina (NEA) groups in the Simple Sequence Repeat (SSR) analyses was in agreement with the ancient structuring pattern reported by Freitas et al [7], we examined the microsatellite allele types at the Adh2 locus in a subset of NEA and Northwestern Argentina (NWA) landraces from the SSR data matrix and analysed it in conjunction with previous data as described in the NEA Flours (N = 344)
Summary
Maize landraces from South America have traditionally been assigned to two main categories: Andean and Tropical Lowland germplasm. Based on the cytogenetic analysis of South American landraces, McClintock et al [6] suggested that different types of maize were early introduced into two initial centres of cultivation: northern South America and the central Andean highlands They proposed that maize germplasm from the northern region had a vast influence on the races of the Caribbean Islands and on those in eastern South America, whereas races from the Andean centre spread extensively throughout the southwest. This hypothesis is consistent with the analysis of a microsatellite repeat within the alcohol dehydrogenase 2 gene (Adh2) in archaeological maize specimens, which revealed an east–west partitioning of allele frequencies. These studies provided evidence of two separate expansion events in South America; one occurring from the highlands of Central America into the Andean region, and the other from the alluvial regions of Panama into the lowlands and along the northeast coast of the continent [7]
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