Relationships among American popcorn and their links with landraces conserved in a microcenter of diversity

This study describes the genetic diversity and the population structure of 66 popcorn landrace collections in a recently discovered microcenter of maize ( Zea mays L.) diversity in southern Brazil. Furthermore, we elucidate their phylogenetic relationships with a diverse panel of 575 popcorn populations from 18 countries of American Continent. The germplasm, representing distinct landraces, was genetically characterized as population bulks using single nucleotide polymorphism markers (SNPs). Five main groups were identified for the popcorn germplasm of the southern region of Brazil. This pattern was associated with phenotypic diversity in grain shape and color. The germplasm of the American Continent was structured in nine groups associated with geographical region with significant differences in terms of genetic diversity and expansion capacity or popping expansion (the formation of large flakes after the kernels explode in response to heating). The popping expansion pattern of the American groups displayed a North-South geographical continuum, in which the average values increased with geographical distance from the center of origin in Mexico. The highest averages were obtained for the Lowland groups of South America. These results raise two hypotheses: the first one is the possibility of a continuous evolution of this characteristic, which is popcorn did disperse North - South, but interbreeding with local varieties drove up the diversity of the gene pool, and at the same time allowed more efficient selection on the trait of expansion capacity. The second is the possibility of a second domestication event of Zea mays ssp. mays L., in South America, which would assume an independent origin of popcorn, but not an independent domestication of maize. Both hypotheses would be based on popcorn populations brought from Mexico by human migrations. The germplasm collected in the microcenter of diversity in southern Brazil, most of them originated in the Lowlands of South America, a part of germplasm established phylogenetic relationships closer to the popcorn populations of Brazil collected in the last century. The study contributes to improved understanding of the origin and phylogenetics of this maize group and as such to conservation of those valuable genetic resources and future breeding efforts. This is valuable genetic research which led to better insight into grouping and dispersion of studied genotypes in the target region, but also its place vis-à-vis origin and migration of popcorn genotypes in South America in general.