Genetic and phenotypic diversity in 2000 years old maize (Zea mays L.) samples from the Tarapacá region, Atacama Desert, Chile

Abstract

The evolution of maize (Zea mays L.) is highly controversial given the discrepancies related to the phenotypic and genetic changes suffered by the species, the incidence of human groups and the times in which these changes occurred. Also, morphological and genetic traits of crops are difficult to evaluate in the absence of fossils macro-botanical remains. In contrast in the Tarapacá region (18–21° S), Atacama Desert of Chile, prehispanic settlements (ca. 2500–400 yr BP) displayed extensive maize agriculture. The presence of archaeological macro-botanical remains of maize provided a unique opportunity to study the evolution of this crop, covering a temporal sequence of at least 2000 years. Thus, in this study, we ask how the morphological and genetic diversity of maize has varied since its introduction during prehispanic times in the Tarapacá region. To answer this, we measured and compared morphological traits of size and shape between archaeological cobs and kernels and 95 ears from landraces. To established genetic diversity eight microsatellite markers (SSR) were analyzed in archaeological and modern kernels. Genetic diversity was estimated by allelic frequency rates, the average number of alleles per locus, observed heterozygosity (Ho) and expected heterozygosity (He). Differences between populations and genetic structure were estimated by fixation index FST and STRUCTURE analysis. Our results indicate significant phenotypic differences and genetic distance between archaeological maize and landraces. This result is suggestive of an introduction of new varieties or drastic selective changes in modern times in Tarapacá. Additionally, archaeological maize shows a low genetic diversity and a progressive increase in the size of ears and kernels. These results suggest a human selection during prehispanic times and establish that prehispanic farmers played an important role in maize development. They also provide new clues for understanding the evolutionary history of maize in hyperarid conditions.

Geohistorical records of the Anthropocene in Chile

“…Poaceae) to the detriment of foraging herbaceous taxa (Baied and Wheeler, 1993). At the same time, several Andean and Mesoamerican crops were introduced into riparian/wetland ecosystems including maize, Chenopodium quinoa, Cucurbita ssp, Lagenaria sp, Oxalis tuberosa, Canna edulis, Capsicum spp, Phaseolus spp, Solanum spp, Manihot spp, Amaranthus spp, Ipomoea spp, among others (García et al, 2014;Núñez and Grosjean, 2003;Vidal-Elgueta et al, 2019). The exploitation of wild plants, however, did not cease, and the use of byproducts from native species intensified systematically (Núñez and Santoro, 2011).…”

“…Indeed, the agricultural land area over the Pampa del Tamarugal expanded by implementing terraced and flat maize crops, several kilometers of perched and stone-lined irrigation canals were developed over the surface, Prosopis trees agroforestry peaked and new exotic species were introduced (Garcia and Uribe, 2012;Gayo et al, 2012;McRostie et al, 2017). Morphological and genetic evidences indicate that the crop yield of maize was increased through artificial selection of regional varieties to produce large cobs and kernels (Vidal-Elgueta et al, 2019). Nitrogen isotope ratios from local human remains suggest that this process was apparently accompanied by the formation of anthropogenic soils through incipient sediment fertilization with camelid manure and/or seabird guano (Figure 2c, Santana-Sagredo et al, 2015).…”

“…The third population event coincides with a pluvial multidecadal period (Morales et al, 2012) and corresponds to intensified resource production through the Inca Andean territorial expansion (1450-1520 AD) followed by the Spanish colonization (1533 AD), and postcolonial industrial growth (Figure 2c). Aside from reorganizing the socio-political structure, the Inca regime intensified the irrigated-agriculture over the region to produce crop surplus either for paying tribute to the empire or for provisioning armies and workforces involved in mining and construction industries (Núñez et al, 2010;Salazar et al, 2013;Santoro et al, 2010;Troncoso et al, 2016;Uribe and Sanchez, 2016;Vidal-Elgueta et al, 2019;Zori et al, 2017). Since 1450 AD, peoples from the low-elevation Atacama Desert engaged in selecting highly productive maize varieties (e.g.…”

“…Since 1450 AD, peoples from the low-elevation Atacama Desert engaged in selecting highly productive maize varieties (e.g. with large kernel sizes), which represent the nearest predecessors of traditional landraces currently cropped in the Atacama areas (Vidal-Elgueta et al, 2019). This production system also supplied prized domestic camelids used in the Inca Road, which represents the first pan-South American vial network (∼23000 km of roads, bridges, waystations) that connected different subcontinental ecoregions including the Pacific coast, Altiplano, Atacama Desert, central Chile and the upper Amazon basin (Berenguer et al, 2007).…”

El Formativo en Tarapacá (3000-1000 aP): Arqueología, naturaleza y cultura en la Pampa del Tamarugal, Desierto de Atacama, norte de Chile

“…Sin embargo, en contextos funerarios sólo se registraron mazorcas grandes, lo cual sugiere que estos ejemplares fueron seleccionados como ofrendas (Mandakovic 2017). Los análisis discriminantes de corontas y granos del Formativo temprano y del Formativo tardío en adelante muestran diferencias significativas (Vidal-Elgueta et al 2019). Los análisis genéticos de microsatélites de estos ejemplares indican baja diversidad y una heterocigosidad observada menor a la heterocigosidad esperada.…”