Genetic Resources in the “Calabaza Pipiana” Squash (Cucurbita argyrosperma) in Mexico: Genetic Diversity, Genetic Differentiation and Distribution Models

Abstract

Analyses of genetic variation allow understanding the origin, diversification and genetic resources of cultivated plants. Domesticated taxa and their wild relatives are ideal systems for studying genetic processes of plant domestication and their joint is important to evaluate the distribution of their genetic resources. Such is the case of the domesticated subspecies C. argyrosperma ssp. argyrosperma, known in Mexico as calabaza pipiana, and its wild relative C. argyrosperma ssp. sororia. The main aim of this study was to use molecular data (microsatellites) to assess the levels of genetic variation and genetic differentiation within and among populations of domesticated argyrosperma across its distribution in Mexico in comparison to its wild relative, sororia, and to identify environmental suitability in previously proposed centers of domestication. We analyzed nine unlinked nuclear microsatellite loci to assess levels of diversity and distribution of genetic variation within and among populations in 440 individuals from 19 populations of cultivated landraces of argyrosperma and from six wild populations of sororia, in order to conduct a first systematic analysis of their genetic resources. We also used species distribution models (SDMs) for sororia to identify changes in this wild subspecies’ distribution from the Holocene (∼6,000 years ago) to the present, and to assess the presence of suitable environmental conditions in previously proposed domestication sites. Genetic variation was similar among subspecies (HE = 0.428 in sororia, and HE = 0.410 in argyrosperma). Nine argyrosperma populations showed significant levels of inbreeding. Both subspecies are well differentiated, and genetic differentiation (FST) among populations within each subspecies ranged from 0.152 to 0.652. Within argyrosperma we found three genetic groups (Northern Mexico, Yucatan Peninsula, including Michoacan and Veracruz, and Pacific coast plus Durango). We detected low levels of gene flow among populations at a regional scale (<0.01), except for the Yucatan Peninsula, and the northern portion of the Pacific Coast. Our analyses suggested that the Isthmus of Tehuantepec is an effective barrier isolating southern populations. Our SDM results indicate that environmental characteristics in the Balsas-Jalisco region, a potential center of domestication, were suitable for the presence of sororia during the Holocene.

Phylogeographic and population genetic analyses ofCucurbita moschatareveal divergence of two mitochondrial lineages linked to an elevational gradient

“…argyrosperma and C. argyrosperma ssp. sororia ( H E = 0.41 and H E = 0.43, respectively; Sánchez‐de la Vega et al., ), but slightly lower than values reported for other annual plants in microsatellite studies (i.e., H E = 0.46; Nybom, ) and for other squash crops, for instance C. pepo ssp. pepo ( H E = 0.500–0.572; Enríquez et al., ; Castellanos‐Morales et al., ), C. maxima ( H E = 0.56; Kaźmińska et al., ), and hybrid Cucurbita rootstocks ( H E = 0.46; Kong et al., ).…”

“…pepo : A = 3.43–3.8 [Gong et al., , ; Formisano et al., ; Enríquez et al., ; Castellanos‐Morales et al., ]; C. argyrosperma ssp. argyrosperma : A = 2.7 [Sánchez‐de la Vega et al., ]; a hybrid Cucurbita rootstock [ C. moschata × C. maxima ]: A = 2.2 [Kong et al., ]). Total genetic variation of C. moschata ( H E = 0.395) was similar to values reported for sister taxa C. argyrosperma ssp.…”

“…In addition, seed exchange and fruit selection within and among localities might also affect levels of nuclear genetic variation in C. moschata , as has been reported for C. argyrosperma ssp. argyrosperma (Sánchez‐de la Vega et al., ).…”

“…To define whether the distribution of genetic variance is better explained by geography or elevation for both molecular markers, we used Arlequin to perform an analysis of molecular variance (AMOVA). We tested two hypotheses: (1) test for genetic differentiation by elevation (highland and lowland); and (2) test for genetic differentiation by geography, considering the Isthmus of Tehuantepec as a geographic barrier as has been described for the closely related C. argyrosperma (Sánchez‐de la Vega et al., ).…”

“…), and squashes ( Cucurbita spp. )—have been associated with differences among human cultural management (Casas et al., ; Blancas et al., ; Sánchez‐de la Vega et al., ) and to fruit or seed selection, with favored traits resulting in a particular morphology (Aguirre‐Dugua et al., ; Blancas et al., ). In addition, patterns of genetic structure have been attributed to ecological factors such as elevation gradients in the case of barley and maize (Tanto et al., ; Van Heerwaarden et al., ).…”