Traditional Management of Maize in the Sierra Sur, Oaxaca, Maintains Moderate Levels of Genetic Diversity and Low Population Differentiation Among Landraces

Abstract

Oaxaca is one of the areas of early maize diversification, with 28 to 35 recognized agronomic races. Campesinos (farmers) cultivate at least ten different races in the Los Loxicha region. There is evidence of introgression between them; however, some traditional practices have promoted further differentiation evidenced by the presence of different landraces, clearly recognized by various morphological and phenological characteristics. Therefore, moderate levels of genetic diversity and low levels of population differentiation are expected. Seven communities belonging to two municipalities were selected in 2005. In total, 140 semistructured interviews were administered, inquiring about cultural practices, which included questions about maize seed, ear, and cob management and selection and maize cultivation. Based on interviews and permissions, 30 maize plants from each of 28 fields (milpas) with maize crop belonging to five landraces (Conejo, Hueso, Tablita, Tepecente, and Pinto) were sampled. From the 82 ISSR loci amplified, Hueso and Conejo exhibited the highest and lowest number of loci (70 and 56 loci, respectively); 72% of loci analyzed in the five populations were polymorphic. The Hueso landrace presented the highest percentage of polymorphism (73%) and Conejo the lowest (54%). The expected heterozygosity (HeT) for the five landraces was 0.225 ± 0.032, and the genetic diversity of Nei (HST) was 0.225 ± 0.034. The population differentiation values obtained with GST indicated that Tablita and Hueso were the least differentiated, while the most differentiated were Pinto and Tepecente. STRUCTURE analysis identified two genetically differentiated groups (k = 2), consistent with the length of their agricultural cycle (number of days until flowering). UPGMA analysis showed that milpas belonging to the same landrace and with a specific agricultural cycle length were grouped, regardless of their geographic distribution. These results indicate that traditional management of maize in the Los Loxicha region, particularly as it relates to the selection of ears and seeds and control of the agricultural cycle, maintains a balance between morphological differentiation, moderate levels of genetic diversity and gene flow, and low levels of population differentiation. This has contributed to the high richness and low evenness of agronomic races reported for the mountainous regions of the Sierra Madre del Sur and for Oaxaca.