Search

Phaseolus lunatusL., Lima bean, has been domesticated at least twice in the Americas, once in the Andean region and at another time in Mesoamerica; however, the domestication history of this crop in the latter region remains unclear. In this study, a phylogeographic analysis of DNA polymorphisms in the internal transcribed spacer (ITS) region of the ribosomal DNA from a collection of wild and domesticated accessions was applied to establish where and how many times in Mesoamerica Lima bean was domesticated. The results showed evidence for two wild Mesoamerican gene pools with contrasting geographical distributions. While the MI gene pool occurs in central western Mexico, including the Pacific coastal range, the MII gene pool is widespread and occurs toward the Gulf of Mexico, the Yucatan peninsula, and Central and South America. In a cluster analysis, all Mesoamerican landraces clustered together with wild accessions from the MI gene pool (L haplotype) suggesting a unique domestication event in central western Mexico. The most likely domestication region is an area of the states of Nayarit–Jalisco or Guerrero–Oaxaca and not areas such as the Peninsula of Yucatan where the crop is currently widespread and diverse. A strong founder effect due to domestication was quantified and several recently diversified haplotypes were identified. A hypothesis about possible dispersal routes of the crop within Mesoamerica is proposed as well as an apparent late adoption of the crop into the milpasystem.

The role of gene flow in autogamous domesticated species diversity and their wild relatives is an issue that requires more field data. Using nine microsatellite loci, an analysis was done of the magnitude and direction of gene flow in the wild–weedy–domesticated complex of Phaseolus lunatusL. under traditional agricultural conditions in four regions on the Yucatan Peninsula, Mexico, its center of domestication and diversity in Mesoamerica. Two complementary methods were used. The Bayesian genotype assignment approach showed that recent gene flow was low at both the intraregional and interregional levels. The same was found with the frequency method for long‐term gene flow (Nmintraregional from 0.31 to 0.51, and Nminterregional = 0.44). In addition, the gene flow from domesticate to wild populations was three times higher than in the opposite direction. This asymmetry can be explained by regional agricultural practices and seed selection criteria. Domesticate alleles were shown to be entering wild populations of different agricultural regions, suggesting exchange of domesticated seed between farmers of different regions. These results are very important because they show that P. lunatuson the Yucatan Peninsula has a predominantly domesticate to wild gene flow. This situation may lead to genetic assimilation of wild lima bean by its domesticated counterpart and may lead to the possible escape of transgenes in this center of origin and diversity.

This study was conducted to determine the genetic diversity, structure and gene flow of 11 wild populations of Phaseolus lunatusL. in four regions of traditional agriculture in the Yucatan Peninsula, Mexico, part of the putative domestication area of its Mesoamerican gene pool. Analyzing eight microsatellite loci, the populations showed high values of diversity: observed heterozygosity (Ho) 0.46 to 0.9; Nei's index of diversity (H) 0.35 to 0.59 and average number of alleles per locus (A) 2.37 to 3.38. Both Nei's index of populations differentiation (Gst) and analysis of molecular variance (AMOVA) indicated strong differentiation. The Bayesian analysis of grouping and the Mantel test suggested isolation among agricultural regions as a major factor for population differentiation. Even though a low long‐term gene flow (Nm = 0.66) and low rates of recent migration among populations were observed, there were some cases where the accidental transport of seeds could be favoring a gene flow at a long distance. Data found in this study suggest a positive correlation between agricultural intensification and increase in diversity, suggesting that wild populations are favored by the intensification of disturbance in situations involving at least 3 yr of fallow. However, the opposite could be true at higher levels of intensification as has been reported in the Central Valley of Costa Rica, where the diversity is diminishing.