Lima beans were domesticated at least twice, according to a new genetic diversity study by Colombian scientists. Big seeded varieties known as "Big Lima" were domesticated in the Andean Mountains, while small seeded "Sieva" and "Potato" varieties originated in central-western Mexico.The researchers also discovered a "founder effect," which is a severe reduction in genetic diversity due to domestication. This means that today's Lima bean varieties contain only a small fraction of the genetic diversity present in their respective wild ancestors.
They study was conducted by a team of Colombian scientists at the Universidad Nacional de Colombia-UNAL, Universidad Industrial de Santander-UIS and the International Center for Tropical Agriculture-CIAT. The results are reported in the September-October 2010 edition of Crop Science, published by the Crop Science Society of America.
The Andean domestication likely occurred in southern Ecuador-northwestern Peru, whereas the second domestication, in central Mexico, likely occurred somewhere north and northwest of the Isthmus of Tehuantepec.
The scientific team analyzed DNA sequences in a sample of wild and cultivated varieties of Lima bean from Mesoamerica and the Andes. They applied a population genetics and phylogeographical approach, which allowed them to identify unique genetic markers that distinguished groups both in geographic space and genetic similarity.
Initially, the team carried out a pilot study to identify genes with enough variation to be useful for domestication studies in Lima bean. They chose two non-coding segments of chloroplast DNA, the photosynthesizing parts of the cell, and non-functional ribosomal DNA segments to carry out the comparative analysis.
These types of DNA segments are often used in genetic analysis, because they often show a great deal of difference between species, because since they don't code for any essential life-processes, they mutate regularly.
"These findings call our attention over the conservation not only of [domesticated populations] but more importantly of wild populations of Lima bean in order to preserve the genetic diversity of the species," said Maria Chacon, one of the researchers from Universidad Nacional de Colombia.
Lima bean is a crop widely distributed in Mesoamerica and the Andes, where several human groups have used it as food resource since pre-Columbian times. In spite of that, scientists have yet to describe its evolutionary history.
Genetic studies, mainly those based on molecular techniques, have proved useful in unraveling the domestication history of many of our major crops such as maize and in pinpointing where early humans made the transition from hunting-gathering to agriculture. Additionally, genetic studies may help measure the effect of domestication activities over the genetic diversity of crops, which may have implications for conservation and improvement efforts.
Research is ongoing to establish where within Mexico one group of Lima beans has been brought into cultivation. The study was funded by La Fundación para la Promoción de la Investigación y la Tecnología of the Banco de la República in Colombia.
- Motta-Aldana et al. Multiple Origins of Lima Bean Landraces in the Americas: Evidence from Chloroplast and Nuclear DNA Polymorphisms. Crop Science, 2010; 50 (5): 1773 DOI: 10.2135/cropsci2009.12.0706
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