ScienceDaily (Nov. 12, 2005) — In a refreshing twist, humans have been shown to be part of the solution to the issue of decreasing genetic diversity in our world rather than part of the problem. Global genetic diversity is being eradicated through any number of human-driven activities, the removal of large scale forests key among them.
Now researchers at Washington University in St. Louis report that farmers and families in Central America actually have saved genetic variation in the jocote (ho-CO-tay), (Spondias purpurea), a small tree that bears fruit similar to a tiny mango. And they've done this by taking the plants out of the forest, their wild habitat, and growing them close to home for family and local consumption.
Allison Miller, Ph.D., a post-doctoral researcher at the University of Colorado, and former graduate student at Washington University, and Spencer T. Olin Professor of Biology Barbara Schaal, Ph.D., from Washington University, in conjunction with Peter Raven, Ph.D. Engelmann Professor of Botany and Director of the Missouri Botanical Garden, have shown multiple domestications of the jocote in Central America in the midst of large-scale deforestation, a practice that endangers genetic diversity.
Weeding out genetic diversity
One effect of modern-day agriculture is the eradication of genetic diversity, as growers select hardy plants that grow vigorously, and continually "weed out" genetic diversity through the selection process.
"Many of the crops are so highly domesticated that they don't have much genetic variation, and we are kind of looking at them after they've been highly domesticated and produced these elite varieties," Schaal explained.
In a paper published in the Proceedings of the National Academy of Science (2005, Aug. 26), Miller identifies the various wild and cultivated jocote species and indicates that cultivation of the jocote has preserved genetic diversity. Genetic diversity has been estimated to have decreased by as much as 80 percent in cultivated populations through the last century, so it's quite a remarkable occurrence when domestication is identified as being a process for preserving genetic diversity, rather than limiting it.
With less than two percent of the Central American tropical dry forests remaining, jocotes would be significantly limited if it were not for the cultivation of the species.
Miller, primary author on the study, collected over 96 samples of S. purpurea through field studies in Costa Rica, El Salvador, Guatemala, Honduras, Mexico, Nicaragua, and Panama. In each of eleven geographic regions, samples were taken from wild and cultivated habitats. Polymerase chair reaction amplification of DNA extracted from the jocote samples allowed for analysis of the chloroplast spacer, a commonly used molecular marker in botanical studies.
The authors say that, through multiple domestications in arenas such as living fences — fences made of plants like jocotes — crops, orchards, trees cultivated in backyards and forests, genetic diversity in the jocote has been preserved.
This is the "first phylogeographic evidence of multiple domestications of a cultivated fruit tree in the Mesoamerican center of domestication," said Miller. With at least 180 common names in various languages for the jocote, the fact that the mature fruits can be green, yellow, orange, red or violet, have varying lengths of a few centimeters, and varying textures (chalky, juicy) and tastes (sweet to acidic), it can be said that there is considerable variation in the species.
The wild fruits are generally bright red, smaller, and more acidic than cultivated varieties. In contrast to cultivated varieties, which reproduce through cuttings, wild jocotes reproduce by seed, indicating that domestication has altered the species.
By taking the jocote out of its natural, wild habitat and planting them in living fences and other means of cultivation, farmers in the Mesoamerican region have helped to preserve the jocote's diversity, the authors note.
"I think it is really amazing to consider that the food we eat today, the foods we find in grocery stores, originated in all different parts of the globe," said Miller. "For me, it is interesting to think that every crop species, including even a little-known fruit tree from Mexico and Central America, has an involved and unique evolutionary history."