Genetic diversity in jocote trees is saved by growing them locally

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, WUSTL researchers have reported that farmers and families in Central America actually have saved genetic variation in the jocote (ho-CO-tay), or Spondias purpurea, a small tree that bears fruit similar to a tiny mango.

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 postdoctoral researcher at the University of Colorado and a former WUSTL graduate student, worked with two faculty members from the Department of Biology in Arts & Sciences: Barbara A. Schaal, Ph.D., the Spencer T. Olin Professor in Arts & Sciences; and Peter H. Raven, Ph.D., the Engelmann Professor of Botany and director of the Missouri Botanical Garden. Also assisting were WUSTL undergraduates Erin O’Mahoney Cubbison and Anna Paschke.

They 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

Modern-day agriculture entails growers selecting hardy plants that grow vigorously and continually “weeding 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 said.

In a paper recently published in Proceedings of the National Academy of Science, 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 2 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 nearly 100 samples of S. purpurea through field studies in Costa Rica, El Salvador, Guatemala, Honduras, Mexico, Nicaragua and Panama. In each of 11 geographic regions, samples were taken from wild and cultivated habitats.

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, 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,” Miller said.

There is considerable variation in the jocote species. The mature fruit can be green, yellow, orange, red or violet, and they can have varying lengths, textures and tastes.

The wild fruits are generally bright red, smaller and more acidic than cultivated varieties. Wild jocotes reproduce by seed, whereas cultivated varieties reproduce through cuttings — indicating that domestication has altered the species.

“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,” Miller said. “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.”