Genetic marker tells squash domestication story

January 07, 2002

In the January 8 Proceedings of the National Academy of Sciences, researchers from the Smithsonian Tropical Research Institute (STRI), The Cucurbit Network and the University of Puerto Rico establish mitochondrial DNA analysis as a powerful tool for understanding relationships among flowering plants. A comparison of mtDNA from cultivated squash, pumpkins, gourds and their wild ancestors strongly supports hypotheses based on archeological and ethnobotanical evidence for six, independent domestication events in the New World. Even Oris Sanjur, who conducted the genetic analysis was "surprised by the resolution" offered by the nad1 gene as a genetic marker.

As excellent sources of edible, protein-rich seeds, members of the genus Cucurbita were among the first plants to be domesticated in the New World. As one of the three sisters (corn, beans and squash) cropping system, squash is prominent in the creation stories of indigenous Americans. In the Iroquois version, corn sprouts from the chest of Sky Woman, squash springs from her belly and beans grow from her hand.

For the last two decades, scientists have pushed to fill in many gaps in the history of agriculture in the humid tropics where archaeological evidence is extremely hard to come by. Last year, Dolores Piperno, archaeologist and staff scientist at the Smithsonian Tropical Research Institute in Panama and colleagues reported the first use of Cucurbita in the Americas at the end of the last ice age (10,000 years ago) based on the presence of tiny plant remains called phytoliths at a site in Ecuador.

Whereas archaeological plant remains tell us that cultivated plants were present at a given time and place, mtDNA sequence data showing similarities among a domesticate and a specific wild population can unambiguously identify the geographic region where prehistoric gardeners first planted the wild ancestors in their gardens, a "promising breakthrough" according to Kenneth Olsen, plant population geneticist at North Carolina State University, who has studied the origins of manioc by sequencing nuclear genes. The silver-seeded squash found in U.S. markets, according to this analysis, may have been domesticated in the area of Mexico where indigenous Americans first domesticated corn. Olsen finds the paper's identification of two separate domestications of Cucurbita pepo, "particularly intriguing" as the relationships of wild and domesticated C. pepo populations have long been a source of contention. The new mtDNA evidence also shows that northeastern Mexico should be seriously considered as a possible region of domestication for one of the lineages of C. pepo squash, which was formerly thought to have originated in eastern North America.

Mitochondrial DNA genetic markers, widely used to gain insight into the relationships among animal species, will surely continue to add to the story of the evolution of crop plants and their wild relatives.

Copies of the article are now available to reporters from the PNAS news office, tel. (202) 334-2138, or e-mail pnasnews@nas.edu .
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The Smithsonian Tropical Research Institute (STRI), with headquarters in Panama City, Panama, is one of the world's leading centers for basic research on the ecology, behavior and evolution of tropical organisms. http://www.stri.org

Smithsonian Tropical Research Institute

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