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June 7, 2017

Male squash bee (Peponapis pruinosa)

A male squash bee (Peponapis pruinosa) gathers nectar or pollen from pumpkin flowers.

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Researchers at North Carolina State University (NC State), using genetic markers, have for the first time shown how cultivating a specific crop can lead to the expansion of a pollinator species. Specifically, the researchers found that the spread of a bee species in pre-Columbian Central and North America was tied to the spread of Native American agriculture.

"We wanted to understand what happens when the range of a bee expands," says Margarita López-Uribe, a postdoctoral researcher at NC State and lead author of a paper describing the work. "What does that mean for its genetic variability? And if the genetic variability declines, does that harm the viability of the species?"

For the study, researchers examined the squash bee (Peponapis pruinosa), a bee indigenous to what is now central Mexico and the southwestern U.S. and specialists at collecting pollen solely from the flowers of plants in the genus Cucurbita, such as squash, zucchini and pumpkins.

Before contact with Europeans, Native American peoples had begun cultivating Cucurbita crops and over time, these agricultural practices spread to the north and east. The researchers sought to learn whether P. pruinosa spread along with those crops.

The researchers looked at DNA from individual squash bees, collected throughout their range, which now runs from southern Mexico to California and Idaho in the west, and from Georgia in the southeast to Quebec in the north. They assessed genetic markers in each bee’s DNA and identified genetic signatures associated with when and where the species expanded. The researchers found that genetic diversity decreased depending on how "new" the species was to a given territory. For example, genetic diversity of squash bees in Mexico was greater than the diversity in the Midwest; and diversity in the Midwest was greater than that of populations on the East Coast.

Given the declining genetic variability, the researchers expected to see adverse effects in the "newer" populations of P. pruinosa; however, they did not.

López-Uribe says they were "specifically expecting to see an increased rate of sterile males in populations with less genetic variability; however, they did not find this. But, says López-Uribe, "We did find genetic 'bottlenecks' in all of the populations -- even in Mexico."

"Because P. pruinosa makes its nests in the ground near squash plants, we think modern farming practices -- such as mechanically tilling the soil -- is causing the species to die out in local areas," López-Uribe says. "And we think that is causing these more recent genetic bottlenecks."

López-Uribe hopes to work on this question in the near future, "because it’s important to helping understand the relevant bee’s population dynamics in modern agricultural systems, as well as what it may mean for Cucurbita crops."

This research was supported by the National Science Foundation (grants DEB 0814544 and DEB 0742998).

To learn more about this research, see the NC State news story How Native American agriculture spread bees in Pre-Columbian North America. (Date image taken: June 2015; date originally posted to NSF Multimedia Gallery: June 6, 2017)

Credit: Elsa Youngsteadt

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