Tiny Bees Loom Large In Tree Reproduction, Texas Scientists Say

AUSTIN, TX — Long-distance love affairs can be tough, especially for trees. But University of Texas at Austin scientists have found that tiny bees play a much larger role in keeping the flames of arbor romance alive than previously believed.

The size of ladybugs, those tiny bees play a critical role in what's essentially sex between plants. In a finding that might make Maury Povich proud, UT-Austin researchers discovered their key role following one of the most detailed paternity tests in wild trees ever conducted, university officials said Monday.

The research yields new insight into how the pollinating insects promote genetic diversity essential for plants' adaptation in the face of disease, climate change and other threats relevant for agriculture and reforestation efforts worldwide, officials said.

In the scientists study published this week in the Proceedings of the National Academy of Sciences, researchers identified pathways that hundreds of wild bees traveled to and from the trees they pollinated across a forest area of more than 2.5 square miles, officials explained.

To biologists' great surprise, the smallest bees managed to cover distances just as far as their larger pollinating cousins, frequently acting as tiny matchmakers for trees separated by more than a mile, scientists acknowledged. Moreover, in half of the cases where a pair of trees separated by a significant distance created a fruit together, the pollen had been carried by a bee no bigger than a grain of rice, scientists found.

"Size isn't everything," said Shalene Jha, an associate professor of integrative biology and the primary investigator on the study. "These little bees are responsible for major beneficial impacts in terms of reproduction and gene flow."

The mapping of trees, bees and genetic markers took roughly four years and spanned three sections of a tropical rainforest near the Panama Canal, scientists explained. The area of study was 10 times the size of most previous research projects attempting to map pollen movement mediated by bees, researchers noted.

"If you work in a small portion of forest, you're only capable of measuring pollen movement in a small area," Jha said. "We picked up the signal about how far these little bees move because we started doing work that was commensurate with the scale at which they're actually flying."

More than 85 percent of plants, including nearly two-thirds of food crops, rely on bees and other pollinating insects for sexual reproduction. UT-Austin scientists noted that most studies of pollinating insects have focused primarily on larger bees, which carry the most pollen and, thus, help plants produce the most fruit and seeds.

But consider this: There are tens of thousands of bee species. Jha and her team wanted to understand more about the function of smaller bees among the dizzying array of species. Among their discoveries, UT-Austin scientists found that tiny bees pay more visits to flowers than their larger counterparts, and they cover remarkable distances as they pollinate.

Stated another way to illustrate their pollinating prowess: Bees as small as a few millimeters move pollen across areas more than a million times their body size, scientists learned. To put that in human perspective, an average adult person would need to travel from spots about as far apart as Los Angeles and Chicago to cover an equivalent distance.

Because these long-distance matches help prevent inbreeding among closely related plants, the far-flying bees aid trees in having a thriving new generation of offspring to replace them, scientists said.

"We haven't traditionally asked how pollinators influence the way genes are passed on across generations, but it's incredibly important for long-term population persistence," Jha said. "We typically look at seeds or the number of seeds when we want to know a bee's role in pollination service, but not 'who are the fathers of these seeds?' "

But really, is lineage information important to know? Are such paternity tests really necessary? In a word, yes. Jha said that knowing the parents of any given seed or plant is essential. Farmers and foresters have long used this sort of information to plan not only for quantity in output but also quality — juicier or larger fruits in a crop, for example, or faster-growing trees in a forest.

That knowledge is especially critical among biodiversity hot spots, such as Panama where Miconia trees play a central role in the nation's rainforests. In the past half-century alone, deforestation has reduced the proportion of forest in the country by almost 30 percent, causing erosion and other challenges in an economically vital watershed surrounding the Panama Canal.

Jha believes the new study sheds light both on how pollinators of all sizes can help to promote plant reproduction and reforestation in places like Panama, and how small pollinators help plants in ecosystems worldwide, she said.

Bee species in most places, including the U.S. tend to vary in size and in pollinating behaviors in ways that are similar to the species studied in Panama, Jha noted. Because smaller bees have often shown more resilience than their larger cousins to threats such as habitat destruction, it's helpful to know their potential as pollen dispersers in the face of other bees' decline, she added.

Postdoctoral researcher Antonio Castilla was first author on the paper with Jha. The study's co-authors were UT Austin graduate students Nathaniel Pope and Megan O'Connell, UT Austin undergraduate Maria Rodriguez, Jha lab outreach coordinator Laurel Trevino, and the University of Panama's Alonso Santos.

Partial funding support was provided by the National Science Foundation.

>>> Image: A stingless bee visits a Miconia tree near Soberania National Park, Panama. Researchers from The University of Texas at Austin spent nearly four years mapping trees, bees, and pollen to reveal how different pollinators aid in the sexual reproduction of trees in one of the most detailed pollinator-mediated paternity tests in wild plants. Antonio Castilla/UT-Austin.