全球蜂群大衰退 科學家揭南瓜蜂族群逆勢成長的秘密

Numbers of bees and other pollinator species have declined worldwide, as these insects suffer due to pesticides and pathogens, likely amplified by climate change. Yet one bee species is showing rapid population expansion. The keys to this insect’s success are its attraction to pumpkins, zucchinis, marrows and gourds, and the massive increase in cultivation of these crops across North America over the last 1,000 years.

A new study, led by scientists at Pennsylvania State University and published April 3 in the journal “Proceedings of the National Academy of Sciences,” found that the specialized squash bee, Eucera pruinosa, has evolved in response to intensifying cultivation of squashes in the genus Curcurbita. Many different cucurbits are consumed around the world, and the majority of the plants are medicinally valuable.

“When we think of insects benefiting from and adapting to widespread agriculture, we tend to think of pests such as certain kinds of moths, flies and beetles,” said Margarita López-Uribe, associate professor of entomology at Penn State. “But the impact of agricultural intensification on the evolution of beneficial pollinators is poorly understood. We found that agriculture facilitated increases in population size of this squash bee, and this may be the case for other insect pollinators, as well.”

The research is the first to demonstrate the role of agriculture as an evolutionary force acting on a wild insect pollinator and may have implications for food security.

Squash bees are found only in the Americas and the greatest number of species is in Mexico. Historically, the squash bee’s primary source of pollen was the wild buffalo gourd, Curcurbita foetidissima, a baseball-sized squash that grew in the deserts of Mexico and the southwestern United States.

As one of the “Three Sisters,” which includes corn and beans, squash was an important crop of Indigenous Peoples in the Americas, and around 5,000 years ago, Indigenous Peoples of the Eastern Woodlands began to domesticate a relative of C. foetidissima, called C. pepo.

Widespread cultivation of the resulting crops – which included pumpkins, squash, and gourds – began later, around 3,000 years ago, and was intensified around 1,000 years ago with the introduction of corn, also called maize, to the agricultural systems of North America.

“By planting squash all over North America, humans created habitat for the squash bee, and that allowed its population to explode,” said López-Uribe. “Today, the squash bee occurs throughout the United States and southeastern Canada – far beyond the range of its original food source.”

To investigate the evolution of the squash bee in response to the intensification of Curcurbita agriculture, the team sequenced the bee’s genome, examined its genetic structure – the amount and distribution of genetic diversity within and among the bee’s various populations – and searched for signatures of adaptation.

Decreases in genetic diversity, explained López-Uribe, can indicate “selective sweeps,” the process by which new beneficial mutations increase in frequency and become fixed.

The researchers found that the bee’s transition from wild host plants in deserts to temperate agricultural habitats was associated with selective sweeps resulting in substantial reductions in genetic diversity in some parts of the genome.

Particularly linked to these selective sweeps were changes in genes associated with chemosensation – the translation of chemical signals from the environment to neurological signals that can be interpreted by an organism. In the case of the squash bee, chemosensation refers to its ability to interpret odor compounds produced by flowers.

“Domesticated Curcurbita plants produce floral blends that are simpler than those of the wild Curcurbita plant,” said López-Uribe. “It is likely that E. pruinosa adapted to a new sensory environment in agricultural habitats, which enabled it to expand its range and significantly increase its population size.”