Honeybee queens vulnerable to heat-related fertility loss

Excessive heat can kill sperm cells in many animal species, but there are few ways to gauge the impact of heat on pollinators like honeybees.

Now, research conducted at the University of British Columbia has found five proteins that are activated when queen honeybees are exposed to temperatures characteristic of climate change.

These proteins not only serve as indicators of bee fertility but also as a bellwether for the survival of insect species in the wider landscape where, in some regions, populations are in sharp decline.

The research team used mass spectrometry to study queen bees under simulated heat-stressed and non-heat stressed conditions. Understanding the impact of excessive heat to the health of queen bees is critical since, if the sperm stored by queen bees in the spermatheca is damaged, the queens may not have enough live sperm to produce drones and workers essential for the colony.

“Mass spectrometry is a technique that lets us take a snapshot of all the different proteins present in a cell or tissue sample at any given time,” said Alison McAfee, biochemist at the Michael Smith Laboratories at UBC, and post-doctoral fellow at North Carolina State University. “It tells us which proteins are present and how many of them there are. So, by comparing heat-stressed spermatheca to non-heat-stressed spermatheca, I can see which proteins become activated.”

McAfee said that all five of the proteins activated were heat-shock proteins, a family of proteins well known for their roles in stress response.

“But it gets more interesting,” said McAfee. “There are dozens of different heat-shock proteins, some of which use up ATP (energy carrying molecules) and some of which don’t. Only the kind that don’t are the ones that are activated in the spermatheca, which makes sense because (previous) research has shown that this is an energy-limited environment. Other tissues in the queens activated different kinds of heat-shock proteins, so this particular signature seems to be unique to the spermatheca.”

The research team needed to know how much heat the queen bee could tolerate before the sperm it has stored is threatened and the colony is put at risk.

“Queens don’t recover from heat stress,” said McAfee. “We are talking about sperm cells dying from excessive heat. There’s no coming back from that. Since queens only have one mating period early in life, if their stored sperm starts to die, that’s a permanent change in her reproductive quality.”

The data showed that temperatures between 15 C and 38 C were safe for the queens, but above 38 C, the percentage of live sperm dropped to or below the level they would see in failed queens.

Honeybees are often shipped to various agricultural locations for pollination services, so researchers placed temperature loggers in seven domestic queen shipments via ground and one by air. One container recorded a temperature of 38 C, while another dropped to 4 C. The data provided shipment guidelines for buyers and sellers moving quality, healthy bees.

Other studies looked at heat regulation inside colonies.

“Colonies don’t exactly trap heat,” said McAfee. “The worker bees bring in water and fan it to cool down the hive through evaporation. But under extreme conditions, like during a heat wave in California, that ability to thermoregulate does start to break down. We saw temperatures spike above 40 C inside hives, and one study in the Middle East linked extreme heat to colony losses.”

The California test site in El Centro consisted of three hives where the ambient temperature at ground level reached close to 45 C in the shade. Temperatures in the hives spiked in excess of 38 C and 40 C.

“In extreme heat, foragers will gather more and more water and bring this back to the hive to evaporate and try to cool everything down,” she said. “But, as our hive temperature data showed, even this isn’t 100 percent effective in extreme heat.”

McAfee said climate data shows the threat of increased frequency, severity, and duration of heat waves both now and in the future.

“The more we pay attention, the more I think we will realize that climate change is doing more to wildlife than changing habitable longitudes and elevations. In extreme conditions, like severe heat waves, insects could be partially sterilized, or die. I think heat stress could turn into an underappreciated driver for declining species abundance as climate change worsens. Terrestrial insect populations are already declining, and climate change will probably only make that worse.”

McAfee says they plan to expand their work to include markers for cold stress and pesticide exposure.

In addition, future research will include the use of honeybee protein signatures to monitor how climate change may reduce insects’ abilities to reproduce. If patterns of heat shock start emerging among honeybees, they could be indicators of what is happening to other insect species.

“We think honeybees can help us track how climate change is making it harder for insects to reproduce,” she said. “We are looking for signs of heat stress in queens as an indicator of what’s going on in the environment.”

The study was published in Nature Sustainability.

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