As we breed crops to grow bigger and faster, they’re less delicious to the bees they depend on

Date:11-05-2018

Bees prefer flowers that offer significant rewards, like high quality pollen. If we expect bees to continue spreading pollen on our farms, they need to get something in return. And CRISPR could help.

Domestication of plants has been a triumph for humanity; shaping crops to fit our needs made farming possible, which in turn laid the groundwork for civilization as we know it. But we’ve also discovered that our history with domestication is something of a pyrrhic victory. As we breed our crops to grow bigger, faster and more palatable than their wild counterparts, we might also be causing other essential traits to erode. We know, for example, that domestication has weakened the chemical defenses of certain crops against pathogens and plant-eating insects. Now there’s evidence that our tinkering may have caused other crops to become less attractive and beneficial to bees.

Blueberries and bees

Paul Egan from the Swedish University of Agricultural Sciences and a team of fellow researchers recently completed a study of blueberries and bumble bees. They had a hunch: Because the chemicals that help a plant defend itself are widely found in both pollen and nectar, the weakening of these compounds might also have an effect on the plant’s connection with pollinators.

When they compared domesticated and wild cultivars of blueberries, they found that several functions were significantly reduced in the floral rewards of the domesticated plants. Specifically, the domesticated cultivars appeared to be less nutritious, less tasty, and lacking some disease-fighting qualities. To verify these results, the team fed bumble bees an artificial diet containing different doses of a natural antimicrobial found in blueberries; only at levels occurring in wild nectar was this compound effective in helping the bees to fight infection by a common gut pathogen.

Domesticated plants may be less nutritious and healthful to pollinators like bees. Credit: Jojan / Wikimedia Commons, October 2018

Bees are critical to the success of our crops. But in reality, the connection flows both ways. Our crops are also critical to the success of bees

According to Egan, this work is the first evidence of trait erosion in plants connected to the well-being of pollinators. Which certainly makes it an ah-ha moment, but is still just a single study.

How widespread might the erosion of pollinator-friendly traits actually be?

“I don’t know other really clear examples,” Egan says. “It is only possible to know what happens to traits that are monitored, which are usually only those in some way related to yield.” Breeding programs are almost always focused on traits like productivity, drought resistance, and disease resistance. The flowers just haven’t been a priority.

“We hope this research helps to put floral rewards on the radar for breeders,” says Egan.

Give and take

Whenever we talk about the connection between bees and our food system, the conversation inevitably flows one way: Bees are critical to the success of our crops. Over 80 percent of the world’s leading food crops depend on pollination by insects and other animals. The value of crops in this country that are directly dependent on insect pollination is estimated to be around $15 billion. So on and so on. But in reality, the connection flows both ways. Our crops are also critical to the success of bees. Or they should be, otherwise bees would have little reason to visit them in the first place. There’s a wealth of research showing that, when given a choice, bees prefer flowers that offer significant rewards, such as high quality pollen. If we expect bees to continue spreading pollen on our farms, then they need to get something in return.

Emily Bailes, a researcher at the School of Biological Sciences at Royal Holloway, University of London, has done some recent work on making faba beans—commonly known as broad beans—more attractive to bees. Specifically, she’s explored how the flowers of these beans might be bred to offer just the right amount of sugar in their nectar to match a bumble bee’s taste. The result, of course, would be better food sources for the bees and improved pollination for the beans.

The trouble with work like this is (and always has been) altering just the one trait you want to change without affecting anything else about the plant. Trait erosion is a perfect example of this problem.

“In some cases it is very difficult to isolate one trait from another,” says Bailes. “Because they are controlled by the same gene, or more often because those genes are located physically close together on the chromosome and therefore are most likely to be selected together.”

Which is why CRISPR—precision gene editing technology—has garnered so much interest. Instead of guiding the natural reproductive process of plants over generations, as we’ve been doing in various ways for centuries, or using a shotgun blast of foreign genetic material like previous methods of genetic engineering, CRISPR is a tool being used to precisely target and edit the genetic sequence of a plant within a single generation. Joyce Van Eck from Cornell University and her colleagues recently used CRISPR to facilitate the domestication of groundcherries, giving this otherwise wild fruit several characteristics that make it much friendlier to farm production and harvesting.

She could definitely see CRISPR being used alongside other breeding approaches to shape plants to be more attractive and beneficial to pollinators.

“To start, you would need the genetic road map—or what is referred to as the genome sequence of a plant species—or at least a very close relative,” says Van Eck. That map is essential to the precision of CRISPR. “You would also need to know some basic information about what makes plant species more attractive to pollinators and identify those genes and their functions.”

This is why wild cultivars, heirloom varieties, and seed banks are still so important to us: their genetic material preserves a range of traits that might be missing in modern crops. They can help us build the genetic road map we need for new priorities in domestication.

Emily Bailes, the researcher from London, agrees with Van Eck. She says that, even though traditional breeding methods could accomplish the same results with the same genetic road map, CRISPR would certainly speed up the process. And CRISPR makes it far less likely that you would unintentionally alter or erode other traits. “As long as your target is specific then you should only be changing the gene you are interested in, so there won’t be off-target effects,” says Bailes.

Priorities and attention

We have a range of tools and the vast collective experience necessary to make our crops more attractive and beneficial to pollinators. Now we just need to do it. So where do we start?

Van Eck’s response is direct and honest: “I don’t know the traits to focus on because I’m not very familiar with genetic control over the cues for pollinators.”

Today’s breeders are already focused on a multitude of different characteristics and teasing out how they’re all connected in a particular plant’s genome, so getting started isn’t as simple as just adding pollinator health to the wish list. It can take years of work by a single team of experts focused on a single trait of a single crop to move things in the right direction. Tying breeding efforts to a dynamic relationship between multiple organisms—even if you’re focused on just one type of blueberry and one type of bumble bee—means managing at least twice as many moving parts. Creating the necessary genetic road maps could take a year or more each.

“To truly make this a priority,” says Paul Egan from Swedish University, “close collaboration is needed between breeders and scientific experts on pollinator health, nutrition, and ecology.” He thinks it might make sense to prioritize pollinator-dependent crops that are crucial to food security in developing countries, and those that contribute the most to alleviating human nutritional deficiencies.

Faba plants grow in small pots on a plot of land. For researchers, determining which traits of a plant to enhance also means being very clear about which types of pollinators we’re hoping will be attracted to it

To start, two traits worth focusing on could be flower color and scent. Those are the first characteristics that attract pollinators to a particular plant. But what about other, less-obvious floral rewards, the kinds that encourage pollinators to visit time and time again? For example, Egan and his team were particularly concerned when they discovered a type of amino acid—a compound essential to stimulating the bumble bee’s sense of taste and getting it interested in the plant—lacking in domesticated blueberries.

Determining which traits to enhance also means being very clear about which types of pollinators we’re hoping will be attracted to and benefit from a particular plant. Obviously, the characteristics that would appeal to a bee versus a bat versus a hummingbird will be quite different. Even among bees, there are important details (some might say, minutiae) to consider. In her work with faba beans, Bailes looked at how much effort was required to open the flowers to get at the tasty stuff inside, and how this characteristic might make it possible (or impossible) for different types of bees to get at the same resources. Dialing in different combinations of all these little traits—characteristics that we might not even fully understand yet—will ultimately make the difference.

“I think it’s important to remember that the world is constantly changing,” says Bailes. “Changes in pollinator numbers and communities might mean that we need to change the flower traits in the future, although the same principles will apply.”

In other words, ensuring that our crops remain attractive and beneficial to pollinators isn’t going to be a one-and-done deal. It’s a moving target that will require our constant attention.