Some of the earliest plants attracted pollinators by producing heat that made these plants glow with infrared light, according to a new set of experiments.
The work, published in the journal Science, suggests that long before brightly colored flowers evolved, these ancient plants would metabolically rev themselves up when they had pollen at the ready. Nocturnal insects such as beetles could then see that heat from afar and home in on the target.
These heat-producing plants, called cycads, exist today in tropical forests around the world, although they're one of the most endangered plant groups.
"Some people call them dinosaur plants because they were much more dominant when the dinosaurs were around," says Wendy Valencia-Montoya, a cycad expert at Harvard University.
Fossils from over 200 million years ago, compared to cycads that exist today, show that "the plants look exactly the same," she says. "So they haven't changed much in hundreds of millions of years."
They're related to pines, and male and female plants each produce fleshy, pine-cone-like structures that contain the pollen and the seeds. "That's something very unique among this group of ancient plants," says Valencia-Montoya, who says these are the oldest known plants that have pollen.
A couple of centuries ago, botanists noticed that these plants produced heat in their reproductive structures. Compared to the ambient air temperature, they can be hotter by 15 to 25 degrees Fahrenheit, or even more.
"We think of producing heat as something that mammals do, or birds do, but in fact, plants can do it too," she says, although it's not common in the plant world and takes a lot of energy.
In these plants, the heat was thought to be a byproduct of metabolic processes. Or, maybe the heat volatilized the scent of the plant, much like plugging an air freshener into an electrical outlet to generate heat that sends scent around a room. Research done in the last couple of decades has shown that male and female cones heat up at slightly different times, which could send pollinators moving from one cone to the other.
To try to understand more about what was going on, Valencia-Montoya and her colleagues painted some pollinating beetles with fluorescent markers to watch when the beetles went to the plant. And they found that the beetles were clearly going to the plant cones when they heated up.
To decouple the heat from other potential signals that these plants produce, like scent and humidity, her team used 3-D printing to make some simulated pollen cones. These fake plant structures were hollow inside, and got filled up with heated sand.
The fake cones glowed in the infrared much like real plants. "It was quite striking, when we saw it in the field," says Valencia-Montoya, who says the fake plants were deployed outside at the Montgomery Botanical Center in Coral Gables, Florida.
Using these fake plants, the researchers figured out a couple of tricks to see if insects were attracted to the infrared glow, rather than the tactile sensation of heat itself. For example, in one experiment, they wrapped the cone with plastic wrap that is transparent to infrared light. Insects could see the infrared light but not get close enough to the cone to actually feel the heat.
What they found is that the infrared glow, by itself, was enough to attract hundreds of pollinating beetles.
And when they looked at those beetles, they found that they have specialized antennae that have evolved to detect slight differences in temperature — similar to the heat-sensing receptors used by snakes to detect prey.
What's more, it turns out that the beetles' antennae seem to be tuned to the exact temperature range deployed by their host plant, since different beetle species frequent different cycad species.
"Infrared radiation is perhaps the oldest discovered pollination signal," says Nicholas Bellono of Harvard University, one of the authors of the new study.
Back when plants first evolved pollen, the insects that were around were nocturnal and had poor vision, says Valencia-Montoya. "So it makes a lot of sense that a signal like heat was guiding them."
But as new groups emerged that were active in the day and had better vision, like butterflies and bees, "it makes more sense for plants to change their signaling strategy to also tap into the sensory systems of these more recent pollinators," she says, adding that in evolution, there's a constant dance back and forth between plants and their pollinators. Once color became a possible signal, flowering plants had an immense range of color combinations at their disposal, allowing them to rapidly diversify.
These researchers used "a powerhouse of techniques" to prove that the temperature of cycad's heat-producing cones is intimately related to attracting pollinators, and that this association is ancient, says Roger Seymour, with the University of Adelaide in Australia, who wasn't part of the research team: "This is an important contribution."
Seymour thinks that heat could have more than one role, however, and the chance for a warm-up may be a bonus for beetles that require high body temperatures for activity. "Heat can be a direct energy reward to insect pollinators which may remain inside a thermogenic flower for much longer than insects visiting non-thermogenic species," he says.
Irene Terry at Tthe University of Utah, an ecologist who specializes in cycads and their insect interactions but wasn't part of this research team, says it's only been relatively recently that people realized these ancient plants even had pollinators, rather than just spreading their pollen with the wind.
Particular odorants seem to be important pollinator attractants, she says, noting that these plants can smell like everything from bubblegum to bell peppers. Still, this study shows "in the most beautiful way" that infrared is also key.
"I was not surprised at all that infrared was involved," says Terry, given that other insects like mosquitoes use infrared to find their way to a target. "There's a range of things that insects can see that we don't."
Imagine, says Bellono, being a prehistoric beetle trying to navigate the darkened environment in the early evening, and seeing various plants start to glow red.
"The infrared is an entirely different world that we don't experience," he says. "I think that's a cool thing to think about, going back in time, that this signal was around when the dinosaurs were there, long before us. And the beetles still use it to this day and are still experiencing that world."
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