Heat signaling from plants is an ancient pollinator signal

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Long before flowers dazzled pollinators with brilliant colors and sweet scents, ancient plants used another feature to signal insects: heat. The findings, based on an analysis of the biology and relationship between modern cycad plants and the rare beetle species that pollinate them, offer new insights into what shaped the earliest eras of plant-animal co-evolution.

Thermal image of a male cone of the cycad Zamia furfuracea during pollen release. Image credit: Photo by Wendy Valencia-Montoya

Plants have evolved a remarkable array of strategies to attract pollinators, including not only color and scent, but also the production of heat. Thermogenic plants generate heat through intense cellular respiration. It’s thought that in some cases, this heat, via infrared radiation, may serve as a direct signal to pollinating insects. However, the ecological and functional role of plant thermogenesis remains speculative.

Beetles of the species Rhopalotria furfuracea on a male cone of the cycad Zamia furfuracea. Image credit: Photo by Michael Calonje

Cycads, the oldest lineage of animal-pollinated seed plants, account for over half of all thermogenic species and rely on specialized beetle pollinators. Fossil evidence indicates that cycad-beetle interactions date back at least 200 million years, making them an ideal system to investigate whether the production of thermal infrared radiation functions as a sensory cue for pollinators and to explore early plant-pollinator evolution.

Wendy Valencia-Montoya and colleagues used a suite of methods, combining field observations from across the Americas with molecular biology, electrophysiology, protein structural studies, and controlled behavioral experiments, to understand cycad thermogenesis and how it relates to beetle pollinators.

Valencia-Montoya et al. found that mitochondrial adaptation and circadian genes drive rhythmic heat production in the plant’s reproductive structures, causing cycads to emit a single daily burst of heat production starting in the afternoon and peaking in the early evening. This infrared radiation alone is sufficient to attract beetle pollinators.

Nicholas Bellono, Wendy Valencia Montoya, and Naomi Pierce. Image credit: Veasey Conway/Harvard Staff Photographer

The authors also show that pollinator beetles have specialized infrared-sensing organs in their antennae, which contain extremely thermosensitive receptors whose structural variants across species align with the specific thermal output of the plants they pollinate. This suggests co-evolution between plant thermogenesis and beetle sensory systems.

Evolutionary comparisons further show that infrared signaling predates the rise of widespread color-based pollination cues. “Infrared is most easily detectable at night, largely limiting cycads to pollination by night-flying beetles,” wrote Beverly Glover and Alex Webb in a related Perspective.

“Perhaps by evolving a signal only detectable by a single receptor carried by a nocturnal insect group, the insect-pollinated cycads limited their speciation opportunities – the moon dance between cycads and beetles may have destined the cycads for limited evolutionary radiation.”

Citation
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The study Infrared radiation is an ancient pollination signal was published on Science journal. Authors: Wendy A. Valencia-Montoya, Marjorie A. Liénard, Neil Rosser, Michael Calonje, Shayla Salzman, Cheng-Chia Tsai, Nanfang Yu, John R. Carlson, Rodrigo Cogni, Naomi E. Pierce, and Nicholas W. Bellono.

Funding
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This research was funded in part by the USA National Institutes of Health.
You may read the article First, male gets heated up, then female, and then, you know, written by Kermit Pattison and published on The Harvard Gazette