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. 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. 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. 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," write 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."
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