Subtle ecophysiological divergences in a deceptive strategy to attract pollinators in two sympatric Arum

Most Angiosperms rely on insect pollinators for their reproduction, implying the evolution of attractive signals. While most plants reward their pollinators, others use a deceptive method where no direct benefit is offered to pollinators. Integrative approaches are mandatory to comprehend the evolution of deceptive strategies, by combining the co-variations in different floral traits and the behavior of the insects. However, they were rarely applied to compare species competing for the same pollinators. We applied an ecophysiological approach to compare floral traits – scent and thermogenesis – in two deceptive sympatric arums (Arum maculatum and Arum italicum) that are pollinated mainly by Psychoda moth flies. We tested the hypothesis that floral traits diverged between the two arums to minimize their interference as they attract locally the same insects. We measured the thermogenesis sequence of both species in their natural environment and volatile organic compounds were analyzed. As expected, the two arums displayed a similar thermogenesis sequence. However, the main thermogenesis peak, which is actively involved in pollinator attraction was warmer in A. italicum and it happened earlier during the day for A. maculatum. For scents, we confirmed a strong differentiation between the two arums when they co-occur. Then, we determined the effect of the different scents on the activity level of their main moth fly pollinator (Psychoda sigma) at our study site to reveal that the odor of A. maculatum tended to stimulate the insect activity more than the odor of A. italicum. This moth fly is nocturnal and therefore A. maculatum, which flowers right before nighttime may need to overstimulate the insect activity to get visited. Our integrative approach highlights that the two sympatric arums have slightly shifted daily phenologies, which implied a fine-tune variation in the attractive volatiles and the thermogenesis sequence in relation to the behavior of the pollinating moth fly. Our results suggest that a tradeoff may exist between odor specificity and the ability to maximize volatile emission. These findings show how evolutionarily close species living in sympatry can diverge finely to lower competing interference and attract the same pollinators.