Postdoctoral Researcher Clemson University, South Carolina, United States
Background/Question/Methods
The thermal environment exhibits wide variation across the ranges of most taxa, establishing the potential for local adaptation of thermal performance optima and tolerance. However, in the absence of local adaptation of thermal performance metrics, selection should favor thermoregulatory mechanisms to reach thermal optima and remain within tolerance zones. Floral temperature can have strong impacts on reproductive success in flowering plants, yet the degree to which the thermal performance of pollen and ovules varies among natural populations spanning thermal gradients is unknown. We characterized long-term and operative flower-level temperature of the widespread herb, Argentina anserina, at extremes of its elevational range in the Southern Rocky Mountains (low elevation, ~2500m, high elevation, ~3400m). We then measured thermal optima and tolerance breadths for pollen germination, pollen tube growth, and seed set in low and high elevation populations using a growth chamber experiment. If thermal performance of gametes is locally adapted, we predicted that thermal optima should be positively correlated with temperature experienced by flowers, and that tolerance breadth should be positively correlated with temperature variability.
Results/Conclusions
As expected, flowers in high elevation populations experienced cooler long-term and operative temperatures than low elevation populations. However, temperature variability was similar between elevation extremes. Contrary to expectations, the thermal optimum for pollen performance was significantly higher in high elevation populations than low. In fact, thermal optima for pollen performance displayed an anticline with field temperatures. Moreover, high elevation optima deviated more from field temperatures than did low elevation optima. The thermal optima for ovule viability did not differ between low and high elevation, but low elevation populations were more strongly negatively impacted by temperature extremes. These results are inconsistent with local adaptation of gametic thermal performance to local thermal environments, and suggest that thermoregulatory mechanisms should be favored by selection across the elevation gradient. Additionally, selection favoring thermoregulation should be stronger in high elevation populations where gametic thermal optima are more mismatched from with experienced temperature. This work suggests that warming will differentially affect gamete performance in low versus high elevation populations, and sets the stage for examining whether floral thermoregulatory mechanisms are locally adapted to reach gametic thermal optima.
