The evolution of mutualisms is currently poorly understood. In theory, if the benefits to both species are positive, their populations should grow infinitely. In nature, however, this is not observed, but why it isn’t has proven elusive. One potential explanation is that organisms have a finite amount of resources to invest in fitness-enhancing interactions, but often have many such interactions in which to invest. Thus, it seems probable that organisms use some set of priority rules to decide in which interactions to invest. However, what those rules are is unknown. To explore these priority rules, two sympatric species of Datura (Solanaceae), the perennial D. wrightii and the annual D. discolor, were grown under nutrient-rich and nutrient-poor conditions. Both species produce large hawkmoth-pollinated flowers. Their seeds have elaiosomes (fat bodies) that act as food rewards for seed-dispersing ants. Flowers of both species were self- or cross-pollinated by hand. Plant growth (leaf number/size, above/belowground biomass), investments into two reproductive mutualisms, pollination (flower size/number, nectar content) and seed dispersal (elaiosome size/caloric content), as well as seed production, were measured for each plant. We hypothesized that investment would be maintained in fitness-enhancing interactions while investment in other interactions and plant growth would decrease.
Results/Conclusions
In both species, significantly fewer seeds were produced in self-pollinated flowers compared to outcrossed ones, and nutrient-stressed plants compared to controls. Under low nutrient conditions, both species produced significantly fewer and smaller leaves, as well as lower biomass both above- and belowground, compared to high nutrient conditions. Flower size and nectar content were constant in both species regardless of nutrient conditions. However, in both species, significantly fewer flowers were produced when resources were more limited. Elaiosome sizes and caloric content remained the same regardless of nutrient availability. Whether the species was perennial or annual did not appear to impact investment priority. The fact that elaiosome size was constant but flower number decreased when nutrients were limiting may indicate either that dispersal has higher priority or that adaptations for dispersal require lower investment than do adaptations for pollination. Future studies are necessary to test these two alternatives.