Counting Calories: Energy investment in elaiosomes in two species of Datura

Background/Question/Methods

Elaiosomes (nutrient-rich fat bodies) act to entice and reward seed-dispersing ants; they have evolved independently over 80 times. The cost of producing an elaiosome can be high, up to 10% of the total energy invested into seed production and dispersal< ![if !supportAnnotations] >[JB1]< ![endif] > . The effects of resource scarcity and plant life history on the size and energy content of these ecologically critical structures are essentially unknown. To address this gap, two sympatric, elaisome-producing species of Datura (Solanaceae), the perennial D. wrightii and the annual D. discolor, were grown under nutrient-rich and nutrient-poor conditions. The average mass per seed and per elaiosome were measured on ten haphazardly chosen seeds from each matured fruit (~50 fruits). The caloric content of the seeds and elaiosomes was also quantified using bomb calorimetry. As ants are believed to prefer seeds with larger elaiosomes and elaiosome size is generally correlated with seed size, we hypothesized that when plants were grown in high-resource environments, they would produce larger, more calorie-dense seeds and elaiosomes. We also hypothesized that D. discolor, which only has one season in which to reproduce and disperse seeds, would prioritize dispersal structures and produce similarly-sized elaiosomes in both nutrient treatments.

Results/Conclusions

Nutrient availability had no effect on mass or caloric content of seeds of the perennial D. wrightii. However, D. discolor (the annual) grown with fewer resources produced lighter-weight, less energy-rich seeds. Nutrient availability had no effect on the mass or caloric content of elaiosomes in either species, failing to support the hypothesis that when plants have more resources they will produce larger, more calorie-rich elaiosomes. This indicates that resource investment into elaiosome production is not resource-limited and suggests that an increase in this trait might not benefit the plant. Although D. wrightii produced heavier elaiosomes than D. discolor, the ratio of elaiosome mass to seed mass was larger in D. discolor. This suggests that investment into elaiosomes and seed dispersal may be more important to the annual D. discolor. Current work is examining preferences of a local seed-dispersing ant in order to determine the ultimate effect of resource availability on investment into elaiosomes and seed dispersal, and to test whether seeds with larger elaiosomes are more likely to be dispersed.