Morphological and chemical attributes of seeds in myrmecochorous (ant-dispersed) plants have been shown to affect both quantitative and qualitative aspects of seed dispersal effectiveness, but the relative importance of these attributes in determining seed attractiveness and dispersal success is poorly understood. We explored whether differences in diaspore (seed + elaiosome) morphology, elaiosome (seed-coat appendage) fatty acids, or elaiosome phytochemical profiles explain the differential attractiveness of the seeds of five Trillium species to eastern North American forest ants. Trillium species were ranked from least- to most-attractive based on empirically-derived seed dispersal probabilities in our study system, and this comparative framework was used to identify interspecific differences in diaspore morphology and elaiosome chemistry. Laboratory measurements and gas- and liquid-chromatography mass spectrometry were used to test our hypotheses that more attractive species of Trillium would have larger diaspores, greater concentrations of elaiosome fatty acids, and distinct elaiosome phytochemistry compared to the less attractive species.
Results/Conclusions
Diaspore length, width, mass, and elaiosome length were significantly greater in the more attractive species of Trillium than in their less-attractive congeners. We found significantly higher concentrations of oleic, linoleic, hexadecenoic, stearic, palmitoleic, and total fatty acids in elaiosomes of the more attractive species. Multivariate assessments revealed that elaiosome phytochemical profiles were more homogeneous for the more attractive species. Random forest classification models (RFCM) identified several elaiosome phytochemicals that differed significantly among species. Random forest regression models revealed that some of the compounds identified by RFCM, including methylhistidine (-amino acid) and d-glucarate (carbohydrate), were positively related to seed dispersal probabilities, while others, including salicylate (salicylic acid) and citrulline (L--amino acid), were negatively related. These results supported our hypotheses that the more attractive species of Trillium—which are geographically widespread compared to their less-attractive, endemic congeners—are characterized by larger diaspores, greater concentrations of fatty acids, and distinct elaiosome phytochemistry. Further advances in our understanding of seed dispersal effectiveness in myrmecochorous systems will benefit from a portrayal of dispersal-unit chemical and physical traits, and their combined responses to selection pressures.