Effects of seed morphology and elaiosome chemical composition on attractiveness of five Trillium species to seed-dispersing ants

Morphological and chemical attributes of diaspores in myrmecochorous
plants have been shown to affect seed dispersal by ants, but the relative
importance of these attributes in determining seed attractiveness and
dispersal success is poorly understood. We explored whether differences in
diaspore morphology, elaiosome fatty acids, or elaiosome phytochemical
profiles explain the differential attractiveness of five species in the
genus Trillium to eastern North American forest ants. Species were ranked
from least- to most-attractive based on empirically-derived seed dispersal
probabilities in our study system, and we compared diaspore traits to test
our hypotheses that more attractive species will have larger diaspores,
greater concentrations of elaiosome fatty acids, and distinct elaiosome
phytochemistry compared to the less attractive species. Diaspore length,
width, mass, and elaiosome length were significantly greater in the more
attractive species. Using gas-chromatography mass spectrometry, 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, identified through liquid-chromatography mass
spectrometry, 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 (α -amino acid) and d-glucarate (carbohydrate),
were positively related to seed dispersal probabilities, while others,
including salicylate (salicylic acid) and citrulline (L-α -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.