Understanding the factors maintaining species endemism is a fundamental question in ecology and is critical to the success of conservation efforts. In southern Appalachia, a global hotspot for myrmecochorous (ant-dispersed) trilliums, several Trillium species are range-restricted endemics that co-occur with more widespread congeners. Previous work has noted that endemic Trillium species have lower seed dispersal effectiveness than widespread trilliums. Field observations revealed significantly lower rates of seed removal by ants for endemic trilliums, and choice experiments in the laboratory confirmed that Aphaenogaster ants, the genus of keystone seed dispersing ants in eastern North America, display a significant preference for seeds of widespread trilliums. Here, we investigate the chemical and morphological mechanisms driving Aphaenogaster preference for widespread trilliums by comparing the chemistry of elaiosomes (ant-attracting seed coat appendages) and the morphology of diaspores (the seed-elaiosome complex) for geographically widespread and narrowly endemic Trillium species in the laboratory. We broadly characterized elaiosome chemical profiles using liquid- chromatography mass spectrometry, and we quantitatively compared concentrations of key and total fatty acids for endemic and widespread trilliums using gas-chromatography mass spectrometry.
Results/Conclusions
Results indicate that there are no consistent morphological differences between the seeds of widespread and endemic Trillium species that would explain Aphaenogaster preference. However, the general elaiosome chemical profiles of widespread trilliums are more tightly clustered in multivariate space than those of endemic species. Widespread species have significantly higher total free fatty acids [X2(1)=4.6, P=0.03], total fatty acids in diglyceride fractions [X2(1)=4.7, P=0.03], and total fatty acids in triglyceride fractions [X2(1)=4.7, P=0.03] than endemics. The elaiosomes of widespread species also have significantly higher concentrations of specific fatty acids within fractions than endemics, in particular, the important ant-signaling compounds oleic acid (averaged across fractions [X2 (1)=17.3, P<0.01]) and linoleic acid (in diglyceride fractions [X2 (1)=8.3, P<0.01]). Our results may explain ant preference for widespread Trillium species, and potentially signify dispersal limitation for endemics. This suggests that endemic Trillium species might benefit from conservation efforts aimed at overcoming dispersal limitation.