CONVERGENCE OF ELAIOSOMES AND INSECT PREY - EVIDENCE FROM ANT FORAGING BEHAVIOR AND FATTY-ACID COMPOSITION

Abstract

1. Elaiosomes are lipid-rich appendages on the seeds of some plant species that promote dispersal of the seeds by ants. In this study we investigated the hypothesis that elaiosomes on seeds have converged in composition and attractiveness with the insect prey of ants. 2. The fatty acid compositions of 12 species of elaiosomes were compared to seeds of the same species and to seven orders of insects. The fatty acid compositions of the elaiosomes were more like those of insects than seeds; the levels of palmitic, palmitoleic, stearic and oleic acids in elaiosomes and insects were particularly similar. 3. The response of six ant species to the diglyceride 1,2-diolein was compared. This lipid is purported to be the principal attractant in elaiosomes and a review of the literature revealed that it is also an important component of insect haemolymph. The ant species tested include different amounts of elaiosomes and insect prey in their diets. 4. Response to pith impregnated with 1,2-diolein was greatest in the three species that are the most active elaiosome collectors, Aphaenogaster longiceps, Rhytidoponera metallica and Pheidole sp. 1. These species all include large amounts of insect material in their diets and two of them, Aphaenogaster longiceps and R. metallica, rarely collect any plant material other than elaiosomes. A fourth species, Iridomyrmex 'anceps', a scavenger which is an occasional elaiosome collector, was attracted to diolein pith but did not distinguish between it and control pith. The two nectar-collecting species that do not normally collect elaiosomes, Polyrhachis sp. 1 and Camponotus consobrinus, did not respond to diolein. 5. The results of this study are consistent with the hypothesis that the chemical composition and the behavioural releaser in elaiosomes have converged with the invertebrate prey of ants. The attraction of carnivorous and omnivorous ant species may be an important adaptive advantage of elaiosomes.