Myrmecochory in Lepidosperma (Cyperaceae): perianth members provide a lipid-rich reward for ants.

The diaspores of Lepidosperma Labill. (Cyperaceae) have thickened perianth members persistent at the base of the fruit, which are generally assumed to form a unique type of elaiosome but this assumption has not been tested rigorously. We tested whether the perianth provides a lipid-rich food reward and improves diaspore removal by ants in three species of Lepidosperma. We measured (i) the lipid and fatty acid composition of the fruit and perianth of two species of Lepidosperma, and of the seed and aril of two species of Acacia (known myrmecochores), (ii) the relative attractiveness to ants of the perianth in three species of Lepidosperma, and (iii) the relative attractiveness to ants of the diaspores of the same species of Lepidosperma compared with the two species of Acacia. We found that (i) Lepidosperma diaspores show traits consistent with myrmecochory, particularly the high lipid content (~12-60%) and fatty acid profile of perianth material, (ii) isolated perianth members were removed in greater numbers over all times than were diaspores only (i.e. with perianth removed) in two species of Lepidosperma (n = 100, P = 0.035 and 0.047), whereas the difference was equivocal in the third species (n = 100, P > 0.05), and (iii) 0.05), and (iii) Lepidosperma and Acacia diaspores were removed in similar numbers over time (n = 100, P > 0.5). Seven species of diaspore-removing ant were observed, with 0.5). Seven species of diaspore-removing ant were observed, with Rhytidoponera metallica accounting for ~50% of observed removals. We conclude that the perianth members of Lepidosperma are an elaiosome that provides a lipid-rich food reward