Ant dispersal of diaspores

Worldwide more than 70 plant families are known, the fruits and seeds (= diaspores) of which are adapted for ant dispersal (myrmecochory) (BEATTIE 1983). The diaspores of these plants possess appendages ("elaiosomes") of various shapes, whose attractiveness for ants was first tested in experimental studies by SERNANDER (1906).His monograph of the Central European ant dispersed plants is still the most comprehensive work in Europe. Various studies demonstrate that elaiosomes contain lipids, proteins, sugar and vitamins and thus constitute a valuable nutritional source for ants. However, only a single Australian study compares the chemical differences in the lipid composition between diaspores and their elaiosomes (HUGHES et al. 1994). The most significant chemical differences in elaiosome and seed composition provide important clues for understanding to which extent ant dispersed diaspores are adapted to the nutritional requirements of their dispersers. This information subsequently plays a key role in understanding the evolution of plant/ant interactions. A study focusing on the following questions could shed light on the evolution of this mode of dispersal: (1) What differences occur in the amino acid, lipid, protein and sugar composition between seeds and elaiosomes of Central European plants? (2) Which chemical components are (a) typical of the elaiosomes as compared to the seeds and (b) do the elaiosomes display significant seasonal differences? (3) What adaptive value do elaiosome components have which are different from the seeds? (4) What is the fate of the diaspores? What proportion of seeds and fruits (a) remain in the nest, (b) are eaten, (c) are destroyed or (d) are discarded on the soil surface again? What role does (a) the removal of the elaiosome and (b) contact with the antifungal substances secreted by ants have on the germination process? The proposed study is undertaken to give some answers in our understanding of the evolution of ant/plant interactions.

Among invertebrates, ants are the major group that disperses seeds and fruits of plants. Most cases of ant dispersal involve diaspores with a variously shaped lipid rich fleshy appendage, called "elaiosome" (elaion = oil, soma = body). Typically, ants take the intact fruit or seed and carry them to their nests, where the elaiosome is removed and eaten without harming the embryo. The fruits or seeds themselves are then usually abandoned, intact and viable either in the nest, or outside the nest on waste piles, or close to the ant midden. Ant-dispersal is known from more than 80 plant species involving over 3000 species worldwide. It is rather common in Europe and eastern North America, but most species occur in the dry heath and sclerophyll forests of Australia and South Africa. It is clear, that the for ants attractive appendage has evolved independently several times. The elaiosome is not only a convergent structure in different plant species and families, but also develops from different parts of seeds and fruits. In our study we investigated the chemical differences between seed and elaiosomes and found, that elaiosomes contain (1) a higher quantity of free amino acids which can be quickly metabolised, (2) a considerably higher percentage of essential fatty acids (oleic, palmitic, palmitoic, stearic acid) than seeds, and (3) have more nitrogen containing amino acids than seeds and are, therefore, a valuable nitrogen source for ants. The plants seem to offer what the ants need for their nutrition. We could further demonstrate, that elaiosomes serve as "baby food" for the ant larvae, which may be very important for the colonies above all in springtime when insect prey and aphid exudates are still rare. This may have been one of the selective forces for the evolution of the high percentage of spring-flowering myrmecochorous species in the European flora.