Dispersal by ants in Centaurea corymbosa (Asteraceae):: What is the elaiosome for?

This paper reports on different observations and experiments examining the role of ants in the seed dispersal of the rare and endemic species Centaurea corymbosa. Direct observations of seed manipulation by ants in natural populations showed that dispersal distances are very limited, and many ants did not disperse seeds at all. Cafeteria experiments failed to demonstrate: (i) that the elaiosome represents the attractive part of the achene; and (ii) that seeds of Euphorbia characias, a common species occurring in the same habitat, are more attractive than achenes of C. corymbosa despite a 10-fold difference in elaiosome size. However, ants show a foraging behavior to collect seeds of C. corymbosa because seed removal is greater at the foot of a fruiting plant than further away. Experiments in controlled conditions were also carried out to investigate the secondary benefits of seed manipulation by ants. Two independent experiments showed that, in contrast to many other myrmecochorous species, elaiosome removal decreases germination rate. Finally, the effects of burial at low depth (2 cm) on seed emergence are limited; however, as ants rarely move seeds to their nest, this secondary benefit of myrmecochory is unlikely. Therefore, it can be concluded that the functional role of an elaiosome in this species is not evident. However, ants clearly contribute to seed dispersal in C. corymbosa.