Case Studies

Plants: Elaiosomes and seed dispersal by ants

Elaiosomes are fleshy appendages that are found on many sorts of seeds. No one is quite sure why they evolved. Because ants often carry such seeds back to their nests and eat the elaiosomes, leaving the seed apparently unharmed, one suggestion is that they evolved as a ‘reward’ for ants, encouraging them to disperse the seeds. An alternative proposal is that elaiosomes may help the seed to absorb and retain moisture for germination.

In this activity, students build a phylogenetic tree of plants from the Polygalaceae. Many of the species in this family produce seeds with elaiosomes. From the dated tree, students infer when elaiosomes might have evolved and compare this with when ants are known to have evolved and become abundant in ecosystems. This casts light on the possible reasons for the evolution of elaiosomes.

Case study written by: Félix Forest, Royal Botanic Gardens, Kew.
Edited by Dean Madden, NCBE, University of Reading.