Convergent evolution of an ant-plant mutualism across plant families, continents and time

Abstract

Questions: How often has dispersal of seeds by ants evolved in monocots and is the timing of origins associated with changes in the ant community or instead with the rise of forests? Are patterns in the origin of elaiosomes (the trait associated with the dispersal of seeds by ants) through time similar to those for the origins of fleshy fruits? Data studied: We estimate the timing of the origin of elaiosomes and fleshy fruits respectivelyby mapping seed morphology onto a recent phylogeny based on ndhF sequence data forthe monocots (Givnish et al., 2005). For comparison, we use fossil data on ant relativeabundance through time and phylogenetic data for the timing of the origin of seed-dispersing ant lineages. Search method: We mapped origins of both elaiosomes and fleshy fruits onto the phylogenyusing parsimony in the program Mesquite (Maddison and Maddison, 2005). We analysed therelationship between ant relative abundances, the number of origins of seed-dispersing ants, and the rate of origination of elaiosomes using randomization-based Monte Carlo regression in the program R (Cliff and Ord, 1981). Using the program Discrete (Pagel, 2006), we test whether fleshy fruits or elaiosomes and shaded forest understoreys show correlated evolution.Conclusions: Morphological features for the dispersal of seeds by ants (myrmecochory) have evolved at least twenty times within the monocots. Origins of myrmecochory are not associated with the rise of forests during the Cretaceous or with subsequent transitions of plant lineages into closed canopy habitats, nor are they contemporaneous with the origins of fleshy fruits. Instead, the origins of myrmecochory are closely associated with the rise in relative abundanceof ants (proportion of all individual insects in fossils) towards the end of the Eocene and more recently.