The Role of Biotic and Abiotic Factors in Evolution of Ant Dispersal in the Milkwort Family (Polygalaceae)

Abstract

A phylogenetic approach was taken to investigate the evolutionary history of seed appendages in the plant family Polygalaceae (Fabales) and determine which factors might be associated with evolution of elaiosomes through comparisons to abiotic (climate) and biotic (ant species number and abundance) timelines. Molecular datasets from three plastid regions representing 160 species were used to reconstruct a phylogenetic tree of the order Fabales, focusing on Polygalaceae. Bayesian dating methods were used to estimate the age of the appearance of ant-dispersed elaiosomes in Polygalaceae, shown by likelihood optimizations to have a single origin in the family. Topology-based tests indicated a diversification rate shift associated with appearance of caruncular elaiosomes. We show that evolution of the caruncular elaiosome type currently associated with ant dispersal occurred 54.0-50.5 million year ago. This is long after an estimated increase in ant lineages in the Late Cretaceous based on molecular studies, but broadly concomitant with increasing global temperatures culminating in the Late Paleocene-Early Eocene thermal maxima. These results suggest that although most major ant clades were present when elaiosomes appeared, the environmental significance of elaiosomes may have been an important factor in success of elaiosome-bearing lineages. Ecological abundance of ants is perhaps more important than lineage numbers in determining significance of ant dispersal. Thus, our observation that elaiosomes predate increased ecological abundance of ants inferred from amber deposits could be indicative of an initial abiotic environmental function.

Dinosaurs and the Cretaceous Terrestrial Revolution

“…To an extent, of course, one can expect statistically significant diversification shifts to occur at the base of the tree, as founding taxa within the clade split and major branches become established. Bats, for example, show a similar early diversification pattern ( Jones et al 2005), but ants do not (Forest et al 2007). The reason is that clades do not stop diversifying once they have become established.…”

“…A completely different approach is to use tree shape to search for and date perturbations in diversification patterns that diverge from a simple birth-death model (e.g. Forest et al 2007;Ruta et al 2007). Here we use both approaches to test whether dinosaurs responded to the KTR, by comparing the magnitude and rates of their diversification in the Cretaceous with their diversification patterns in the Triassic and Jurassic.…”

A treatment for Polygala of northern Australia

“…Regional treatments and recent molecular and morphological analyses of inter-and intra-family phylogeny, conducted by Eriksen (1993aEriksen ( , 1993b, Persson (2001), Monro (2003) and Forest et al (2007), have questioned the classification of Polygalaceae and sectional treatment of Polygala presented by Chodat (1896). Although these authors were unable to provide a fully resolved phylogeny for all Polygalaceae, they were able to highlight issues requiring further research, such as polyphyly in several genera such as Bredemeyera and Polygala.…”

“…Arils and caruncles are mechanisms for attracting and rewarding dispersal vectors (Berg 1975), or may assist in seed hydration (Forest et al 2007). Many types of dispersal have been noted for Polygala (van der Meijden 1988), including myrmecochory (ant dispersal), ornithochory (bird dispersal), anemochory (wind dispersal), diplochory (dispersal by two separate vectors, e.g.…”

“…However, these characters are not exclusive to Leptaleae and could also apply to series Chloropterae subsection Deltoideae ( Table 2). Given that some aspects of Chodat's classification are not supported by recent phylogenetic analyses (Eriksen 1993a(Eriksen , 1993bPersson 2001;Monro 2003;Forest et al 2007) and can be too ambiguous to apply, an infrageneric classification is not provided. However, it is possible to speculate on putative natural groups within the Australian material.…”