Evolution of obligate pollination mutualism in the tribe Phyllantheae (Phyllanthaceae)

We investigate the evolution of host association in a cryptic complex of mutualistic Crematogaster (Decacrema) ants that inhabits and defends Macaranga trees in Southeast Asia. Previous phylogenetic studies based on limited samplings of Decacrema present conflicting reconstructions of the evolutionary history of the association, inferring both cospeciation and the predominance of host shifts. We use cytochrome oxidase I (COI) to reconstruct phylogenetic relationships in a comprehensive sampling of the Decacrema inhabitants of Macaranga. Using a published Macaranga phylogeny, we test whether the ants and plants have cospeciated. The COI phylogeny reveals 10 well-supported lineages and an absence of cospeciation. Host shifts, however, have been constrained by stem traits that are themselves correlated with Macaranga phylogeny. Earlier lineages of Decacrema exclusively inhabit waxy stems, a basal state in the Pachystemon clade within Macaranga, whereas younger species of Pachystemon, characterized by nonwaxy stems, are inhabited only by younger lineages of Decacrema. Despite the absence of cospeciation, the correlated succession of stem texture in both phylogenies suggests that Decacrema and Pachystemon have diversified in association, or codiversified. Subsequent to the colonization of the Pachystemon clade, Decacrema expanded onto a second clade within Macaranga, inducing the development of myrmecophytism in the Pruinosae group. Confinement to the aseasonal wet climate zone of western Malesia suggests myrmecophytic Macaranga are no older than the wet forest community in Southeast Asia, estimated to be about 20 million years old (early Miocene). Our calculation of COI divergence rates from several published arthropod studies that relied on tenable calibrations indicates a generally conserved rate of approximately 1.5% per million years. Applying this rate to a rate-smoothed Bayesian chronogram of the ants, the Decacrema from Macaranga are inferred to be at least 12 million years old (mid-Miocene). However, using the extremes of rate variation in COI produces an age as recent as 6 million years. Our inferred timeline based on 1.5% per million years concurs with independent biogeographical events in the region reconstructed from palynological data, thus suggesting that the evolutionary histories of Decacrema and their Pachystemon hosts have been contemporaneous since the mid-Miocene. The evolution of myrmecophytism enabled Macaranga to radiate into enemy-free space, while the ants' diversification has been shaped by stem traits, host specialization, and geographic factors. We discuss the possibility that the ancient and exclusive association between Decacrema and Macaranga was facilitated by an impoverished diversity of myrmecophytes and phytoecious (obligately plant inhabiting) ants in the region.

Active pollination, although rare, has been documented in a few pollination mutualisms. Such behaviour can only evolve if it benefits the pollinator in some way. The wasps that pollinate Ficus inflorescences can be active or passive pollinators. They lay their eggs in fig flowers, so that a proportion of flowers will host a wasp larva instead of a seed. We show in an actively pollinated monoecious fig that lack of pollination does not induce fig abortion or affect wasp offspring size but results in smaller numbers of offspring. Hence, conversely to other active pollination systems, seed formation is not obligatory to sustain developing pollinator larvae; however there is a direct fitness cost to active pollinators not to pollinate. We then compared the locations of eggs and fertilised flowers of three actively pollinated Ficus species and one passively pollinated species. We found that more flowers containing wasp eggs were fertilised in the actively pollinated species relative to those of the passively pollinated one. These results along with comparison with similar studies on dioecious figs, support the hypothesis that active pollination has evolved in fig wasps to ensure that more flowers containing wasp eggs are fertilised as this may increase the chances of successful gall development. The stigmatic platform characterising actively pollinated figs is probably an adaptation to increase pollen dispersion within the fig.