CONVERGENCE AND COEVOLUTION IN A MUTUALISM: EVIDENCE FROM A MOLECULAR PHYLOGENY OF FICUS

TLDR

The highly specialized mutualism between Ficus trees and their species‑specific pollinating wasps shows extensive biological variation, and morphological traits linked to growth habit and breeding system may evolve convergently. The study investigates Ficus life‑history trait evolution and fig‑wasp coadaptation using a well‑resolved molecular phylogeny, and discusses how observed correlations inform the coevolutionary process. The authors constructed a ribosomal DNA phylogeny and mapped Ficus life‑history and inflorescence traits onto it to identify parallel transitions and reconstruct adaptations to pollinators. The phylogenetic analysis uncovered multiple convergent transitions in growth habit, breeding system, and inflorescence morphology, with statistical correlations linking fig traits to pollinator head shape and behavior, supporting reciprocal adaptations between figs and their wasp partners.

Abstract

The interaction between Ficus (Moraceae) and their pollinating wasps (Chalcidoidea: Agaonidae; more than 700 species-specific couples) is one of the most specialized mutualisms found in nature. Both partners of this interaction show extensive variation in their respective biology. Here we investigate Ficus life-history trait evolution and fig/fig wasp coadaptation in the context of a well-resolved molecular phylogeny. Mapping out variations in Ficus life-history traits on an independently derived phylogeny constructed from ribosomal DNA sequences (external and internal transcribed spacer) reveals several parallel transitions in Ficus growth habit and breeding system. Convergent trait evolution might explain the discrepancies between morphological analyses and our molecular reconstruction of the genus. Morphological characters probably correlate with growth habit and breeding system and could therefore be subject to convergent evolution. Furthermore, we reconstruct the evolution of Ficus inflorescence characters that are considered adaptations to the pollinators. Our phylogeny reveals convergences in ostiole shape, stigma morphology, and stamen:ovule ratio. Statistical tests taking into account the phylogenetic relationship of the species show that transitions in ostiole shape are correlated with variation in wasp pollinator head shape, and evolutionary changes in stigma morphology and stamen:ovule ratio correlate with changes in the pollination behavior of the associated wasp. These correlations provide evidence for reciprocal adaptations of morphological characters between these mutualistic partners that have interacted over a long evolutionary time. In light of previous ecological studies on mutualism, we discuss the adaptive significance of these correlations and what they can tell us about the coevolutionary process occurring between figs and their pollinators.