Pollination along an elevational gradient mediated both by floral scent and pollinator compatibility in the fig and fig‐wasp mutualism

Abstract

1. In the fig (Moraceae) and fig-wasp (Agaonidae) mutualism, scent is believed to be of primary importance in pollinator attraction and maintenance of species specificity. Scent divergence between closely related Ficus species seems sufficient in promoting reproductive isolation through pollinator behaviour, starting the process of speciation. 2. We investigated volatile organic compound (VOC) variation from figs in several Ficus species endemic to Papua New Guinea. Sister species of section Papuacyse and subspecies of Ficus trichocerasa substitute each other along the continuously forested Mt. Wilhelm elevational gradient. We placed these species in a phylogenetic context to draw conclusions of scent divergence between close relatives. In addition, pollinator response to VOCs emitted by figs of different species was tested. 3. Volatile profiles differed significantly between focal species, although with a varying degree of overlap between (sub)species and elevations. Pollinators were generally attracted to VOCs emitted only by their hosts except in one case where pollinating fig wasps were also attracted to the sister species of its host. Wasp morphological traits, however, indicate that it is mechanically impossible for this species to oviposit in figs of this atypical encounter. 4. Synthesis. This study demonstrates that while scent is an effective signal for partner recognition, there are multiple barriers which help maintain prepollination isolation in fig and pollinating fig-wasp interactions. Speciation along this elevational gradient is reinforced by divergence in key reproductive isolation mechanisms on both sides of the mutualism. K E Y W O R D S character divergence, evolutionary ecology, fig pollination, fig-wasp attraction, reproductive isolation, sister species, speciation, volatile organic compounds (VOCs) | 2257 Journal of Ecology SOUTO-VILARÓS eT AL.

Faster speciation of fig‐wasps than their host figs leads to decoupled speciation dynamics: Snapshots across the speciation continuum

“…pleioclada (Figure a) and their corresponding fig‐wasps (Figure b). Ficus trichocerasa has two clear and genetically defined clusters which correspond to the described ranges of both subspecies (Berg & Corner, ; Souto‐Vilarós et al, ). F ST values between subspecies show high genetic dissimilarity ( F ST > 0.24) while there appears to be very little dissimilarity within subspecies at different sites ( F ST < 0.07; Figure c).…”

“…Pollinating wasps of both subspecies are undescribed. Finally, we studied a monophyletic complex of three species (Souto‐Vilarós et al, ) belonging to the section Papuacyse that follow a lowland, mid‐elevation, and highland distribution, henceforth referred to as F. itoana species complex which includes: F. itoana Diels pollinated by Ceratosolen armipes Wiebes; F. microdictya Diels pollinated by C . sp.…”

“…“IMI” and Ceratosolen sp. “IMI” (see also Souto‐Vilarós et al, ). This complex represents a further step along the speciation continuum where each species‐pair has differentiated enough to constitute a separate entity.…”

“…We performed DNA extractions from leaf discs using the CTAB protocol (Doyle & Doyle, ) followed by an additional cleaning step using a silica column (Souto‐Vilarós et al, ). From each individual fig, a single female pollinating wasp stored and dried in silica gel was used for DNA extraction.…”

“…Because of their tight mutualism with pollinating fig wasps, our choice of host plant species therefore predetermined the choice of pollinating wasps, following the classic one‐to‐one relationship characteristic of this mutualism. We placed the fig species along the putative continuum according to existing phylogenetic, chemical (Souto‐Vilarós et al, ), population genetic (Segar et al, ), and morphological information (Berg & Corner, ). Further, the species were selected to be representative of an identifiable pattern of wide‐ranging species (e.g., Ficus dammaropsis Diels), and recognized lowland and highland subspecies pairs (e.g., F. saccata Corner/ F. megalophylla Diels and F. trachypison K. Schum./ F. quercetorum Corner), in line with extensive field observations (Segar et al, ; L. Sam, unpublished data) and literature surveys (e.g., Berg & Corner, ).…”