Asymmetric sharing of pollinator fig wasps between two sympatric dioecious fig trees: a reflection of supply and demand or differences in the size of their figs?

Abstract

Background: Host specificity among pollinator fig wasps (Agaonidae) depends on host plant specific volatile cues, but fig wasps must also pass through a narrow physical barrier (the ostiole) if they are to pollinate and oviposit. Across South East Asia the dioecious shrub Ficus hirta is associated with at least ten pollinator species allied to Valisia javana. Ficus triloba has a single recorded pollinator, Valisia esquirolianae. Receptive figs of F. hirta are usually much smaller than those of F. triloba, but at a mainland site where F. hirta has atypically large figs we identified both V. esquirolianae and V. javana from both Ficus species using COI and ITS2 sequencing. To investigate whether this host overlap was exceptional we reared fig wasps from the two trees elsewhere and recorded features that may facilitate host transfer between them, including attractant volatiles, reproductive phenology and the sizes of their figs and fig wasps.

Results: The two Ficus species were found to support both Valisia species at several of the sites, suggesting that the differences we detected in volatile profiles, ostiole sizes and pollinator head sizes are not strict barriers to host sharing. Valisia javana colonised F. triloba more frequently than V. esquirolianae colonised F. hirta.

Conclusions: This asymmetric sharing of pollinators may reflect the relative abundance of the two species of fig wasps and differences in host reproductive phenology. Asynchronous flowering of individual F. hirta may favor local retention of pollinators, in contrast to the tree-wide synchrony of F. triloba figs, which can generate local shortages of V. esquirolianae. If the pollinator sharing by male figs of F. triloba and F. hirta also occurs in female figs then this could result in gene flow between them.

Keywords: Host specificity; Hybrid; Mutualism; Pollinator sharing; Size-matching.

Fig. 1

Sample sites and distributions of the pollinator fig wasp species associated with Ficus hirta and F. triloba

Fig. 2

The syconia of F. hirta showing the measure of the ostiole diameter. The length of the blue line represent the ostiole diameter (mm)

Fig. 3

a The reproductive phenology of nine male and eight female F. triloba at Dinghu Mountain. Each thick bar shows a particular developmental stage present on at least one of the plants. Emerging (D phase sensu Galil and Eisikowitch 1968) phase male figs release pollinator fig wasps, which enter receptive (B) phase male and female figs where they pollinate and attempt oviposition. In contrast to F. triloba, fig production on each male and female F. hirta individual is asynchronous and small numbers of receptive figs and male figs that are releasing pollinators were present almost throughout the year at this site. b Comparison of ostiole diameter in male receptive syconia between F. hirta and F. triloba, and head width between their specific pollinating wasps. c Comparison of style length in male receptive syconia between F. hirta and F. triloba, and ovipositor length between their specific pollinating wasps. d Non-metric multidimensional scaling of receptive floral volatiles of F. hirta (black circles) and F. triloba (grey squares) based on Bray–Curtis dissimilarity index (stress = 0.128).

Fig. 4

a ITS2 neighbour joining tree of the Valisia fig wasp pollinators associated with Ficus hirta and Ficus triloba. b COI neighbour-joining tree of the pollinators associated, with sequences of Ceratosolen and Kradibia species as outgroups. Node support rates are shown. Haplotypes (V. jav_sp1_H1-H12 for Valisia javana complex sp 1 and V. esq_ H1-H16 for V. esquirolianae) are also listed together with their host figs (hir = F. hirta (black triangles), tri = F. triloba (black stars), hirt/tri = both F. hirta and F. triloba). The collecting sites of their hosts listed by haplotypes. For haplotypes of Valisia javana complex sp 1, those sequenced in this study is marked as sp 1, the others extracted from Yu et al. (2019) are marked as SP1