Tracking the elusive history of diversification in plant–herbivorous insect–parasitoid food webs: insights from figs and fig wasps

Abstract

The food webs consisting of plants, herbivorous insects and their insect parasitoids are a major component of terrestrial biodiversity. They play a central role in the functioning of all terrestrial ecosystems, and the number of species involved is mind-blowing (Nyman et al. 2015). Nevertheless, our understanding of the evolutionary and ecological determinants of their diversity is still in its infancy. In this issue of Molecular Ecology, Sutton et al.(2016) open a window into the comparative analysis of spatial genetic structuring in a set of comparable multitrophic models, involving highly species-specific interactions: figs and fig wasps. This is the first study to compare genetic structure using population genetics tools in a fig-pollinating wasp (Pleistodontes imperialis sp1) and its main parasitoid (Sycoscapter sp.A). The fig-pollinating wasp has a discontinuous spatial distribution that correlates with genetic differentiation, while the parasitoid bridges the discontinuity by parasitizing other pollinator species on the same host fig tree and presents basically no spatial genetic structure. The full implications of these results for our general understanding of plant-herbivorous insect-insect parasitoids diversification become apparent when envisioned within the framework of recent advances in fig and fig wasp biology.

Multiple parapatric pollinators have radiated across a continental fig tree displaying clinal genetic variation

“…This apparent contradiction suggests that factors which facilitate speciation, such as a short generation time, infection by Wolbachia and assortative mating, can compensate for extensive gene flow and be decisive factors facilitating pollinating fig wasp speciation. Some nonpollinating fig wasps (NPFW, belonging to several families of Chalcidoidea) may disperse as widely as pollinator fig wasps (Kjellberg & Proffit, ; Sutton et al, ). NPFW have a largely similar biology to the pollinators, but exhibit varying intensities of local mate competition due to varying patterns of oviposition and mating sites.…”

Field studies reveal a close relative of C. elegans thrives in the fresh figs of Ficus septica and disperses on its Ceratosolen pollinating wasps

“…Figs are also a key resource for over a thousand bird and mammal species, who in turn aid in seed dispersal [ 78 ]. As a consequence of this microcosm complexity, this remains an influential and active system for study in ecology and evolution [ 79 – 82 ]. However, none of the species in these communities are particularly amenable to functional genetics and laboratory studies—both of which are crucial for refining the explanatory power of evolutionary science.…”

Diversification and spatial structuring in the mutualism between Ficus septica and its pollinating wasps in insular South East Asia

“…The two co-occurring species differ in precocity of emergence from figs after over-wintering, suggesting differences in temperature thresholds. This ecological differentiation may facilitate the coexistence of competing species [ 50 ], while the parapatric distribution of the two wide-ranging species is probably best explained by competitive exclusion in a context of strong gene flow limiting local adaptation in widespread species [ 51 ]. Finally, F. rubiginosa provides the most complex pattern of distribution of diverse pollinator species documented to date, a feature that may result from the range of the host tree encompassing a combination of two climatic gradients, a progressive North-South gradient and a steep East-West one [ 42 , 46 , 52 ].…”