One step ahead: a parasitoid disperses farther and forms a wider geographic population than its fig wasp host

Parasitoid ecology along geographic gradients: lessons for climate change studies

One method to study the impact of climate change on host-parasitoid relationships is to compare populations along geographical gradients in latitude, altitude, or longitude. Indeed, temperatures, which vary along geographic gradients, directly shape the life traits of parasitoids and indirectly shift their populations through trophic interactions with hosts and plants. We explored the pros and cons of using these comparisons along gradients. We highlighted that the longitudinal gradients, although understudied, are well correlated to winter warming and summer heat waves and we draw attention to the impact of the increase in extreme events, which will probably be the determining parameters of the effect of climate change on host-parasitoid relationships.

Source-sink dynamics assists the maintenance of a pollinating wasp

Dispersal that unites spatially subdivided populations into a metapopulation with source-sink dynamics is crucial for species persistence in fragmented landscapes. Understanding such dynamics for pollinators is particularly urgent owing to the ongoing global pollination crisis. Here, we investigated the population structure and source-sink dynamics of a pollinating wasp (Wiebesia sp. 3) of Ficus pumila in the Zhoushan Archipelago of China. We found significant asymmetry in the pairwise migrant numbers for 22 of 28 cases on the historical timescale, but only two on the contemporary timescale. Despite a small population size, the sole island not colonized by a superior competitor wasp (Wiebesia sp. 1) consistently behaved as a net exporter of migrants, supplying large sinks. Comparable levels of genetic diversity, with few private alleles and low genetic differentiation (total F_st : 0.03; pairwise F_st : 0.0005-0.0791), were revealed among all the islands. There was a significant isolation-by-distance pattern caused mainly by migration between the competition-free island and other islands, otherwise the pattern was negligible. The clustering analysis failed to detect multiple gene pools for the whole region. Thus, the sinks were most probably organized into a patchy population. Moreover, the estimates of effective population sizes were comparable between the two timescales. Thus the source-sink dynamics embedded within a well-connected population network may allow Wiebesia sp. 3 to persist at a competitive disadvantage. This study provides evidence that metapopulations in the real world may be complicated and changeable over time, highlighting the necessity to study such metapopulations in detail.

Low host specificity and broad geographical ranges in a community of parasitic non-pollinating fig wasps (Sycoryctinae; Chalcidoidea)

Plants, phytophagous insects and their parasitoids form the most diverse assemblages of macroscopic organisms on earth. Enclosed assemblages in particular represent a tractable system for studying community assembly and diversification. Communities associated with widespread plant species are especially suitable as they facilitate a comparative approach. Pantropical fig-wasp communities represent a remarkably well-replicated system, ideal for studying these historical processes. We expect high dispersal ability in non-pollinating fig wasps to result in lower geographical turnover in comparison to pollinating fig wasps. The ability of non-pollinating wasps to utilise a number of hosts (low host specificity) is a key determinant of overall geographical range, with intraspecific competition becoming a constraining factor should diet breadth overlap among species. Finally, we expect conserved community structure throughout the host range. We aim to test these expectations, derived from population genetic and community studies, using the multi-trophic insect community associated with Ficus hirta throughout its 3,500 km range across continental and insular Asia. We collect molecular evidence from one coding mitochondrial gene, one non-coding nuclear gene and multiple microsatellites across 25 geographical sites. Using these data, we establish species boundaries, determine levels of host specificity among non-pollinating fig wasps and quantify geographical variation in community composition. We find low host specificity in two genera of non-pollinating fig wasps. Functional community structure is largely conserved across the range of the host fig, despite limited correspondence between the ranges of non-pollinator and pollinator species. While nine pollinators are associated with Ficus hirta, the two non-pollinator tribes developing in its figs each contained only four species. Contrary to predictions, we find stronger isolation by distance in non-pollinators than pollinators. Long-lived non-pollinators may disperse more gradually and be less reliant on infrequent long-distance dispersal by wind currents. Segregation among non-pollinating species across their range is suggestive of competitive exclusion and we propose that this may be a result of increased levels of local adaptation and moderate, but regular, rates of dispersal. Our findings provide one more example of lack of strict codiversification in the geographical diversification of plant-associated insect communities.

Low-coverage genomic data resolve the population divergence and gene flow history of an Australian rain forest fig wasp

Population divergence and gene flow are key processes in evolution and ecology. Model-based analysis of genome-wide data sets allows discrimination between alternative scenarios for these processes even in nonmodel taxa. We used two complementary approaches (one based on the blockwise site frequency spectrum [bSFS], the second on the pairwise sequentially Markovian coalescent [PSMC]) to infer the divergence history of a fig wasp, Pleistodontes nigriventris. Pleistodontes nigriventris and its fig tree mutualist Ficus watkinsiana are restricted to rain forest patches along the eastern coast of Australia and are separated into The Northern population is to the north of the Southern populations by two dry forest corridors (the Burdekin and St. Lawrence Gaps). We generated whole genome sequence data for two haploid males per population and used the bSFS approach to infer the timing of divergence between northern and southern populations of P. nigriventris, and to discriminate between alternative isolation with migration (IM) and instantaneous admixture (ADM) models of postdivergence gene flow. Pleistodontes nigriventris has low genetic diversity (π = 0.0008), to our knowledge one of the lowest estimates reported for a sexually reproducing arthropod. We find strongest support for an ADM model in which the two populations diverged ca. 196 kya in the late Pleistocene, with almost 25% of northern lineages introduced from the south during an admixture event ca. 57 kya. This divergence history is highly concordant with individual population demographies inferred from each pair of haploid males using PSMC. Our analysis illustrates the inferences possible with genome-level data for small population samples of tiny, nonmodel organisms and adds to a growing body of knowledge on the population structure of Australian rain forest taxa.

Plant geographic phenotypic variation drives diversification in its associated community of a phytophagous insect and its parasitoids

BACKGROUND

While the communities constituted by phytophageous insects and their parasites may represent half of all terrestrial animal species, understanding their diversification remains a major challenge. A neglected idea is that geographic phenotypic variation in a host plant may lead to heterogeneous evolutionary responses of the different members of the associated communities. This could result in diversification on a host plant by ecological speciation in some species, leading to geographic variation in community composition. In this study we investigated geographic variation of inflorescence receptacle size in a plant, Ficus hirta, and how the hymenopteran community feeding in the inflorescences has responded. Our predictions were: 1) Inflorescence size variation affects wasp species differently depending on how they access oviposition sites. 2) In some affected lineages of wasps, we may observe vicariant, parapatric species adapted to different inflorescence sizes.

RESULTS

We show that fig (the enclosed inflorescence of Ficus) wall thickness varies geographically. The fig-entering pollinating wasp was not affected, while the parasites ovipositing through the fig wall were. Two parapatric species of Philotrypesis, exhibiting strikingly different ovipositor lengths, were recorded. One species of Sycoscapter was also present, and it was restricted, like the shorter-ovipositor Philotrypesis, to the geographic zone where fig walls were thinner.

CONCLUSIONS

Previous work on fig wasps suggested that parapatric geographic ranges among congenerics were due to adaptation to variation in abiotic factors, complemented by interspecific competition. Our results show that parapatric ranges may also result from adaptation to variation in biotic factors. Within an insect community, differences among species in their response to geographic phenotypic variation of their host plant may result in geographically heterogeneous community structure. Such heterogeneity leads to heterogeneous interaction networks among sites. Our results support the hypothesis that plant geographic phenotypic variation can be a driver of diversification in associated insect communities, and can complement other diversification processes.

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

BACKGROUND

Biotic interactions are ubiquitous and require information from ecology, evolutionary biology, and functional genetics in order to be understood. However, study systems that are amenable to investigations across such disparate fields are rare. Figs and fig wasps are a classic system for ecology and evolutionary biology with poor functional genetics; Caenorhabditis elegans is a classic system for functional genetics with poor ecology. In order to help bridge these disciplines, here we describe the natural history of a close relative of C. elegans, Caenorhabditis inopinata, that is associated with the fig Ficus septica and its pollinating Ceratosolen wasps.

RESULTS

To understand the natural context of fig-associated Caenorhabditis, fresh F. septica figs from four Okinawan islands were sampled, dissected, and observed under microscopy. C. inopinata was found in all islands where F. septica figs were found. C.i nopinata was routinely found in the fig interior and almost never observed on the outside surface. C. inopinata was only found in pollinated figs, and C. inopinata was more likely to be observed in figs with more foundress pollinating wasps. Actively reproducing C. inopinata dominated early phase figs, whereas late phase figs with emerging wasp progeny harbored C. inopinata dauer larvae. Additionally, C. inopinata was observed dismounting from Ceratosolen pollinating wasps that were placed on agar plates. C. inopinata was not found on non-pollinating, parasitic Philotrypesis wasps. Finally, C. inopinata was only observed in F. septica figs among five Okinawan Ficus species sampled.

CONCLUSION

These are the first detailed field observations of C. inopinata, and they suggest a natural history where this species proliferates in early phase F. septica figs and disperses from late phase figs on Ceratosolen pollinating fig wasps. While consistent with other examples of nematode diversification in the fig microcosm, the fig and wasp host specificity of C. inopinata is highly divergent from the life histories of its close relatives and frames hypotheses for future investigations. This natural co-occurrence of the fig/fig wasp and C. inopinata study systems sets the stage for an integrated research program that can help to explain the evolution of interspecific interactions.

Whole-genome data reveal the complex history of a diverse ecological community

How widespread ecological communities assemble remains a key question in ecology. Trophic interactions between widespread species may reflect a shared population history or ecological fitting of local pools of species with very different population histories. Which scenario applies is central to the stability of trophic associations and the potential for coevolution between species. Here we show how alternative community assembly hypotheses can be discriminated using whole-genome data for component species and provide a likelihood framework that overcomes current limitations in formal comparison of multispecies histories. We illustrate our approach by inferring the assembly history of a Western Palearctic community of insect herbivores and parasitoid natural enemies, trophic groups that together comprise 50% of terrestrial species. We reject models of codispersal from a shared origin and of delayed enemy pursuit of their herbivore hosts, arguing against herbivore attainment of "enemy-free space." The community-wide distribution of species expansion times is also incompatible with a random, neutral model of assembly. Instead, we reveal a complex assembly history of single- and multispecies range expansions through the Pleistocene from different directions and over a range of timescales. Our results suggest substantial turnover in species associations and argue against tight coevolution in this system. The approach we illustrate is widely applicable to natural communities of nonmodel species and makes it possible to reveal the historical backdrop against which natural selection acts.

Conserved community structure and simultaneous divergence events in the fig wasps associated with Ficus benjamina in Australia and China

Background

Localised patterns of species diversity can be influenced by many factors, including regional species pools, biogeographic features and interspecific interactions. Despite recognition of these issues, we still know surprisingly little about how invertebrate biodiversity is structured across geographic scales. In particular, there have been few studies of how insect communities vary geographically while using the same plant host. We compared the composition (species, genera) and functional structure (guilds) of the chalcid wasp communities associated with the widespread fig tree, Ficus benjamina, towards the northern (Hainan province, China) and southern (Queensland, Australia) edges of its natural range. Sequence data were generated for nuclear and mtDNA markers and used to delimit species, and Bayesian divergence analyses were used to test patterns of community cohesion through evolutionary time.

Results

Both communities host at least 14 fig wasp species, but no species are shared across continents. Community composition is similar at the genus level, with six genera shared although some differ in species diversity between China and Australia; a further three genera occur in only China or Australia. Community functional structure remains very similar in terms of numbers of species in each ecological guild despite community composition differing a little (genera) or a lot (species), depending on taxonomic level. Bayesian clustering analyses favour a single community divergence event across continents over multiple events for different ecological guilds. Molecular dating estimates of lineage splits between nearest inter-continental species pairs are broadly consistent with a scenario of synchronous community divergence from a shared “ancestral community”.

Conclusions

Fig wasp community structure and genus-level composition are largely conserved in a wide geographic comparison between China and Australia. Moreover, dating analyses suggest that the functional community structure has remained stable for long periods during historic range expansions. This suggests that ecological interactions between species may play a persistent role in shaping these communities, in contrast to findings in some comparable temperate systems.

Electronic supplementary material

The online version of this article (10.1186/s12898-018-0167-y) contains supplementary material, which is available to authorized users.

Life-history strategy, resource dispersion and phylogenetic associations shape dispersal of a fig wasp community

BACKGROUND

The combined influence of life-history strategy and resource dispersion on dispersal evolution of a biological community, and by extension, on community assemblage, has received sparse attention. Highly specialized fig wasp communities are ideal for addressing this question since the life-history strategies that affect their pace of life and the dispersion of their oviposition resources vary. We compared dispersal capacities of the wasp community of a widespread tropical fig, Ficus racemosa, by measuring flight durations, somatic lipid content and resting metabolic rates.

RESULTS

Wasp species exhibiting greater flight durations had higher energy reserves and resting metabolic rates. "Fast"-paced species showed higher dispersal capacities reflecting requirements for rapid resource location within short adult lifespans. Longer-lived "slow"-paced species exhibited lower dispersal capacities. Most dispersal traits were negatively related with resource dispersion while their variances were positively related with this variable, suggesting that resource dispersion selects for dispersal capacity. Dispersal traits exhibited a phylogenetic signal.

CONCLUSIONS

Using a combination of phylogeny, trait functionality and community features, we explain how dispersal traits may have co-evolved with life-history strategies in fig wasps and influenced a predisposition for dispersal. We speculate how processes influencing dispersal trait expression of community members may affect resource occupancy and community assemblage.

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

BACKGROUND

Interspecific interactions have long been assumed to play an important role in diversification. Mutualistic interactions, such as nursery pollination mutualisms, have been proposed as good candidates for diversification through co-speciation because of their intricate nature. However, little is known about how speciation and diversification proceeds in emblematic nursery pollination systems such as figs and fig wasps. Here, we analyse diversification in connection with spatial structuring in the obligate mutualistic association between Ficus septica and its pollinating wasps throughout the Philippines and Taiwan.

RESULTS

Ceratosolen wasps pollinating F. septica are structured into a set of three vicariant black coloured species, and a fourth yellow coloured species whose distribution overlaps with those of the black species. However, two black pollinator species were found to co-occur on Lanyu island. Microsatellite data on F. septica indicates the presence of three gene pools that broadly mirrors the distribution of the three black clades. Moreover, receptive fig odours, the specific message used by pollinating wasps to locate their host tree, varied among locations.

CONCLUSIONS

F. septica and its black pollinator clades exhibited similar geographic structuring. This could be due originally to geographic barriers leading to isolation, local adaptation, and finally co-structuring. Nevertheless, the co-occurrence of two black pollinator species on Lanyu island suggests that the parapatric distribution of the black clades is now maintained by the inability of migrating individuals of black pollinators to establish populations outside their range. On the other hand, the distribution of the yellow clade strongly suggests an initial case of character displacement followed by subsequent range extension: in our study system, phenotypic or microevolutionary plasticity has allowed the yellow clade to colonise hosts presenting distinct odours. Hence, while variation in receptive fig odours allows specificity in the interaction, this variation does not necessarily lead to coevolutionary plant-insect diversification. Globally, our results evidence evolutionary plasticity in the fig-fig wasp mutualism. This is the first documentation of the presence of two distinct processes in pollinating fig wasp diversification on a host species: the formation of vicariant species and the co-occurrence of other species over large parts of their ranges probably made possible by character displacement.

Different genetic structures revealed resident populations of a specialist parasitoid wasp in contrast to its migratory host

Genetic comparisons of parasitoids and their hosts are expected to reflect ecological and evolutionary processes that influence the interactions between species. The parasitoid wasp, Cotesia vestalis, and its host diamondback moth (DBM), Plutella xylostella, provide opportunities to test whether the specialist natural enemy migrates seasonally with its host or occurs as resident population. We genotyped 17 microsatellite loci and two mitochondrial genes for 158 female adults of C. vestalis collected from 12 geographical populations, as well as nine microsatellite loci for 127 DBM larvae from six separate sites. The samplings covered both the likely source (southern) and immigrant (northern) areas of DBM from China. Populations of C. vestalis fell into three groups, pointing to isolation in northwestern and southwestern China and strong genetic differentiation of these populations from others in central and eastern China. In contrast, DBM showed much weaker genetic differentiation and high rates of gene flow. TESS analysis identified the immigrant populations of DBM as showing admixture in northern China. Genetic disconnect between C. vestalis and its host suggests that the parasitoid did not migrate yearly with its host but likely consisted of resident populations in places where its host could not survive in winter.

Unravelling mummies: cryptic diversity, host specificity, trophic and coevolutionary interactions in psyllid - parasitoid food webs

BACKGROUND

Parasitoids are hyperdiverse and can contain morphologically and functionally cryptic species, making them challenging to study. Parasitoid speciation can arise from specialisation on niches or diverging hosts. However, which process dominates is unclear because cospeciation across multiple parasitoid and host species has rarely been tested. Host specificity and trophic interactions of the parasitoids of psyllids (Hemiptera) remain mostly unknown, but these factors are fundamentally important for understanding of species diversity, and have important applied implications for biological control.

RESULTS

We sampled diverse parasitoid communities from eight Eucalyptus-feeding psyllid species in the genera Cardiaspina and Spondyliaspis, and characterised their phylogenetic and trophic relationships using a novel approach that forensically linked emerging parasitoids with the presence of their DNA in post-emergence insect mummies. We also tested whether parasitoids have cospeciated with their psyllid hosts. The parasitoid communities included three Psyllaephagus morphospecies (two primary and, unexpectedly, one heteronomous hyperparasitoid that uses different host species for male and female development), and the hyperparasitoid, Coccidoctonus psyllae. However, the number of genetically delimited Psyllaephagus species was three times higher than the number of recognisable morphospecies, while the hyperparasitoid formed a single generalist species. In spite of this, cophylogenetic analysis revealed unprecedented codivergence of this hyperparasitoid with its primary parasitoid host, suggesting that this single hyperparasitoid species is possibly diverging into host-specific species. Overall, parasitoid and hyperparasitoid diversification was characterised by functional conservation of morphospecies, high host specificity and some host switching between sympatric psyllid hosts.

CONCLUSIONS

We conclude that host specialisation, host codivergence and host switching are important factors driving the species diversity of endoparasitoid communities of specialist host herbivores. Specialisation in parasitoids can also result in heteronomous life histories that may be more common than appreciated. A host generalist strategy may be rare in endoparasitoids of specialist herbivores despite the high conservation of morphology and trophic roles, and endoparasitoid species richness is likely to be much higher than previously estimated. This also implies that the success of biological control requires detailed investigation to enable accurate identification of parasitoid-host interactions before candidate parasitoid species are selected as biological control agents for target pests.

Local coexistence and genetic isolation of three pollinator species on the same fig tree species

Molecular tools increasingly reveal cryptic lineages and species that were previously unnoticed by traditional taxonomy. The discovery of cryptic species in sympatry prompts the question of how they coexist in the apparent absence of ecological divergence. However, this assumes first that the molecular taxonomy used to identify cryptic lineages delimits species boundaries accurately. This issue is important, because many diversity studies rely heavily or solely on data from mitochondrial DNA sequences for species delimitation, and several factors may lead to poor identification of species boundaries. We used a multilocus population genetics approach to show that three mtDNA-defined cryptic lineages of the fig wasp Pleistodontes imperialis Saunders, which pollinate Port Jackson figs (Ficus rubiginosa) in north-eastern Australia, represent reproductively isolated species. These species coexist locally, with about 13% of figs (where mating occurs) containing wasps from two or three species. However, there was no evidence for gene flow between them. Confirmed cases of coexisting cryptic species provide excellent opportunities for future studies of the ecological and evolutionary forces shaping both species coexistence and fig/pollinator coevolution.

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

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.

Diversity, population structure, and individual behaviour of parasitoids as seen using molecular markers

Parasitoids have long been models for host-parasite interactions, and are important in biological control. Neutral molecular markers have become increasingly accessible tools, revealing previously unknown parasitoid diversity. Thus, insect communities are now seen as more speciose. They have also been found to be more complex, based on trophic links detected using bits of parasitoid DNA in hosts, and host DNA in adult parasitoids. At the population level molecular markers are used to determine the influence of factors such as host dynamics on parasitoid population structure. Finally, at the individual level, they are used to identify movement of individuals. Overall molecular markers greatly increase the value of parasitoid samples collected, for both basic and applied research, at all levels of study.