A test of trade-offs in dispersal and reproduction within and between a sister species pair of specialist insect herbivores

Understanding the drivers of trade-offs among traits is vital for comprehending the evolution and maintenance of trait variation. Theoretical frameworks propose that evolutionary mechanisms governing trade-offs frequently exhibit a scale-dependent nature. However, empirical tests of whether trade-offs exhibited across various biological scales (i.e. individuals, populations, species, genera, etc.) remains scarce. In this study, we explore trade-off between dispersal and reproductive effort among sympatric sister species of wasps in the genus Belonocnema (Hymenoptera: Cynipini: Cynipidae) that form galls on live oaks: B. fossoria, which specializes on Quercus geminata, and B. treatae, which specializes on Q. virginiana. Specifically, our results suggest that B. fossoria has evolved reduced flight capability and smaller wings, but a larger abdomen and greater total reproductive effort than B. treatae, which has larger wings and is a stronger flier, but has a smaller abdomen and reduced total reproductive effort. These traits and the relationships among them remain unchanged when B. fossoria and B. treatae are transplanted and reared onto the alternative host plant, suggesting that trait divergence is genetically based as opposed to being a plastic response to the different rearing environments. However, when looking within species, we found no evidence of intraspecific trade-offs between wing length and reproductive traits within either B. fossoria or B. treatae. Overall, our results indicate that observed trade-offs in life history traits between the two gall former species are likely a result of independent adaptations in response to different environments as opposed to the amplified expression of within species intrinsic tradeoffs.

The role of divergent host use and geography in the evolution of habitat isolation and sexual isolation among sister species of Belonocnema gall wasps

Ecology and geography can play important roles in the evolution of reproductive isolation across the speciation continuum, but few studies address both at the later stages of speciation. This notable gap in knowledge arises from the fact that traditional ecological speciation studies have predominantly focused on the role of ecology in initiating the speciation process, while many studies exploring the effect of geography (e.g., reinforcement) concentrate on species pairs that lack divergent ecological characteristics. We simultaneously examine the strength of habitat isolation and sexual isolation among three closely related species of Belonocnema gall-forming wasps on two species of live oaks, Quercus virginiana and Q. geminata, that experience divergent selection from their host plants and variable rates of migration due to their geographic context. We find that the strength of both habitat isolation and sexual isolation is lowest among allopatric species pairs with the same host plant association, followed by allopatric species with different host plant associations, and highest between sympatric species with different host-plant associations. This pattern suggests that divergent selection due to different host use interacts with geography in the evolution of habitat isolation and sexual isolation during the later stages of speciation of Belonocnema wasps.

Speciation in Nearctic oak gall wasps is frequently correlated with changes in host plant, host organ, or both

Quantifying the frequency of shifts to new host plants within diverse clades of specialist herbivorous insects is critically important to understand whether and how host shifts contribute to the origin of species. Oak gall wasps (Hymenoptera: Cynipidae: Cynipini) comprise a tribe of ∼1000 species of phytophagous insects that induce gall formation on various organs of trees in the family Fagacae-primarily the oaks (genus Quercus; ∼435 sp.). The association of oak gall wasps with oaks is ancient (∼50 my), and most oak species are galled by one or more gall wasp species. Despite the diversity of both gall wasp species and their plant associations, previous phylogenetic work has not identified the strong signal of host plant shifting among oak gall wasps that has been found in other phytophagous insect systems. However, most emphasis has been on the Western Palearctic and not the Nearctic where both oaks and oak gall wasps are considerably more species rich. We collected 86 species of Nearctic oak gall wasps from most of the major clades of Nearctic oaks and sequenced >1000 Ultraconserved Elements (UCEs) and flanking sequences to infer wasp phylogenies. We assessed the relationships of Nearctic gall wasps to one another and, by leveraging previously published UCE data, to the Palearctic fauna. We then used phylogenies to infer historical patterns of shifts among host tree species and tree organs. Our results indicate that oak gall wasps have moved between the Palearctic and Nearctic at least four times, that some Palearctic wasp clades have their proximate origin in the Nearctic, and that gall wasps have shifted within and between oak tree sections, subsections, and organs considerably more often than previous data have suggested. Given that host shifts have been demonstrated to drive reproductive isolation between host-associated populations in other phytophagous insects, our analyses of Nearctic gall wasps suggest that host shifts are key drivers of speciation in this clade, especially in hotspots of oak diversity. Although formal assessment of this hypothesis requires further study, two putatively oligophagous gall wasp species in our dataset show signals of host-associated genetic differentiation unconfounded by geographic distance, suggestive of barriers to gene flow associated with the use of alternative host plants.

Context-Dependent Reproductive Isolation: Host Plant Variability Drives Fitness of Hybrid Herbivores

AbstractThe role of divergent selection between alternative environments in promoting reproductive isolation (RI) between lineages is well recognized. However, most studies view each divergent environment as homogenous, thereby overlooking the potential role within-environment variation plays in RI between differentiating lineages. Here, we test the importance of microenvironmental variation in RI by using individual trees of two host plants, each harboring locally adapted populations of the cynipid wasp Belonocnema treatae. We compared the fitness surrogate (survival) of offspring from hybrid crosses with resident crosses across individual trees on each of two primary host plants, Quercus virginiana and Q. geminata. We found evidence of weak hybrid inviability between host-associated lineages of B. treatae despite strong genomic differentiation. However, averaging across environments masked great variation in hybrid fitness on individual trees, where hybrids performed worse than, equal to, or better than residents. Thus, considering the environmental context of hybridization is critical to improving the predictability of divergence under variable selection.

One hundred and sixty years of taxonomic confusion resolved: Belonocnema (Hymenoptera: Cynipidae: Cynipini) gall wasps associated with live oaks in the USA

Gall wasps (Hymenoptera: Cynipidae) in the genus Belonocnema induce galls on live oaks (Quercus series Virentes), forming multilocular root galls in the sexual generation and unilocular leaf galls in the asexual generation. Using morphological characters, host records, museum specimens, flight propensity and phylogenetic analysis of published cytochrome c oxidase subunit I (COI) and nuclear SNP data, we resolve the long-standing taxonomic confusion within Belonocnema and recognize three distinct species that are distributed throughout the southern and south-eastern USA: B. fossoria (rev. stat.), B. kinseyi (rev. stat.) and B. treatae, while B. quercusvirens is treated as species inquirenda. The presence of mitonuclear discordance results in the failure of a mitochondrial DNA barcode region to distinguish between B. fossoria and B. treatae, while recognizing B. kinseyi, despite the three species being clearly separated based on morphology and phylogenetic analysis of SNP data. We provide re-descriptions and an updated dichotomous key for both asexual and sexual generations of these widespread species. Finally, as Belonocnema has emerged as a model organism for ecological and evolutionary studies, we clarify the species examined in published studies to date.

Asymmetric, but opposing reductions in immigrant viability and fecundity promote reproductive isolation among host-associated populations of an insect herbivore

Immigrant inviability can contribute to reproductive isolation (RI) during ecological speciation by reducing the survival of immigrants in non-native environments. However, studies that assess the fitness consequence of immigrants moving from native to non-native environments typically fail to explore the potential role of concomitant reductions in immigrant fecundity despite recent evidence suggesting its prominent role during local adaptation. Here, we evaluate the directionality and magnitude of both immigrant viability and fecundity to RI in a host-specific gall-forming wasp, Belonocnema treatae. Using reciprocal transplant experiments replicated across sites, we measure immigrant viability and fecundity by comparing differences in the incidence of gall formation (viability) and predicted the number of eggs per female (fecundity) between residents and immigrants in each of two host-plant environments. Reduced immigrant viability was found in one host environment while reduced immigrant fecundity was found in the other. Such habitat-dependent barriers resulted in asymmetric RI between populations. By surveying recent literature on local adaptation, we find that asymmetry in immigrant viability and fecundity are widespread across disparate taxa, which highlights the need to combine estimates of both common and overlooked barriers in cases of potential bi-directional gene flow to create a more comprehensive view of the evolution of RI.

Temporal isolation between sympatric host plants cascades across multiple trophic levels of host-associated insects

Phenological differences between host plants can promote temporal isolation among host-associated populations of insects with life cycles tightly coupled to plant phenology. Divergence in the timing of spring budbreak between two sympatric sister oak species has been shown to promote temporal isolation between host plants and their host-associated populations of a cynipid gall wasp. Here, we examined the generality of this mechanism by testing the hypothesis of cascading temporal isolation for five additional gall-formers and three natural enemy species associated with these same oak species. The timing of adult emergence from galls differed significantly between host-associated populations for all nine species and parallels the direction of the phenological differences between host plants. Differences in emergence timing can reduce gene flow between host-associated populations by diminishing mating opportunities and/or reducing the fitness of immigrants due to differences in the availability of ephemeral resources. Our study suggests that cascading temporal isolation could be a powerful 'biodiversity generator' across multiple trophic levels in tightly coupled plant-insect systems.

Cynipid gall wasps

Egan et al. introduce the reader to gall wasps, including a description of their life cycle and complex ecological interactions with host plants and natural enemies.

Diversity and distribution of Wolbachia in relation to geography, host plant affiliation and life cycle of a heterogonic gall wasp

BACKGROUND

The maternally inherited endosymbiont Wolbachia is widespread in arthropods and nematodes and can play an important role in the ecology and evolution of its host through reproductive manipulation. Here, we survey Wolbachia in Belonocnema treatae, a widely distributed North American cynipid gall forming wasp that exhibits regional host specialization on three species of oaks and alternation of sexually and asexually reproducing generations. We investigated whether patterns of Wolbachia infection and diversity in B. treatae are associated with the insect's geographic distribution, host plant association, life cycle, and mitochondrial evolutionary history.

RESULTS

Screening of 463 individuals from 23 populations including sexual and asexual generations from all three host plants across the southern U.S. showed an average infection rate of 56% with three common Wolbachia strains: wTre1-3 and an additional rare variant wTre4. Phylogenetic analysis based on wsp showed that these strains are unrelated and likely independently inherited. We found no difference in Wolbachia infection frequency among host plant associated populations or between the asexual and sexual generations, or between males and females of the sexual generation. Partially incomplete Wolbachia transmission rates might explain the occurrence of uninfected individuals. A parallel analysis of the mitochondrial cytochrome oxidase I gene in B. treatae showed high mtDNA haplotype diversity in both infected and uninfected populations suggesting an ancestral infection by Wolbachia as well as a clear split between eastern and western B. treatae mtDNA clades with a sequence divergence of > 6%. The strain wTre1 was present almost exclusively in the western clade while wTre2 and wTre3 occur almost exclusively in eastern populations. In contrast, the same strains co-occur as double-infections in Georgia and triple-infections in two populations in central Florida.

CONCLUSIONS

The diversity of Wolbachia across geographically and genetically distinct populations of B. treatae and the co-occurrence of the same strains within three populations highlights the complex infection dynamics in this system. Moreover, the association of distinct Wolbachia strains with mitochondrial haplotypes of its host in populations infected by different Wolbachia strains suggests a potential role of the endosymbiont in reproductive isolation in B. treatae.

Chromosomes of Belonocnematreatae Mayr, 1881 (Hymenoptera, Cynipidae)

Chromosomes of the asexual and sexual generation of the gall wasp Belonocnematreatae Mayr, 1881 (Cynipidae) were analyzed. Females of both generations have 2n = 20, whereas males of the sexual generation have n = 10. Cyclical deuterotoky is therefore confirmed in this species. All chromosomes are acrocentric and form a continuous gradation in size. This karyotype structure is probably ancestral for many gall wasps and perhaps for the family Cynipidae in general. Chromosome no. 7 carries a characteristic achromatic gap that appears to represent a nucleolus organizing region.

Interspecific patterns of phenotypic selection do not predict intraspecific patterns

We estimated linear (β) and nonlinear (γ) selection gradients to quantify host plant-mediated selection on the trait gall size in each of 22 unequally sampled subpopulations of the cynipid gall wasp Belonocnema treatae. We characterized the relationship between variation in subpopulation sample size and the magnitude of and the variance among selection gradients. We then tested the hypothesis that the intraspecific patterns we observed would follow two patterns that have emerged from published estimates of linear and nonlinear selection gradients compiled across species, namely that the average magnitude of β and γ and the variance among estimated β and γ decrease with increasing sample size. For both β and γ, intraspecific patterns of phenotypic selection in relation to sample size were not predicted by interspecific patterns. Thus, our results suggest that when selection is heterogeneous among subpopulations, variation in the biological basis for selection is more important in influencing estimates of selection than is variation in study size. Our study highlights the value of inspecting selection in relation to sampling effort at the level at which understanding the sources of variation in selection is most important, among populations within species.

Schmidt decompositions of parametric processes II: vector four-wave mixing

In vector four-wave mixing, one or two strong pump waves drive two weak signal and idler waves, each of which has two polarization components. In this paper, vector four-wave mixing processes in a randomly-birefringent fiber (modulation interaction, phase conjugation and Bragg scattering) are studied in detail. For each process, the Schmidt decompositions of the coupling matrices facilitate the solution of the signal-idler equations and the Schmidt decomposition of the associated transfer matrix. The results of this paper are valid for arbitrary pump polarizations.

Divergent host-plant use promotes reproductive isolation among cynipid gall wasp populations

Ecological speciation occurs when reproductive isolation evolves as a consequence of divergent natural selection among environments. A direct prediction of this process is that ecologically divergent pairs of populations will exhibit greater reproductive isolation than ecologically similar pairs of populations. By comparing allopatric populations of the cynipid gall wasp Belonocnema treatae infesting Quercus virginiana and Quercus geminata, we tested the role that divergent host use plays in generating ecological divergence and sexual isolation. We found differences in body size and gall structure associated with divergent host use, but no difference in neutral genetic divergence between populations on the same or different host plant. We observed significant assortative mating between populations from alternative host plants but not between allopatric populations on the same host plant. Thus, we provide evidence that divergent host use promotes speciation among gall wasp populations.

Full and semi-analytic analyses of two-pump parametric amplification with pump depletion

This paper solves the four coupled equations describing non-degenerate four-wave mixing, with the focus on amplifying a signal in a fiber optical parametric amplifier (FOPA). Based on the full analytic solution, a simple approximate solution describing the gain is developed. The advantage of this new approximation is that it includes the depletion of the pumps, which is lacking in the usual quasi-linearized approximation. With the proposed model it is thus simple to predict the gain of a FOPA, which we demonstrate with a highly nonlinear fiber to show that an undepleted FOPA can produce a flat gain spectrum with a bandwidth in the 100-nm range, centered on the zero-dispersion wavelength. When running the FOPA in depletion, this range can be slightly increased.

Quantum interference and entanglement induced by multiple scattering of light

We report on the effects of quantum interference induced by the transmission of an arbitrary number of optical quantum states through a multiple-scattering medium. We identify the role of quantum interference on the photon correlations and the degree of continuous variable entanglement between two output modes. It is shown that quantum interference survives averaging over all ensembles of disorder and manifests itself as increased photon correlations due to photon antibunching. Furthermore, the existence of continuous variable entanglement correlations in a volume speckle pattern is predicted. Our results suggest that multiple scattering provides a promising way of coherently interfering many independent quantum states of light of potential use in quantum information processing.

Self-pulsation threshold of Raman amplified Brillouin fiber cavities

An implicit equation for the oscillation threshold of stimulated Brillouin scattering from Raman amplified signals in fibers with external feedback is derived under the assumption of no depletion. This is compared to numerical investigations of Raman amplification schemes showing good agreement for high reflectivities. For low reflectivities and high attenuation or long fibers, the assumption of no depletion is shown not to be valid. In these cases the effects of the depletion on the self-pulsation is examined.

Host plant quality and local adaptation determine the distribution of a gall-forming herbivore

Herein we report results of transplant experiments that link variation in host plant quality to herbivore fitness at the local scale (among adjacent plants) with the process of local (demic) adaptation at the landscape scale to explain the observed distribution of the specialist gall former Belonocnema treatae (Hymenoptera: Cynipidae) within populations of its host plant, Quercus fusiformis. Field surveys show that leaf gall densities vary by orders of magnitude among adjacent trees and that high-gall-density trees are both rare (< 5%) and patchily distributed. B. treatae from each of five high-gall-density trees were reared on (1) the four nearest low-gall-density trees, (2) the four alternative high-gall-density trees, and (3) their natal trees (control). Each treatment (source X rearing site) was replicated three times. Nine components of performance that sequentially contribute to fitness were evaluated with over 21000 galls censused across the 25 experimental trees. When reared on their natal trees and compared with low-gall-density neighbors, transplanted gall formers had higher gall initiation success (P < 0.05), produced more (P < 0.001) and larger galls (P < 0.001), and produced a higher proportion of galls that exceeded the threshold size for natural enemy avoidance (P < 0.05). Comparison of gall-former performance on natal vs. alternative high-gall-density trees demonstrated significant (P < 0.001) differences in six performance measures with five differing in the direction predicted by the hypothesis of local adaptation. Overall, these linked experiments document direct and indirect effects of host plant variation on gall-former performance and demonstrate convincingly that (1) high-gall-density trees equate to high-quality trees that are surrounded by trees of relatively lower quality to the herbivore and (2) gall-former populations have become locally adapted to individual trees.

Effects of community composition and growth rate on aquifer biofilm bacteria and their susceptibility to betadine disinfection

Biofilm formation and function was studied in mixed culture using 20 bacterial strains isolated from a karst aquifer. When co-cultured in a glucose-limited chemostat, Vogesella indigofera and Pseudomonas putida were the dominant planktonic and biofilm organisms respectively. Biofilm formation and resistance to the iodine disinfectant betadine were then studied with monoculture and binary cultures of V. indigofera and P. putida and a 20-strain community. Biofilm population size [measured as colony-forming units (CFU) cm(-2)] increased with increasing species diversity. Significantly larger populations formed at dilution rates (DRs) of 0.0083 h(-1) than at 0.033 h(-1). P. putida populations were higher and V. indigofera lower in binary than in monoculture biofilms, suggesting that P. putida outcompeted V. indigofera. In binary biofilms, V. indigofera, a betadine-resistant organism, enhanced the survival of P. putida, a betadine-susceptible organism. In the 20-strain biofilms, this protective effect was not observed because of low concentrations of V. indigofera (< 1% of the total population), suggesting that resistant organisms contribute to overall biofilm disinfectant resistance. Growth at 0.033 h(-1) enhanced survival of V. indigofera biofilms against betadine. Although DR did influence survival of the other communities, its effects were neither consistent nor significant. All told, biofilm formation and betadine resistance are complex phenomena, influenced by community composition, growth rate and betadine concentration.