HOST-ASSOCIATED GENETIC DIFFERENTIATION IN PHYTOPHAGOUS INSECTS: GENERAL PHENOMENON OR ISOLATED EXCEPTIONS? EVIDENCE FROM A GOLDENROD-INSECT COMMUNITY

Molecular and morphometric analyses reveal host-specific cryptic speciation in a mite species, Tetranychus neocaledonicus (Andre, 1933) (Acari: Tetranychidae)

Host- and habitat-induced morphological shape and size variations are common in phytophagous and parasitic taxa. Several integrated morphological and molecular techniques have been commonly used to understand host-induced morpho-cryptic species forms. Compared to other arthropods, cryptic speciation was more common in Acari. This study focused on the host-specific morphological cryptic shape and size variations of Tetranychus neocaledonicus, collected from moringa and cassava hosts. We used geometric morphometric analysis to uncover the shape and size of inter-and intra-spider mite populations, and discovered that host-specific shape and size variations existed in spider mites regardless of sex. Interestingly, there was no phylogenetic signal in spider mites, implying that the morpho-cryptic speciation of T. neocaledonicus is solely based on the host-induced selection. The molecular clock hypothesis was accepted in our CO1 and 18s rRNA phylogeny analyses, and spider mites collected from both hosts were genetically less diverse. We conclude that T. neocaledonicus exhibited morphologically detectable cryptic population diversity in each host but that these populations are evolutionarily young form. Apart from these host-induced variations, we also monitored the impact of the clearing agent (lactic acid) on the shape and size of T. neocaledonicus; from this study, we proved that the clearing agent significantly alters the taxonomically important morphological traits of spider mites irrespective of the mites' sex, as confirmed by multivariate statistical analysis. This is the first study report to investigated the host-induced morphological variations of spider mites and the impact of a clearing agent.

Host-plant adaptation in xylophagous insect-microbiome systems: Contributionsof longicorns and gut symbionts revealed by parallel metatranscriptome

Adaptation to host plants is of great significance in the ecology of xylophagous insects. The specific adaptation to woody tissues is made possible through microbial symbionts. We investigated the potential roles of detoxification, lignocellulose degradation, and nutrient supplementation of Monochamus saltuarius and its gut symbionts in host plant adaptation using metatranscriptome. The gut microbial community structure of M. saltuarius that fed on the two plant species were found to be different. Plant compound detoxification and lignocellulose degradation genes have been identified in both beetles and gut symbionts. Most differentially expressed genes associated with host plant adaptations were up-regulated in larvae fed on the less suitable host (Pinus tabuliformis) compared to larvae fed on the suitable host (Pinus koraiensis). Our findings indicated that M. saltuarius and its gut microbes respond to plant secondary substances through systematic transcriptome responses, allowing them to adapt to unsuitable host plants.

The role of plants in the formation of species-specific features in grass flies (Diptera, Chloropidae, Meromyza)

In the current manuscript, we present the results of comparative analysis of seven species of Meromyza flies in the "variegata" cluster and of the evolutionary close species M.inornata, based the following criteria: 1) 14 external key features; 2) shape and area of the anterior processes of postgonites; 3) mtDNA CO1 region and 4) host plant diversity data. We could demonstrate the primary role of host plants in species formation inside genus Meromyza and calculated the timing of the divergence of M.inornata and the species of "variegata" cluster. Based on our estimates of evolution rate for mtDNA CO1 gene, we could conclude that that divergence of herbs happened before the speciation of grass flies Meromyza. Meromyza species, close to the ancestral species of the cluster, are adapted to the wide range of host plants. We revealed the most informative variables h1, S and Plant analysing data with the following statistical methods: linear discriminant analysis - LDA, regularised discriminant analysis - RDA, flexible discriminant analysis - FDA and probabilistic neural network - PNN. The highest classification accuracy was achieved using PNN (99%) and the lowest when using LDA (95.8%). When the Plant trait was excluded, the classification accuracy decreased by 14%. We revealed the significant trends in size change of the anterior process of the postgonite amongst studies species. This morphological structure is an element of male reproductive apparatus critical for the restriction of interspecies mating. We determined three branches of speciation in the "variegata" cluster and five trends in the evolution of this cluster, based on the external morphological features. We showed that M.variegata and especially M.mosquensis, the species closest to the ancestral haplotype, have the largest number of features typical of those of M.inornata. Based on the external features and the area of the anterior process of the postgonite, we reconstructed the phylogenetic position of M.elbergi in the cluster. In accordance with the obtained outcomes, we could conclude that the distribution, species diversity and the adaptation of the grass flies to narrow oligophagy were directly connected to host plant diversity. The adaptation to different host plants could be the main factor in divergence of grass flies and their evolution started later than the diversification in the Pooideae subfamily of grasses.

Plant pathogen-mediated rapid acclimation of a host-specialized aphid to a non-host plant

Polyphagous aphids often consist of host-specialized lineages, which have greater fitness on their native hosts than on others. The underlying causes are important for understanding of the evolution of diet breadth and host shift of aphids. The cotton-melon aphid Aphis gossypii Glover is extremely polyphagous with many strict host-specialized lineages. Whether and how the lineage specialized on the primary host hibiscus shifts to the secondary host cucumber remains elusive. We found that the hibiscus-specialized lineage suffered high mortality and gave birth to very few nymphs developing into yellow dwarfs on fresh cucumber leaves, and did not inflict any damage symptoms on cucumber plants. The poor performance did not improve with prolonged exposure to cucumber; however, it did significantly improve when the cucumber leaves were pre-infected with a biotrophic phytopathogen Pseudoperonospora cubensis. More importantly, the hibiscus-specialized lineage with two-generation feeding experience on pre-infected cucumber leaves performed as well as the cucumber-specialized lineage did on fresh cucumber leaves, and inflicted typical damage symptoms on intact cucumber plants. Electrical penetration graph (EPG) indicated that the hibiscus-specialized lineage did not ingest phloem sap from fresh cucumber leaves but succeeded in ingesting phloem sap from pre-infected cucumber leaves, which explained the performance improvement of the hibiscus-specialized lineage on pre-infected cucumber leaves. This study revealed a new pathway for the hibiscus-specialized lineage to quickly acclimate to cucumber under the assistance of the phytopathogen. We considered that the short feeding experience on pre-infected cucumber may activate expression of effector genes that are related to specific host utilization. We suggest to identify host-specific effectors by comparing proteomes or/and transcriptomes of the hibiscus-specialized lineage before and after acclimating to cucumber.

Hind wing variation in Leptura annularis complex among European and Asiatic populations (Coleoptera, Cerambycidae)

The ability to quantify morphological variation is essential for understanding the processes of species diversification. The geometric morphometrics approach allows reliable description of variation in animals, including insects. Here, this method was used to quantify the morphological variation among European and Asiatic populations of Lepturaannularis Fabricius, 1801 and its closely related species L.mimica Bates, 1884, endemic for Japan and Sakhalin islands. Since the taxonomic status of these two taxa is differently interpreted by taxonomists, they are collectively called “Lepturaannularis complex” in this paper. The analysis was based on the measurements of hind wings of 269 specimens from six populations from Europe and Asia. The level of morphological divergence between most of continental European and Asiatic populations was relatively small and proportional to the geographic distance between them. However, distinct morphotype was detected in Sakhalin Is. and Japan. These data confirm the morphological divergence of the endemic L.mimica species. Obtained results highlight the potential of the geometric morphometric method in studying morphological variation in beetles.

Genetic Variability of Two Leaffooted Bugs, Leptoglossus clypealis and Leptoglossus zonatus (Hemiptera: Coreidae) in the Central Valley of California

Leaffooted plant bugs (LFPBs) (Leptoglossus spp., Guérin-Méneville) (Hemiptera: Coreidae) are large seed-feeding bugs native to the Western Hemisphere. In California, several Leptoglossus spp. feed on almonds, pistachios, and pomegranate and are occasional pests. The objective of this study was to survey the different species of Leptoglossus present in almond, pistachio, and pomegranate orchards in the Central Valley of California. We used two molecular markers, amplified fragment length polymorphisms (AFLPs) and mitochondrial DNA COI, to determine the number of species or strains of each species, and to infer whether individuals of each species move and possibly interbreed with populations from the other host plants. Two species of leaffooted bugs were abundant, Leptoglossus clypealis Heidemann, and Leptoglossus zonatus (Dallas). L. clypealis was collected in almond and pistachio, while L. zonatus was found on all three host plants, but was the dominant species in pomegranate. The AFLP results indicated that L. clypealis consisted of one species, which suggests it moves between almonds and pistachios during the growing season. Mitochondrial DNA COI for L. clypealis found 1-2% divergence between sequences, and a high haplotype diversity of 0.979 with 17 haplotypes. The AFLP results for L. zonatus found two genetically divergent populations which were morphologically similar. The mtDNA COI sequences for L. zonatus were used for haplotype analysis; three haplotypes were found in California, with one haplotype shared with collections from Brazil. The importance of genetic variability and cryptic species for pest management are discussed.

Relative Abundance and Strain Diversity in the Bacterial Endosymbiont Community of a Sap-Feeding Insect Across Its Native and Introduced Geographic Range

Most insects are associated with bacterial symbionts. The bacterial diversity and community composition within hosts may play an important role in shaping insect population biology, ecology and evolution. We focussed on the bacterial microbiome of the Australian fig homotomid Mycopsylla fici (Hemiptera: Psylloidea), which can cause defoliation of its only host tree, Ficus macrophylla. This sap-feeding insect is native to mainland Australia and Lord Howe Island (LHI) but also occurs where its host has been planted, notably in New Zealand. By using a high-throughput 16S rDNA amplicon sequencing approach, we compared the bacterial diversity and community composition in individual adult males of four host populations, Sydney, Brisbane, LHI and Auckland. We also compared males, females and nymphs of the Sydney population. The microbiome of M. fici was simple and consisted mostly of the following three maternally inherited endosymbiont species: the primary endosymbiont Carsonella, a secondary (S-) endosymbiont and Wolbachia. However, the relative abundance of their sequence reads varied between host populations, except for similarities between Sydney and Auckland. In addition, insects from Sydney and Auckland had identical bacterial strains supporting the hypothesis that Sydney is the source population for Auckland. In contrast, mainland and LHI populations harboured the same S-endosymbiont, co-diverged Carsonella but different Wolbachia strains. Besides detecting endosymbiont-specific patterns of either co-evolution or horizontal acquisition, our study highlights that relative abundance of maternally inherited endosymbionts should also be taken into account when studying bacterial communities across host populations, as variations in bacterial density may impact host biology and ecology.

Differential Host Plant-Associated Genetic Variation Between Sympatric Mite Species of the Genus Oligonychus (Acari: Tetranychidae)

Adaptation to different host plants can lead to host-associated differentiation (HAD). The mites Oligonychus perseae and Oligonychus punicae have a broad range of host plants, but, to date, records of them coexisting sympatrically had only been reported on avocado. However, our field observations showed both species coexisting on host plants other than avocado. The lack of previous records of these mites on the host plants studied here suggests only recent divergence to new host plant species. Previous studies showed that O. punicae had a limited migration capacity compared with O. perseae, suggesting that O. punicae is more likely to develop a close host plant relationship leading to HAD. Adults of both species were collected from trees hosting both mite species. Three genera of host plants considered were Persea, Salix, and Alnus; two species within one genus were Alnus jorullensis and Alnus acuminata; and three varieties within one species were Persea americana var. Fuerte, var. Hass, and var. Criollo, a noncommercial variety. Using sequence data from a segment of the mitochondrial cytochrome oxidase subunit I, the phylogenetic relationships and genetic population structure of both mite species in relation to the host plant were determined. Oligonychus perseae populations showed a significant population structure in relation to host plant at the species and genus level, but there was no effect of variety. In contrast, host plant explained none of the genetic variation among O. punicae populations. The potential role of coexistence mechanisms in the contrasting genetic population structure of both mite species is discussed.

Enemy-free space promotes maintenance of host races in an aphid species

The enormous biodiversity of herbivorous insects may arise from ecological speciation via continuous host-plant switches. Whether such switches are successful depends on the trade-off between different selection pressures that act on herbivores. Decreased herbivore performance due to suboptimal nutrition might be compensated for by a reduced natural enemy pressure. As a consequence, an "enemy-free space" on a certain plant might facilitate host-plant switches and maintain biotypes. To test this hypothesis, we used the pea aphid (Acyrthosiphon pisum) complex, which consists of at least 11 genetically distinct host races that are native to specific legume host plants but can all develop on the universal host plant Vicia faba. Three A. pisum host races native to Trifolium pratense, Pisum sativum, and Medicago sativa were investigated in experiments on their respective host plants and on the universal host plant V. faba. We found that hoverflies preferred to oviposit on P. sativum and the universal host V. faba. Since feeding by hoverfly larvae suppressed aphid population growth on these host plants, the native hosts M. sativa and T. pratense provided enemy-free space for the respective A. pisum races. Mobile predators, such as ants and ladybird beetles, preferred Pisum race aphids on V. faba over P. sativum. Thus, all three of the native host plants studied supply enemy-free space for A. pisum compared to the universal host V. faba. Reducing encounters between aphid races on V. faba would reduce gene flow among them and could contribute to maintaining the host races.

Cryptic speciation in the Acari: a function of species lifestyles or our ability to separate species?

There are approximately 55,000 described Acari species, accounting for almost half of all known Arachnida species, but total estimated Acari diversity is reckoned to be far greater. One important source of currently hidden Acari diversity is cryptic speciation, which poses challenges to taxonomists documenting biodiversity assessment as well as to researchers in medicine and agriculture. In this review, we revisit the subject of biodiversity in the Acari and investigate what is currently known about cryptic species within this group. Based on a thorough literature search, we show that the probability of occurrence of cryptic species is mainly related to the number of attempts made to detect them. The use of, both, DNA tools and bioassays significantly increased the probability of cryptic species detection. We did not confirm the generally-accepted idea that species lifestyle (i.e. free-living vs. symbiotic) affects the number of cryptic species. To increase detection of cryptic lineages and to understand the processes leading to cryptic speciation in Acari, integrative approaches including multivariate morphometrics, molecular tools, crossing, ecological assays, intensive sampling, and experimental evolution are recommended. We conclude that there is a demonstrable need for future investigations focusing on potentially hidden mite and tick species and addressing evolutionary mechanisms behind cryptic speciation within Acari.

Transcriptome analysis of host-associated differentiation in Bemisia tabaci (Hemiptera: Aleyrodidae)

Host-associated differentiation is one of the driving forces behind the diversification of phytophagous insects. In this study, host induced transcriptomic differences were investigated in the sweetpotato whitefly Bemisia tabaci, an invasive agricultural pest worldwide. Comparative transcriptomic analyses using coding sequence (CDS), 5′ and 3′ untranslated regions (UTR) showed that sequence divergences between the original host plant, cabbage, and the derived hosts, including cotton, cucumber and tomato, were 0.11–0.14%, 0.19–0.26%, and 0.15–0.21%, respectively. In comparison to the derived hosts, 418 female and 303 male transcripts, respectively, were up-regulated in the original cabbage strain. Among them, 17 transcripts were consistently up-regulated in both female and male whiteflies originated from the cabbage host. Specifically, two ESTs annotated as Cathepsin B or Cathepsin B-like genes were significantly up-regulated in the original cabbage strain, representing a transcriptomic response to the dietary challenges imposed by the host shifting. Results from our transcriptome analysis, in conjunction with previous reports documenting the minor changes in their reproductive capacity, insecticide susceptibility, symbiotic composition and feeding behavior, suggest that the impact of host-associated differentiation in whiteflies is limited. Furthermore, it is unlikely the major factor contributing to their rapid range expansion/invasiveness.

Genetic and morphological diversity of Trisetacus species (Eriophyoidea: Phytoptidae) associated with coniferous trees in Poland: phylogeny, barcoding, host and habitat specialization

Eriophyoid species belonging to the genus Trisetacus are economically important as pests of conifers. A narrow host specialization to conifers and some unique morphological characteristics have made these mites interesting subjects for scientific inquiry. In this study, we assessed morphological and genetic variation of seven Trisetacus species originating from six coniferous hosts in Poland by morphometric analysis and molecular sequencing of the mitochondrial cytochrome oxidase subunit I gene and the nuclear D2 region of 28S rDNA. The results confirmed the monophyly of the genus Trisetacus as well as the monophyly of five of the seven species studied. Both DNA sequences were effective in discriminating between six of the seven species tested. Host-dependent genetic and morphological variation in T. silvestris and T. relocatus, and habitat-dependent genetic and morphological variation in T. juniperinus were detected, suggesting the existence of races or even distinct species within these Trisetacus taxa. This is the first molecular phylogenetic analysis of the Trisetacus species. The findings presented here will stimulate further investigations on the evolutionary relationships of Trisetacus as well as the entire Phytoptidae family.

Parallel patterns of morphological and behavioral variation among host-associated populations of two gall wasp species

A powerful approach to address the general factors contributing to ecological speciation is to compare distantly related taxa that inhabit the same selective environments. In this design, similarities among taxa can elucidate general mechanisms of the process whereas differences may uncover specific factors important to the process for individual taxa. Herein, we present evidence of parallel patterns of morphological and behavioral variation among host-associated populations of two species of cynipid gall wasps, Belonocnema treatae and Disholcaspis quercusvirens, that each exhibit a life cycle intimately tied to the same two host plant environments, Quercus geminata and Q. virginiana. Across both gall-former species we find consistent differences in body size and gall morphology associated with host plant use, as well as strong differences in host plant preference, a measure of habitat isolation among populations. These consistent differences among taxa highlight the important role of host plant use in promoting reproductive isolation and morphological variation among herbivorous insect populations–a prerequisite for ecological speciation.

The gall midge Asphondylia borrichiae (Diptera: Cecidomyiidae): an indigenous example of host-associated genetic divergence in sympatry

Speciation usually is conceptualized as occurring via three biogeographic modes: allopatry, parapatry, and sympatry. Sympatric speciation has been the most controversial because of the difficulty of developing plausible theoretical models in which the homogenizing effects of gene flow are sufficiently overcome to permit genetic divergence to occur in the absence of geographic barriers restricting gene flow. Recently, a number of hypothetical models for sympatric speciation have been advanced and several candidate study systems have provided evidence of sympatric divergence, although many of the systems so identified involve introduced species, especially in the cases of host-race formation in phytophagous insects, which expand their host range and use a novel host. Although these cases demonstrate the reality of sympatric divergence, they do not address which mode of speciation predominates in indigenous communities. Asphondylia borrichiae Rossi & Strong has been proposed as a potential example of sympatric divergence in a fully indigenous system, based on the results of a host-choice experiment involving three host-plant species. In the current study, we report significant differences in the genetic composition of midge populations collected from each host in situ, supporting the hypothesis of sympatric genetic divergence among the morphologically identical host-associated populations of A. borrichiae and consistent with host fidelity in oviposition choice.

Community genetic interactions mediate indirect ecological effects between a parasitoid wasp and rhizobacteria

Indirect ecological effects (IEEs) clearly influence species dynamics and abundance, yet relatively little is known about how they influence the evolution of species involved. While genetic variation in the species causing and responding to the IEE has obvious effects, the influence of genetic variation in intermediate species remains unexamined. Given the often counterintuitive responses of populations to IEEs this seems a significant omission. Following a community genetics approach, we used a model tetra-trophic system (parasitoid wasp, aphid, barley, and rhizobacteria) to investigate the effect of genetic interactions within the two linking species (aphids and barley) on the IEE of rhizobacteria on wasps. We show that 12.4% of the variation in wasp size, a proxy for fitness, is explained by higher-order interactions between aphid genotype (A), barley genotype (B), and presence or absence of rhizobacteria (R) (Genotype[B] x Genotype[A] x Environment[R]). Thus, the IEE of rhizobacteria on the parasitoid wasp is influenced by the specific combination of aphid and barley genotypes that mediate the interactions. In some cases changes in the genotypes of the intermediate species completely reverse the effect of rhizobacteria on wasp size. Our work demonstrates that within-species genetic variation is important in shaping IEEs in communities, an essential component of community evolutionary processes.

Infestation of a novel host plant by Tephritis conura (Diptera: Tephritidae) in northern Britain: host-range expansion or host shift?

The addition of a novel host plant to a phytophagous insect's diet may result in subsequent host-plant specialisation, and is believed to be a key cause for speciation in this trophic group. In northern Britain, the tephritid fly Tephritis conura has experienced a unique host-plant expansion, from the melancholy thistle Cirsium heterophyllum to the marsh thistle C. palustre. Here, we examine whether the incorporation of C. palustre in the repertoire of British T. conura flies has caused genetic divergence between populations infesting the old host and the novel host, and how British populations differ from populations infesting C. heterophyllum in continental Europe where C. palustre is not infested. No evidence for restricted gene flow among British C. palustre and C. heterophyllum flies was found. Significant differentiation between British and continental T. conura was found at only one allozyme locus, hexokinase, and caused by a new allele, Hex_95. Hexokinase is related to host-race formation in continental European flies infesting C. heterophyllum and C. oleraceum, and might be linked to loci determining host choice. Based on morphological and phenological data from previous studies, we suggest that T. conura in Britain has adapted to the novel host but that host-race formation is impeded by similar plant phenologies.