Effects of host plant on life‐history traits in the polyphagous spider mite Tetranychus urticae

Studying antagonistic coevolution between host plants and herbivores is particularly relevant for polyphagous species that can experience a great diversity of host plants with a large range of defenses. Here, we performed experimental evolution with the polyphagous spider mite Tetranychus urticae to detect how mites can exploit host plants. We thus compared on a same host the performance of replicated populations from an ancestral one reared for hundreds of generations on cucumber plants that were shifted to either tomato or cucumber plants. We controlled for maternal effects by rearing females from all replicated populations on either tomato or cucumber leaves, crossing this factor with the host plant in a factorial design. About 24 generations after the host shift and for all individual mites, we measured the following fitness components on tomato leaf fragments: survival at all stages, acceptance of the host plant by juvenile and adult mites, longevity, and female fecundity. The host plant on which mite populations had evolved did not affect the performance of the mites, but only affected their sex ratio. Females that lived on tomato plants for circa 24 generations produced a higher proportion of daughters than did females that lived on cucumber plants. In contrast, maternal effects influenced juvenile survival, acceptance of the host plant by adult mites and female fecundity. Independently of the host plant species on which their population had evolved, females reared on the tomato maternal environment produced offspring that survived better on tomato as juveniles, but accepted less this host plant as adults and had a lower fecundity than did females reared on the cucumber maternal environment. We also found that temporal blocks affected mite dispersal and both female longevity and fecundity. Taken together, our results show that the host plant species can affect critical parameters of population dynamics, and most importantly that maternal and environmental conditions can facilitate colonization and exploitation of a novel host in the polyphagous T. urticae, by affecting dispersal behavior (host acceptance) and female fecundity.     

The evolution of novel host use is unlikely to be constrained by trade‐offs or a lack of genetic variation

The genetic and ecological factors that shape the evolution of animal diets remain poorly understood. For herbivorous insects, the expectation has been that trade‐offs exist, such that adaptation to one host plant reduces performance on other potential hosts. We investigated the genetic architecture of alternative host use by rearing individual Lycaeides melissa butterflies from two wild populations in a crossed design on two hosts (one native and one introduced) and analysing the genetic basis of differences in performance using genomic approaches. Survival during the experiment was highest when butterfly larvae were reared on their natal host plant, consistent with local adaptation. However, cross‐host correlations in performance among families (within populations) were not different from zero. We found that L. melissa populations possess genetic variation for larval performance and variation in performance had a polygenic basis. We documented very few genetic variants with trade‐offs that would inherently constrain diet breadth by preventing the optimization of performance across hosts. Instead, most genetic variants that affected performance on one host had little to no effect on the other host. In total, these results suggest that genetic trade‐offs are not the primary cause of dietary specialization in L. melissa butterflies.     

Host plant adaptation during contemporary range expansion in the monarch butterfly

Herbivores that have recently expanded their host plant ranges provide opportunities to test hypotheses about the evolution of host plant specialization. Here, we take advantage of the contemporary global range expansion of the monarch butterfly (Danaus plexippus) and conduct a reciprocal rearing experiment involving monarch populations with divergent host plant assemblages. Specifically, we ask the following questions: (1) Do geographically disparate populations of monarch butterflies show evidence for local adaptation to their host plants? If so, what processes contribute to this pattern? (2) How is dietary breadth related to performance across multiple host species in monarch populations? (3) Does the coefficient of variation in performance vary across sympatric versus allopatric hosts? We find evidence for local adaptation in larval growth rate and survival based on sympatric/allopatric contrasts. Migratory North American monarchs, which have comparatively broad host breadth, have higher mean performance than derived nonmigratory populations across all host plant species. Monarchs reared on their sympatric host plants show lower coefficient of variation in performance than monarchs reared on allopatric hosts. We focus our discussion on possible mechanisms contributing to local adaptation to novel host plants and potential explanations for the reduction in performance that we observed in derived monarch populations.     

Oviposition preference and larval performance within a diverging lineage of lycaenid butterflies

Abstract. 1. The butterfly genus Mitoura in Northern California includes three nominal species associated with four host plants having parapatric or interdigitated ranges. Genetic analyses have shown the taxa to be very closely related, and adults from all host backgrounds will mate and produce viable offspring in the laboratory. Oviposition preference and larval performance were investigated with the aim of testing the hypothesis that variation in these traits can exist in a system in which non‐ecological barriers to gene flow (i.e. geographic barriers and genetic incompatibilities) appear to be minimal. 2. Females were sampled from 12 locations throughout Northern California, including sympatric and parapatric populations associated with the four different host‐plant species. Oviposition preference was assayed by confining wild‐caught females with branches of all four host species and counting the number of eggs laid on each. Offspring were reared on the same host species and two measures of larval success were taken: per cent survival and pupal weight. 3. For populations associated with one of the hosts, incense cedar, the preference–performance relationship is simple: the host that females chose is the plant which results in the highest pupal weights for offspring. The preference–performance relationship for populations associated with the other hosts is more complex and may reflect different levels of local adaptation. The variation in preference and performance reported here suggests that these traits can evolve when non‐ecological barriers to gene flow are low, and that differences in these traits may be important for the evolution of reproductive isolation within Mitoura.     

You stay,but I Hop: Host shifting near and far co‐dominated the evolution of Enchenopa treehoppers

The importance and prevalence of phylogenetic tracking between hosts and dependent organisms caused by co‐evolution and shifting between closely related host species have been debated for decades. Most studies of phylogenetic tracking among phytophagous insects and their host plants have been limited to insects feeding on a narrow range of host species. However, narrow host ranges can confound phylogenetic tracking (phylogenetic tracking hypothesis) with host shifting between hosts of intermediate relationship (intermediate hypothesis). Here, we investigated the evolutionary history of the Enchenopa binotata complex of treehoppers. Each species in this complex has high host fidelity, but the entire complex uses hosts across eight plant orders. The phylogenies of E. binotata were reconstructed to evaluate whether (1) tracking host phylogeny; or (2) shifting between intermediately related host plants better explains the evolutionary history of E. binotata. Our results suggest that E. binotata primarily shifted between both distant and intermediate host plants regardless of host phylogeny and less frequently tracked the phylogeny of their hosts. These findings indicate that phytophagous insects with high host fidelity, such as E. binotata, are capable of adaptation not only to closely related host plants but also to novel hosts, likely with diverse phenology and defense mechanisms.     

Local adaptation and ecological genetics of host-plant specialization in a leaf beetle

The tendency of insect species to evolve specialization to one or a few plant species is probably a major reason for the remarkable diversity of herbivorous insects. The suggested explanations for this general trend toward specialization include a range of evolutionary mechanisms, whose relative importance is debated. Here we address two potentially important mechanisms: (i) how variation in the geographic distribution of host use may lead to the evolution of local adaptation and specialization; (ii) how selection for specialization may lead to the evolution of trade‐offs in performance between different hosts. We performed a quantitative genetic experiment of larval performance in three different populations of the alpine leaf beetle Oreina elongata reared on two of its main host plants. Due to differences in host availability, each population represents a distinctly different selective regime in terms of host use including selection for specialization on one or the other host as well as selection for utilizing both hosts during the larval stage. The results suggest that selection for specialization has lead to some degree of local adaptations in host use: both single‐host population had higher larval growth rate on their respective native host plant genus, while there was no difference between plant treatments in the two‐host population. However, differences between host plant treatments within populations were generally small and the degree of local adaptation in performance traits seems to be relatively limited. Genetic correlations in performance traits between the hosts ranged from zero in the two‐host population to significantly positive in the single‐host populations. This suggests that selection for specialization in single host populations typically also increased performance on the alternative host that is not naturally encountered. Moreover, the lack of a positive genetic correlation in the two host‐population give support for the hypothesis that performance trade‐offs between two host plants may typically evolve when a population have adapted to both these plants. We conclude that although there is selection for specialization in larval performance traits it seems as if the genetic architecture of these traits have limited the divergence between populations in relative performance on the two hosts.     

Rapid specialization of counter defenses enables two-spotted spider mite to adapt to novel plant hosts

Genetic adaptation, occurring over a long evolutionary time, enables host-specialized herbivores to develop novel resistance traits and to efficiently counteract the defenses of a narrow range of host plants. In contrast, physiological acclimation, leading to the suppression and/or detoxification of host defenses, is hypothesized to enable broad generalists to shift between plant hosts. However, the host adaptation mechanisms used by generalists composed of host-adapted populations are not known. Two-spotted spider mite (TSSM; Tetranychus urticae) is an extreme generalist herbivore whose individual populations perform well only on a subset of potential hosts. We combined experimental evolution, Arabidopsis thaliana genetics, mite reverse genetics, and pharmacological approaches to examine mite host adaptation upon the shift of a bean (Phaseolus vulgaris)-adapted population to Arabidopsis. We showed that cytochrome P450 monooxygenases are required for mite adaptation to Arabidopsis. We identified activities of two tiers of P450s: general xenobiotic-responsive P450s that have a limited contribution to mite adaptation to Arabidopsis and adaptation-associated P450s that efficiently counteract Arabidopsis defenses. In approximately 25 generations of mite selection on Arabidopsis plants, mites evolved highly efficient detoxification-based adaptation, characteristic of specialist herbivores. This demonstrates that specialization to plant resistance traits can occur within the ecological timescale, enabling the TSSM to shift to novel plant hosts.

Mites can evolve highly efficient detoxification-based adaptation in approximately 25 generations on an initially unfavorable plant host, revealing that specialization can occur within the ecological timescale.     

Host-associated fitness trade-offs do not limit the evolution of diet breadth in the small milkweed bug Lygaeus kalmii (Hemiptera: Lygaeidae)

Theoretical models of evolution in a temporally variable environment predict that genotypes with low variance in fitness across generations will be favored. When host use varies temporally and fitness trade-offs exist among hosts, such that an increase in performance on one host results in a correlated decrease on the other, selection for low variance in fitness across generations will favor genotypes which are generalists. Before predictions such as this can be extended to natural herbivore populations, however, it is necessary to understand the extent to which performance trade-offs limit simultaneous adaptation to multiple hosts. The experiment reported here compares two populations of the common milkweed bug, Lygaeus kalmii (Hemiptera: Lygaeidae) which differ in patterns of host usage. One population is largely restricted to milkweed (Asclepias spp.) when milkweed seeds are available, but becomes a scavenger on a large assortment of available seeds when milkweed seeds are unavailable. The second population is restricted largely to dandelion (Taraxacum officinale), without access to milkweed. We examine these populations to test for host-associated genetic trade-offs between specialization on dandelion (Taraxacum) and two species of milkweed, Asclepias fascicularis, which is low in cardiac glycoside content, and A. speciosa, which is high in cardiac glycoside content. Despite the difference in patterns of host use of the two L. kalmii populations, the populations did not differ in their performance on any of the host plants. Within each population, bugs performed nearly as well on each host, except that bugs had significantly lower survivorship on dandelion than on either milkweed species. Trade-offs in performance among hosts were not present in either population: estimated genetic correlations across hosts were strongly positive. The inability of this study to detect host-associated fitness trade-offs is consistent with most published data on this topic.     

Parallel evolution in sympatric, hybridizing species: performance of Colias butterflies on their introduced host plants

Shared ancestry and introgression can contribute to genetic similarity between hybridizing species, and it is generally difficult to disentangle these causes. However, shared ancestry plays a more limited role in traits that have recently undergone parallel directional selection in the two species, permitting the role of introgression to be better understood. The butterflies Colias eurytheme (Boisduval) and Colias philodice (Godart) (Lepidoptera, Pieridae) are native to North America and have shifted their host ranges in parallel onto several introduced weedy and agricultural legumes. These butterflies hybridize at moderate rates throughout their range, and there is a strong possibility that they could be sharing host‐associated adaptations. We split families of each species among nine introduced, prospective hosts and measured survivorship, larval duration, pupal weight, and a new variable, effective daily growth rate (DGR), analogous to a compound daily interest rate in economics. We found strong effects of host, sex, and family (species), but negligible effects of the host*species interaction that would indicate species‐specific differences in performance on different hosts. We found species‐specific life‐history differences: C. eurytheme matured significantly later and reached a significantly larger body size than C. philodice while growing at the same DGR. Protandry was strong, and males, in addition to pupating sooner than females, grew significantly faster than females as measured by DGR. We measured broad‐sense heritabilities and genetic correlations for host‐associated performance variables. Most pairwise comparisons of performance among hosts and most pairwise comparisons between performance variables showed positive genetic correlations, except survivorship where little heritability was found. Nevertheless, a factorial multivariate analysis of variance of G‐matrices showed highly significant species, host, and host*species interactions, suggesting differentially evolving genetic architectures underlying host adaptation in these two species, despite the small differences in overall performance. At least some of the genes affecting host performance in Colias are likely to be in the small, species‐diagnostic regions and not shared via introgression between these hybridizing species. For biologists interested in the evolutionary ecology of their host associations, including applied biologists managing their agricultural pest potential, C. eurytheme and C. philodice are most usefully studied as if they were a single polymorphic species wherever they co‐exist. In studying species that hybridize readily with a sympatric congener, it may often be necessary to include the second species in the experimental design.     

Local adaptation of a holoparasitic plant,Cuscuta europaea: variation among populations

Locally adapted parasites have higher infectivity and/or fitness on sympatric than on allopatric hosts. We tested local adaptation of a holoparasitic plant, Cuscuta europaea, to its host plant, Urtica dioica. We infected hosts from five sites with holoparasites from the same five sites and measured local adaptation in terms of infectivity and parasite performance (biomass) in a reciprocal cross‐infection experiment. The virulence of the parasite did not differ between sympatric and allopatric hosts. Overall, parasites had higher infectivity on sympatric hosts but infectivity and parasite performance varied among populations. Parasites from one of the populations showed local adaptation in terms of performance, whereas parasites from one of the populations had higher infectivity on allopatric hosts compared with sympatric hosts. This among‐population variation may be explained by random variation in parasite adaptation to host populations or by time‐lagged co‐evolutionary oscillations that lead to fluctuations in the level of local adaptation.     

Symbionts in excess? No correlation between symbiont density and the ability of mealybug hosts to exploit plant species or tolerate insecticide stress

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Host plant preference and performance of the vine weevil Otiorhynchus sulcatus

Abstract 1 The relationship between reproductive performance and preference for potential host plants of the vine weevil is investigated, as shown in tests on contact (or feeding) preference, presented herein, and tests on olfactory preference, published elsewhere. 2 Assessment of reproductive performance shows that the host‐plant range of the adult vine weevil Otiorhynchus sulcatus in Europe is limited to one gymnosperm genus (Taxus sp.) and a broad range of angiosperm plants in two subclasses of the Dicotyledonae, namely Dilleniidae and Rosidae. The successful reproduction on very distantly related plant taxa suggests that the original weevil‐ and plant‐habitat has mediated the current host‐plant range of the vine weevil. 3 Contact‐preference tests with equally suitable hosts, such as Aronia, Fragaria, Euonymus and Taxus, and one less suitable host, Humulus, indicate a mismatch between contact preference and performance and, as far as olfactory preferences are known, these match neither the contact preferences nor the performance. This mismatch may arise because (i) host plant species offered do not occur in weevil habitat in Europe (e.g. Aronia and the cultivated Fragaria come from North America) and (ii) predation (or disease) risks differ among host plants, thereby altering effective reproductive performance. 4 With respect to performance on novel hosts (Thuja, Prunus) and bad hosts (Rhododendron), some between‐individual variation is found within a single population, suggesting that local populations harbour (possibly genetic) variation for adaptation to new hosts. How this variation is maintained in the face of strong selection pressures on local populations of flightless and thelytokous weevils, is an important question for understanding the broad host plant range in the vine weevil.     

Experimental evidence of host race formation in Mitoura butterflies (Lepidoptera: Lycaenidae)

A population of herbivorous insects that shifts to a novel host can experience selection pressures that result in adaptation to the new resource. Host race formation, considered an early stage of the speciation process, may result. The current study investigates host shifts and variation in traits potentially involved in the evolution of reproductive isolation among populations of the juniper hairstreak butterfly, Mitoura gryneus. Mitoura are closely associated with their host trees (Cupressaceae) and exhibit host plant fidelity: in addition to larval development and oviposition, host trees support male leks and mating. Female oviposition preference for the natal host, and differential fitness of larvae when reared on natal versus alternate hosts, are indications that specialization and local adaptation to the natal host plant are occurring. Populations with single host plant associations (Juniperus ashei, J. pinchotii and J. virginiana) as well as populations with multiple hosts (both J. ashei and J. pinchotii) were examined. Concordance between female preference and larval performance was found for J. ashei‐associated populations. Population‐level variation in the patterns of female preference and larval performance, both within and among host associations, may reflect differences in the timing and direction of colonization of hosts. For a single nominal species that otherwise exhibits no morphological or phenological differences, the experimental assessment of specialization and host fidelity in M. gryneus provides strong support for the hypothesis of ongoing host race formation in these butterflies.     

MULTITRAIT,HOST‐ASSOCIATED DIVERGENCE AMONG SETS OF BUTTERFLY POPULATIONS: IMPLICATIONS FOR REPRODUCTIVE ISOLATION AND ECOLOGICAL SPECIATION

When populations use different resources, they tend to diverge in traits that affect performance on those resources. The extent and complexity of divergence that is achieved will depend on gene flow, genetic constraints, and the character of divergent selection. We describe divergent host adaptation among Californian populations of the Melitaeine butterfly, Euphydryas editha. Divergence in seven traits created parallel phenotypic suites, each suite associated with the use of a different host species, either Collinsia torreyi or Pedicularis semibarbata. The suites involved alighting responses of adults (probably to visual stimuli), chemosensory responses to leaf surfaces, vertical positioning of adults and larvae (probably due to geotaxis), partitioning of reproductive effort among clutches, and larval performance. Remarkably, the divergent suites did not occur sympatrically, despite ubiquitous co‐occurrence of the hosts, and we know of only one site where any Collinsia species is used sympatrically with another host. In contrast, E. editha often uses two host genera sympatrically when neither of them is Collinsia. We suggest that adaptation to Collinsia is incompatible with adaptation to other hosts and may generate extrinsic postzygotic reproductive isolation among populations. Despite the apparent rarity of host‐shift‐associated speciation in Melitaeine butterflies, adoption of Collinsia as a host may lead to allopatric ecological speciation.     

Evolutionary adaptation to host plants in a laboratory population of the phytophagous mite Tetranychus urticae Koch

Summary For evolutionary expansion of host range to occur in an herbivore population, genetic variation in ability to survive on and/or accept new hosts must be present. To determine whether a population of the phytophagous mite Tetranychus urticae contained such variation, I established lines from the population on two hosts on which mites initially showed both high juvenile mortality and low acceptance, tomato and broccoli. In less than ten generations, mites from the line kept on each host showed both lower mortality and greater acceptance on it than mites from a control line kept on lima bean, a favorable host for T. urticae. Host acceptance was measured by the proportion of mites attempting to disperse from leaves of the host. The line kept on tomato but not the one kept on broccoli also increased in development rate on its host. These results and those of a similar previous experiment on cucumber indicate that T. urticae populations can adapt to a diversity of initially unfavorable hosts. T. urticae populations therefore should be able to respond to temporal and spatial variation in host availability by adapting to the most abundant hosts.     

The evolution of climate tolerance in conifer‐feeding aphids in relation to their host's climatic niche

Climate adaptation has major consequences in the evolution and ecology of all living organisms. Though phytophagous insects are an important component of Earth's biodiversity, there are few studies investigating the evolution of their climatic preferences. This lack of research is probably because their evolutionary ecology is thought to be primarily driven by their interactions with their host plants. Here, we use a robust phylogenetic framework and species‐level distribution data for the conifer‐feeding aphid genus Cinara to investigate the role of climatic adaptation in the diversity and distribution patterns of these host‐specialized insects. Insect climate niches were reconstructed at a macroevolutionary scale, highlighting that climate niche tolerance is evolutionarily labile, with closely related species exhibiting strong climatic disparities. This result may suggest repeated climate niche differentiation during the evolutionary diversification of Cinara. Alternatively, it may merely reflect the use of host plants that occur in disparate climatic zones, and thus, in reality the aphid species' fundamental climate niches may actually be similar but broad. Comparisons of the aphids' current climate niches with those of their hosts show that most Cinara species occupy the full range of the climatic tolerance exhibited by their set of host plants, corroborating the hypothesis that the observed disparity in Cinara species' climate niches can simply mirror that of their hosts. However, 29% of the studied species only occupy a subset of their hosts' climatic zone, suggesting that some aphid species do indeed have their own climatic limitations. Our results suggest that in host‐specialized phytophagous insects, host associations cannot always adequately describe insect niches and abiotic factors must be taken into account.     

Parasite host range and the evolution of host resistance

Parasite host range plays a pivotal role in the evolution and ecology of hosts and the emergence of infectious disease. Although the factors that promote host range and the epidemiological consequences of variation in host range are relatively well characterized, the effect of parasite host range on host resistance evolution is less well understood. In this study, we tested the impact of parasite host range on host resistance evolution. To do so, we used the host bacterium Pseudomonas fluorescens SBW25 and a diverse suite of coevolved viral parasites (lytic bacteriophage Φ2) with variable host ranges (defined here as the number of host genotypes that can be infected) as our experimental model organisms. Our results show that resistance evolution to coevolved phages occurred at a much lower rate than to ancestral phage (approximately 50% vs. 100%), but the host range of coevolved phages did not influence the likelihood of resistance evolution. We also show that the host range of both single parasites and populations of parasites does not affect the breadth of the resulting resistance range in a naïve host but that hosts that evolve resistance to single parasites are more likely to resist other (genetically) more closely related parasites as a correlated response. These findings have important implications for our understanding of resistance evolution in natural populations of bacteria and viruses and other host–parasite combinations with similar underlying infection genetics, as well as the development of phage therapy.     

Adaptation to host plants may prevent rapid insect responses to climate change

We must consider the role of multitrophic interactions when examining species' responses to climate change. Many plant species, particularly trees, are limited in their ability to shift their geographic ranges quickly under climate change. Consequently, for herbivorous insects, geographic mosaics of host plant specialization could prohibit range shifts and adaptation when insects become separated from suitable host plants. In this study, we examined larval growth and survival of an oak specialist butterfly (Erynnis propertius) on different oaks (Quercus spp.) that occur across its range to determine if individuals can switch host plants if they move into new areas under climate change. Individuals from Oregon and northern California, USA that feed on Q. garryana and Q. kelloggii in the field experienced increased mortality on Q. agrifolia, a southern species with low nutrient content. In contrast, populations from southern California that normally feed on Q. agrifolia performed well on Q. agrifolia and Q. garryana and poorly on the northern, high elevation Q. kelloggii. Therefore, colonization of southern E. propertius in higher elevations and some northern locales may be prohibited under climate change but latitudinal shifts to Q. garryana may be possible. Where shifts are precluded due to maladaptation to hosts, populations may not accrue warm‐adapted genotypes. Our study suggests that, when interacting species experience asynchronous range shifts, historical local adaptation may preclude populations from colonizing new locales under climate change.     

Genetic structure and host–parasite co‐divergence: evidence for trait‐specific local adaptation

Host–parasite co‐evolution can lead to genetic differentiation among isolated host–parasite populations and local adaptation between parasites and their hosts. However, tests of local adaptation rarely consider multiple fitness‐related traits although focus on a single component of fitness can be misleading. Here, we concomitantly examined genetic structure and co‐divergence patterns of the trematode Coitocaecum parvum and its crustacean host Paracalliope fluviatilis among isolated populations using the mitochondrial cytochrome oxidase I gene (COI). We then performed experimental cross‐infections between two genetically divergent host–parasite populations. Both hosts and parasites displayed genetic differentiation among populations, although genetic structure was less pronounced in the parasite. Data also supported a co‐divergence scenario between C. parvum and P. fluviatilis potentially related to local co‐adaptation. Results from cross‐infections indicated that some parasite lineages seemed to be locally adapted to their sympatric (home) hosts in which they achieved higher infection and survival rates than in allopatric (away) amphipods. However, local, intrinsic host and parasite characteristics (host behavioural or immunological resistance to infections, parasite infectivity or growth rate) also influenced patterns of host–parasite interactions. For example, overall host vulnerability to C. parvum varied between populations, regardless of parasite origin (local vs. foreign), potentially swamping apparent local co‐adaptation effects. Furthermore, local adaptation effects seemed trait specific; different components of parasite fitness (infection and survival rates, growth) responded differently to cross‐infections. Overall, data show that genetic differentiation is not inevitably coupled with local adaptation, and that the latter must be interpreted with caution in a multi‐trait context.