Host Race Formation Research Articles

Evidence for adaptation of Rhagoletis pomonella (Diptera: Tephritidae) on large-thorn hawthorn, Crataegus macracantha, in Okanogan County, Washington State, USA.

The apple maggot fly, Rhagoletis pomonella (Walsh) (Diptera: Tephritidae), was introduced from eastern North America into western North America via infested apples (Malus domestica Borkhausen) about 44 yr ago, where it subsequently adapted to 2 hawthorn species, Crataegus douglasii Lindley and C. monogyna Jacquin. Here, we test whether R. pomonella has also adapted to large-thorn hawthorn, Crataegus macracantha Loddiges ex Loudon, in Okanogan County, Washington State, USA. In 2020, 2021, and 2022, fruit of C. macracantha were shown to ripen in late September and were infested at rates from 0.7% to 3.0%. In laboratory rearing studies, large-thorn hawthorn flies from C. macracantha eclosed on average 9-19 days later than apple flies from earlier ripening apple (August-early September), consistent with large-thorn hawthorn flies having adapted to the later fruiting phenology of its host. In a laboratory no-choice test, significantly fewer (64.8%) large-thorn hawthorn than apple flies visited apples. In choice tests, greater percentages of large-thorn hawthorn than apple flies resided on and oviposited into C. macracantha versus apple fruit. Large-thorn hawthorn flies were also smaller in size than apple flies. Our results provide further support for the recursive adaptation hypothesis that R. pomonella has rapidly and independently specialized phenologically and behaviorally to different novel hawthorn hosts since its introduction into the Pacific Northwest of the USA, potentially leading to host race formation.

Phylogenomic inference of two widespread European leaf miner species complexes suggests mechanisms for sympatric speciation (Lepidoptera: Nepticulidae: Ectoedemia)

AbstractLeafmining insects have extraordinarily intimate relationships with their host plants and are therefore prime candidates for potential ecological speciation in sympatry, but how commonly this mode of speciation occurs in any group of life remains debated. Prior research on the pygmy moth (Nepticulidae) genus Ectoedemia using morphology, DNA barcoding, and multi‐marker phylogenetic approaches left two unresolved species complexes, each with each four described species. In this study, we thoroughly sampled each complex across a range of host plants and localities across Europe. We used a double digest restriction‐site associated DNA (ddRAD) sequencing approach to clarify species boundaries. In the E. rubivora complex, ddRAD data resolved all four species, contrary to morphological and COI data, which supports a potential scenario of host plant‐driven speciation where the host plant specialization provides an ecological barrier to hybridization. However, we found no indication of host race formation within the oligophagous E. atricollis (Stainton). In the E. subbimaculella complex, SNP data only partly distinguishes between the parapatric E. subbimaculella (Haworth) and E. heringi (Toll), but with some statistical overlap, suggesting incomplete lineage sorting which may represent early phases of host‐based ecological speciation, or admixture following a period of isolation.

Theme: Recent work in speciation research by women authors.

The Society for the Study of Evolution was established in 1943 by merging the National Research Council with the Society for the Study of Speciation (Evolution, 2021). Two leaders of this group, Mayr and Dobzhansky, set the agenda for decades of speciation research. Mayr's Biological Species Concept (1942), see also (Dobzhansky, 1935) put the focus of speciation research squarely on the evolution of reproductive isolation, and his vehement arguments against the plausibility of sympatric speciation stoked decades of debates over whether or not reproductive isolation could evolve in the absence of geographical isolation. Likewise, Dobzhansky's list of “isolating mechanisms” (1951) and model for the evolution of intrinsic postzygotic isolation (the eponymous Bateson‐Dobzhansky‐Muller model; [Dobzhansky, 1935]) laid the groundwork for comparative and genetic speciation studies. Several prominent contributions to our understanding of speciation were published during the following decades: Bush's empirical study of sympatric host race formation in the apple maggot fly (1969), Felsenstein's modeling work demonstrating how recombination halts progress toward speciation in sympatry (1981), and Coyne and Orr's comparative analyses of the evolution of prezygotic and postzygotic isolation (Coyne & Orr, 1989, 1997). These papers, in turn, have had an enormous influence on modern speciation research, which continues to feature prominently in the pages of Evolution (Matute & Cooper, 2021).

Genomic Traces of the Fruit Fly Anastrepha obliqua Associated with Its Polyphagous Nature.

Simple SummaryIndividuals of a polyphagous species, such as Anastrepha obliqua, that plague different host plants, present differences at the genome level; these differences are associated with adaptive processes related to the exploitation of the resource and can lead to speciation in sympatry, first with the formation of host races. In this studio, we used pooled RAD-seq to assess genomic differentiation and population structure in sympatric populations of Anastepha obliqua that infest three different host plants. The results obtained support the effect of host plants on genomic differentiation in populations of the species A. obliqua. We identified a small group of candidate genes that could be under divergent selection, a product of the selective pressure mediated by the host plants.Anastrepha obliqua (Macquart) (Diptera: Tephritidae) is an important pest in the neotropical region. It is considered a polyphagous insect, meaning it infests plants of different taxonomic families and readily colonizes new host plants. The change to new hosts can lead to diversification and the formation of host races. Previous studies investigating the effect of host plants on population structure and selection in Anastrepha obliqua have focused on the use of data from the mitochondrial DNA sequence and microsatellite markers of nuclear DNA, and there are no analyses at the genomic level. To better understand this issue, we used a pooled restriction site-associated DNA sequencing (pooled RAD-seq) approach to assess genomic differentiation and population structure across sympatric populations of Anastrepha obliqua that infest three host plants—Spondias purpurea (red mombin), Mangifera indica (mango) of the family Anacardiaceae and Averrhoa carambola (carambola) of the family Oxalidaceae—in sympatric populations of the species Anastrepha obliqua of Inter-Andean Valley of the Cauca River in southwestern Colombia. Our results show genomic differentiation of populations from carambola compared to mango and red mombin populations, but the genetic structure was mainly established by geography rather than by the host plant. On the other hand, we identified 54 SNPs in 23 sequences significantly associated with the use of the host plant. Of these 23 sequences, we identified 17 candidate genes and nine protein families, of which four protein families are involved in the nutrition of these flies. Future studies should investigate the adaptive processes undergone by phytophagous insects in the Neotropics, using fruit flies as a model and state-of-the-art molecular tools.

Fundamental and specialized questions on host race formation and speciation – an introduction

AbstractHost race formation and speciation are at the core of the enormous insect diversification. Insect–host relationships can take many forms, ranging from parasitic to mutualistic. Despite its long history of study, many questions remain about host race formation and speciation. They are addressed in eight original papers of this special issue, including two review‐type articles. In line with the scope of Entomologia Experimentalis et Applicata, they include both fundamental and applied studies. They reveal that host race formation and speciation can take many forms and that they are still active topics of entomological research.

A test of adaptive divergence in a newly discovered host association of the soapberry bugJadera haematolomaon Mexican buckeye,Ungnadia speciosa

AbstractHost races represent an important step in the speciation process of phytophagous insects as they reflect the maintenance of genetically divergent host‐associated populations in the face of appreciable gene flow. The red‐shouldered soapberry bug,Jadera haematoloma(Herrich‐Schäffer) (Hemiptera: Rhopalidae), is an oligophagous seed predator with a history of host race evolution on plant associations in the (soapberry) family Sapindaceae. Soapberry bugs are a model group for understanding rapid ecological adaptation to their hosts, and hence good candidates for investigating evolutionary divergence in host associations over short timescales. Here, we describe the recent discovery of Mexican buckeye,Ungnadia speciosaEndl., as a host ofJ. haematolomain a region of the Chihuahuan desert including west Texas and southeastern New Mexico, USA. This host differs fromJ. haematoloma’s previously recorded hosts in the Sapindaceae in seed chemistry, ecology, and phylogeny. The tendency toward rapid, host‐associated adaptations by populations ofJ. haematolomaand the unique biology of the newly discoveredUngnadiahost create the opportunity for potential host race formation, as it overlaps geographically with two previously recorded host plants in this region – the native western soapberry tree,Sapindus saponariavar.drummondii(Hook & Arn.), and the non‐native goldenrain tree,Koelreuteria paniculataLaxm. We explore the possibility of host race formation onUngnadia‐associated insects by testing for host‐associated differentiation in morphology and feeding behaviors. We find evidence of differentiation in the length of the mouth parts, which is an ecologically relevant feeding trait between host plant species with larger or smaller seed capsules. This divergence is maintained in the face of potential gene flow by reproductive isolation in the form of habitat isolation, which we detect in host preference trials. Together, our results demonstrate that soapberry bugs associated with this newly discovered host exhibit morphological and behavioral traits consistent with host race formation, but additional work is required to confirm its state along the speciation continuum.

Genomic evidence for host‐associated differentiation in an animal parasite, Dermacentor variabilis, the American dog tick

AbstractThe process responsible for the formation of genetically distinct populations associated with different host species is known as host‐associated differentiation (HAD). Many insect parasites of plants have been shown to exhibit HAD but there have been fewer studies of HAD in parasites of vertebrate animals. Previous to this study, HAD has been documented in at least three species of ticks. The American dog tick, Dermacentor variabilis (Say) (Acari: Ixodidae) was chosen as the focal species for this study due to its importance as the vector of tularemia and Rocky Mountain spotted fever. Previous population genetic studies of this tick found the existence of various haplotypes but the tick’s host origins were unknown. In this study, ticks were collected from 15 vertebrate host species to test for HAD using single nuclear polymorphisms (SNPs). In total, 136 individual D. variabilis ticks were sequenced using ddRADseq. Genomic evidence was found to point to D. variabilis exhibiting HAD on eight different hosts. A STRUCTURE analysis showed that the highest posterior probability was obtained with a population size of eight and these populations correlated with host species. Pairwise FST values were as high as 0.622 and indicated a range of genetic distinction between host groups. In addition, ticks collected from the vegetation appeared as one homogenous distinct genotype suggesting the existence of nidicolous (nest dwelling) and non‐nidicolous genotypes. The identification of host race formation occurring in this animal parasite has implications for the understanding of D. variabilis pathogen transmission and targeted control efforts because genetically distinct populations can differ in traits relevant to these applications.

Recursive adaptation in action: allochronic isolation and divergence of host‐associated populations of the apple maggot fly, Rhagoletis pomonella, following its recent introduction to the western USA

AbstractAn outstanding issue in the study of insect host races concerns the idea of ‘recursive adaptive divergence’, whereby adaptation can occur repeatedly across space and/or time, and the most recent adaptive episode is defined by one or more previously similar cases. The host plant shift of the apple maggot fly, Rhagoletis pomonella (Walsh) (Diptera: Tephritidae, Carpomyini), from ancestral downy hawthorn [Crataegus mollis (Torr. & A. Gray) Scheele] to introduced, domesticated apple (Malus domestica Borkh.) in the eastern USA has long served as a model system for investigating ecologically driven host race formation in phytophagous insect specialists. Here, we report results from an annual geography survey of eclosion time demonstrating a similar ecological pattern among nascent host‐associated populations of the fly recently introduced ca. 40 years ago from its native range in the east into the Pacific Northwest (PNW) region of the USA. Specifically, using data collected from 25 locations across 5 years, we show that apple‐infesting fly populations in the PNW have rapidly and repeatedly shifted (and maintained differences in) their adult eclosion life‐history timing to infest two novel hawthorn hosts with different fruiting phenologies – a native species (Crataegus douglasii Lindl.) and an introduced species (Crataegus monogyna Jacq.) – generating partial allochronic reproductive isolation in the process. The shifts in the PNW parallel the classic case of host race formation in the eastern USA, but have occurred bi‐directionally to two hawthorn species with phenologies slightly earlier (black hawthorn) and significantly later (ornamental hawthorn) than apple. Our results imply that R. pomonella can both possess and retain extensive‐standing variation (i.e., ‘adaptive memory’) in diapause traits, even following introductions, to rapidly and temporally track novel phenological host opportunities when they arise. Thus, ‘specialized’ host races may not constitute evolutionary dead ends. Rather, adaptive phenotypic and genetic memory may carry over from one host shift to the next, recursively facilitating host race formation in phytophagous insects.

Adaptive divergence and post-zygotic barriers to gene flow between sympatric populations of a herbivorous mite

Plant-herbivore interactions promote the generation and maintenance of both plant and herbivore biodiversity. The antagonistic interactions between plants and herbivores lead to host race formation: the evolution of herbivore types specializing on different plant species, with restricted gene flow between them. Understanding how ecological specialization promotes host race formation usually depends on artificial approaches, using laboratory experiments on populations associated with agricultural crops. However, evidence on how host races are formed and maintained in a natural setting remains scarce. Here, we take a multidisciplinary approach to understand whether populations of the generalist spider mite Tetranychus urticae form host races in nature. We demonstrate that a host race co-occurs among generalist conspecifics in the dune ecosystem of The Netherlands. Extensive field sampling and genotyping of individuals over three consecutive years showed a clear pattern of host associations. Genome-wide differences between the host race and generalist conspecifics were found using a dense set of SNPs on field-derived iso-female lines and previously sequenced genomes of T. urticae. Hybridization between lines of the host race and sympatric generalist lines is restricted by post-zygotic breakdown, and selection negatively impacts the survival of generalists on the native host of the host race. Our description of a host race among conspecifics with a larger diet breadth shows how ecological and reproductive isolation aid in maintaining intra-specific variation in sympatry, despite the opportunity for homogenization through gene flow. Our findings highlight the importance of explicitly considering the spatial and temporal scale on which plant-herbivore interactions occur in order to identify herbivore populations associated with different plant species in nature. This system can be used to study the underlying genetic architecture and mechanisms that facilitate the use of a large range of host plant taxa by extreme generalist herbivores. In addition, it offers the chance to investigate the prevalence and mechanisms of ecological specialization in nature.

Host-dependent dispersal demonstrates both-sex host specificity in cuckoos

Abstract In a parasite species, the dispersal of individuals should be dependent on the host species to which they are specialized; thus, any sexual/individual difference in host specificity may influence their dispersal patterns and, hence, population genetic structures. However, such predictions remain poorly verified in generalist avian brood parasites that are composed of multiple lineages of host-specific races. Here, we show the dispersal consequences inferred from spatial genetic structures and their association with host specificity in brood parasitic common cuckoos Cuculus canorus in which female-specific host race formation has been widely accepted. Genetic sampling from adult cuckoos confirmed restricted dispersal in both sexes and resultant genetic structures between populations where different host species breed allopatrically, whereas it was not the case between distant areas inhabited by the same host species. Contrary to the female host race hypothesis, our results demonstrate that male cuckoos may also have host specificity and disperse accordingly, conclusively allowing us to hypothesize the formation of a host race including both sexes.

Common pheromone use among host-associated populations of the browntail moth, Euproctis chrysorrhoea, displaying different adult phenologies

The diversity of herbivorous insects may arise from colonization and subsequent specialization on different host plants. Such specialization requires changes in several insect traits, which may lead to host race formation if they reduce gene flow among populations that feed on different plants. Behavioural changes may play a relevant role in host race formation, for example if different races evolve distinct sexual communication signals or adult phenology. Previous research has revealed differences in larval phenology in different host-associated populations of the browntail moth, Euproctis chrysorrhoea (Lepidoptera: Erebidae). Here, sex pheromones among populations of this species are compared, and pheromone trapping data obtained is used in the field to build a phenological model that tests whether populations that feed on different plants differ in their adult flight period. The chemical and electrophysiological analyses revealed that two E. chrysorrhoea populations (on Prunus and on Arbutus unedo) use the same sex pheromone component for mate finding. Our trapping data, however, showed that males fly on average 25 days earlier in populations whose larvae feed on A. unedo compared to those whose larvae feed on Quercus species. Although the shifted phenology described here may underlie host-plant specialization in E. chrysorrhoea, and adults of this species are short-lived, the use of a common sexual pheromone and a large overlap in flight periods suggest that host race formation via allochronic isolation is unlikely in this moth. (Less)

Rapid and repeatable host plant shifts drive reproductive isolation following a recent human-mediated introduction of the apple maggot fly, Rhagoletis pomonella.

Ecological speciation via host-shifting is often invoked as a mechanism for insect diversification, but the relative importance of this process is poorly understood. The shift of Rhagoletis pomonella in the 1850s from the native downy hawthorn, Crataegus mollis, to introduced apple, Malus pumila, is a classic example of sympatric host race formation, a hypothesized early stage of ecological speciation. The accidental human-mediated introduction of R. pomonella into the Pacific Northwest (PNW) in the late 1970s allows us to investigate how novel ecological opportunities may trigger divergent adaptation and host race formation on a rapid timescale. Since the introduction, the fly has spread in the PNW, where in addition to apple, it now infests native black hawthorn, Crataegus douglasii, and introduced ornamental hawthorn, Crataegus monogyna. We use this "natural experiment" to test for genetic differentiation among apple, black, and ornamental hawthorn flies co-occurring at three sympatric sites. We report evidence that populations of all three host-associations are genetically differentiated at the local level, indicating that partial reproductive isolation has evolved in this novel habitat. Our results suggest that conditions suitable for initiating host-associated divergence may be common in nature, allowing for the rapid evolution of new host races when ecological opportunity arises.

Increases in Genistein in Medicago sativa Confer Resistance against the Pisum Host Race of Acyrthosiphon pisum.

Interspecific interaction with host plants have important consequences for the host race formation of herbivorous insects. Plant secondary metabolites, particularly those that are involved in host races specializing on plants, warrant the theory of host specialization. Acyrthosiphon pisum comprises various host races that adapt to different Fabaceae plants, which provides an ideal system for determining the behavioral and physiological mechanisms underlying host-adaptive diversification. The current study evaluated the effects of host transfer on population fitness, feeding behavior and the transcriptome-wide gene expression of the two host races of A. pisum, one of which was originally from Medicago sativa and the other from Pisum sativum. The results showed that the Pisum host race of A. pisum had a lower population abundance and feeding efficiency than the Medicago host race in terms of a longer penetration time and shorter duration times of phloem ingestion when fed on M. sativa. In contrast, few differences were found in the population abundance and feeding behavior of A. pisum between the two host races when fed on P. sativum. Meanwhile, of the nine candidate phenolic compounds, only genistein was significantly affected by aphid infestation; higher levels of genistein were detected in M. sativa after feeding by the Pisum host race, but these levels were reduced relative to uninfested controls after feeding by the Medicago host race, which suggested that genistein may be involved in the specialization of the aphid host race on M. sativa. Further exogenous application of genistein in artificial diets showed that the increase in genistein reduced the survival rate of the Pisum host race but had little effect on that of the Medicago host race. The transcriptomic profiles indicated that the transcripts of six genes with functions related to detoxification were up-regulated in the Pisum host race relative to the Medicago host race of A. pisum. These results suggested that the inducible plant phenolics and associated metabolic process in aphids resulted in their differential adaptations to their Fabaceae host.

Are Eurosta solidaginis on Solidago rugosa a divergent host-associated race?

The ball-gall fly Eurosta solidaginis is considered a classic example of host-race formation in herbivorous insects, with host-associated races evolving at least twice, including the well-known pair on Solidago altissima and S. gigantea. Yet E. solidaginis has been occasionally observed galling other Solidago species. Here, we explore the origins of E. solidaginis on Solidago rugosa. We hypothesize that flies associated with S. rugosa are derived from the S. altissima host-race, and ask whether S. rugosa-associated flies have initiated host-race formation. We compared genetic variation among seventeen E. solidaginis populations collected from S. rugosa, S. altissima, and S. gigantea, in an adjacent COI/COII region of the mitochondrial genome. Across the study area, E. solidaginis flies from S. rugosa were as diverged from S. altissima-flies as they were from S. gigantea-flies (pairwise ΦPT 0.075 and 0.78 respectively) but S. altissima and S. gigantea-flies appeared considerably less diverged (0.002). This pattern was driven by the majority of flies, regardless of host-plant, sharing the same haplotype across the study area. However, we detected several site/region-specific haplotypes, not shared among host species. At the local site scale we were able to distinguish S. gigantea-associated fly haplotypes from either S. altissima or S. rugosa haplotypes, but the majority of S. altissima and S. rugosa-flies shared the same haplotype locally. These patterns of haplotype diversity support existing evidence of host-associated divergence in S. altissima- and S. gigantea-associated flies, and suggest that S. rugosa flies are either the same host-race as, or are recently derived from, S. altissima flies. Successful development on S. rugosa and the use of S. rugosa in the absence of use on sympatric S. altissima suggests that E. solidaginis either has a single oligophagous race using both S. altissima and S. rugosa or is in the earliest stages of host-associated differentiation.

Standing geographic variation in eclosion time and the genomics of host race formation in Rhagoletis pomonella fruit flies.

Taxa harboring high levels of standing variation may be more likely to adapt to rapid environmental shifts and experience ecological speciation. Here, we characterize geographic and host‐related differentiation for 10,241 single nucleotide polymorphisms in Rhagoletis pomonella fruit flies to infer whether standing genetic variation in adult eclosion time in the ancestral hawthorn (Crataegus spp.)‐infesting host race, as opposed to new mutations, contributed substantially to its recent shift to earlier fruiting apple (Malus domestica). Allele frequency differences associated with early vs. late eclosion time within each host race were significantly related to geographic genetic variation and host race differentiation across four sites, arrayed from north to south along a 430‐km transect, where the host races co‐occur in sympatry in the Midwest United States. Host fruiting phenology is clinal, with both apple and hawthorn trees fruiting earlier in the North and later in the South. Thus, we expected alleles associated with earlier eclosion to be at higher frequencies in northern populations. This pattern was observed in the hawthorn race across all four populations; however, allele frequency patterns in the apple race were more complex. Despite the generally earlier eclosion timing of apple flies and corresponding apple fruiting phenology, alleles on chromosomes 2 and 3 associated with earlier emergence were paradoxically at lower frequency in the apple than hawthorn host race across all four sympatric sites. However, loci on chromosome 1 did show higher frequencies of early eclosion‐associated alleles in the apple than hawthorn host race at the two southern sites, potentially accounting for their earlier eclosion phenotype. Thus, although extensive clinal genetic variation in the ancestral hawthorn race exists and contributed to the host shift to apple, further study is needed to resolve details of how this standing variation was selected to generate earlier eclosing apple fly populations in the North.

Genomic transitions during host race and species formation.

Darwin recognized species as discontinuous, yet considered them to be formed by an incremental process of natural selection. Recent theoretical work on 'genome-wide congealing' is bridging this gap between the gradualism of divergent selection and rapid genome-wide divergence, particularly during ecological speciation-with-gene-flow. Host races and species of phytophagous insects, displaying a spectrum of divergence and gene flow among member taxa, provide model systems for testing predicted non-linear transitions from 'genic' divergence at a few uncoupled loci to 'genomic' divergence with genome-wide coupling of selected loci and strong reproductive isolation. Integrating across natural history, genomics, and evolutionary theory, emerging research suggests a tipping point from 'genic' to 'genomic' divergence between host races and species, during both sympatric speciation and secondary contact.

Genetic and chemical divergence among host races of a socially parasitic ant.

Host–parasite associations facilitate the action of reciprocal selection and can drive rapid evolutionary change. When multiple host species are available to a single parasite, parallel specialization on different hosts may promote the action of diversifying natural selection and divergence via host race formation. Here, we examine a population of the kidnapper ant (Polyergus mexicanus) that is an obligate social parasite of three sympatric ant species: Formica accreta, F. argentea, and F. subaenescens (formerly F. fusca). Behavioral and ecological observations of P. mexicanus have shown that individual colonies parasitize only one species of host and that new Polyergus queens maintain host fidelity when establishing new colonies. To successfully adapt to a particular host, Polyergus ants may mimic or camouflage themselves with the species‐specific chemical cues (cuticular hydrocarbons) that their hosts use to ascertain colony membership. To investigate the extent of host specialization, we collected both genetic and chemical data from P. mexicanus that parasitize each of the three different Formica species in sympatry. We show that host‐associated genetic structure exists for both maternally inherited mitochondrial DNA data and biparentally inherited microsatellite markers. We also show that P. mexicanus can be distinguished by chemical profile according to host due to partial matching with their host. Our results support the hypothesis that host race formation is occurring among lineages of P. mexicanus that use different Formica hosts. Thus, this system may represent a promising model for illuminating the early steps of divergence, accumulation of reproductive isolation, and speciation.

Morphological variation of Aphidius ervi Haliday (Hymenoptera: Braconidae) associated with different aphid hosts.

BackgroundParasitoids are frequently used in biological control due to the fact that they are considered host specific and highly efficient at attacking their hosts. As they spend a significant part of their life cycle within their hosts, feeding habits and life history of their host can promote specialization via host-race formation (sequential radiation). The specialized host races from different hosts can vary morphologically, behaviorally and genetically. However, these variations are sometimes inconspicuous and require more powerful tools in order to detect variation such as geometric morphometrics analysis.MethodsWe examined Aphidius ervi, an important introduced biological control agent in Chile associated with a great number of aphid species, which are exploiting different plant hosts and habitats. Several combinations (biotypes) of parasitoids with various aphid/host plant combinations were analyzed in order to obtain measures of forewing shape and size. To show the differences among defined biotypes, we chose 13 specific landmarks on each individual parasitoid wing. The analysis of allometric variation calculated in wing shape and size over centroid size (CS), revealed the allometric changes among biotypes collected from different hosts. To show all differences in shape of forewings, we made seven biotype pairs using an outline-based geometric morphometrics comparison.ResultsThe biotype A. pis_pea (Acyrthosiphon pisum on pea) was the extreme wing size in this study compared to the other analyzed biotypes. Aphid hosts have a significant influence in the morphological differentiation of the parasitoid forewing, splitting biotypes in two groups. The first group consisted of biotypes connected with Acyrthosiphon pisum on legumes, while the second group is composed of biotypes connected with aphids attacking cereals, with the exception of the R. pad_wheat (Rhopalosiphum padi on wheat) biotype. There was no significant effect of plant species on parasitoid wing size and shape.DiscussionAlthough previous studies have suggested that the genotype of parasitoids is of greater significance for the morphological variations of size and shape of wings, this study indicates that the aphid host on which A. ervi develops is the main factor to alter the structure of parasitoid forewings. Bigger aphid hosts implied longer and broader forewings of A. ervi.

The influence of climatic niche preferences on the population genetic structure of a mistletoe species complex.

The prevalent view on genetic structuring in parasitic plants is that host-race formation is caused by varying degrees of host specificity. However, the relative importance of ecological niche divergence and host specificity to population differentiation remains poorly understood. We evaluated the factors associated with population differentiation in mistletoes of the Psittacanthus schiedeanus complex (Loranthaceae) in Mexico. We used genetic data from chloroplast sequences and nuclear microsatellites to study population genetic structure and tested its association with host preferences and climatic niche variables. Pairwise genetic differentiation was associated with environmental and host preferences, independent of geography. However, environmental predictors appeared to be more important than host preferences to explain genetic structure, supporting the hypothesis that the occurrence of the parasite is largely determined by its own climatic niche and, to a lesser degree, by host specificity. Genetic structure is significant within this mistletoe species complex, but the processes associated with this structure appear to be more complex than previously thought. Although host specificity was not supported as the major determinant of population differentiation, we consider this to be part of a more comprehensive ecological model of mistletoe host-race formation that incorporates the effects of climatic niche evolution.

Geographic variation and trade-offs in parasitoid virulence.

Host-parasitoid systems are characterized by a continuous development of new defence strategies in hosts and counter-defence mechanisms in parasitoids. This co-evolutionary arms race makes host-parasitoid systems excellent for understanding trade-offs in host use caused by evolutionary changes in host immune responses and parasitoid virulence. However, knowledge obtained from natural host-parasitoid systems on such trade-offs is still limited. In this study, the aim was to examine trade-offs in parasitoid virulence in Asecodes parviclava (Hymenoptera: Eulophidae) when attacking three closely related beetles: Galerucella pusilla, Galerucella calmariensis and Galerucella tenella (Coleoptera: Chrysomelidae). A second aim was to examine whether geographic variation in parasitoid infectivity or host immune response could explain differences in parasitism rate between northern and southern sites. More specifically, we wanted to examine whether the capacity to infect host larvae differed depending on the previous host species of the parasitoids and if such differences were connected to differences in the induction of host immune systems. This was achieved by combining controlled parasitism experiments with cytological studies of infected larvae. Our results reveal that parasitism success in A.parviclava differs both depending on previous and current host species, with a higher virulence when attacking larvae of the same species as the previous host. Virulence was in general high for parasitoids from G.pusilla and low for parasitoids from G.calmariensis. At the same time, G.pusilla larvae had the strongest immune response and G.calmariensis the weakest. These observations were linked to changes in the larval hemocyte composition, showing changes in cell types important for the encapsulation process in individuals infected by more or less virulent parasitoids. These findings suggest ongoing evolution in parasitoid virulence and host immune response, making the system a strong candidate for further studies on host race formation and speciation.