Host Races Research Articles

Ecological divergence despite common mating sites: Genotypes and symbiotypes shed light on cryptic diversity in the black bean aphid species complex.

Different host plants represent ecologically dissimilar environments for phytophagous insects. The resulting divergent selection can promote the evolution of specialized host races, provided that gene flow is reduced between populations feeding on different plants. In black bean aphids belonging to the Aphis fabae complex, several morphologically cryptic taxa have been described based on their distinct host plant preferences. However, host choice and mate choice are largely decoupled in these insects: they are host-alternating and migrate between specific summer host plants and shared winter hosts, with mating occurring on the shared hosts. This provides a yearly opportunity for gene flow among aphids using different summer hosts, and raises the question if and to what extent the ecologically defined taxa are reproductively isolated. Here, we analyzed a geographically and temporally structured dataset of microsatellite genotypes from A. fabae that were mostly collected from their main winter host Euonymus europaeus, and additionally from another winter host and fourteen summer hosts. The data reveals multiple, strongly differentiated genetic clusters, which differ in their association with different summer and winter hosts. The clusters also differ in the frequency of infection with two heritable, facultative endosymbionts, separately hinting at reproductive isolation and divergent ecological selection. Furthermore, we found evidence for occasional hybridization among genetic clusters, with putative hybrids collected more frequently in spring than in autumn. This suggests that similar to host races in other phytophagous insects, both prezygotic and postzygotic barriers including selection against hybrids maintain genetic differentiation among A. fabae taxa, despite a common mating habitat.

Colonization of a Novel Host Plant Reduces Phenotypic Variation

Understanding the evolution of evolvability—the evolutionary potential of populations—is key to predicting adaptation to novel environments. Despite growing evidence that evolvability structures adaptation, it remains unclear how adaptation to novel environments in turn influences evolvability. Here we address the interplay between adaptation and evolvability in the peacock fly Tephritis conura, which recently underwent an adaptive change in ovipositor length following a host shift. We compared the evolvability of morphological traits, including ovipositor length, between the ancestral and the derived host race. We found that mean evolvability was reduced in females of the derived host race compared to the ancestral host race. However, patterns of multivariate evolvability (considering trait covariances) were very similar in both host races, and populations of the derived host race had diverged from the ancestral host race in directions of greater-than-average evolvability. Exploration of phenotypic integration patterns further revealed relatively high levels of independent variation in ovipositor length compared to other measured traits, allowing some degree of independent divergence. Our findings suggest that adaptation to novel environments can reduce mean evolvability without major changes in patterns of variational constraints, and that trait autonomy helps facilitate divergence of functionally important traits.

Phylogeography and host‐shift speciation in the vampirecups, an enigmatic clade of endophytic holoparasitic plants

AbstractAimDiversification of obligate parasites is often closely associated with historical shifts among host species. This phenomenon is well‐documented in animal parasites but still poorly understood for parasitic plants. We studied the associations between host shifts, biogeography and diversification in Cytinus, a genus of endophytic holoparasitic plants which show a remarkable disjunct distribution.LocationMediterranean Basin, southern Africa, Madagascar.TaxonCytinus.MethodsWe sampled numerous populations in South Africa and the western Mediterranean Basin, as well as a few populations in Madagascar where the genus is rare. We amplified two nuclear (ITS and SSU) and two mitochondrial (matR and nad1) loci, to infer its evolutionary history using maximum likelihood and time‐calibrated Bayesian analyses, to elucidate the factors responsible for its current distribution and to assess whether host switching is a main driver of speciation.ResultsAll genes recover lineages corresponding to the three geographic areas of Cytinus and sublineages associated with specific host plants, although they disagree on their deep‐level relationships. Ribosomal nuclear genes support an initial split of the Malagasy lineage and an Afrotropical origin of Cytinus before colonization of warm temperate latitudes. Mitochondrial genes support an initial split of the Mediterranean and Afro‐Malagasy lineages and a later split between the South African and Malagasy clades. Introgressive hybridization is the most likely explanation for this discrepancy, while horizontal gene transfers from hosts is less likely. We found strong support for cryptic host‐specific diversity, with five and eight host races in the Mediterranean and South Africa respectively, and a strong phylogenetic signal in the host–parasite associations suggestive of parasite diversification via phylogenetic host tracking.Main ConclusionsThese dated phylogenies indicate that radiation through long‐distance dispersal events and adaptation to new hosts best explain the disjunct distribution of Cytinus. We emphasize the importance of host specificity and host‐switching in the diversification and biogeography of plant endoparasites.

Hybridisation between host races broadens the host range of offspring in Eotetranychus asiaticus (Acari: Tetranychidae).

Host adaptation in herbivorous arthropods is one of the first steps to sympatric speciation, and spider mites (Acari: Tetranychidae) are useful model organisms for studying this phenomenon. Many researchers have studied the process of host adaptation via artificial selection experiments. Recent analyses suggest that hybridisation has diversified host ranges, although empirical evidence is scarce. We explored the host exploitation of two strains of Eotetranychus asiaticus established from Ternstroemia gymnanthera (Pentaphylacaceae) and Japanese cinnamon, Cinnamomum yabunikkei (Lauraceae), and evaluated the effect of hybridisation on offspring host use. Transplant experiments showed that females oviposited and immature mites developed only on their native hosts, suggesting specialisation to the secondary metabolites of each host plant. However, F1 hybrids from reciprocal crosses developed on both host plants (survival rate: 92-100%) with normal female-biased sex ratios. Furthermore, all backcrosses to the parental strains yielded B1 offspring that were also viable on both host plants with normal sex ratios (69-87% and39-92%femalesonT. gymnanthera andC. yabunikkei, respectively). B1 survival rates in interstrain crosses were varied (11-63%) and lower than those in intrastrain crosses (88-93%). We could not detect any reproductive barriers in these experiments, and host preference may be the sole factor determining pre-mating isolation. The survival rates and sex ratios we observed suggest cytochromosome interactions. In conclusion, hybridisation, which results in heterozygotes and recombination, is an underexplored way to provide spider mites with a novel host plant.

Retrospective use of integrative taxonomy in classical biological control: The unintentional introduction of the weevil Rhinusa dieckmanni to North America

A seed-feeding weevil introduced to North America (NA) as a biological control agent of the invasive toadflax Linaria dalmatica (L.) Mill., identified then as Gymnetron antirrhini “Dalmatian host race” and subsequently confirmed as established, was revealed through our study to be a separate species, i.e., Rhinusa dieckmanni (Behne) (Coleoptera: Curculionidae). This weevil species was presumed to be endemic in its native range, with a distribution restricted to Mount Rila in southwestern Bulgaria. We conducted a comprehensive study of seed-feeding weevils associated with L. dalmatica, L. dalmatica ssp. macedonica (Griseb.) D.A. Sutton, L. genistifolia (L.) Mill., and L. grandiflora Desf. across a broad geographic area of their native range. Those results revealed that all four host plants were used by R. dieckmanni and thus the native geographic range of the species is wider than expected, encompassing the Balkans and the Anatolian Plateau. Our observations suggest that phenotypes of this weevil are highly variable and dependent on the seed capsule size of the Linaria host population. The haplotype network based on mitochondrial COII, 16S genes, and nuclear EF 1-α gene genealogy confirmed the conspecific nature of geographically distant weevil populations, that is, R. dieckmanni phenotypes utilizing L. genistifolia, L. dalmatica, and L. grandiflora for larval development. Specimens collected from L. dalmatica in the northwestern USA shared the same haplotypes as samples from L. dalmatica ssp. macedonica in southwestern North Macedonia, supporting the known introduction history of the North American population. Females from these populations have relatively short rostrums, which may limit their reproductive success on North American invasive L. dalmatica with larger seed capsules.

Pea Aphid (Acyrthosiphon pisum) Host Races Reduce Heat-Induced Forisome Dispersion in Vicia faba and Trifolium pratense.

Although phloem-feeding insects such as aphids can cause significant damage to plants, relatively little is known about early plant defenses against these insects. As a first line of defense, legumes can stop the phloem mass flow through a conformational change in phloem proteins known as forisomes in response to Ca2+ influx. However, specialized phloem-feeding insects might be able to suppress the conformational change of forisomes and thereby prevent sieve element occlusion. To investigate this possibility, we triggered forisome dispersion through application of a local heat stimulus to the leaf tips of pea (Pisum sativum), clover (Trifolium pratense) and broad bean (Vicia faba) plants infested with different pea aphid (Acyrthosiphon pisum) host races and monitored forisome responses. Pea aphids were able to suppress forisome dispersion, but this depended on the infesting aphid host race, the plant species, and the age of the plant. Differences in the ability of aphids to suppress forisome dispersion may be explained by differences in the composition and quantity of the aphid saliva injected into the plant. Various mechanisms of how pea aphids might suppress forisome dispersion are discussed.

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.

Diversification linked to larval host plant in the butterfly Eumedonia eumedon.

It is widely accepted that the relationship between phytophagous insects and their host plants influences insect diversification. However, studies addressed at documenting host-associated genetic differentiation (HAD) and the mechanisms that drive reproductive isolation in host-associated lineages (or host races) are still scarce relative to insect diversity. To uncover further evidence on the HAD processes in Lepidoptera, we investigated the genetic structure of the geranium argus butterfly (Eumedonia eumedon) and tested for isolation by ecology (IBE) vs. isolation by distance (IBD). Genomic data revealed an array of host races (three of them in the same mountain range, the Cantabrian Mountains, northern Iberia) at apparently distinct levels of reproductive isolation. We found a pattern of IBE mediated by HAD at both local and European scales, in which genetic differentiation between populations and individuals correlated significantly with the taxonomic relatedness of the host plants. IBD was significant only when considered at the wider European scale. We hypothesize that, locally, HAD between Geranium-feeding populations was caused (at least partially) by allochrony, that is via adaptation of adult flight time to the flowering period of each host plant species. Nevertheless, the potential reproductive isolation between populations using Erodium and populations using Geranium cannot be explained by allochrony or IBD, and other mechanisms are expected to be at play.

Oviposition stimulants underlying different preferences between host races in the leaf-mining moth Acrocercops transecta (Lepidoptera: Gracillariidae)

The importance of plant chemistry in the host specialization of phytophagous insects has been emphasized. However, only a few chemicals associated with host shifting have been characterized. Herein, we focus on the leaf-mining moth Acrocercops transecta (Gracillariidae) consisting of ancestral Juglans (Juglandaceae)- and derived Lyonia (Ericaceae)-associated host races. The females of the Lyonia race laid eggs on a cover glass treated with an L. ovalifolia leaf extract; the extract was fractionated using silica gel and ODS column chromatography to isolate the oviposition stimulants. From a separated fraction, two analogous Lyonia-specific triterpenoid glycosides were characterized as oviposition stimulants. Furthermore, we observed probable contact chemosensilla on the distal portion of the female antennae. Lyonia race females laid their eggs on the non-host Juglans after the leaves were treated with a Lyonia-specific oviposition stimulant, although they do not lay eggs on Juglans. These results suggest that Lyonia race females do not lay eggs on Juglans leaves because the leaves do not contain specific oviposition stimulant(s). Otherwise, the activity of the oviposition stimulants overcomes oviposition deterrents contained in Juglans leaves. This paper describes the roles of plant chemicals in the different preferences between host races associated with distantly related plant taxa.

Sympatric speciation by allochrony?

Sympatric speciation was once thought most improbable, but careful study of some systems, particularly the apple maggot (Rhagoletis pomonella) and related Rhagoletis species, has led to its reinstatement as a likely mode of speciation in some cases. Different species and host races in this clade of flies often have highly specialized host preference, and along with frequent evolutionary shifts to different fruit species between sister taxa, there is a likely effect of the timing of adult emergence that follows host fruiting phenology. This is known as "allochronic" isolation (from the Greek meaning "different timing"). This overview covers recent discoveries by Inskeep et al. (Molecular Ecology, 2021) showing how allochrony is a major factor in preventing gene flow between a pair of sister species of Rhagoletis on different host fruits. Although the authors do not claim to prove sympatric speciation, it does seem very likely, and the work clearly underscores how readily host shifts via allochrony can aid sympatric speciation.

Non-parallel morphological divergence following colonization of a new host plant

Adaptation to new ecological niches is known to spur population diversification and may lead to speciation if gene flow is ceased. While adaptation to the same ecological niche is expected to be parallel, it is more difficult to predict whether selection against maladaptive hybridization in secondary sympatry results in parallel divergence also in traits that are not directly related to the ecological niches. Such parallelisms in response to selection for reproductive isolation can be identified through estimating parallelism in reproductive character displacement across different zones of secondary contact. Here, we use a host shift in the phytophagous peacock fly Tephritis conura, with both host races represented in two geographically separate areas East and West of the Baltic Sea to investigate convergence in morphological adaptations. We asked (i) if there are consistent morphological adaptations to a host plant shift and (ii) if the response to secondary sympatry with the alternate host race is parallel across contact zones. We found surprisingly low and variable, albeit significant, divergence between host races. Only one trait, the length of the female ovipositor, which serves an important function in the interaction with the hosts, was consistently different between host races. Instead, co-existence with the other host race significantly affected the degree of morphological divergence, but the divergence was largely driven by different traits in different contact zones. Thus, local stochastic fixation or reinforcement could generate trait divergence, and additional evidence is needed to conclude whether divergence is locally adaptive.

Ecological speciation by sympatric host shifts in a clade of herbivorous sea slugs, with introgression and localized mitochondrial capture between species

Host shifting in insect-plant systems was historically important to the development of ecological speciation theory, yet surprisingly few studies have examined whether host shifting drives diversification of marine herbivores. When small-bodied consumers feed and also mate on a preferred host, disruptive selection can split a population into host races despite gene flow. Support for host shifts is notably lacking for invertebrates associated with macroalgae, where the scale of dispersal by planktonic larvae often far exceeds the grain of host patchiness, and adults are typically less specialized than terrestrial herbivores. Here, we present a candidate example of ecological speciation in a clade of sea slugs that primarily consume green algae in the genus Caulerpa, including highly invasive species. Ancestral character state reconstructions supported ‘sea grapes’ (C. racemosa, C. lentillifera) as the ancestral host for a tropical radiation of 12 Elysia spp., with one shift onto alternative Caulerpa spp. in the Indo-Pacific. A Caribbean radiation of three species included symaptric host shifts to Rhipocephalus brevicaulis in the ancestor of E. pratensis Ortea & Espinosa, 1996, and to C. prolifera in E. hamanni Krug, Vendetti & Valdes 2016, plus a niche expansion to a range of Caulerpa spp. in E. subornata Verrill, 1901. All three species are broadly sympatric across the Caribbean but are host-partitioned at a fine grain, and distinct by morphology and at nuclear loci. However, non-recombining mtDNA revealed a history of gene flow between E. pratensis and E. subornata: COI haplotypes from E. subornata were 10.4% divergent from E. pratensis haplotypes from four sites, but closely related to all E. pratensis haplotypes sampled from six Bahamian islands, indicating historical introgression and localized “mitochondrial capture.” Disruptive selective likely fueled divergence and adaptation to distinct host environments, indicating ecological speciation may be an under-appreciated driver of diversification for marine herbivores as well as epibionts and other resource specialists.

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.

Genomically correlated trait combinations and antagonistic selection contributing to counterintuitive genetic patterns of adaptive diapause divergence in Rhagoletis flies.

Adaptation to novel environments can result in unanticipated genomic responses to selection. Here, we illustrate how multifarious, correlational selection helps explain a counterintuitive pattern of genetic divergence between the recently derived apple- and ancestral hawthorn-infesting host races of Rhagoletis pomonella (Diptera: Tephritidae). The apple host race terminates diapause and emerges as adults earlier in the season than the hawthorn host race, to coincide with the earlier fruiting phenology of their apple hosts. However, alleles at many loci associated with later emergence paradoxically occur at higher frequencies in sympatric populations of the apple compared to the hawthorn race. We present genomic evidence that historical selection over geographically varying environmental gradients across North America generated genetic correlations between two life history traits, diapause intensity and diapause termination, in the hawthorn host race. Moreover, the loci associated with these life history traits are concentrated in genomic regions in high linkage disequilibrium (LD). These genetic correlations are antagonistic to contemporary selection on local apple host race populations that favours increased initial diapause depth and earlier, not later, diapause termination. Thus, the paradox of apple flies appears due, in part, to pleiotropy or linkage of alleles associated with later adult emergence and increased initial diapause intensity, the latter trait strongly selected for by the earlier phenology of apples. Our results demonstrate how understanding of multivariate trait combinations and the correlative nature of selective forces acting on them can improve predictions concerning adaptive evolution and help explain seemingly counterintuitive patterns of genetic diversity in nature.

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.

Cuticular hydrocarbon variation among Rhagoletis fruit flies (Diptera: Tephritidae): implications for premating reproductive isolation and ecological speciation

1. Fruit flies in the genus Rhagoletis are a model for ecological speciation via sympatric host plant shifting. Rhagoletis mate on or near the fruit of their respective host plants, generating premating reproductive isolation between taxa specialised on different hosts. However, non‐host‐related premating isolation has been observed between some Rhagoletis species associated with morphological differences in body colour and wing patterns.2. Here, the extent of epicuticular hydrocarbon (CHC) variation as a possible, additional determinant of mate choice in adults of six Rhagoletis taxa, including the apple and hawthorn‐infesting host races of R. pomonella is investigated.3. Gas‐chromatography‐mass‐spectrometry revealed 36 repeatable and quantifiable hydrocarbon components present on the epicuticles of all six fly taxa, comprised of at least 53 different CHC compounds, with chain lengths varying from 27 to 34 carbon atoms, consisting of n‐alkanes, mono‐, dimethyl‐, and trimethyl‐alkanes, alkenes, and alkadienes. There were significant differences in the relative proportions of CHCs between adult R. cingulata, R. cornivora, R. zephyria, R. mendax, and R. pomonella, as well as between the apple‐ and hawthorn‐infesting host races of R. pomonella. Furthermore, within the R. pomonella host races and R. mendax, significant CHC differences were observed between the sexes and across collecting sites.4. The results are consistent with variation in CHCs potentially contributing to observed patterns of premating isolation between Rhagoletis taxa, possibly due to combination of sexual and host‐related selection, which will necessitate further, in depth chemical analyses and future mating trials to substantiate.

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.