Rapid and repeatable shifts in life‐history timing of Rhagoletis pomonella (Diptera: Tephritidae) following colonization of novel host plants in the Pacific Northwestern United States

Host shifts of phytophagous insect specialists to novel plants can result in divergent ecological adaptation, generating reproductive isolation and potentially new species. Rhagoletis pomonella fruit flies in eastern North America underwent a host shift ~160 ya from native downy hawthorn (Crataegus mollis) to introduced, domesticated apple (Malus domestica). Divergent selection on diapause phenology related to the earlier fruiting time of apples versus downy hawthorns resulted in partial allochronic reproductive isolation between the fly races. Here, we test for how rapid and repeatable shifts in life‐history timing are driving ecological divergence of R. pomonella in the Pacific Northwestern USA. The fly was introduced into the region via larval‐infested apples 40–65 ya and now attacks native black hawthorn (Crataegus douglasii) and introduced ornamental hawthorn (Crataegus monogyna), in addition to early‐ and late‐maturing apple varieties in the region. To investigate the life‐history timing hypothesis, we used a field‐based experiment to characterize the host‐associated eclosion and flight activity patterns of adults, and the feeding times of larvae at a field site in Vancouver, Washington. We also assessed the degree to which differences in host‐fruiting time generate allochronic isolation among apple‐, black hawthorn‐, and ornamental hawthorn‐associated fly populations. We conclude that host‐associated fly populations are temporally offset 24.4% to 92.6% in their seasonal distributions. Our results imply that R. pomonella possesses the capacity for rapid and repeatable shifts in diapause life history to match host‐fruiting phenology, which can generate ecologically based reproductive isolation, and potentially biodiversity in the process.     

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.     

Ecological adaptation and reproductive isolation in sympatry: genetic and phenotypic evidence for native host races of Rhagoletis pomonella

Ecological speciation with gene flow may be an important mode of diversification for phytophagous insects. The recent shift of Rhagoletis pomonella from its native host downy hawthorn (Crataegus mollis) to introduced apple (Malus domestica) in the northeastern United States is a classic example of sympatric host race formation. Here, we test whether R. pomonella has similarly formed host races on four native Crataegus species in the southern United States: western mayhaw (C. opaca), blueberry hawthorn (C. brachyacantha), southern red hawthorn (C. mollis var. texana) and green hawthorn (C. viridis). These four southern hosts differ from each other in their fruiting phenology and in the volatile compounds emitted from the surface of their fruits. These two traits form the basis of ecological reproductive isolation between downy hawthorn and apple flies in the north. We report evidence from microsatellite population surveys and eclosion studies supporting the existence of genetically differentiated and partially reproductively isolated host races of southern hawthorn flies. The results provide an example of host shifting and ecological divergence involving native plants and imply that speciation with gene flow may be commonly initiated in Rhagoletis when ecological opportunity presents itself.     

The effects of climate, host plant phenology and host fidelity on the genetics of apple and hawthorn infesting races ofRhagoletis pomonella

True fruit flies belonging to theRhagoletis pomonella (Walsh) sibling species complex have been proposed to speciate sympatrically by shifting and adapting to new host plants. Here, we report the results from a series of ecological and genetic experiments conducted at a study site near Grant, Michigan, U.S.A., aimed at clarifying the relationship between host specialization and reproductive isolation for these flies. Our findings indicate that apple (Malus pumila) and hawthorn (Crataegus mollis) infesting populations ofR. pomonella are partially allochronically isolated. Differences in the timing of adult eclosion account for part of the allochronic divergence, as apple adults emerge approximately ten days earlier than hawthorn flies in the field. Genetic analyses across different life history stages of the fly show that adults do not randomly move between apple and hawthorn trees, but trend to attack the same species of plant that they infested as larvae. Estimates of interhost migration from the allozyme data suggest that from 2.8 to 10% of the apple population is of hawthorn origin and that over 20% of the hawthorn population is of apple origin. The length and quality of the growing season appear to affect the genetic composition of the host races, as allele frequencies in the hawthorn population are correlated with ambient temperature and rainfall during the spring of the preceding year. Finally, allele frequencies for six allozyme loci displaying host associated differentiation also show significant linear regressions with the timing of adult eclosion within both races. These regressions establish a link between allozyme loci displaying inter-host differentiation and a developmental trait (adult eclosion) responsible for partially isolating the races. The slopes of the regressions are paradoxical, however, as they suggest that apple adults should eclose later, not earlier, than hawthorn flies. We conclude by discussing potential resolutions to the eclosion time paradox.     

BEHAVIORAL EVIDENCE FOR FRUIT ODOR DISCRIMINATION AND SYMPATRIC HOST RACES OF RHAGOLETIS POMONELLA FLIES IN THE WESTERN UNITED STATES

The recent shift of Rhagoletis pomonella (Diptera: Tephritidae) from its native host downy hawthorn, Crataegus mollis, to introduced domesticated apple, Malus domestica, in the eastern United States is a model for sympatric host race formation. However, the fly is also present in the western United States, where it may have been introduced via infested apples within the last 60 years. In addition to apple, R. pomonella also infests two hawthorns in the West, one the native black hawthorn, C. douglasii, and the other the introduced English ornamental hawthorn, C. monogyna. Here, we test for behavioral evidence of host races in the western United States. through flight tunnel assays of western R. pomonella flies to host fruit volatile blends. We report that western apple, black hawthorn, and ornamental hawthorn flies showed significantly increased levels of upwind‐directed flight to their respective natal compared to nonnatal fruit volatile blends, consistent with host race status. We discuss the implications of the behavioral results for the origin(s) of western R. pomonella, including the possibility that western apple flies were not introduced, but may represent a recent shift from local hawthorn fly populations.     

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.     

It''s about time: the evidence for host plant-mediated selection in the apple maggot fly, Rhagoletis pomonella, and its implications for fitness trade-offs in phytophagous insects

The apple maggot fly, Rhagoletis pomonella, Walsh (Diptera: Tephritidae), provides a unique opportunity to address the issue of host-related fitness trade-offs for phytophagous insects. Rhagoletis pomonella has been controversial since the 1860's when Benjamin Walsh cited the fly's shift from hawthorn (Crataegus spp.) to apple (Malus pumila) as an example of an incipient sympatric speciation event. Allozyme and mark-release-recapture studies have subsequently confirmed the status of apple and hawthorn flies as partially reproductively isolated and genetically differentiated host races, the hypothesized initial stage in sympatric divergence. Here, we review the ecological and genetic evidence for host-plant mediated selection in R. pomonella. We reach the following three major conclusions: First, although developmental timing is not everything, it is a good deal of the story. Differences in the fruiting phenologies of apple and hawthorn trees exert different selection pressures on the diapause and eclosion time characteristics of the host races. In particular, the 3-week earlier mean fruiting phenology of apples in eastern North America appears to select for a slower rate of metabolism or deeper pupal diapause in apple than hawthorn flies. Second, host-related fitness trade-offs for R. pomonella may not be due to disruptive selection affecting any one specific life-history stage. Rather, it is the sum total of directional selection pressures acting across different life-stages that generates divergent selection on apple and hawthorn flies. For example, selection favors the alleles Me 100, Acon-2 95 and Mpi 37 (or linked genes) in the larval stage in both host races. However, these same alleles are disfavored in the pupal stage to follow, where they correlate with early adult eclosion, and by inference premature diapause termination. Because apple trees fruit an average of 3 weeks earlier than hawthorn trees, this counter-balancing selection is stronger on apple-fly pupae. The net result is that the balance of selective forces is different between apple and hawthorn flies, helping to maintain the genetic integrity of the host races in sympatry in the face of gene flow. Finally, natural R. pomonella populations harbor a good deal of genetic variation for development-related traits. This variation allows fly populations to rapidly respond to temporal vagaries in local environmental conditions across years, as well as to broad-scale geographic differences that exist across the range of the species. Perhaps most importantly, this variation gives R. pomonella the flexibility to explore and adapt to novel plants. Taken together, our results underscore how difficult it can be to document host plant-related fitness trade-offs for phytophagous insects due to the need to consider details of the entire life-cycle of a phytophagous insect. Our findings also show how reproductive isolation can arise as a by-product of host-associated adaptation in insects, a central theme for models of sympatric speciation via host shifts.     

Intra- and interspecific competition and host race formation in the apple maggot fly,Rhagoletis pomonella (Diptera: Tephritidae)

Intra- and interspecific resource competition are potentially important factors affecting host plant use by phytophagous insects. In particular, escape from competitors could mediate a successful host shift by compensating for decreased feeding performance on a new plant. Here, we examine the question of host plant-dependent competition for apple (Malus pumila)- and hawthorn (Crataegus mollis)-infesting larvae of the apple maggot fly, Rhagoletis pomonella (Diptera: Tephritidae) at a field site near Grant, Michigan, USA. Interspecific competition from tortricid (Cydia pomonella, Grapholita prunivora, and Grapholita packardi) and agonoxenid (subfamily Blastodacninae) caterpillars and a curculionid weevil (Conotrachelus crataegi) was much stronger for R. pomonella larvae infesting the ancestral host hawthorn than the derived host apple. Egg to pupal survivorship was estimated as 52.8% for fly larvae infesting hawthorn fruit without caterpillars and weevils compared to only 27.3% for larvae in harthorns with interspecific insects. Survivorship was essentially the same between fly larvae infesting apples in the presence (44.8%) or absence (42.6%) of interspecific insects. Intraspecific competition among maggots was also stronger in hawthorns than apples. The order or time that a larva exited a hawthorn fruit was a significant determinant of its pupal mass, with earlier emerging larvae being heavier than later emerging larvae. This was not the case for larvae in apples, as the order or time that a larva exited an apple fruit had relatively little influence on its pupal mass. Our findings suggest that decreased performance related to host plant chemistry/nutrition may restrict host range expansion and race formation in R. pomonella to those plants where biotic/ecological factors (i.e. escape from competitors and parasitoids) adequately balance the survivorship equation. This balance permits stable fly populations to persist on novel plants, setting the stage for the evolution of host specialization under certain mitigating conditions (e.g. when mating is host specific and host-associated fitness trade-offs exist).     

Geographic variation in fruit volatiles emitted by the hawthorn Crataegus mollis and its consequences for host race formation in the apple maggot fly,Rhagoletis pomonella

The shift of the apple (AP) maggot fly, Rhagoletis pomonella (Walsh) (Diptera: Tephritidae), from its ancestral host downy hawthorn, Crataegus mollis (DH) (Torr. & A. Gray) Scheele, to introduced domesticated AP, Malus domestica Borkh. (both Rosaceae), is a model for ecological divergence and incipient sympatric speciation with gene flow. However, a portion of the variation contributing to the sympatric host shift from DH to AP appears to have a different biogeographic history, pre‐dating the shift. One potential source of standing variation may trace to a number of different native hawthorn species infested by R. pomonella in the southern USA, where the AP‐attacking race is absent. Herein, we investigate this possibility for the southern red hawthorn (SR) endemic to Texas, Crataegus mollis var. texana (Buckl.), which has been described as a member of the Molles series that includes the more northern distributed DH. We report results from chemical analyses of host fruit volatiles, fly behavioural responses to synthetic fruit blends, and microsatellite surveys of fly populations, implying that R. pomonella infesting SR may behaviourally and genetically represent a native host race differing from the DH‐infesting fly. No fly reared from SR responded to AP fruit volatiles in flight tunnel assays. However, coupled gas chromatographic‐electroantennographic detection (GC‐EAD) profiles for SR fruit contain all five of the component esters that comprise the standard AP volatile blend inducing behavioural orientation for AP‐infesting flies, compounds that appear to be largely missing from volatile profiles for DH fruit. Thus, SR‐infesting flies do not represent a source for a preassembled AP‐accepting phenotype. However, they may help explain why the ancestral DH race that shifted to AP in the northeastern USA had the ability to recognize AP fruit esters, potentially enabling the shift to AP. Our results highlight how categorizing speciation into different geographic modes may not adequately describe the evolutionary origins of important genetic variation fuelling adaptive radiation and the genesis of new biodiversity.     

Genetic variation among apple and hawthorn host races of Rhagoletis pomonella across an ecological transition zone in the Mid-Western United States

In previous studies, we have shown that apple and hawthorn populations of Rhagoletis pomonella (Diptera: Tephritidae) represent partially reproductively isolated and genetically differentiated host races; a result consistent with predictions of sympatric speciation models. The geographic pattern of allozyme variation for these flies is complex, however, as inter-host differences are superimposed on latitudinal allele frequency clines within the races. In addition, pronounced allele frequency shifts exist among R. pomonella populations across three major ecological transition zones in the mid-western United States. This suggests that selection related to environmental heterogeneity is responsible for the allele frequency shifts, but does not rule out secondary contact as an alternative possibility. Resolution of this issue is important, because if secondary contact is involved, then we would have to reassess the relationship host race formation has with speciation in the R. pomonella group.Here, we present results from a detailed genetic analysis of fly populations spanning the deciduous/prairie transition zone near the border between the states of Wisconsin and Illinois. Allele frequencies for hawthorn populations within the zone formed spikes, rather than the expected steps, and these frequency peaks correlated with variation in local ambient temperature conditions. Ambient temperature, and not secondary contact, therefore appears to be an important determinant of the shape of R. pomonella allele frequency clines. Allele frequency heterogeneity was also observed among apple populations, but was less pronounced compared to that for hawthorn flies. This suggests that ambient temperature differentially affects the host races, possibly through differences in the fruiting phenologies of apple and hawthorn trees. Several pairs of linked loci displayed concordant allele frequency changes and were in disequilibrium among both apple and hawthorn populations along the Wisconsin/Illinois transect. Although we do not know the reason for the observed pattern of disequilibrium, site to site variation in levels of inter-host migration, coupled with selection, seem the most likely explanations. We conclude by discussing how host specific adaptations, such as those associated with ambient temperature, may interact with host recognition traits to drive the sympatric speciation process for R. pomonella group flies.     

THE EFFECTS OF WINTER LENGTH ON THE GENETICS OF APPLE AND HAWTHORN RACES OF RHAGOLETIS POMONELLA (DIPTERA: TEPHRITIDAE)

Host plant-associated fitness trade-offs are central to models of sympatric speciation proposed for certain phytophagous insects. But empirical evidence for such trade-offs is scant, which has called into question the likelihood of nonallopatric speciation. Here, we report on the second in a series of studies testing for host-related selection on pupal life-history characteristics of apple- (Malus pumila L.) and hawthorn- (Crataegus mollis L. spp.) infesting races of the Tephritid fruit fly, Rhagoletis pomonella (Walsh). In particular, we examine the effects of winter length on the genetics of these flies. We have previously found that the earlier fruiting phenology of apple trees exposes apple-fly pupae to longer periods of warm weather preceding winter than hawthorn-fly pupae. Because R. pomonella has a facultative diapause, we hypothesized that this selects for pupae with more recalcitrant pupal diapauses (or slower metabolic/development rates) in the apple-fly race. A study in which we experimentally manipulated the length of the prewintering period for hawthorn-origin pupae supported this prediction. If the period preceding winter is important for apple- and hawthorn-fly pupae, then so too should be the length (duration) of winter; the rationale for this prediction is that “fast developing” pupae that break diapause too early will deplete their energy reserves and disproportionately die during long winters. To test this possibility, we chilled apple- and hawthorn-origin pupae collected from a field site near Grant, Michigan, in a refrigerator at 4°C for time periods ranging from one week to two years. Our a priori expectation was that longer periods of cold storage would select against allozyme markers that were associated with faster rates of development in our earlier study. Since these electromorphs are typically found at higher frequencies in hawthorn flies, extending the overwintering period should favor “apple-fly alleles” in both races. The results from this “overwinter” experiment supported the diapause hypothesis. The anticipated genetic response was observed in both apple and hawthorn races, as allele frequencies became significantly more “apple-fly-like” in eclosing adults surviving longer chilling periods. This indicates that it is the combination of environmental conditions before and during winter that selects on the host races. Many tests for trade-offs fail to adequately consider the interplay between insect development, host plant phenology, and local climatic conditions. Our findings suggest that such oversight may help to explain the paucity of reported fitness trade-offs.     

The diapause response of Rhagoletis pomonella to varying environmental conditions and its significance for geographic and host plant‐related adaptation

The recent shift of Rhagoletis pomonella Walsh (Diptera: Tephritidae) from its ancestral host hawthorn to apple is a model for incipient sympatric speciation in action. Previous studies have shown that changes in the over‐wintering pupal diapause are critical for differentially adapting R. pomonella flies to a difference in the fruiting times of apples vs. hawthorns, generating ecologically based reproductive isolation. Here, we exposed pupae of the hawthorn race to various combinations of pre‐ and over‐wintering rearing conditions and analyzed their effects on eclosion time and genetics. We report certain unexpected results in regards to a combination of brief pre‐winter and over‐wintering periods indicative of gene*environment interactions requiring a reassessment of our current understanding of R. pomonella diapause. We present a hypothesis that involves physiological factors related to stored energy reserves in pupae that influences the depth and duration of Rhagoletis diapause. This ‘pupal energy reserve’ hypothesis can account for our findings and help clarify the role host plant‐related life history adaptation plays in phytophage biodiversity.     

A FIELD TEST FOR HOST-PLANT DEPENDENT SELECTION ON LARVAE OF THE APPLE MAGGOT FLY,RHAGOLETIS POMONELLA

Host-plant dependent fitness trade-offs refer to traits that enhance the performance of an insect on one plant species to its detriment on others. Such trade-offs are central to models of sympatric speciation via host shifts, but have proven difficult to empirically demonstrate. Here, we test for host-plant dependent selection on larvae of apple (Malus pumila L.)- and hawthorn (Crataegus mollis L. spp.)-infesting races of Rhagoletis pomonella (Walsh). Samples of larvae were reared in the field and under protective conditions in a garage. Our rationale was that the garage should slow rates of fruit rot relative to the field, relaxing selection pressures associated with declining fruit quality. Four findings emerged from the study. (1) Larvae suffered higher mortality in fruits in the field than the garage. (2) The increase in mortality was greater for larvae in haws. (3) Larvae possessing the alleles Me 100, Acon-2 95, and Mpi 37, three allozymes displaying host-related differentiation in R. pomonella that map to linkage group II in the fly, left fruits earlier than other genotypes. (4) Allele frequencies for Me 100, Acon-2 95, and Mpi 37 were significantly higher in both apple and haw larvae surviving the field versus the garage treatment. Our results suggested that field conditions favored larvae that rapidly developed and left rotting fruits. Since these individuals tended to possess the alleles Me 100, Acon-2 95, and Mpi 37, frequencies of these allozymes were higher in the field. Selection on larvae was directional for Me 100, Acon-2 95, and Mpi 37 (or linked genes) in both host races. We previously showed that these same alleles can be disfavored in the pupal stage, especially in the apple race, where they correlate with premature diapause termination. Fitness trade-offs in Rhagoletis may therefore be due as much to differences in the relative strengths of directional selection pressures acting on different life stages as to disruptive selection affecting any one particular stage. The necessity to consider details of the entire life-cycle highlights one of the many challenges posed to documenting fitness trade-offs for phytophagous insects.     

Variability in response specificity of apple, hawthorn, and flowering dogwood-infesting Rhagoletis flies to host fruit volatile blends: implications for sympatric host shifts

Rhagoletis pomonella Walsh (Diptera: Tephritidae) originating from domesticated apple (Malus pumila), hawthorn (Crataegus mollis) (Rosaceae), and flowering dogwood (Cornus florida) (Cornaceae) were tested sequentially in flight‐tunnel assays to volatile blends previously identified from the three fruit types. The majority of flies flew to odor sources containing their natal blend (68–83%). Some flies from each fruit type also flew to non‐natal fruit blends (11–39%), but of these non‐natal responders the vast majority were flies that responded to their natal blend as well. The results indicate that individual flies within R. pomonella populations infesting different host types have different degrees of specificity with respect to discriminating among fruit volatile blends, and that a moderate proportion of apple, hawthorn, and dogwood flies (10–30%) are broad responders, with the capacity to recognize and orient to more than one blend. The observed variability in response specificity could facilitate sympatric shifts to new host plants.     

Evidence for inversion polymorphism related to sympatric host race formation in the apple maggot fly,Rhagoletis pomonella

Evidence suggests that the apple maggot, Rhagoletis pomonella (Diptera: Tephritidae) is undergoing sympatric speciation (i.e., divergence without geographic isolation) in the process of shifting and adapting to a new host plant. Prior to the introduction of cultivated apples (Malus pumila) in North America, R. pomonella infested the fruit of native hawthorns (Crataegus spp.). However, sometime in the mid-1800s the fly formed a sympatric race on apple. The recently derived apple-infesting race shows consistent allele frequency differences from the hawthorn host race for six allozyme loci mapping to three different chromosomes. Alleles at all six of these allozymes correlate with the timing of adult eclosion, an event dependent on the duration of the overwintering pupal diapause. This timing difference differentially adapts the univoltine fly races to an approximately 3- to 4-week difference in the peak fruiting times of apple and hawthorn trees, partially reproductively isolating the host races. Here, we report finding substantial gametic disequilibrium among allozyme and complementary DNA (cDNA) markers encompassing the three chromosomal regions differentiating apple and hawthorn flies. The regions of disequilibrium extend well beyond the previously characterized six allozyme loci, covering substantial portions of chromosomes 1, 2, and 3 (haploid n = 6 in R. pomonella). Moreover, significant recombination heterogeneity and variation in gene order were observed among single-pair crosses for each of the three genomic regions, implying the existence of inversion polymorphism. We therefore have evidence that genes affecting diapause traits involved in host race formation reside within large complexes of rearranged genes. We explore whether these genomic regions (inversions) constitute coadapted gene complexes and discuss the implications of our findings for sympatric speciation in Rhagoletis.     

Rhagoletis sibling species and host races differ in host odor recognition

Electronantennograms (EAG) were recorded from the apple and hawthorn host race of the apple maggot fly, Rhagoletis pomonella (Walsh), and from the blueberry maggot fly, R. mendax (Curran) (Diptera: Tephritidae), in response to host fruit extracts and nine volatile host fruit odor compounds at six concentrations. Mean relative EAG response to apple odor is the same in both species, but in respect to blueberry odor, it is significantly stronger in R. mendax than in both host races of R. pomonella (P<0.05), indicating that antennal sensitivity is selectively adapted to species specific host fruit odors. Differences in antennal response to several host fruit odor compounds were found between both species as well as between the host races. This indicates differences in antennal receptor cell types and/or numbers between species and host races. The flies had no prior host fruit experience which indicates that the measured differences are genetically based. Because Rhagoletis fruit flies are highly host specific parasites which meet and mate on their respective host plants, the results suggest that antennal sensitivity plays an important role in host shifts and speciation in this genus.

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Zusammenfassung Elektroantennogramme (EAG) der Apfel- und der Weissdorn-Wirtsrasse der Apfelfruchtfliege, Rhagoletis pomonella, und der Blaubeerenfruchtfliege, Rhagoletis mendax (Diptera: Tephritidae), wurden aufgezeichnet als Reizantworten auf Wirtsfruchtextrakte und auf neun flüchtige Duftkomponenten ihrer Wirtsfrüchte in sechs Konzentrationen. Die Sensitivität der Antennen bezüglich Apfelduft ist in beiden Arten gleich, aber die Reizantwort von R. mendax auf den Duft ihrer spezifischen Wirtsfrucht, Blaubeeren, ist signifikant stärker als diejenige beider Wirtsrassen von R. pomonella (P<0.05), was darauf hinweist, dass die antennale Sensitivität möglicherwiese an den artspezifischen Wirtsfruchtduft adaptiert ist. Unterschiede in der antennalen Reizantwort auf mehrere Duftkomponenten der Wirtsfrüchte konnten sowohl zwischen den Arten als auch Wirtsrassen gefunden werden. Dies deutet auf Unterschiede in antennalen Rezeptorzelltypen und/oder Rezeptorzellzahl zwischen Arten und Wirtsrassen. Die Unterschiede wurden von Individuen aufgezeichnet, die keine vorherige Erfahrung mit den Wirtsfrüchten hatten und sind daher genetischen Ursprungs. Fruchtfliegen der Gattung Rhagoletis sind stark wirtsspezifische Pflanzenparasiten, die ihre Wirtspflanze als Treffpunkt und Paarungsort benützen. Diese Resultate deuten daher darauf hin, dass antennale Sensitivität eine wichtige Rolle in Wirtswechseln und Speziation in diesem Genus spielt.

     

A second case of genetic host races in Rhagoletis? A population genetic comparison of sympatric host populations in the European cherry fruit fly,Rhagoletis cerasi

Despite an increasing acceptance in the biological community for sympatric speciation as a mode of species formation, well documented examples of sympatrically evolved ‘incipient species’ remain rare. The sympatric host races of apple maggot, Rhagoletis pomonella (Walsh), represent one of the most prominent case studies for sympatric speciation via a host shift. The European cherry fruit fly, R. cerasi (L.), shows strong ecological similarities to R. pomonella: (1) infestation of two different host plants, Lonicera xylosteum L. and Prunus avium L., and (2) divergent phenological and behavioral adaptations of flies on different hosts. The population genetic study presented here addresses whether the host associated populations of R. cerasi also represent genetically differentiated true host races. Out of a total of 29 allozyme loci examined, six were polymorphic and used to analyze six sympatric pairs of R. cerasi populations on Lonicera and Prunus from Switzerland and Germany. A direct comparison of allele frequencies between sympatric sites showed no pattern indicative of host races in R. cerasi. However, the hierarchical F‐statistic for one locus, mannose 6‐phosphate isomerase (Mpi), showed significant population differentiation that was in accordance with host race differentiation. Mpi is one of several loci that are also diagnostic for host race differentiation in R. pomonella. Results from Mpi suggest the formation of sympatric host races in R. cerasi, but additional polymorphic markers are necessary.     

Behavioral evidence for host races in Rhagoletis pomonella flies

Summary One of the most controversial putative cases of host race formation in insects is that of the apple maggot fly, Rhagoletis pomonella (Diptera: Tephritidae). A principal cause of the controversy is lack of relevant data. In laboratory and field enclosure experiments, we compared the host acceptance behavior of sympatric populations of flies originating from naturally infested hawthorn (the native host) and apple (an introduced host) in Amherst, Massachusetts or East Lansing, Michigan. In general, hawthorn fruit were accepted for ovipositional attempts nearly equally by apple and hawthorn origin females, whereas apples were accepted much more often by apple than hawthorn origin females. Similarly, males of apple and hawthorn origin exhibited about equal duration of residence on hawthorn fruits as sites at which to acquire potential mates, while males of apple origin tended to reside substantially longer than males of hawthorn origin on apples. Irrespective of fly origin, both sexes always responded more positively to hawthorn fruit than to apples. Because all flies assayed were naive (ruling out effects of prior host experience of adults) and because tests revealed no influence of pre-imaginal fruit exposure on pattern of host fruit acceptance by females, the combined evidence suggests the phenotypic differences we observed in host response pattern between hawthorn and apple origin flies may have an underlying genetic basis. Further tests showed that while larval progeny of flies of each origin survived better in naturally growing hawthorn fruit than in naturally growing apples, there was no differential effect of fly origin on larval survival ability in either host. We discuss our findings in relation to restriction in gene flow between sympatric populations of R. pomonella and in relation to current models of host shifts in insects.     

GENETIC DIVERGENCE ALONG THE SPECIATION CONTINUUM: THE TRANSITION FROM HOST RACE TO SPECIES IN RHAGOLETIS (DIPTERA: TEPHRITIDAE)

Studies of related populations varying in their degrees of reproductive isolation can provide insights into speciation. Here, the transition from partially isolated host races to more fully separated sibling species is investigated by comparing patterns of genetic differentiation between recently evolved (～150 generations) apple and ancestral hawthorn‐infesting populations of Rhagoletis pomonella to their sister taxon, the undescribed flowering dogwood fly attacking Cornus florida. No fixed or diagnostic private alleles differentiating the three populations were found at any of 23 microsatellites and 10 allozymes scored. Nevertheless, allele frequency differences were sufficient across loci for flowering dogwood fly populations from multiple localities to form a diagnosable genotypic cluster distinct from apple and hawthorn flies, indicative of species status. Genome‐wide patterns of differentiation were correlated between the host races and species pair comparisons along the majority of chromosomes, suggesting that similar disruptive selection pressures affect most loci. However, differentiation was more pronounced, with some additional regions showing elevated divergence, for the species pair comparison. Our results imply that Rhagoletis sibling species such as the flowering dogwood fly represent host races writ large, with the transition to species status primarily resulting from increased divergence of the same regions separating apple and hawthorn flies.