The role of sexual isolation during rapid ecological divergence: Evidence for a new dimension of isolation in Rhagoletis pomonella

The pace of divergence and likelihood of speciation often depends on how and when different types of reproductive barriers evolve. Questions remain about how reproductive isolation evolves after initial divergence. We tested for the presence of sexual isolation (reduced mating between populations due to divergent mating preferences and traits) in Rhagoletis pomonella flies, a model system for incipient ecological speciation. We measured the strength of sexual isolation between two very recently diverged (~170 generations) sympatric populations, adapted to different host fruits (hawthorn and apple). We found that flies from both populations were more likely to mate within than between populations. Thus, sexual isolation may play an important role in reducing gene flow allowed by early-acting ecological barriers. We also tested how warmer temperatures predicted under climate change could alter sexual isolation and found that sexual isolation was markedly asymmetric under warmer temperatures – apple males and hawthorn females mated randomly while apple females and hawthorn males mated more within populations than between. Our findings provide a window into the early speciation process and the role of sexual isolation after initial ecological divergence, in addition to examining how environmental conditions could shape the likelihood of further divergence.     

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.     

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.     

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.     

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.     

Experimental evidence of genome‐wide impact of ecological selection during early stages of speciation‐with‐gene‐flow

Theory predicts that speciation‐with‐gene‐flow is more likely when the consequences of selection for population divergence transitions from mainly direct effects of selection acting on individual genes to a collective property of all selected genes in the genome. Thus, understanding the direct impacts of ecologically based selection, as well as the indirect effects due to correlations among loci, is critical to understanding speciation. Here, we measure the genome‐wide impacts of host‐associated selection between hawthorn and apple host races of Rhagoletis pomonella (Diptera: Tephritidae), a model for contemporary speciation‐with‐gene‐flow. Allele frequency shifts of 32 455 SNPs induced in a selection experiment based on host phenology were genome wide and highly concordant with genetic divergence between co‐occurring apple and hawthorn flies in nature. This striking genome‐wide similarity between experimental and natural populations of R. pomonella underscores the importance of ecological selection at early stages of divergence and calls for further integration of studies of eco‐evolutionary dynamics and genome divergence.     

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.     

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.     

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.     

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.     

Prior experience influences the fruit residence of male apple maggot flies,Rhagoletis pomonella

Male apple maggot flies spend considerable time residing on individual host fruit as territories on which they force-copulate arriving females in search of oviposition sites. Here, we present evidence from investigations in nature and the laboratory that shows the propensity of males to reside on a hawthorn or apple fruit as a territory is significantly modifiable through prior experience with fruit and, hence, involves learning. Previous studies revealed that after a female apple maggot fly, Rhagoletis pomonella, arrived on a host hawthorn or apple fruit, its propensity to accept or reject that fruit for egg-laying was similarly modifiable through prior fruit-exposure experience and also involved learning. We discuss how host fruit learning in males and females, in concert with genetic-based differences in host fruit residence and acceptance behavior between populations of flies originating from hawthorn and apple, could give rise to a reduction in gene flow between populations of flies on these two host types.     

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.     

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

An 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.     

On the scent of standing variation for speciation: behavioral evidence for native sympatric host races of rhagoletis pomonella (Diptera: tephritidae) in the southern United States

Standing variation can be critical for speciation. Here, we investigate the origins of fruit odor discrimination for Rhagoletis pomonella underlying the fly's sympatric shift in the northeastern United States from downy hawthorn (Crataegus mollis) to apple (Malus domestica). Because R. pomonella mate on host fruit, preferences for natal fruit volatiles generate prezygotic isolation. Apples emit volatiles that appear to be missing from gas chromatography/electroantennographic detection profiles for flies infesting downy hawthorns, raising the question of how R. pomonella evolved a preference for apple. In the southern United States, R. pomonella attacks several native hawthorns. Behaviorally active volatile blends for R. pomonella infesting southern hawthorns contain the missing apple volatiles, potentially explaining why downy hawthorn flies could have evolved to be sensitive to a blend of apple volatiles. Flight tunnel assays imply that southern hawthorn populations were not the antecedent of a preassembled apple race, as southern flies were not attracted to the apple volatile blend. Instead, behavioral evidence was found for southern host races on native hawthorns, complementing the story of the historical sympatric shift to introduced apple in the North and illustrating how R. pomonella may evolve novel combinations of agonist and antagonist responses to volatiles to use new fruit resources.     

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.     

Microgeographic genetic variation in the apple maggot rhagoletis pomonella

We examined electrophoretic variability at five enzyme loci in the apple maggot fly, Rhagoletis pomonella, on a microgeographic scale. Treating flies from individual hawthorn trees as separate populations, we estimated F(ST) values from allele frequencies. The results indicate that there is significant allele frequency heterogeneity among fly populations over a small spatial scale at some loci but not at others. This variation among loci in degree of differentiation is itself statistically significant, casting doubt on the role of genetic drift in maintaining the heterogeneity. There is also heterogeneity between years in flies from a given tree. These data provide a baseline with which future work on genetic differentiation among apple maggot populations associated with different species of host plants may be compared.     

Cascading reproductive isolation: Plant phenology drives temporal isolation among populations of a host‐specific herbivore

All organisms exist within a complex network of interacting species, thus evolutionary change may have reciprocal effects on multiple taxa. Here, we demonstrate “cascading reproductive isolation,” whereby ecological differences that reduce gene flow between populations at one trophic level affect reproductive isolation (RI) among interacting species at the next trophic level. Using a combination of field, laboratory and common‐garden studies and long‐term herbaria records, we estimate and evaluate the relative contribution of temporal RI to overall prezygotic RI between populations of Belonocnema treatae, a specialist gall‐forming wasp adapted to sister species of live oak (Quercus virginiana and Q. geminata). We link strong temporal RI between host‐associated insect populations to differences between host plant budbreak phenology. Budbreak initiates flowering and the production of new leaves, which are an ephemeral resource critical to insect reproduction. As flowering time is implicated in RI between plant species, budbreak acts as a “multitrophic multi‐effect trait,” whereby differences in budbreak phenology contribute to RI in plants and insects. These sister oak species share a diverse community of host‐specific gall‐formers and insect natural enemies similarly dependent on ephemeral plant tissues. Thus, our results set the stage for testing for parallelism in a role of plant phenology in driving temporal cascading RI across multiple species and trophic levels.     

THE GEOGRAPHIC PATTERN OF GENETIC DIFFERENTIATION BETWEEN HOST ASSOCIATED POPULATIONS OF RHAGOLETIS POMONELLA (DIPTERA: TEPHRITIDAE) IN THE EASTERN UNITED STATES AND CANADA

True fruit flies in the Rhagoletis pomonella sibling species group are at the center of a long-standing debate concerning modes of speciation. The allopatric separation of populations is widely thought to be a prerequisite for speciation in sexually reproducing animals. However, speciation in the R. pomonella group appears to have occurred sympatrically as a consequence of these flies shifting and adapting to new host plants. The sympatric shift of R. pomonella from its native host hawthorn to introduced domestic apple, which occurred approximately 150 years ago, provides a test of whether host specialization is sufficient to allow populations to differentiate in the absence of geographic barriers to gene flow. We report the results of a geographic study of allozyme variation for hawthorn and apple infesting populations of R. pomonella across the eastern United States and Canada. Six loci consistently show significant allele frequency differences at paired apple and hawthorn sites. These six loci map to three different regions of the genome, and linkage disequilibrium exists between non-allelic genes within each of these regions. Allele frequencies for five of the six loci displaying host associated differences also co-vary significantly with latitude. Inter-host divergence is, therefore, superimposed on north-south clinal patterns of intra-host variation such that the magnitude of genetic divergence between hawthorn and apple flies is a function of latitude. The findings suggest that partially reproductively isolated “host races” can evolve in sympatry as a consequence of R. pomonella infesting new host plants. Host recognition and host associated developmental traits are discussed as important factors differentiating apple and hawthorn flies.     

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).