The genetic structure of hawthorn-infesting Rhagoletis pomonella populations in Mexico: implications for sympatric host race formation

The genetic origins of species may not all trace to the same time and place as the proximate cause(s) for population divergence. Moreover, inherent gene-flow barriers separating populations may not all have evolved under the same geographical circumstances. These considerations have lead to a greater appreciation of the plurality of speciation: that one geographical mode for divergence may not always be sufficient to describe a speciation event. The apple maggot fly, Rhagoletis pomonella , a model system for sympatric speciation via host-plant shifting, has been a surprising contributor to the concept of speciation mode plurality. Previous studies have suggested that past introgression of inversion polymorphism from a hawthorn-fly population in the trans-Mexican volcanic belt (EVTM) introduced diapause life-history variation into a more northern fly population that subsequently contributed to sympatric host race formation and speciation in the United States (US). Here, we report results from a microsatellite survey implying (i) that volcanic activity in the eastern EVTM may have been responsible for the initial geographical isolation of the Mexican and northern hawthorn-fly populations c . 1.57 mya; and (ii) that flies in the Sierra Madre Oriental Mountains (SMO) likely served as a conduit for past gene flow from the EVTM into the US. Indeed, the microsatellite data suggest that the current US population may represent a range expansion from the northern SMO. We discuss the implications of these findings for sympatric race formation in Rhagoletis and speciation theory.     

The population genetics of the apple maggot fly,Rhagoletis pomonella and the snowberry maggot,R. zephyria: implications for models of sympatric speciation

True fruit flies belonging to the Rhagoletis pomonella (Diptera:Tephritidae) sibling species complex possess several attributes consistent with a history of sympatric divergence via host plant shifts. Here, we investigate whether hybridization and genetic introgression is occurring between two members of the group, Rhagoletis pomonella (Walsh), whose primary hosts are domestic apples (Malus pumila) and hawthorns (Crataegus spp., and R. zephyria (Snow) whose host is snowberries (Symphoricarpos spp.). These flies are important because they appear to be at a transition stage between taxa reproductively isolated solely on the basis of host plant-related adaptations and those that have evolved additional non-host dependent sterility and inviability. Observing extensive hybridization and introgression between R. pomonella and R. zephyria would have major repercussions for current models of sympatric speciation.In a survey of allozyme and mitochondrial DNA (mtDNA) variation for 1105 flies collected throughout the northern United States we found two results suggesting that low level hybridization occurs between R. pomonella and R. zephyria. (1) Two flies reared from snowberries and one fly reared from hawthoms had genotypes indicative of them being R. pomonella and R. zephyria, respectively. Rhagoletis pomonella and R. zephyria adults may therefore occasionally frequent each others host plant, providing the opportunity for hybridization. (2) Four flies collected from hawthorns and one from snowberries had genotypes that made them likely to be F1 hybrids.Likelihood analysis revealed the data were also consistent with an hypothesis of shared ancestral alleles (i.e., the pattern of genetic variation could also be explained by R. pomonella and R. zephyria sharing alleles/haplotypes whose origins date to a common ancestor). We estimated that, in the absence of interspecific mating, random assortment of genes within R. pomonella and R. zephyria populations would produce an average of 5.4 flies with genotypes suggesting they were F1 hybrids – a number equivalent to the 5 putative F1 hybrids observed in the study. Our results therefore underscore the difficulty in distinguishing between hypotheses of low level introgression and shared ancestral polymorphism. But even if hybridization is occurring, the data suggest that it is happening at a very low and probably evolutionarily insignificant level (perhaps 0.09% per generation), consistent with sympatric speciation theory. Future tests are discussed that could help resolve the hybridization issue for R. pomonella and R. zephyria.     

Divergence and evolution of reproductive barriers among three allopatric populations of Rhagoletis cingulata across eastern North America and Mexico

Geography is often a key factor facilitating population divergence and speciation. In this regard, the geographic distributions of flies in the genus Rhagoletis (Diptera: Tephritidae) in temperate North America have been affected by cycles of Pleistocene glaciation and interglacial periods. Fluctuations in climatic conditions may have had their most dramatic effects on geographically isolating Rhagoletis flies in the central highland region of Mexico. During past periods of allopatry, a degree of post‐zygotic reproductive isolation appears to have evolved between hawthorn‐infesting populations of Rhagoletis pomonella (Walsh) in the central Eje Volcanico Trans Mexicano (EVTM) and those from the Sierra Madre Oriental Mountains (SMO) of Mexico, as well as hawthorn flies from the eastern USA. Here, we investigate the generality of this finding in the genus Rhagoletis by testing for reproductive isolation among populations of Rhagoletis cingulata (Loew) (Diptera: Tephritidae) collected from infested domesticated sweet cherry (Prunus avium L.) in the USA and black cherry [Prunus serotina Ehrh. (both Rosaceae)] from the SMO and EVTM. We report evidence for marked post‐mating reproductive isolation among certain R. cingulata populations. The high levels of reproductive isolation were observed between R. cingulata flies from populations in the USA and SMO differed from the pattern seen for R. pomonella, primarily involving the EVTM. In addition, egg hatch was significantly reduced for crosses between SMO males and EVTM females, but not greatly in the opposite direction. We discuss potential causes for the different patterns of post‐mating reproductive isolation among Rhagoletis flies.     

Evolution of intrinsic reproductive isolation among four North American populations of Rhagoletis pomonella (Diptera: Tephritidae)

Across its range in North America, four geographically separated, ecologically and genetically diverged populations of hawthorn (Crataegus)‐infesting Rhagoletis pomonella (Diptera: Tephritidae) flies inhabit the Eje Volcánico Trans Mexicano (EVTM), the Sierra Madre Oriental (SMO), the Chiapas Highlands (CHIS) and the USA. Here, we tested whether these four populations are reproductively isolated by any intrinsic, nonhost‐related, pre‐ or postmating barriers to gene flow. Crossing experiments suggested that a low level of host‐independent prezygotic isolation may exist between hawthorn flies from EVTM and the three other populations, but only with respect to a slight reduction in copulation duration in EVTM matings. Some evidence for postmating isolation was found, again primarily involving EVTM crossed to SMO, CHIS and US flies. Certain crosses produced no (SMO male × EVTM female) or few (EVTM male × CHIS female; CHIS male × SMO female) F1 hybrid offspring. F2 crosses were generally fertile, except for US male × CHIS female matings. Inherent reproductive isolation therefore appears to be quantitative rather than absolute between populations, as the possibility for gene flow exists through at least some combinations of mating among EVTM, SMO, CHIS and US flies. Our results are consistent with a recently advanced hypothesis that episodic introgression from Mexico into the USA has played a role in providing genetic variation, facilitating sympatric host race formation and the adaptive radiation of the R. pomonella sibling species' complex in the USA. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 213–223.     

Host plant and latitude-related diapause variation in Rhagoletis pomonella: a test for multifaceted life history adaptation on different stages of diapause development

Variation in the overwintering pupal diapause of Rhagoletis pomonella appears to adapt sympatric populations of the fly to seasonal differences in the fruiting times of their host plants, generating ecological reproductive isolation. Here, we investigate what aspects of diapause development are differentially affected (1) by comparing the propensities of apple vs. hawthorn-infesting host races of R. pomonella to forgo an initially deep diapause and directly develop into adults, and (2) by determining the chronological order that R. pomonella races and sibling species break diapause and eclose when reared under standardized environmental conditions. The results imply that factors affecting initial diapause depth (and/or differential mortality during the prewintering period) and those determining the timing of diapause termination or rates of post-diapause development are both under differential selection and are to some degree genetically uncoupled in flies. The modular nature of diapause life history adaptation in Rhagoletis suggests that phenology may involve multiple genetic changes and represent a stronger ecological barrier separating phytophagous specialists than is generally appreciated.     

HABITAT AVOIDANCE: OVERLOOKING AN IMPORTANT ASPECT OF HOST‐SPECIFIC MATING AND SYMPATRIC SPECIATION?

Understanding speciation requires discerning how reproductive barriers to gene flow evolve between previously interbreeding populations. Models of sympatric speciation for phytophagous insects posit that reproductive isolation can evolve in the absence of geographic isolation as a consequence of an insect shifting and ecologically adapting to a new host plant. One important adaptation contributing to sympatric differentiation is host-specific mating. When organisms mate in preferred habitats, a system of positive assortative mating is established that facilitates sympatric divergence. Models of host fidelity generally assume that host choice is determined by the aggregate effect of alleles imparting positive preferences for different plant species. But negative effect genes for avoiding nonnatal plants may also influence host use. Previous studies have shown that apple and hawthorn-infesting races of Rhagoletis pomonella flies use volatile compounds emitted from the surface of fruit as key chemosensory cues to recognize and distinguish between their host plants. Here, we report results from field trials indicating that in addition to preferring the odor of their natal fruit, apple and hawthorn flies, and their undescribed sister species infesting flowering dogwood (Cornus florida), also avoid the odors of nonnatal fruit. We discuss the implications of nonnatal fruit avoidance for the evolutionary dynamics and genetics of sympatric speciation. Our findings reveal an underappreciated role for habitat avoidance as a potential postmating, as well as prezygotic, barrier to gene flow.     

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.     

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.     

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.     

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.     

Evidence for spatial clines and mixed geographic modes of speciation for North American cherry‐infesting Rhagoletis (Diptera: Tephritidae) flies

An important criterion for understanding speciation is the geographic context of population divergence. Three major modes of allopatric, parapatric, and sympatric speciation define the extent of spatial overlap and gene flow between diverging populations. However, mixed modes of speciation are also possible, whereby populations experience periods of allopatry, parapatry, and/or sympatry at different times as they diverge. Here, we report clinal patterns of variation for 21 nuclear‐encoded microsatellites and a wing spot phenotype for cherry‐infesting Rhagoletis (Diptera: Tephritidae) across North America consistent with these flies having initially diverged in parapatry followed by a period of allopatric differentiation in the early Holocene. However, mitochondrial DNA (mtDNA) displays a different pattern; cherry flies at the ends of the clines in the eastern USA and Pacific Northwest share identical haplotypes, while centrally located populations in the southwestern USA and Mexico possess a different haplotype. We hypothesize that the mitochondrial difference could be due to lineage sorting but more likely reflects a selective sweep of a favorable mtDNA variant or the spread of an endosymbiont. The estimated divergence time for mtDNA suggests possible past allopatry, secondary contact, and subsequent isolation between USA and Mexican fly populations initiated before the Wisconsin glaciation. Thus, the current genetics of cherry flies may involve different mixed modes of divergence occurring in different portions of the fly''s range. We discuss the need for additional DNA sequencing and quantification of prezygotic and postzygotic reproductive isolation to verify the multiple mixed‐mode hypothesis for cherry flies and draw parallels from other systems to assess the generality that speciation may commonly involve complex biogeographies of varying combinations of allopatric, parapatric, and sympatric divergence.     

Radiation and divergence in the Rhagoletis pomonella species complex: inferences from DNA sequence data

Here, we investigate the evolutionary history and pattern of genetic divergence in the Rhagoletis pomonella (Diptera: Tephritidae) sibling species complex, a model for sympatric speciation via host plant shifting, using 11 anonymous nuclear genes and mtDNA. We report that DNA sequence results largely coincide with those of previous allozyme studies. Rhagoletis cornivora was basal in the complex, distinguished by fixed substitutions at all loci. Gene trees did not provide reciprocally monophyletic relationships among US populations of R. pomonella, R. mendax, R. zephyria and the undescribed flowering dogwood fly. However, private alleles were found for these taxa for certain loci. We discuss the implications of the results with respect to identifiable genetic signposts (stages) of speciation, the mosaic nature of genomic differentiation distinguishing formative species and a concept of speciation mode plurality involving a biogeographic contribution to sympatric speciation in the R. pomonella complex.     

Reproductive character displacement and environmental filtering shape floral variation between sympatric sister taxa

Divergence in reproductive traits between closely related species that co‐occur contributes to speciation by reducing interspecific gene flow. In flowering plants, greater floral divergence in sympatry than allopatry may reflect reproductive character displacement (RCD) by means of divergent pollinator‐mediated selection or mating system evolution. However, environmental filtering (EF) would prevail for floral traits under stronger selection by abiotic factors than pollination, and lead to sympatric taxa being more phenotypically similar. We determine whether floral UV pigmentation and size show signatures of RCD or EF using a biogeographically informed sister taxa comparison. We determine whether 35 sister pairs in the Potentilleae tribe (Rosaceae) are allopatric or sympatric and confirm that sympatric sisters experience more similar bioclimatic conditions, an assumption of the EF hypothesis. We test whether interspecific differences are greater in allopatry or sympatry while accounting for divergence time. For UV pigmentation, sympatric sisters are more phenotypically similar than allopatric ones. For flower size, sympatric sisters show increased divergence with time since speciation but allopatric ones do not. We conclude that floral UV pigmentation shows a signature of EF, whereas flower size shows a signature of RCD. Discordant results between the traits suggest that the dominant selective agent differs between them.     

Evidence for divergence and adaptive isolation in post‐glacially derived bimodal allopatric and sympatric rainbow smelt populations

The roles of ecological speciation and reinforcement in the formation of contemporary diversity remain contentious. In the present study, we contrast phenotypic and molecular divergence within morphologically diverged bimodal sympatric and allopatric pairs of rainbow smelt, Osmerus mordax. We hypothesize that, in sympatry, evidence of selection associated with resource partitioning will be visible through strong divergence, reinforcement, and greater character displacement. Parallel morphological divergence was observed between the two trophic forms (macrophagous and microphagous), with several examples of greater trait divergence in sympatry than allopatry. Mitochondrial DNA sequence analysis indicated no association between historical clades and morphology; however, Bayesian clustering using microsatellites supported the isolation of these morphs under both allopatry and sympatry. Estimates of genetic isolation were one order of magnitude lower than measures of morphological divergence, consistent with a hypothesis of strong contemporary selection. Using experimental crosses, we obtained similar rates of fertilization success among the allopatric hybrid and pure crosses; whereas, in the sympatric hybrid crosses, fertilization rates dropped by 30–50%, suggesting a clear role for reinforcement through prezygotic incompatibilities. The present study supports the hypothesis that processes of post‐glacial radiation and diversification differ between sympatry and allopatry, and indicates a role for reinforcement and ecological processes in recent sympatric diversification. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101 , 583–594.     

Speciation in the apple maggot fly: a blend of vintages?

The importance of speciation without the complete geographical separation of diverging populations (sympatric speciation) has become increasingly accepted. One of the textbook examples of recent speciation in sympatry is the apple maggot fly Rhagoletis pomonella, in which genetically differentiated host races feed on either hawthorn or apple. Three recent articles by Feder and collaborators show that the history of these host races is more complicated than was previously realized. Genes that differentiate races of flies that feed on either apple or hawthorn are located in chromosomal rearrangements. This variation forms a latitudinal cline that must have been established long before apples were available as hosts. Furthermore, there is also new evidence for the very recent evolution of a derived preference for volatile chemicals that are typical of apple fruits among apple-feeding flies. These results show that adaptation to apple populations has involved both the sorting of ancestral adaptive variation and the selection of novel mutations.     

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.     

The role of geographic setting on the diversification process among Tephritis conura (Tephritidae) host races

We address the controversy over the processes causing divergence during speciation. Host races of the fruit fly Tephritis conura attack the thistles Cirsium oleraceum and Cirsium heterophyllum. By studying the genetic divergence of T. conura in areas where host plants are sympatric, parapatric and allopatric, we assessed the contribution of geography in driving host-race divergence. We also evaluated the relative importance of genetic drift and selection in the diversification process, by analysis of the geographic distribution of genetic variation. Host races were significantly diverged at five out of 13 polymorphic allozyme loci. Variance at two loci, Hex and Pep D, was almost exclusively attributable to host-plant affiliation in all geographic settings. However, Hex was significantly more differentiated between host races in sympatry/parapatry than in allopatry. This result might be explained by selection against hybridisation or against incorrect host choice in contact areas. Linkage disequilibrium tests suggest the latter: gene flow in contact areas may occur from males of the host-race C. heterophyllum to females of the host-race C. oleraceum, whereas incorrect oviposition events were never observed. The distinctive patterns of genetic differentiation at the two highly differentiated loci implicate the action of selection (acting directly or on linked loci) rather than genetic drift. Despite their restricted interactions in sympatry, we conclude that host races are stable and that the major diversification process took place before species arrived in today's geographical settings.     

Radiation and divergence in the Rhagoletis pomonella species group: inferences from allozymes

The Rhagoletis pomonella species group has for decades been a focal point for debate over the possibility of sympatric speciation via host shift. Here I present the first extensive analysis of genetic (allozyme) divergence in the pomonella group, including all known taxa/populations except the allopatric Mexican population of R. pomonella. The phylogeny is estimated for all four described species (pomonella, mendax, zephyria, and cornivora) plus two undescribed species (the "flowering dogwood fly" and "sparkleberry fly"). Allozyme data for two additional populations of uncertain status (the "plum fly" and "mayhaw fly") are presented for the first time. Two data sets were analyzed, one for 17 loci from 77 populations and one for an additional 12 loci for a subset of 12 of these populations, with more than 4000 flies analyzed in total. Interspecific Nei unbiased genetic distances were generally small, being as low as 0.040. No fixed autapomorphic alleles beyond those already known for R. cornivora and R. zephyria were revealed in the new data, but several loci displaying frequency patterns useful in discriminating the species were discovered. The phylogenetic placement of the flowering dogwood fly differed depending on whether a molecular clock was assumed (UPGMA of Nei distance) or not assumed (frequency parsimony) for tree building. Other than this, however, trees under either assumption were essentially identical. The best tree was used to test the prediction of the sympatric speciation hypothesis that sister taxa should be broadly sympatric. This prediction was not rejected, but the best tree was weakly supported by bootstrap analysis. An unexpected finding was that R. pomonella populations representing ends of its strong latitudinal clines did not cluster together. One possible explanation is that the current R. pomonella is the result of a genetic fusion of two previously isolated, genetically differentiated populations. Such a fusion prior to the origin of the other species in the group could contribute to the poor resolution of the phylogeny.     

Gene flow,divergent selection and resistance to introgression in two species of morning glories (Ipomoea)

Gene flow is thought to impede genetic divergence and speciation by homogenizing genomes. Recent theory and research suggest that sufficiently strong divergent selection can overpower gene flow, leading to loci that are highly differentiated compared to others. However, there are also alternative explanations for this pattern. Independent evidence that loci in highly differentiated regions are under divergent selection would allow these explanations to be distinguished, but such evidence is scarce. Here, we present multiple lines of evidence that many of the highly divergent SNPs in a pair of sister morning glory species, Ipomoea cordatotriloba and I. lacunosa, are the result of divergent selection in the face of gene flow. We analysed a SNP data set across the genome to assess the amount of gene flow, resistance to introgression and patterns of selection on loci resistant to introgression. We show that differentiation between the two species is much lower in sympatry than in allopatry, consistent with interspecific gene flow in sympatry. Gene flow appears to be substantially greater from I. lacunosa to I. cordatotriloba than in the reverse direction, resulting in sympatric and allopatric I. cordatotriloba being substantially more different than sympatric and allopatric I. lacunosa. Many SNPs highly differentiated in allopatry have experienced divergent selection, and, despite gene flow in sympatry, resist homogenization in sympatry. Finally, five out of eight floral and inflorescence characteristics measured exhibit asymmetric convergence in sympatry. Consistent with the pattern of gene flow, I. cordatotriloba traits become much more like those of I. lacunosa than the reverse. Our investigation reveals the complex interplay between selection and gene flow that can occur during the early stages of speciation.     

Divergence in the calling songs between sympatric and allopatric populations of the southern wood cricket Gryllus fultoni (Orthoptera: Gryllidae)

In the eastern United States the wood cricket Gryllus fultoni (Orthoptera: Gryllidae) occurs in sympatry with G. vernalis in an area between eastern Kansas and west of the Appalachian Mountains. Calling songs were recorded from 13 sympatric and allopatric localities. Both field and laboratory recordings showed that chirp rate (CR) and pulse rate (PR) overlapped extensively between allopatric populations of G. fultoni and sympatric populations of G. vernalis; by contrast, there was little or no overlap in these variables between sympatric populations of these two species. Divergence in PR and CR between the two species was thus greater in areas of sympatry than in areas of allopatry. Our field and laboratory studies of G. fultoni calling songs thus demonstrate the pattern expected of character displacement and support the genetic assumptions of this hypothesis. Other possible explanations for the sympatric divergence such as ecological character displacement and clinal variation are discussed.