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1.
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.  相似文献   

2.
The present study investigates morphological differentiation among host races of the fruit fly Tephritis conura Loew (Diptera: Tephritidae) for two fitness‐related traits and whether these traits are host induced or genetically determined. Flies were analyzed from independent sympatric regions, and from one syntopic site where parental host plants [Cirsium heterophyllum (L.) Hill. and Cirsium oleraceum (L.) Scop. (Cardueae)] and hybrid plants (C. heterophyllum×C. oleraceum) co‐occur. As both host races may oviposit on hybrid plants and hybrid plants provide an identical environment for larvae of both host races, flies emerging from C. heterophyllum×C. oleraceum hybrids were used to assess whether host‐race morphological differences are genetically determined or due to phenotypic plasticity. No significant size (wing length) differences were found among host races, whereas flies emerging from C. heterophyllum had on average 8.4% longer ovipositors than flies emerging from C. oleraceum. The mean size‐corrected ovipositor length (i.e., the ratio ovipositor/wing length) was 10.3% longer. These proportions were repeated among host races emerging from hybrid plants. Although flies of the C. heterophyllum host race from hybrid plants were smaller than on parental host plants, the ratio ovipositor/wing length was constant. Hybrid flies (which emerged only on hybrid plants) were intermediate in relative and absolute ovipositor length. Thus, ovipositor‐length differences among T. conura host races most likely have a genetic basis. This suggests that host‐related differences in ovipositor length reflect adaptations to the respective host‐plant species, most likely to the host's flower‐head size, whereas both host races experience similar selection regimes on body size.  相似文献   

3.
Host shifts and the formation of insect-host races are likely common processes in the speciation of herbivorous insects. The interactions of goldenrods Solidago (Compositae), the gall fly Eurosta solidaginis (Diptera: Tephritidae) and the beetle Mordellistena convicta (Coleoptera: Mordellidae) provide behavioural, ecological and genetic evidence of host races that may represent incipient species forming via sympatric speciation. We summarize evidence for Eurosta host races and show that M. convicta has radiated from goldenrod stems to Eurosta galls to form host-part races and, having exploited the galler's host shift, has begun to differentiate into host races within galls. Thus, host-race formation has occurred in two interacting, but unrelated organisms representing two trophic levels, resulting in 'sequential radiation' (escalation of biodiversity up the trophic system). Distributions of host races and their behavioural isolating mechanisms suggest sympatric differentiation. Such differentiation suggests host-race formation and subsequent speciation may be an important source of biodiversity.  相似文献   

4.
We studied the inheritance of survival ability in host-associated populations of the tephritid fly, Eurosta solidaginis, to test predictions of sympatric speciation models. Eurosta solidaginis induces galls on two species of goldenrod, Solidago altissima and S. gigantea. The host-associated populations have been hypothesized to be host races that originated in sympatry (Craig et al. 1993). We found evidence for disruptive selection for host use, which is a critical assumption of sympatric speciation models. Each host race had higher survival rates on their host plant than on the alternative host. F1 and backcross hybrids also had lower survival rates than the pure host-race flies on their host plant. Since assortative mating occurs due to host-plant preference (Craig et al. 1993) this would select for divergence in host preference. Low hybrid survival could have been due to strong genetic incompatibilities of the populations or due to host adaptation by each population. Strong genetic incompatibilities would result in poor survival on all host plants, while host adaptation could result in low overall survival with high hybrid survival on some host plants with particularly “benign” environments. High survival of F1, F2, and backcross hybrids on some plant genotypes in some years supported the host adaptation hypothesis. F1 flies mated and oviposited normally and produced viable F2 and backcross hybrids indicating gene flow is possible between the host races. A few flies developed and emerged on the alternative host plant. This demonstrates that genes necessary to utilize the alternative host exist in both host races. This could have facilitated the origin of one of the populations via a host shift from the ancestral host. The inheritance of survival ability appears to be an autosomal trait. We did not find evidence that survival ability was maternally influenced or sex linked.  相似文献   

5.
Phytophagous insects provide useful models for the study of ecological speciation. Much attention has been paid to host shifts, whereas situations where closely related lineages of insects use the same plant during different time periods have been relatively neglected in previous studies of insect diversification. Flies of the genus Strobilomyia are major pests of conifers in Eurasia and North America. They are specialized feeders in cones and seeds of Abies (fir), Larix (larch) ,and Picea (spruce). This close association is accompanied by a large number of sympatric Strobilomyia species coexisting within each tree genus. We constructed a molecular phylogeny with a 1320 base-pair fragment of mitochondrial DNA that demonstrated contrasting patterns of speciation in larch cone flies, as opposed to spruce and fir cone flies; this despite their comparable geographic distributions and similar resource quality of the host. Species diversity is the highest on larch, and speciation is primarily driven by within-host phenological shifts, followed by allopatric speciation during geographical expansion. By contrast, fewer species exploit spruce and fir, and within-host phenological shifts did not occur. This study illustrates within-host adaptive radiation through phenological shifts, a neglected mode of sympatric speciation.  相似文献   

6.
We investigated genetic diversity in West European populations of the fungal pathogen Microbotryum violaceum in sympatric, parapatric and allopatric populations of the host species Silene latifolia and S. dioica, using four polymorphic microsatellite loci. In allopatric host populations, the fungus was highly differentiated by host species, exhibiting high values of F(ST) and R(ST), and revealed clear and distinct host races. In sympatric and parapatric populations we found significant population differentiation as well, except for one sympatric population in which the two host species grew truly intermingled. The mean number of alleles per locus for isolates from each of the host species was significantly higher in sympatric/parapatric than in allopatric populations. This suggests that either gene flow between host races in sympatry, or in case of less neutral loci, selection in a more heterogeneous host environment can increase the level of genetic variation in each of the demes. The observed pattern of host-related genetic differentiation among these geographically spread populations suggest a long-term divergence between these host races. In sympatric host populations, both host races presumably come in secondary contact, and host-specific alleles are exchanged depending on the amount of fungal gene flow.  相似文献   

7.
In North America, the pea aphid Acyrthosiphon pisum encompasses ecologically and genetically distinct host races that offer an ideal biological system for studies on sympatric speciation. In addition to its obligate symbiont Buchnera, pea aphids harbour several facultative and phylogenetically distant symbionts. We explored the relationships between host races of A. pisum and their symbiotic microbiota to gain insights into the historical process of ecological specialization and symbiotic acquisition in this aphid. We used allozyme and microsatellite markers to analyse the extent of genetic differentiation between populations of A. pisum on pea, alfalfa and clover in France. In parallel, we examined: (i) the distribution of four facultative symbionts; and (ii) the genetic variation in the Buchnera genome across host-associated populations of A. pisum. Our study clearly demonstrates that populations of A. pisum on pea, clover and alfalfa in France are genetically divergent, which indicates that they constitute distinct host races. We also found a very strong association between host races of A. pisum and their symbiotic microbiota. We stress the need for phylogeographic studies to shed light on the process of host-race formation and acquisition of facultative symbionts in A. pisum. We also question the effects of these symbionts on aphid host fitness, including their role in adaptation to a host plant.  相似文献   

8.
The addition of a novel host plant to a phytophagous insect’s diet may result in subsequent host-plant specialisation, and is believed to be a key cause for speciation in this trophic group. In northern Britain, the tephritid fly Tephritis conura has experienced a unique host-plant expansion, from the melancholy thistle Cirsium heterophyllum to the marsh thistle C. palustre. Here, we examine whether the incorporation of C. palustre in the repertoire of British T. conura flies has caused genetic divergence between populations infesting the old host and the novel host, and how British populations differ from populations infesting C. heterophyllum in continental Europe where C. palustre is not infested. No evidence for restricted gene flow among British C. palustre and C. heterophyllum flies was found. Significant differentiation between British and continental T. conura was found at only one allozyme locus, hexokinase, and caused by a new allele, Hex_95. Hexokinase is related to host-race formation in continental European flies infesting C. heterophyllum and C. oleraceum, and might be linked to loci determining host choice. Based on morphological and phenological data from previous studies, we suggest that T. conura in Britain has adapted to the novel host but that host-race formation is impeded by similar plant phenologies.  相似文献   

9.
Species of the Heterodoxus octoseriatus group infest five species, comprising eight chromosome races, of Petrogale in Queensland and northern New South Wales, Australia. The precise host and geographic ranges of the 11 species of the H. octoseriatus group were determined. Individual hosts and host populations were infested by single species of lice only. The geographic ranges of lice were discrete. In some cases the geographic ranges of louse species and their hosts were approximately congruent. In others they were completely incongruent and it was unclear which species originally infested which hosts or even which was the most recent colonizer. In at least one case, expansion of the geographic range of one species of louse apparently led to fragmentation of the range of another and subsequently, to allopatric speciation.  相似文献   

10.
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.  相似文献   

11.
Aguin-Pombo D 《Heredity》2002,88(6):415-422
The limited importance ascribed to sympatric speciation processes via host race formation is partially due to the few cases of host races that have been reported among host populations. This work sheds light on the taxonomy of Alebra leafhoppers and examines the possible existence of host races among host-associated populations. The species of this genus show varying degrees of host association with deciduous trees and shrubs and, frequently, host populations of uncertain taxonomic status coexist and occasionally become pests. Allozyme electrophoresis of 21 Greek populations including sympatric, local and geographically distant samples collected on 13 different plant species, show that they represent at least five species: A. albostriella Fallén, A. viridis (Rey) (sensu Gillham), A. wahlbergi Boheman and two new species. Of these, one is associated to Quercus frainetto and other is specific to Crataegus spp. Significant genetic differences among sympatric and local host populations were found only in A. albostriella, between populations on Turkey oak, beech and common alder. It is suggested that the last two of these host populations may represent different host races. The results show that both the host plant and geographical distance affect the patterns of differentiation in the genus. The formation of some species seems to have been the result of allopatric speciation events while, for others, their origin can be equally explained either by sympatric or allopatric speciation.  相似文献   

12.
French populations of the European corn borer consist of two sympatric and genetically differentiated host races. As such, they are well suited to study processes that could be involved in sympatric speciation, but the initial conditions of host-race divergence need to be elucidated. Gene genealogies can provide insight into the processes involved in speciation. We used DNA sequences of four nuclear genes to (1) document the genetic structure of the two French host races previously delineated with allozyme markers, (2) find genes directly or indirectly involved in reproductive isolation between host races, and (3) estimate the time since divergence of the two taxa and see whether this estimate is compatible with this divergence being the result of a host shift onto maize after its introduction into Europe approximately 500 years ago. Gene genealogies revealed extensive shared polymorphism, but confirmed the previously observed genetic differentiation between the two host races. Significant departures from the predictions of neutral molecular evolution models were detected at three loci but were apparently unrelated to reproductive isolation between host races. Estimates of time since divergence between French host races varied from approximately 75,000 to approximately 150,000 years, suggesting that the two taxa diverged recently but probably long before the introduction of maize into Europe.  相似文献   

13.
Categorizing speciation into dichotomous allopatric versus nonallopatric modes may not always adequately describe the geographic context of divergence for taxa. If some of the genetic changes generating inherent barriers to gene flow between populations evolved in geographic isolation, whereas others arose in sympatry, then the mode of divergence would be mixed. The apple maggot fly, Rhagoletis pomonella, has contributed to this emerging concept of a mixed speciation mode "plurality." Genetic studies have implied that a source of diapause life-history variation associated with inversions and contributing to sympatric host race formation and speciation for R. pomonella in the United States may have introgressed from the Eje Volcanico Trans Mexicano (EVTM; a.k.a. the Altiplano) in the past. A critical unresolved issue concerning the introgression hypothesis is how past gene flow occurred given the current 1200-km disjunction in the ranges of hawthorn-infesting flies in the EVTM region of Mexico and the southern extreme of the U.S. population in Texas. Here, we report the discovery of a hawthorn-infesting population of R. pomonella in the Sierra Madre Oriental Mountains (SMO) of Mexico. Sequence data from 15 nuclear loci and mitochondrial DNA imply that the SMO flies are related to, but still different from, U.S. and EVTM flies. The host affiliations, diapause characteristics, and phylogeography of the SMO population are consistent with it having served as a conduit for gene flow between Mexico and the United States. We also present evidence suggesting greater permeability of collinear versus rearranged regions of the genome to introgression, in accord with recent models of chromosomal speciation. We discuss the implications of the results in the context of speciation mode plurality. We do not argue for abandoning the terms sympatry or allopatry, but caution that categorizing divergence into either/or geographic modes may not describe the genetic origins of all species. For R. pomonella in the United States, the proximate selection pressures triggering race formation and speciation stem from sympatric host shifts. However, some of the phenological variation contributing to host-related ecological adaptation and reproductive isolation in sympatry at the present time appears to have an older history, having originated and become packaged into inversion polymorphism in allopatry.  相似文献   

14.
We introduce a general approach for investigating the role of geography in speciation, based on analyzing the geography of sister clades across all nodes in a species-level phylogeny. We examine the predictions of allopatric, sympatric, and peripatric models of speciation in several animal groups, using patterns of range overlap and range size symmetry between sister clades. A simple model of cladogenesis incorporating random movements of species' ranges is used to illustrate the effects of range changes on expected patterns. We find evidence for a predominantly allopatric mode of speciation in our study groups, with sympatry arising through postspeciational range changes. In addition, we find that relatively recent speciation events are characterized by greater asymmetry in range size between sister clades than expected under our null models, providing potential support for the peripatric model of speciation. We discuss the possible confounding effects of postspeciational range changes on our conclusions.  相似文献   

15.
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.  相似文献   

16.
We report behavioral evidence that Eurosta solidaginis, a stem-galling tephritid fly, has formed host races on its two goldenrod hosts, Solidago altissima and S. gigantea. Previous work has shown that flies from each host plant differ electrophoretically at the level of host races. The two host-associated populations were truly sympatric and were frequently found on host plants of the two species growing interdigitated with each other. Each host-associated population demonstrated a strong preference for ovipuncturing its own host. The S. gigantea–associated population emerged 10 to 14 d earlier than the S. altissima–associated population, contributing to the reproductive isolation between populations. Partial reproductive isolation is also maintained by a preference for mating on the host from which the fly emerged. The populations meet the criteria established for host races, suggesting that they may be in an intermediate stage of sympatric speciation.  相似文献   

17.
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.  相似文献   

18.
The origins of hybrid zones between parapatric taxa have been of particular interest for understanding the evolution of reproductive isolation and the geographic context of species divergence. One challenge has been to distinguish between allopatric divergence (followed by secondary contact) versus primary intergradation (parapatric speciation) as alternative divergence histories. Here, we use complementary phylogeographic and population genetic analyses to investigate the recent divergence of two subspecies of Clarkia xantiana and the formation of a hybrid zone within the narrow region of sympatry. We tested alternative phylogeographic models of divergence using approximate Bayesian computation (ABC) and found strong support for a secondary contact model and little support for a model allowing for gene flow throughout the divergence process (i.e. primary intergradation). Two independent methods for inferring the ancestral geography of each subspecies, one based on probabilistic character state reconstructions and the other on palaeo-distribution modelling, also support a model of divergence in allopatry and range expansion leading to secondary contact. The membership of individuals to genetic clusters suggests geographic substructure within each taxon where allopatric and sympatric samples are primarily found in separate clusters. We also observed coincidence and concordance of genetic clines across three types of molecular markers, which suggests that there is a strong barrier to gene flow. Taken together, our results provide evidence for allopatric divergence followed by range expansion leading to secondary contact. The location of refugial populations and the directionality of range expansion are consistent with expectations based on climate change since the last glacial maximum. Our approach also illustrates the utility of combining phylogeographic hypothesis testing with species distribution modelling and fine-scale population genetic analyses for inferring the geography of the divergence process.  相似文献   

19.
Host races play a central part in understanding the role of host plant mediated divergence and speciation of phytophagous insects. Of greatest interest are host-associated populations that have recently diverged; however, finding genetic evidence for very recent divergences is difficult because initially only a few loci are expected to evolve diagnostic differences. The holly leafminer Phytomyza glabricola feeds on two hollies, Ilex glabra and I. coriacea, that are broadly sympatric throughout most of their ranges. The leafminer is often present on both host plants and exhibits a dramatic life history difference on the two hosts, suggesting that host races may be present. We collected 1393 bp of mitochondrial cytochrome oxidase I (COI) sequence and amplified fragment length polymorphism (AFLP) data (45 polymorphic bands) from sympatric populations of flies reared from the two hosts. Phylogenetic and frequency analysis of mitochondrial COI sequence data uncovered considerable variation but no structuring by the host plant, and only limited differentiation among geographical locations. In contrast, analysis of AFLP frequency data found a significant effect with host plant, and a much smaller effect with geographical location. Likewise, neighbour-joining analysis of AFLP data resulted in clustering by host plant. The AFLP data indicate that P. glabricola is most likely comprised of two host races. Because there were no fixed differences in mitochondrial or AFLP data, this host-associated divergence is likely to have occurred very recently. P. glabricola therefore provides a new sympatric system for exploring the role of geography and ecological specialization in the speciation of phytophagous insects.  相似文献   

20.
Elucidating the mechanisms and conditions facilitating the formation of biodiversity are central topics in evolutionary biology. A growing number of studies imply that divergent ecological selection may often play a critical role in speciation by counteracting the homogenising effects of gene flow. Several examples involve phytophagous insects, where divergent selection pressures associated with host plant shifts may generate reproductive isolation, promoting speciation. Here, we use ddRADseq to assess the population structure and to test for host‐related genomic differentiation in the European cherry fruit fly, Rhagoletis cerasi (L., 1758) (Diptera: Tephritidae). This tephritid is distributed throughout Europe and western Asia, and has adapted to two different genera of host plants, Prunus spp. (cherries) and Lonicera spp. (honeysuckle). Our data imply that geographic distance and geomorphic barriers serve as the primary factors shaping genetic population structure across the species range. Locally, however, flies genetically cluster according to host plant, with consistent allele frequency differences displayed by a subset of loci between Prunus and Lonicera flies across four sites surveyed in Germany and Norway. These 17 loci display significantly higher FST values between host plants than others. They also showed high levels of linkage disequilibrium within and between Prunus and Lonicera flies, supporting host‐related selection and reduced gene flow. Our findings support the existence of sympatric host races in R. cerasi embedded within broader patterns of geographic variation in the fly, similar to the related apple maggot, Rhagoletis pomonella, in North America.  相似文献   

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