Browsing by Author "Smith, James J."

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    Evidence for spatial clines and mixed geographic modes of speciation for North American cherry‐infesting Rhagoletis (Diptera: Tephritidae) flies
    (Wiley, 2020) Doellman, Meredith M.; Jean, Gilbert Saint; Egan, Scott P.; Powell, Thomas H.Q.; Hood, Glen Ray; Schuler, Hannes; Bruzzese, Daniel J.; Glover, Mary M.; Smith, James J.; Yee, Wee L.; Goughnour, Robert B.; Rull, Juan; Aluja, Martin; Feder, Jeffrey L.
    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.
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    Genomic Differentiation during Speciation-with-Gene-Flow: Comparing Geographic and Host-Related Variation in Divergent Life History Adaptation in Rhagoletis pomonella
    (MDPI, 2018) Doellman, Meredith M.; Ragland, Gregory J.; Hood, Glen Ray; Meyers, Peter J.; Egan, Scott P.; Powell, Thomas H.Q.; Lazorchak, Peter; Glover, Mary M.; Tait, Cheyenne; Schuler, Hannes; Hahn, Daniel A.; Berlocher, Stewart H.; Smith, James J.; Nosil, Patrik; Feder, Jeffrey L.
    A major goal of evolutionary biology is to understand how variation within populations gets partitioned into differences between reproductively isolated species. Here, we examine the degree to which diapause life history timing, a critical adaptation promoting population divergence, explains geographic and host-related genetic variation in ancestral hawthorn and recently derived apple-infesting races of Rhagoletis pomonella. Our strategy involved combining experiments on two different aspects of diapause (initial diapause intensity and adult eclosion time) with a geographic survey of genomic variation across four sites where apple and hawthorn flies co-occur from north to south in the Midwestern USA. The results demonstrated that the majority of the genome showing significant geographic and host-related variation can be accounted for by initial diapause intensity and eclosion time. Local genomic differences between sympatric apple and hawthorn flies were subsumed within broader geographic clines; allele frequency differences within the races across the Midwest were two to three-fold greater than those between the races in sympatry. As a result, sympatric apple and hawthorn populations displayed more limited genomic clustering compared to geographic populations within the races. The findings suggest that with reduced gene flow and increased selection on diapause equivalent to that seen between geographic sites, the host races may be recognized as different genotypic entities in sympatry, and perhaps species, a hypothesis requiring future genomic analysis of related sibling species to R. pomonella to test. Our findings concerning the way selection and geography interplay could be of broad significance for many cases of earlier stages of divergence-with-gene flow, including (1) where only modest increases in geographic isolation and the strength of selection may greatly impact genetic coupling and (2) the dynamics of how spatial and temporal standing variation is extracted by selection to generate differences between new and discrete units of biodiversity.
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    Geographic and Ecological Dimensions of Host Plant-Associated Genetic Differentiation and Speciation in theﾠRhagoletis cingulataﾠ(Diptera: Tephritidae) Sibling Species Group
    (MDPI, 2019) Doellman, Meredith M.; Schuler, Hannes; Jean Saint, Gilbert; Hood, Glen R.; Egan, Scott P.; Powell, Thomas H.Q.; Glover, Mary M.; Bruzzese, Daniel J.; Smith, James J.; Yee, Wee L.; Goughnour, Robert B.; Rull, Juan; Aluja, Martin; Feder, Jeffrey L.
    Ascertaining the causes of adaptive radiation is central to understanding how new species arise and come to vary with their resources. The ecological theory posits adaptive radiation via divergent natural selection associated with novel resource use; an alternative suggests character displacement following speciation in allopatry and then secondary contact of reproductively isolated but ecologically similar species. Discriminating between hypotheses, therefore, requires the establishment of a key role for ecological diversification in initiating speciation versus a secondary role in facilitating co-existence. Here, we characterize patterns of genetic variation and postzygotic reproductive isolation for tephritid fruit flies in the Rhagoletis cingulata sibling species group to assess the significance of ecology, geography, and non-adaptive processes for their divergence. Our results support the ecological theory: no evidence for intrinsic postzygotic reproductive isolation was found between two populations of allopatric species, while nuclear-encoded microsatellites implied strong ecologically based reproductive isolation among sympatric species infesting different host plants. Analysis of mitochondrial DNA suggested, however, that cytoplasmic-related reproductive isolation may also exist between two geographically isolated populations within R cingulata. Thus, ecology associated with sympatric host shifts and cytoplasmic effects possibly associated with an endosymbiont may be the key initial drivers of the radiation of the R. cingulata group.
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    Hybridization and the spread of the apple maggot fly, Rhagoletis pomonella (Diptera: Tephritidae), in the northwestern United States
    (Wiley, 2015) Arcella, Tracy; Hood, Glen Ray; Powell, Thomas H.Q.; Sim, Sheina B.; Yee, Wee L.; Schwarz, Dietmar; Egan, Scott P.; Goughnour, Robert B.; Smith, James J.; Feder, Jeffrey L.
    Hybridization may be an important process interjecting variation into insect populations enabling host plant shifts and the origin of new economic pests. Here, we examine whether hybridization between the native snowberry-infesting fruit fly Rhagoletis zephyria (Snow) and the introduced quarantine pest R. pomonella (Walsh) is occurring and may aid the spread of the latter into more arid commercial apple-growing regions of central Washington state, USA. Results for 19 microsatellites implied hybridization occurring at a rate of 1.44% per generation between the species. However, there was no evidence for increased hybridization in central Washington. Allele frequencies for seven microsatellites in R. pomonella were more ‘R. zephyria-like’ in central Washington, suggesting that genes conferring resistance to desiccation may be adaptively introgressing from R. zephyria. However, in only one case was the putatively introgressing allele from R. zephyria not found in R. pomonella in the eastern USA. Thus, many of the alleles changing in frequency may have been prestanding in the introduced R. pomonella population. The dynamics of hybridization are therefore complex and nuanced for R. pomonella, with various causes and factors, including introgression for a portion, but not all of the genome, potentially contributing to the pest insect's spread.