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1. Range expansion of Lymantria dispar dispar (L.) (Lepidoptera: Erebidae) along its north‐western margin in North America despite low predicted climatic suitability

   https://doi.org/10.1111/jbi.13474  

   Streifel, Marissa A. ; Tobin, Patrick C. ; Kees, Aubree M. ; Aukema, Brian H. ( November 2018 , Journal of Biogeography)

   The European gypsy moth,Lymantria dispar dispar(L.), (Lepidoptera: Erebidae) is an invasive defoliator that has been expanding its range in North America following its introduction in 1869. Here, we investigate recent range expansion into a region previously predicted to be climatically unsuitable. We examine whether winter severity is correlated with summer trap captures of male moths at the landscape scale, and quantify overwintering egg survivorship along a northern boundary of the invasion edge.

   Northern Minnesota,USA.

   Several winter severity metrics were defined using daily temperature data from 17 weather stations across the study area. These metrics were used to explore associations with male gypsy moth monitoring data (2004–2014). Laboratory‐reared egg masses were deployed to field locations each fall for 2 years in a 2 × 2 factorial design (north/south aspect × below/above snow line) to reflect microclimate variation. Rates of successful egg hatch were assessed the following springs.

   Reductions in summer male moth captures are associated with several metrics of winter severity, such as minimum temperatures. Most egg masses suffered >95% mortality each winter. However, hatching success reached up to 80% in egg masses that had overwintered below the snow line (e.g., <30 cm from the ground).

   Our findings that cold winter temperatures are associated with reduced summer trap captures of European gypsy moth, likely due to increased overwintering mortality of exposed egg masses, are consistent with previous predictions of thermal range boundaries for this species. However, high survival in egg masses deposited close to the ground are consistent with thermal escape in subnivean environs (i.e., below snow cover), and suggest that further northward range expansion will be likely in areas that receive measurable annual snowfall.

    

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2. Climate‐related geographical variation in performance traits across the invasion front of a widespread non‐native insect

   https://doi.org/10.1111/jbi.14005  

   Thompson, Lily M. ; Powers, Sean D. ; Appolon, Ashley ; Hafker, Petra ; Milner, Lelia ; Parry, Dylan ; Agosta, Salvatore J. ; Grayson, Kristine L. ( November 2020 , Journal of Biogeography)

   Invasive species are ideal systems for testing geographical differences in performance traits and measuring evolutionary responses as a species spreads across divergent climates and habitats. The European gypsy moth,Lymantria dispar disparL. (Lepidoptera: Erebidae), is a generalist forest defoliator introduced to Medford, Massachusetts, USA in 1869. The invasion front extends from Minnesota to North Carolina and the ability of this species to adapt to local climate may contribute to its continuing spread. We evaluated the performance of populations along the climatic gradient of the invasion front to test for a relationship between climate and ecologically important performance traits.

   Eastern United States of America

   Lymantria dispar disparL. (Lepidoptera: Erebidae)

   Insects from 14 populations across the US invasion front and interior of the invasive range were reared from hatch to adult emergence in six constant temperature treatments. The responses of survival, pupal mass and larval development time were analysed as a function of source climate (annual mean normal temperature), rearing temperature and their interaction using multiple polynomial regression.

   With the exception of female development time, there were no significant interactions between source climate and rearing temperature, indicating little divergence in the shape of thermal reaction norms among populations. Source population and rearing temperature were significant predictors of survival and pupal mass. Independent of rearing temperature, populations from warmer climates had lower survival than those from colder climates, but attained larger body size despite similar development times. Larval development time was dependent on rearing temperature, but there were not consistent relationships with source climate.

   Thermal adaptation can be an important factor shaping the spread of invasive species, particularly in the context of climate change. Our results suggest thatL. d. disparis highly plastic, but has undergone climate‐related adaptation in thermal performance and life‐history traits as it spread across North America.

    

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3. Worldwide relationships in the fern genus Pteridium (bracken) based on nuclear genome markers

   https://doi.org/10.1002/ajb2.1365  

   Wolf, Paul G. ; Rowe, Carol A. ; Kinosian, Sylvia P. ; Der, Joshua P. ; Lockhart, Peter J. ; Shepherd, Lara D. ; McLenachan, Patricia A. ; Thomson, John A. ( September 2019 , American Journal of Botany)

   Spore‐bearing plants are capable of dispersing very long distances. However, it is not known if gene flow can prevent genetic divergence in widely distributed taxa. Here we address this issue, and examine systematic relationships at a global geographic scale for the fern genusPteridium.

   We sampled plants from 100 localities worldwide, and generated nucleotide data from four nuclear genes and two plastid regions. We also examined 2801 single nucleotide polymorphisms detected by a restriction site‐associatedDNAapproach.

   We found evidence for two distinct diploid species and two allotetraploids between them. The “northern” species (Pteridium aquilinum) has distinct groups at the continental scale (Europe, Asia, Africa, and North America). The northern European subspeciespinetorumappears to involve admixture among all of these. A sample from the Hawaiian Islands contained elements of both North American and AsianP. aquilinum. The “southern” species,P. esculentum, shows little genetic differentiation between South American and Australian samples. Components of African genotypes are detected on all continents.

   We find evidence of distinct continental‐scale genetic differentiation inPteridium. However, on top of this is a clear signal of recent hybridization. Thus, spore‐bearing plants are clearly capable of extensive long‐distance gene flow; yet appear to have differentiated genetically at the continental scale. Either gene flow in the past was at a reduced level, or vicariance is possible even in the face of long‐distance gene flow.

    

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4. Whole-genome resequencing data support a single introduction of the invasive white pine sawfly, Diprion similis

   https://doi.org/10.1093/jhered/esad012  

   Davis, Jeremy S ; Sim, Sheina ; Geib, Scott ; Sheffler, Brian ; Linnen, Catherine R ( February 2023 , Journal of Heredity)

   Abstract Biological introductions are unintended “natural experiments” that provide unique insights into evolutionary processes. Invasive phytophagous insects are of particular interest to evolutionary biologists studying adaptation, as introductions often require rapid adaptation to novel host plants. However, adaptive potential of invasive populations may be limited by reduced genetic diversity—a problem known as the “genetic paradox of invasions”. One potential solution to this paradox is if there are multiple invasive waves that bolster genetic variation in invasive populations. Evaluating this hypothesis requires characterizing genetic variation and population structure in the invaded range. To this end, we assemble a reference genome and describe patterns of genetic variation in the introduced white pine sawfly, Diprion similis. This species was introduced to North America in 1914, where it has rapidly colonized the thin-needled eastern white pine (Pinus strobus), making it an ideal invasion system for studying adaptation to novel environments. To evaluate evidence of multiple introductions, we generated whole-genome resequencing data for 64 D. similis females sampled across the North American range. Both model-based and model-free clustering analyses supported a single population for North American D. similis. Within this population, we found evidence of isolation-by-distance and a pattern of declining heterozygosity with distance from the hypothesized introduction site. Together, these results support a single-introduction event. We consider implications of these findings for the genetic paradox of invasion and discuss priorities for future research in D. similis, a promising model system for invasion biology. 

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5. Host plant‐related genomic differentiation in the European cherry fruit fly, Rhagoletis cerasi

   https://doi.org/10.1111/mec.15239  

   Bakovic, Vid ; Schuler, Hannes ; Schebeck, Martin ; Feder, Jeffrey L. ; Stauffer, Christian ; Ragland, Gregory J. ( October 2019 , Molecular Ecology)

   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,Prunusspp. (cherries) andLoniceraspp. (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 betweenPrunusandLoniceraflies across four sites surveyed in Germany and Norway. These 17 loci display significantly higher F_STvalues between host plants than others. They also showed high levels of linkage disequilibrium within and betweenPrunusandLoniceraflies, supporting host‐related selection and reduced gene flow. Our findings support the existence of sympatric host races inR. cerasiembedded within broader patterns of geographic variation in the fly, similar to the related apple maggot,Rhagoletis pomonella, in North America.

    

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