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ABSTRACT Human disturbance can have profound effects on biodiversity, including increasing hybridization between reproductively isolated species. One approach for understanding how human activity affects hybridization dynamics is to evaluate correlations between disturbance (e.g., urbanisation, temperature change) and hybridization. Because variation in hybridization can also arise from historical factors unrelated to recent human disturbance, it is essential to account for population structure to avoid spurious correlations. Here, we combine environmental and high‐coverage whole‐genome resequencing data to investigate how human disturbance and population structure affect hybridization dynamics between a pair of pine sawflies adapted to different pines,Neodiprion leconteiandNeodiprion pinetum. We find thatN. leconteiandN. pinetumexhibit strikingly different patterns of population structure, which we hypothesise stem from differences in host use. We also find that recent admixture is both asymmetric and geographically variable. Linear regression analyses reveal that admixture proportion is predicted by indirect human disturbance (i.e., climate change) and not direct human disturbance (e.g., urbanisation) in bothN. leconteiandN. pinetum. Lastly, inN. pinetum, we find evidence of a spurious association between admixture and direct human disturbance that disappears when regression models account for population structure via inclusion of genetic principal component scores as covariates. Together, our data suggest that indirect human disturbance and population structure both contribute to geographic variation in admixture betweenN. leconteiandN. pinetum. Our study also highlights the importance of adequately controlling for population structure when attempting to identify environmental predictors (human disturbance‐related or not) of hybridization. 

Abstract Understanding the genetics of adaptation and speciation is critical for a complete picture of how biodiversity is generated and maintained. Heterogeneous genomic differentiation between diverging taxa is commonly documented, with genomic regions of high differentiation interpreted as resulting from differential gene flow, linked selection and reduced recombination rates. Disentangling the roles of each of these non‐exclusive processes in shaping genome‐wide patterns of divergence is challenging but will enhance our knowledge of the repeatability of genomic landscapes across taxa. Here, we combine whole‐genome resequencing and genome feature data to investigate the processes shaping the genomic landscape of differentiation for a sister‐species pair of haplodiploid pine sawflies,Neodiprion leconteiandNeodiprion pinetum. We find genome‐wide correlations between genome features and summary statistics are consistent with pervasive linked selection, with patterns of diversity and divergence more consistently predicted by exon density and recombination rate than the neutral mutation rate (approximated by dS). We also find that both global and local patterns ofF_ST,d_XYandπprovide strong support for recurrent selection as the primary selective process shaping variation across pine sawfly genomes, with some contribution from balancing selection and lineage‐specific linked selection. Because inheritance patterns for haplodiploid genomes are analogous to those of sex chromosomes, we hypothesize that haplodiploids may be especially prone to recurrent selection, even if gene flow occurred throughout divergence. Overall, our study helps fill an important taxonomic gap in the genomic landscape literature and contributes to our understanding of the processes that shape genome‐wide patterns of genetic variation. 

Pine sawflies in the genusNeodiprionRohwer are widely distributed across the Northern Hemisphere and are pests of commercially important conifer trees. While sampling forNeodiprionspecies in eastern North America, two colonies ofNeodiprion warreniRoss were discovered in Tennessee feeding onPinus virginianaMill. These are the first records ofN. warreniin Tennessee and on the hostP. virginiana. Here, we use a combination of larval and adult female morphology to confirm species identification. We also discuss two potential explanations for these observations:N. warreniwas always present in Tennessee and feeding onP. virginianabut, until now, has gone unreported or these new records are attributable to a recent range expansion and host shift. We also discuss potential economic and evolutionary implications of range expansions and host shifts in plant-feeding insect pest species. 

Abstract Pine sawflies (Hymenoptera: Diprionidae) are eruptive herbivores found throughout eastern North America. The Diprionidae family, which contains at least 140 species, constitutes the most persistent threat to conifers as population outbreaks can cause widespread defoliation. Because some species are more prone to large, destructive outbreaks than others, species identification is critical to effective management. Although existing taxonomic keys are primarily based on internal adult morphology, substantial variation among species in larval color traits, geographic location, overwintering strategy, host plant, and egg patterns can be diagnostic at the species level. Here, we focus on the Pinaceae-feeding subfamily Diprioninae, of which there are 25 species in eastern North America. We describe the general biology, life cycle, and host-use ecology of Diprioninae, with an emphasis on the variation among these traits within this subfamily. In addition, we provide tools for species identification, including a taxonomic key that utilizes external diagnostic characteristics. Finally, we discuss available management strategies. 

Abstract When gene flow accompanies speciation, recombination can decouple divergently selected loci and loci conferring reproductive isolation. This barrier to sympatric divergence disappears when assortative mating and disruptive selection involve the same “magic” trait. Although magic traits could be widespread, the relative importance of different types of magic traits to speciation remains unclear. Because body size frequently contributes to host adaptation and assortative mating in plant-feeding insects, we evaluated several magic trait predictions for this trait in a pair of sympatric Neodiprion sawfly species adapted to different pine hosts. A large morphological dataset revealed that sawfly adults from populations and species that use thicker-needled pines are consistently larger than those that use thinner-needled pines. Fitness data from recombinant backcross females revealed that egg size is under divergent selection between the preferred pines. Lastly, mating assays revealed strong size-assortative mating within and between species in three different crosses, with the strongest prezygotic isolation between populations that have the greatest interspecific size differences. Together, our data support body size as a magic trait in pine sawflies and possibly many other plant-feeding insects. Our work also demonstrates how intraspecific variation in morphology and ecology can cause geographic variation in the strength of prezygotic isolation.