Abstract
The mountain pine beetle (MPB; Dendroctonus ponderosae Hopkins), a major pine forest pest native to western North America, has extended its range north and eastward during an ongoing outbreak. Determining how the MPB has expanded its range to breach putative barriers, whether physical (nonforested prairie and high elevation of the Rocky Mountains) or climatic (extreme continental climate where temperatures can be below −40 °C), may contribute to our general understanding of range changes as well as management of the current epidemic. Here, we use a panel of 1,536 single nucleotide polymorphisms (SNPs) to assess population genetic structure, connectivity, and signals of selection within this MPB range expansion. Biallelic SNPs in MPB from southwestern Canada revealed higher genetic differentiation and lower genetic connectivity than in the northern part of its range. A total of 208 unique SNPs were identified using different outlier detection tests, of which 32 returned annotations for products with putative functions in cholesterol synthesis, actin filament contraction, and membrane transport. We suggest that MPB has been able to spread beyond its previous range by adjusting its cellular and metabolic functions, with genome scale differentiation enabling populations to better withstand cooler climates and facilitate longer dispersal distances. Our study is the first to assess landscape-wide selective adaptation in an insect. We have shown that interrogation of genomic resources can identify shifts in genetic diversity and putative adaptive signals in this forest pest species.
Highlights
Rapid range expansion is an extremely important biological phenomenon that is only beginning to be understood at the molecular level
Several of the most robust outlier SNPs that were detected appear to have metabolic functions. These results suggest that signatures of selection exist within mountain pine beetle (MPB) populations, supporting the hypothesis that MPB is undergoing selection, leading to adaptation to the novel environments that it entered during the current range expansion
We have provided strong support for the hypothesis that the current MPB expansion across western Canada arose from multiple sources
Summary
Rapid range expansion is an extremely important biological phenomenon that is only beginning to be understood at the molecular level. A more common alternative is to use ecological modeling to define a “potential” species range by using a suite of bioclimatic parameters from the existing species range to forecast its range expansion into new territory (Urban et al 2007; Van Bocxlaer et al 2010). Both approaches may be confounded by adaptation, sitespecific conditions, phenotypic plasticity, and/or sampling that does not fully represent the total genetic variation present (Urban et al 2007). Molecular estimates of the origin and expected expansion of a key species can provide valuable information about climate and habitat suitabilities (Whittaker and Levin 1975), population management methods (Freeland and Martin 1985), and evolutionary processes (Hardie and Hutching 2010)
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