Weak spatial-genetic structure in a native invasive, the southern pine beetle (Dendroctonus frontalis), across the eastern United States

The southern pine beetle, Dendroctonus frontalis, is a native pest of pine
trees that has recently expanded its range into the northeastern United
States. Understanding its colonization, dispersal, and connectivity will
be critical for mitigating negative economic and ecological impacts in the
newly invaded areas. Characterization of spatial-genetic structure can
contribute to this; however, previous studies have reached different
conclusions about regional population genetic structure, with one study
reporting a weak east-west pattern, and the most recent reporting absence
of structure. Here we systematically assessed several explanations for the
absence of spatial-genetic structure. To do this, we developed nine new
microsatellite markers and combined them with an existing 24-locus data
matrix for the same individuals. We then reanalyzed this full dataset
alongside datasets in which certain loci were omitted with the goal of
creating more favorable signal to noise ratios. We also partitioned the
data based on the sex of D. frontalis individuals, and then employed a
broad suite of genotypic clustering and isolation-by-distance (IBD)
analyses. We found that neither inadequate information content in the
molecular marker set, nor unfavorable signal-to-noise ratio, nor
insensitivity of the analytical approaches could explain the absence of
structure. Regardless of dataset composition, there was little evidence
for clusters (i.e., distinct geo-genetic groups) or clines (i.e.,
gradients of increasing allele frequency differences over larger
geographic distances), with one exception: significant IBD was repeatedly
detected using an individual-based measure of relatedness whenever
datasets included males (but not for female-only datasets). This is
strongly indicative of broad-scale female-biased dispersal, which has not
previously been reported for D. frontalis, in part owing to logistical
limitations of direct approaches (e.g., capture-mark-recapture). Weak
spatial-genetic structure suggests long-distance connectivity and that
gene flow is high, but additional research is needed to understand range
expansion dynamics in this species using alternate approaches.

南部松小蠹（Dendroctonus frontalis）是松树的本土害虫，近年来其分布范围已扩张至美国东北部地区。明确其定殖、扩散与种群连通性，对于减轻该虫在新入侵区域造成的负面经济与生态影响至关重要。对空间遗传结构的解析可为该目标提供支撑，但既往针对其区域种群遗传结构的研究结论存在分歧：一项研究指出存在微弱的东西向格局，而最新研究则未检测到空间遗传结构。本研究针对空间遗传结构缺失的多种潜在成因开展了系统性评估。为此，我们开发了9对全新的微卫星（microsatellite）标记，并将其与已发表的同一组个体的24位点数据矩阵进行整合；随后，我们对完整数据集以及剔除部分位点后的数据集分别进行重分析，以优化信噪比。此外，我们还按南部松小蠹个体的性别对数据进行分组，并采用了涵盖多种基因型聚类与距离隔离（isolation-by-distance, IBD）的分析方法。研究结果显示，分子标记组信息含量不足、信噪比不佳以及分析方法灵敏度欠缺，均无法解释空间遗传结构缺失的现象。无论数据集的组成如何，均未检测到显著的聚类（即明确的地理遗传类群）或渐变群（即随地理距离增加等位基因频率差异逐渐增大的梯度），仅存在一个例外：当数据集包含雄性个体时，基于个体亲缘关系的分析方法可反复检测到显著的IBD信号，而仅包含雌性个体的数据集则无此现象。这一结果强烈表明该虫存在大范围的雌性偏向扩散，而这一现象此前尚未在南部松小蠹中被报道，部分原因是直接观测方法（如标记-重捕法）存在后勤限制。微弱的空间遗传结构暗示该虫存在长距离种群连通性且基因流水平较高，但未来仍需采用其他研究方法，以进一步明确该物种的种群扩张动态。