Data from: Genetic mixture of multiple source populations accelerates invasive range expansion

A wealth of population genetic studies have documented that many successful biological invasions stem from multiple introductions from genetically distinct source populations. Yet, mechanistic understanding of whether and how genetic mixture promotes invasiveness has lagged behind documentation that such mixture commonly occurs. We conducted a laboratory experiment to test the influence of genetic mixture on the velocity of invasive range expansion. The mechanistic basis for effects of genetic mixture could include evolutionary responses (mixed invasions may harbour greater genetic diversity and thus elevated evolutionary potential) and/or fitness advantages of between-population mating (heterosis). If driven by evolution, positive effects of source population mixture should increase through time, as selection sculpts genetic variation. If driven by heterosis, effects of mixture should peak following first reproductive contact and then dissipate. Using a laboratory model system (beetles spreading through artificial landscapes), we quantified the velocity of range expansion for invasions initiated with one, two, four or six genetic sources over six generations. Our experiment was designed to test predictions corresponding to the evolutionary and heterosis mechanisms, asking whether any effects of genetic mixture occurred in early or later generations of range expansion. We also quantified demography and dispersal for each experimental treatment, since any effects of mixture should be manifest in one or both of these traits. Over six generations, invasions with any amount of genetic mixture (two, four and six sources) spread farther than single-source invasions. Our data suggest that heterosis provided a 'catapult effect', leaving a lasting signature on range expansion even though the benefits of outcrossing were transient. Individual-level trait data indicated that genetic mixture had positive effects on local demography (reduced extinction risk and enhanced population growth) during the initial stages of invasion but no consistent effects on dispersal ability. Our work is the first to demonstrate that genetic mixture can alter the course of spatial expansion, the stage of invasion typically associated with the greatest ecological and economic impacts. We suggest that similar effects of genetic mixture may be a common feature of biological invasions in nature, but that these effects can easily go undetected.

诸多种群遗传学研究均已证实，多数成功的生物入侵事件，均源自多个来自遗传分化源种群的引种事件。然而，针对遗传混合是否能够提升入侵性、以及其作用机制的解析，却远滞后于‘此类混合普遍存在’这一现象的记录。本研究开展室内实验，以检验遗传混合对入侵种群扩张速率的影响。其潜在机制可涵盖两类：一是进化响应——混合入侵种群往往拥有更高的遗传多样性，进而具备更强的进化潜力；二是种群间交配带来的适合度优势，即杂种优势（heterosis）。若该效应由进化响应介导，则源种群混合的正向效应应随时间推移逐步增强——因为自然选择会持续塑造种群内的遗传变异；若该效应由杂种优势介导，则混合效应应在首次生殖接触后达到峰值，随后逐渐衰减。

本研究以‘甲虫在人工景观中扩散’为室内模型系统（model system），量化了由1、2、4或6个遗传源种群启动的入侵事件，在6代繁衍周期内的种群扩张速率。本实验旨在检验对应进化响应与杂种优势两种机制的理论预测，探究遗传混合的效应是否出现在种群扩张的早期或后期世代。此外，我们还量化了各实验处理组的种群统计特征（demography）与扩散能力——因为遗传混合的效应，必然会体现在这两类性状的至少一种之中。

历经6代繁衍后，任何程度的遗传混合组（包含2、4、6个源种群）的入侵扩张距离，均显著高于单源种群入侵组。我们的数据分析结果表明，杂种优势带来了‘弹射效应’——即便异交带来的益处仅为短暂存在，仍对种群扩张进程留下了持久的印记。基于个体水平的性状数据，我们发现遗传混合在入侵初始阶段，对本地种群统计特征具有正向影响——可降低灭绝风险、提升种群增长速率，但对扩散能力未产生一致的影响。

本研究首次证实，遗传混合能够改变种群空间扩张的进程——而这一入侵阶段，通常伴随着最为显著的生态与经济影响。我们认为，类似的遗传混合效应，或许是自然界中生物入侵事件的普遍特征，但这类效应往往极易被忽视。