Data from: Discordant introgression in a rapidly expanding hybrid swarm

The erosion of species boundaries can involve rapid evolutionary change. Consequently, many aspects of the process remain poorly understood, including the formation, expansion and evolution of hybrid swarms. Biological invasions involving hybridization present exceptional opportunities to study the erosion of species boundaries because timelines of interactions and outcomes are frequently well known. Here, we examined clinal variation across codominant and maternally inherited genetic markers as well as phenotypic traits to characterize the expansion and evolution of a hybrid swarm between native Cyprinella venusta and invasive C. lutrensis minnows. Discordant introgression of phenotype, microsatellite multilocus genotype and mtDNA haplotype indicate that the observable expansion of the C. venusta x C. lutrensis hybrid swarm is a false invasion front. Both parental and hybrid individuals closely resembling C. lutrensis are numerically dominant in the expansion wake, indicating that the non-native parental phenotype may be selectively favored. These findings show that cryptic introgression can extend beyond the phenotypic boundaries of hybrid swarms, and that hybrid swarms likely expand more rapidly than can be documented from phenotypic variation alone. Similarly, dominance of a single parental phenotype following an introduction event may lead to instances of species erosion being mistaken for species displacement without hybridization.物种边界的侵蚀（erosion of species boundaries）往往伴随快速的进化改变。因此，该过程的诸多方面仍有待深入解析，其中杂交群（hybrid swarm）的形成、扩张与演化均尚未得到充分阐释。涉及杂交的生物入侵为研究物种边界的侵蚀提供了绝佳的研究契机，因为其相互作用的时间线与最终结果往往较为明确。本研究针对本土Cyprinella venusta与入侵性C. lutrensis两种鲤科小鱼之间形成的杂交群，通过分析共显性遗传标记（codominant genetic markers）、母系遗传标记（maternally inherited genetic markers）、梯度变异（clinal variation）以及表型性状（phenotypic traits），对其扩张与演化过程进行了系统表征。研究发现，表型、微卫星多位点基因型（microsatellite multilocus genotype）与线粒体DNA单倍型（mtDNA haplotype）的非协同基因渐渗（discordant introgression）表明，该Cyprinella venusta × C. lutrensis杂交群的可观测扩张实为虚假入侵前沿（false invasion front）。在扩张尾迹（expansion wake）中，形态与C. lutrensis高度相似的亲本个体与杂交个体占据数量优势，这提示非本土亲本的表型可能受到选择偏好（selectively favored）。本研究结果证实，隐秘的基因渐渗（cryptic introgression）可突破杂交群的表型边界，且杂交群的实际扩张速度可能远超仅通过表型变异所能记录的水平。同理，外来物种引入后单一亲本表型占据优势的情况，可能会导致物种边界侵蚀的案例被误判为无杂交参与的物种替代（species displacement）事件。