Maintenance of a narrow hybrid zone between native and introduced red foxes (Vulpes vulpes) despite conspecificity and high dispersal capabilities

Human-facilitated introductions of nonnative populations can lead to secondary contact between previously allopatric lineages, resulting in either homogenization of the lineages or stable hybrid zones that are maintained by pre-zygotic (e.g., behavioral) or post-zygotic (e.g., reduced hybrid fitness) reproductive barriers. We investigated patterns of gene flow between the native Sacramento Valley red fox (Vulpes vulpes patwin) and an introduced conspecific population of captive-bred (fur-farm) origin in California’s Central Valley. Considering their recent divergence (i.e., ~50 kya), we hypothesized that pre-zygotic mechanisms primarily impede gene flow, rather than post-zygotic barriers. Additionally, some genes originating in nonnative foxes may confer higher fitness in the currently human-dominated landscape resulting in selective introgression into the native population. Genetic analysis of 682 red foxes (255 native, 427 nonnative) at both mitochondrial (cytB + Dloop) and nuclear loci (~19,000 SNPs) revealed significantly narrower cline widths than expected under a simulated model of unrestricted gene flow, consistent with the existence of pre- or post-zygotic reproductive barriers. We identified several loci with reduced introgression linked to behavioral divergence in captive bred foxes, which supports pre-zygotic mechanisms as a putative driver of the narrow hybrid zone. Additionally, several loci with elevated gene flow from the nonnative into the native population, were near genes associated with adaptation to human dominated landscapes.  Overall, this study contributes to our understanding of hybrid zone dynamics in vertebrates, particularly in the context of species introductions and landscape changes, underscoring the importance of considering multiple mechanisms that may be at play in maintaining lineages at both the species and subspecies level.

Methods
We obtained both scat (n = 195) and tissue (n = 490) samples as well as 4 hair and swab samples from red foxes in the native SVRF (n = 293) and nonnative ranges (n = 396). Samples were collected between 1982–2021 and many of them (n = 472) were first described in previous studies. Samples were obtained through non-invasive den and transect surveys, roadkill collection, live captures (following American Society of Mammalogy animal care guidelines and with University of California, Davis Animal Care and Use Committee approval, IACUC No.17860), and nonnative species removal programs in the case of the introduced red foxes.

人为介导的外来种群引入事件可导致此前处于异域分布的谱系间发生二次接触，最终要么引发谱系间的遗传同质化，要么形成由合子前（pre-zygotic，如行为隔离）或合子后（post-zygotic，如杂交适合度降低）生殖屏障维持的稳定杂交带。我们针对加州中央谷地本土的萨克拉门托河谷赤狐（Vulpes vulpes patwin）与引入的、源自毛皮养殖场人工繁育的同种种群之间的基因流模式展开了研究。鉴于二者的分化时间较近（约5万年前），我们提出假说：阻碍基因交流的主要机制为合子前隔离，而非合子后隔离。此外，部分源自外来赤狐的基因或可在当前以人类活动为主导的景观中赋予更高的进化适合度，进而通过选择性基因渗入进入本土种群。对682只赤狐（255只本土个体、427只外来个体）的线粒体基因（cytB + Dloop）与核基因座（约19000个单核苷酸多态性（Single Nucleotide Polymorphisms，SNPs））开展遗传分析后发现，其遗传梯度宽度显著窄于无限制基因流模拟模型的预期值，这与合子前或合子后生殖屏障的存在相符。我们鉴定出多个与人工繁育赤狐行为分化相关、基因渗入程度降低的基因座，这支持合子前隔离机制作为窄幅杂交带形成的推定驱动因素。此外，多个基因流水平从外来种群向本土种群升高的基因座，紧邻与人类主导景观适应相关的功能基因。总体而言，本研究增进了我们对脊椎动物杂交带动态演化的认知，尤其是在物种引入与景观变化的背景下，同时强调了在物种及亚种层面维持谱系时，考虑多种潜在作用机制的重要性。

材料与方法
我们从本土萨克拉门托河谷赤狐种群（n=293）与外来分布范围（n=396）的赤狐中采集了粪便样本（n=195）、组织样本（n=490）以及4份毛发与拭子样本。样本采集时间跨度为1982年至2021年，其中472份样本此前已在相关研究中被报道。样本通过以下方式获取：非侵入性巢穴与样线调查、路杀动物采集、活体捕捉（遵循美国哺乳动物学会动物护理指南，并获得加州大学戴维斯分校实验动物护理和使用委员会批准，IACUC编号：17860），以及针对引入赤狐的外来物种清除项目。