Hybridization is an evolutionary process that can generate diverse outcomes, such as reinforcing species boundaries, generating new species, or facilitating the introgression of locally-adapted alleles into new genomic backgrounds. Liolaemus is a highly diverse clade of South American lizards with ~260 species and as many as ten new species are described each year. Previous Liolaemus studies have detected gene flow and introgression among species using phylogenetic network methods and/or through comparisons of nuclear and mitochondrial DNA patterns, yet no study has systematically studied hybrid zones between Liolaemus species. Here, we compared three hybrid zones between four species in the Liolaemus fitzingerii group of lizards in Central Argentina where two species, L. melanops and L. xanthoviridis, each hybridize with two other species (L. shehuen and L. fitzingerii). We sampled three transects that were each approximately 120km in length and sequenced both mitochondrial and genome-wide SNP data for 267 individuals. In our analyses of nuclear DNA, we also compared bi-allelic SNPs to phased alleles (50bp RAD loci). Population structure analyses confirmed that boundaries separating species are sharp, and all clines are <65km wide. Cline center estimates were consistent between SNPs and phased alleles, but cline width estimates were significantly different with the SNPs producing wider estimates. The mitochondrial clines are narrower and shifted 4-20km southward in comparison to the nuclear clines in all three hybrid zones, indicating that either each of the species has sex-biased dispersal (males northward or females southward), the population densities are unequal, or that the hybrid zones are moving north over time. These comparisons indicate that some patterns of hybridization are similar across hybrid zones (mtDNA clines all narrower and shifted to the south), whereas cline width is variable. Hybridization in the L. fitzingerii group is common and geographically localized; further studies are needed to investigate whether hybrid zones act as hard species boundaries or promoters of speciation through processes such as reinforcement. Nonetheless, this study provides insights into both biotic and abiotic mechanisms helping to maintain species boundaries within the speciose Liolaemus system.

中文翻译：

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由三个杂种区链接的四个物种：在一个物种组（Liolaemus属）中发生重复杂交的两个实例

杂交是一个进化过程，可以产生多种结果，例如加强物种边界，产生新物种或促进将局部适应的等位基因渗入新的基因组背景。Liolaemus是南美蜥蜴的一个高度多样化的进化枝，约有260种，每年描述的新物种多达十种。先前的Liolaemus研究已经使用系统进化网络方法和/或通过核和线粒体DNA模式的比较来检测物种之间的基因流动和基因渗入，但还没有系统地研究Liolaemus物种之间的杂交区的研究。在这里，我们比较了阿根廷中部蜥蜴Liolaemus fitzingerii组的四个物种之间的三个杂种区，其中两个物种L. melanops和L.xanthoviridis分别与另外两个物种杂交（L. shehuen和L.fitzingerii）。我们对三个样例进行了采样，每个样例的长度约为120 km，并对267个个体的线粒体和全基因组SNP数据进行了测序。在我们对核DNA的分析中，我们还比较了双等位基因SNP与阶段性等位基因（50bp RAD基因座）。种群结构分析证实，分隔物种的边界很清晰，所有谱线的宽度均小于65公里。CNP中心估计与SNP和分阶段的等位基因一致，但CNP宽度估计与SNP产生更宽的估计有显着差异。与所有三个杂种区的核线相比，线粒体线变窄并且向南移动4-20 km，这表明，每个物种都有性别偏向的分散性（雄性向北或雌性向南），种群密度不相等，或随着时间的推移，混合区向北移动。这些比较表明，杂交的某些模式在整个杂交区相似（mtDNA株系更窄并向南移动），而株系宽度是可变的。L. fitzingerii组中的杂交很普遍，并且在地理上是局部的。需要进行进一步的研究以研究杂种区是否通过强化等过程充当硬物种边界或物种形成的促进剂。但是，这项研究提供了有关生物和非生物机制的见解，有助于维持特定的Liolaemus系统内的物种边界。L. fitzingerii组中的杂交很普遍，并且在地理上是局部的。需要进行进一步的研究以研究杂种区是否通过强化等过程充当硬物种边界或物种形成的促进剂。但是，这项研究提供了有关生物和非生物机制的见解，有助于维持特定的Liolaemus系统内的物种边界。L. fitzingerii组中的杂交很普遍，并且在地理上是局部的。需要进行进一步的研究以研究杂种区是否通过强化等过程充当硬物种边界或物种形成的促进剂。但是，这项研究提供了有关生物和非生物机制的见解，有助于维持特定的Liolaemus系统内的物种边界。