Jockusch Elizabeth L, Hansen Robert W, Fisher Robert N, Wake David B
Department of Ecology and Evolutionary Biology, University of Connecticut, Storrs, CT, USA.
Museum of Vertebrate Zoology, University of California, Berkeley, CA, United States of America.
PeerJ. 2020 Aug 14;8:e9599. doi: 10.7717/peerj.9599. eCollection 2020.
The southern California biodiversity hotspot has had a complex geological history, with both plate tectonic forces and sea level changes repeatedly reconfiguring the region, and likely driving both lineage splittings and extinctions. Here we investigate patterns of genetic divergence in two species of slender salamanders (Plethodontidae: ) in this region. The complex geological history in combination with several organismal traits led us to predict that these species harbor multiple ancient mitochondrial lineages endemic to southern California. These species belong to a clade characterized by fine-scale mitochondrial structure, which has been shown to track ancient splits. Both focal species, and , are relatively widely distributed in southern California, and estimated to have persisted there across millions of years. Recently several extralimital populations of were found in the San Joaquin Valley of California, a former desert area that has been extensively modified for agriculture. The origins of these populations are unknown, but based on morphology, they are hypothesized to result from human-mediated introductions of .
We sequenced the mitochondrial gene from a geographically comprehensive sampling of the mitochondrial lineages of and that are endemic to southern California. We used phylogenetic analyses to characterize phylogeographic structure and identify mitochondrial contact zones. We also included the San Joaquin Valley samples to test whether they resulted from introductions. We used a bootstrap resampling approach to compare the strength of isolation-by-distance in both species and four other salamander species with which they co-occur in southern California.
The northern lineage of harbors at least eight deeply differentiated, geographically cohesive mitochondrial subclades. We identify geographic contact between many of these mtDNA lineages and some biogeographic features that are concordant with lineage boundaries. also has multiple deeply differentiated clades within the region. Comparative analyses highlight the smaller spatial scales over which mitochondrial divergence accumulates in relative to most other salamander species in southern California. The extralimital populations of from the San Joaquin Valley are assigned to and are shown to result from at least two independent introductions from different source populations. We also suggest that on Catalina Island, where it is considered native, may be the result of an introduction. Some of the same traits that facilitate the build-up of deep phylogeographic structure in likely also contribute to its propensity for introductions, and we anticipate that additional introduced populations will be discovered.
南加州生物多样性热点地区有着复杂的地质历史，板块构造力和海平面变化反复重塑该地区，可能推动了谱系分裂和物种灭绝。在此，我们研究该地区两种细趾蝾螈（Plethodontidae: ）的遗传分化模式。复杂的地质历史与多种生物特征共同促使我们预测，这些物种拥有多个南加州特有的古老线粒体谱系。这些物种属于一个以精细线粒体结构为特征的进化枝，该结构已被证明可追踪古老的分裂事件。两种目标物种， 和 ，在南加州分布相对广泛，估计已在那里存续数百万年。最近，在加利福尼亚州的圣华金谷发现了几个 的域外种群，该地区曾是沙漠，现已被大规模改造用于农业。这些种群的起源不明，但根据形态学推测，它们可能是人类介导引入 的结果。
我们对南加州特有的 和 的线粒体谱系进行了地理范围广泛的采样，并对线粒体基因 进行了测序。我们使用系统发育分析来表征系统地理学结构并识别线粒体接触区。我们还纳入了圣华金谷的样本，以测试它们是否是引入的结果。我们使用自展重采样方法比较了 物种以及与它们在南加州共有的其他四种蝾螈物种中距离隔离的强度。
的北部谱系包含至少八个深度分化、地理上连贯的线粒体亚分支。我们确定了许多这些线粒体DNA谱系之间的地理接触以及一些与谱系边界一致的生物地理特征。 在该地区也有多个深度分化的分支。比较分析突出了相对于南加州大多数其他蝾螈物种， 在较小空间尺度上积累线粒体分化的情况。来自圣华金谷的 的域外种群被归为 ，并显示是至少两次来自不同源种群的独立引入的结果。我们还认为，在卡特琳娜岛被视为本地物种的 可能是引入的结果。一些促进 在深层系统地理学结构形成的相同特征可能也导致了其被引入的倾向，我们预计还会发现更多引入的种群。
