Department of Biology, University of Washington, Seattle, WA, USA.
Burke Museum of Natural History and Culture, Seattle, WA, USA.
Mol Ecol. 2020 Mar;29(5):956-969. doi: 10.1111/mec.15377. Epub 2020 Mar 1.
Most species and therefore most hybrid zones have historically been defined using phenotypic characters. However, both speciation and hybridization can occur with negligible morphological differentiation. Recently developed genomic tools provide the means to better understand cryptic speciation and hybridization. The Northwestern Crow (Corvus caurinus) and American Crow (Corvus brachyrhynchos) are continuously distributed sister taxa that lack reliable traditional characters for identification. In this first population genomic study of Northwestern and American crows, we use genomic SNPs (nuDNA) and mtDNA to investigate the degree of genetic differentiation between these crows and the extent to which they may hybridize. Our results indicate that American and Northwestern crows have distinct evolutionary histories, supported by two nuDNA ancestry clusters and two 1.1%-divergent mtDNA clades dating to the late Pleistocene, when glacial advances may have isolated crow populations in separate refugia. We document extensive hybridization, with geographic overlap of mtDNA clades and admixture of nuDNA across >900 km of western Washington and western British Columbia. This broad hybrid zone consists of late-generation hybrids and backcrosses, but not recent (e.g., F1) hybrids. Nuclear DNA and mtDNA clines had concordant widths and were both centred in southwestern British Columbia, farther north than previously postulated. Overall, our results suggest a history of reticulate evolution in American and Northwestern crows, perhaps due to recurring neutral expansion(s) from Pleistocene glacial refugia followed by lineage fusion(s). However, we do not rule out a contributing role for more recent potential drivers of hybridization, such as expansion into human-modified habitats.
大多数物种，因此大多数杂交区，在历史上都是使用表型特征来定义的。然而，物种形成和杂交都可能发生，而形态分化可忽略不计。最近开发的基因组工具为更好地理解隐种形成和杂交提供了手段。西北乌鸦（Corvus caurinus）和美洲乌鸦（Corvus brachyrhynchos）是连续分布的姊妹种，缺乏可靠的传统识别特征。在这项关于西北乌鸦和美洲乌鸦的首次群体基因组研究中，我们使用基因组 SNPs（核 DNA）和 mtDNA 来研究这些乌鸦之间的遗传分化程度，以及它们杂交的程度。我们的研究结果表明，美洲和西北乌鸦有明显的进化历史，这得到了两个核 DNA 祖先群和两个 1.1%分化的 mtDNA 分支的支持，这些分支可以追溯到晚更新世，当时冰川推进可能将乌鸦种群隔离在不同的避难所中。我们记录了广泛的杂交，mtDNA 分支的地理重叠和核 DNA 的杂种混合跨越了华盛顿州西部和不列颠哥伦比亚省西部的 900 多公里。这个广泛的杂交区由晚期杂种和回交组成，但不包括最近的（例如 F1）杂种。核 DNA 和 mtDNA 谱系的宽度一致，均集中在不列颠哥伦比亚省西南部，比以前假设的更靠北。总的来说，我们的研究结果表明，美洲和西北乌鸦可能存在网状进化的历史，这可能是由于更新世冰川避难所的反复中性扩张，然后是谱系融合。然而，我们不能排除最近潜在杂交驱动因素的作用，例如扩展到人类改造的栖息地。
