Genomic distinctness despite shared color patterns among threatened populations of a tiger beetle

Conservation biologists have long debated the value of subspecies, which are morphologically and geographically identifiable but not necessarily evolutionarily distinctive. One example of a controversial subspecies is Cicindela formosa gibsoni, a tiger beetle that is nationally listed as threatened in Canada and whose taxonomic status is based primarily on its unique elytral (forewing) color pattern. To determine whether C. f. gibsoni represents one or more genetically distinctive units, we sampled 14 populations within or near this subspecies' disjunct North American range and assessed their genetic differentiation from neighbouring and phenotypically distinctive populations of C. f. formosa and C. f. fletcheri. Instead of clustering by color pattern, analyses of mitochondrial and nuclear markers recovered three geographically structured genetic groupings: a northern cluster from Canada, a southwestern cluster from northwestern Colorado, and a southeastern cluster of US populations east of the Continental Divide. These data, coupled with previously documented differences in larval morphology, suggest that populations of C. formosa in western Canada and northwestern Colorado may have independently evolved similar color patterns. Thus, we consider C. f. gibsoni to be endemic to Canada and describe the novel subspecies C. f. gaumeri ssp. nov. from northwestern Colorado. Both subspecies are evolutionarily significant units, and each deserves consideration for conservation listing. Collectively, our results reveal general congruence between mitochondrial and nuclear genetic data but conflict with color pattern, the conventional basis for subspecies designations in tiger beetles.

保护生物学家长期以来一直围绕亚种（subspecies）的价值展开争论：这类类群可通过形态与地理特征辨识，但未必具备进化独特性。颇具争议的亚种之一为美丽虎甲吉布森亚种（Cicindela formosa gibsoni），该物种在加拿大被列为国家级受威胁物种，其分类地位主要基于其独特的鞘翅（elytral，即前翅）颜色模式。为明确C. f. gibsoni是否属于一个或多个遗传独特的演化单元，我们在该亚种的北美间断分布区内及邻近区域采集了14个种群，并评估了其与邻域且表型独特的C. f. formosa和C. f. fletcheri种群间的遗传分化。研究结果显示，类群并未按颜色模式聚类；通过线粒体分子标记与核分子标记分析，我们解析得到三个具有地理结构的遗传类群：分别为来自加拿大的北部类群、来自科罗拉多州西北部的西南部类群，以及分布于大陆分水岭以东的美国东南部种群类群。结合此前已报道的幼虫形态差异，上述数据表明，加拿大西部与科罗拉多州西北部的美丽虎甲种群可能独立演化出了相似的颜色模式。据此，我们认为C. f. gibsoni为加拿大特有亚种，并描述了来自科罗拉多州西北部的新亚种盖默美丽虎甲（C. f. gaumeri ssp. nov.）。这两个亚种均为进化显著单元（evolutionarily significant unit, ESU），均应被纳入保护名录进行考量。总体而言，本研究结果揭示出线粒体与核遗传数据整体一致，但与虎甲虫亚种定名的传统依据——颜色模式存在冲突。