Conservation genetic assessment of the paleback darter (Etheostoma pallididorsum), a narrowly distributed endemic in the Ouachita Highlands, Arkansas, USA

The Paleback Darter is considered imperiled and has recently been petitioned for listing under the Endangered Species Act. Prior allozyme-based studies found evidence of small effective population size, warranting conservation concern. The objective of this study was to assess population dynamics and the phylogeographic history of the Paleback Darter, using a multilocus microsatellite approach and mitochondrial DNA.
The predictions of this study were that Paleback Darter populations will exhibit low genetic diversity and minimal gene flow; population structure will correspond to river systems where samples are derived; the reservoir dams impounding the reaches between the Caddo and Ouachita rivers would serve as effective barriers to gene flow; and the Caddo and Ouachita rivers are reciprocally monophyletic.
Microsatellite DNA loci revealed significant structure among sampled localities (global Fst = 0.17, P < 0.001), with evidence of two distinct populations representing the Caddo and Ouachita rivers. However, Bayesian phylogeographic analyses resulted in three distinct clades, Caddo River, Ouachita River, and Mazarn Creek. Divergence from the most recent ancestor shared among the river drainages was estimated at 60-KYA. Population genetic diversity was relatively low (He = 0.65; mean alleles per locus [A] = 6.26), but comparable to close relatives, Slackwater Darter (He = 0.65; A = 6.74) and Tuscumbia Darter (He = 0.57; A = 5.53).
These results have conservation implications for Paleback Darter populations and can be informative for other headwater specialist species. Like other headwater species with population structuring and relatively low genetic diversity, persistence of Paleback Darter populations is likely tied to persistence and connectivity of local breeding and non-breeding habitat. These results do not raise conservation concern for a population decline; however, the restricted distribution and endemic status of the species still renders Paleback Darter populations vulnerable to extirpation or extinction.

Methods
Tissue samples were collected from up to 30 Paleback Darter individuals per locality throughout the Ouachita River basin in southwestern Arkansas. A total of 181 individuals were successfully genotyped for 8 microsatellite DNA loci, with primers designed for the Greenside, Candy, Rainbow, Cherokee, and Orangethroat darters. Up to 10 individuals per sub-drainage were additionally amplified for the mitochondrial cytochrome b gene (cyt b). The cyt b sequences were accessioned with GenBank.

浅背镖鲈（Paleback Darter）被认为处于濒危状态，近期已被提请依据《濒危物种法》列入保护名录。此前基于同工酶的研究发现其有效种群大小（effective population size）较小，值得引起保护关注。本研究旨在通过多位点微卫星方法与线粒体DNA（mitochondrial DNA）分析，探究浅背镖鲈的种群动态与系统地理学历史。
本研究提出以下预测：浅背镖鲈种群将表现出较低的遗传多样性与极弱的基因流；种群结构将与采样所在的河流水系相匹配；拦截卡多河与沃希托河之间河段的水库大坝可作为基因流的有效阻隔屏障；卡多河与沃希托河种群互为单系群。
微卫星DNA位点分析显示，采样点间存在显著的种群结构（全局Fst值=0.17，P<0.001），且存在两个分别对应卡多河与沃希托河的独立种群。然而，贝叶斯系统地理学分析却得到三个独立支系：卡多河支系、沃希托河支系与马扎恩溪支系。各河流水系类群的最近共同祖先的分化时间约为6万年前（60 KYA）。种群遗传多样性相对较低（期望杂合度（He）=0.65；每个位点平均等位基因数（A）=6.26），但与其近缘物种缓水镖鲈（Slackwater Darter，He=0.65，A=6.74）和塔斯坎比亚镖鲈（Tuscumbia Darter，He=0.57，A=5.53）相当。
本研究结果对浅背镖鲈种群的保护工作具有指导意义，同时也可为其他源头溪流特化物种提供参考。与其他具有种群结构且遗传多样性较低的源头溪流物种类似，浅背镖鲈种群的存续大概率依赖于当地繁殖与非繁殖栖息地的留存与连通性。本研究结果并未表明浅背镖鲈存在种群衰退的保护隐患，但该物种分布范围狭窄且为特有种，其种群仍极易面临局部种群灭绝或物种整体灭绝的风险。

材料与方法
研究于美国阿肯色州西南部的沃希托河流域开展，每个采样点采集最多30尾浅背镖鲈的组织样本。最终共成功对181尾个体的8个微卫星DNA位点进行基因分型，所用引物针对绿背镖鲈、糖果镖鲈、彩虹镖鲈、切罗基镖鲈与橙喉镖鲈设计。此外，每个亚流域选取最多10尾个体，对其线粒体细胞色素b基因（cyt b）进行扩增。所得cyt b序列已提交至基因银行（GenBank）。