School of Earth and Environmental Sciences & State Key Laboratory of Grassland Agro-Ecosystem, School of Life Sciences, Lanzhou University, Lanzhou, Gansu, People's Republic of China ; College of Plant Sciences, Xinjiang Production & Construction Corps Key Laboratory of Protection and Utilization of Biological Resources in Tarim Basin, Tarimu University, Alar, Xinjiang, People's Republic of China.
PLoS One. 2013 Aug 19;8(8):e71710. doi: 10.1371/journal.pone.0071710. eCollection 2013.
Populus is an ecologically and economically important genus of trees, but distinguishing between wild species is relatively difficult due to extensive interspecific hybridization and introgression, and the high level of intraspecific morphological variation. The DNA barcoding approach is a potential solution to this problem.
METHODOLOGY/PRINCIPAL FINDINGS: Here, we tested the discrimination power of five chloroplast barcodes and one nuclear barcode (ITS) among 95 trees that represent 21 Populus species from western China. Among all single barcode candidates, the discrimination power is highest for the nuclear ITS, progressively lower for chloroplast barcodes matK (M), trnG-psbK (G) and psbK-psbI (P), and trnH-psbA (H) and rbcL (R); the discrimination efficiency of the nuclear ITS (I) is also higher than any two-, three-, or even the five-locus combination of chloroplast barcodes. Among the five combinations of a single chloroplast barcode plus the nuclear ITS, H+I and P+I differentiated the highest and lowest portion of species, respectively. The highest discrimination rate for the barcodes or barcode combinations examined here is 55.0% (H+I), and usually discrimination failures occurred among species from sympatric or parapatric areas.
CONCLUSIONS/SIGNIFICANCE: In this case study, we showed that when discriminating Populus species from western China, the nuclear ITS region represents a more promising barcode than any maternally inherited chloroplast region or combination of chloroplast regions. Meanwhile, combining the ITS region with chloroplast regions may improve the barcoding success rate and assist in detecting recent interspecific hybridizations. Failure to discriminate among several groups of Populus species from sympatric or parapatric areas may have been the result of incomplete lineage sorting, frequent interspecific hybridizations and introgressions. We agree with a previous proposal for constructing a tiered barcoding system in plants, especially for taxonomic groups that have complex evolutionary histories (e.g. Populus).
杨属是一类生态和经济上都很重要的树种，但由于种间广泛杂交和基因渐渗，以及种内形态变异程度高，因此鉴别野生种相对困难。DNA 条码技术可能是解决这一问题的方法。
方法/主要发现：本研究选取中国西部的 21 种杨属 95 株代表植物，测试了 5 个叶绿体条码和 1 个核基因 ITS 的鉴别能力。在所有单一条码候选者中，核 ITS 的鉴别能力最高，叶绿体条码 matK（M）、trnG-psbK（G）和 psbK-psbI（P）以及 trnH-psbA（H）和 rbcL（R）的鉴别能力逐渐降低；核 ITS（I）的鉴别效率也高于任何两个、三个甚至五个叶绿体条码的组合。在 5 个叶绿体条码加核 ITS 的组合中，H+I 和 P+I 分别能区分最多和最少的物种。本研究中检测的条码或条码组合的最高鉴别率为 55.0%（H+I），通常在来自同域或邻域的物种之间会出现鉴别失败的情况。
结论/意义：在本案例研究中，我们表明在鉴别中国西部的杨属植物时，核 ITS 区比任何母系遗传的叶绿体区或叶绿体区组合更具潜力，同时将 ITS 区与叶绿体区结合可以提高条码的成功率并有助于检测近期的种间杂交。在同域或邻域的几个杨属植物群之间无法鉴别可能是不完全谱系分选、频繁的种间杂交和基因渐渗的结果。我们同意之前提出的在植物中构建分层条码系统的建议，特别是对于具有复杂进化历史的分类群（如杨属）。
