基于叶绿体和 ITS 序列的甘蒙柽柳谱系地理学研究

甘蒙柽柳（Tamarix austromongolica Nakai）是我国黄河流域特有种，具有较强的抗逆性，耐寒耐旱、耐盐碱，是维护岸堤稳定、改良土壤、防风固沙的优良树种。为了探寻甘蒙柽柳居群间的谱系进化关系，本研究利用四条叶绿体基因片段（psbA-trnH、trnT-trnL、trnL-trnF 和 trnQ-rpS16）和一条核基因片段 ITS 序列，通过 PCR 扩增、测序对甘蒙柽柳 20 个居群的叶片样品进行分析，研究了甘蒙柽柳的遗传多样性、谱系地理结构，以及黄河形成对甘蒙柽柳居群分布、遗传结构的影响。同时，通过对甘蒙柽柳的叶绿体全基因组测序分析，研究了甘蒙柽柳的系统进化及其种群的扩散历史。研究结果为甘蒙柽柳种质资源的保护利用提供理论依据。本研究主要结果如下： 　 （1）甘蒙柽柳居群的遗传变异主要发生在居群内（cpDNA：66.2%；nDNA：73.9%）。叶绿体基因和核基因的遗传分化系数N ST （cpDNA: 0.24; nDNA: 0.23）和G ST （cpDNA: 0.22;nDNA: 0.25）差异均不显著（P > 0.05），表明甘蒙柽柳居群间无明显的谱系地理结构。这可能是由于甘蒙柽柳主要分布于黄河流域，其种子小且被毛，易于随风或水传播，促进了居群间的交流。 　 （2）叶绿体单倍型 H1、核基因单倍型 R1 频率最高，分布最广，且位于单倍型分布网络结构的中心位置，可能为古老单倍型，其余单倍型均由 H1、R1 分化而来。甘蒙柽柳居群单倍型网络结构呈星状分布，表明其居群间基因交流频繁，暗示该物种经历过快速扩张。中性检验结果 Tajima’s D 和 Fu’s Fs 均为负值，失配分析结果表明群体的期望扩张分布曲线与实际观测到的分布曲线基本吻合，均呈单峰分布，进一步证实该物种经历过快速扩张。 　 （3）甘蒙柽柳代表性居群及其近缘物种的叶绿体测序结果表明，甘肃省永靖县附近的甘蒙柽柳位于系统发育树基部，暗示了该居群较为古老，该物种随着黄河的流向迅速向中下游迁移扩散。青海省同德县的甘蒙柽柳居群与其他居群的分化时间和黄河溯源加长切穿龙羊峡的时间基本一致，推测黄河的溯源侵蚀使得甘蒙柽柳居群从甘肃省永靖县附近迁移至上游。 综上，本研究通过叶绿体全基因组、基因片段和核基因片段初步揭示了甘蒙柽柳居群间的谱系关系及黄河贯通形成对甘蒙柽柳现有分布格局的影响。研究结果有助于甘蒙柽柳的种质资源保存、优良性状固定以及新品种培育等工作的开展，为该物种的遗传改良及分子育种提供重要参考。 PDF文件:<span><a href="../page/ViewAttach.cbs?ResName=lybs&AttachFile=lkybsslw20190222%2Epdf&foldername=&dismyti=&tpage=1" title="阅读1" target=_blank><img src=../image/homebody/chaX.gif border=0><font color=red>浏览全文</font></a></span>&nbsp;&nbsp;

Tamarix austromongolica Nakai is an endemic species to the Yellow River Basin in China, with strong stress resistance including cold, drought and salt tolerance, and it is an excellent tree species for maintaining bank stability, improving soil, and preventing wind and fixing sand. To explore the phylogenetic relationships among populations of T. austromongolica, this study used four chloroplast gene fragments (psbA-trnH, trnT-trnL, trnL-trnF and trnQ-rpS16) and one nuclear gene fragment ITS sequence, and analyzed leaf samples from 20 populations of T. austromongolica via PCR amplification and sequencing, to investigate the genetic diversity, phylogeographic structure of T. austromongolica, and the effects of Yellow River formation on the distribution and genetic structure of its populations. Meanwhile, through chloroplast whole genome sequencing and analysis of T. austromongolica, the phylogenetic evolution and population dispersal history of this species were studied. The research results provide a theoretical basis for the conservation and utilization of T. austromongolica germplasm resources. The main findings of this study are as follows:

(1) Genetic variation of T. austromongolica populations mainly occurs within populations (cpDNA: 66.2%; nDNA: 73.9%). The differences in genetic differentiation coefficients NST (cpDNA: 0.24; nDNA: 0.23) and GST (cpDNA: 0.22; nDNA: 0.25) between chloroplast genes and nuclear genes were not significant (P > 0.05), indicating that there is no obvious phylogeographic structure among T. austromongolica populations. This may be due to the fact that T. austromongolica is mainly distributed in the Yellow River Basin, and its small seeds with trichomes are easily dispersed by wind or water, promoting gene flow among populations.

(2) Chloroplast haplotype H1 and nuclear gene haplotype R1 have the highest frequency and widest distribution, and are located at the center of the haplotype distribution network, which may be ancestral haplotypes, while the remaining haplotypes are differentiated from H1 and R1. The haplotype network structure of T. austromongolica populations is star-shaped, indicating frequent gene flow among populations, suggesting that this species has experienced rapid expansion. The results of neutral tests Tajima’s D and Fu’s Fs are both negative, and the mismatch distribution analysis shows that the expected expansion distribution curve of the population is basically consistent with the actually observed distribution curve, both showing a unimodal distribution, further confirming that this species has experienced rapid expansion.

(3) Chloroplast sequencing results of representative populations of T. austromongolica and its closely related species show that T. austromongolica near Yongjing County, Gansu Province is located at the base of the phylogenetic tree, suggesting that this population is relatively ancient. This species rapidly migrated and dispersed to the middle and lower reaches along the flow direction of the Yellow River. The differentiation time of T. austromongolica populations in Tongde County, Qinghai Province from other populations is basically consistent with the time when the Yellow River traced its source and elongated to cut through the Longyang Gorge. It is speculated that the headward erosion of the Yellow River caused the migration of T. austromongolica populations from near Yongjing County, Gansu Province to the upper reaches.

In summary, this study preliminarily revealed the phylogenetic relationships among T. austromongolica populations and the impact of the formation and connection of the Yellow River on the current distribution pattern of T. austromongolica through chloroplast whole genome, gene fragments and nuclear gene fragments. The research results are conducive to the conservation of T. austromongolica germplasm resources, the fixation of excellent traits, and the breeding of new varieties, providing an important reference for genetic improvement and molecular breeding of this species.

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