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Strategy of selecting 16S rRNA hypervariable regions for matagenome-phylogenetic marker genes based analysis.

基于分子标记的宏基因组16S rRNA基因高变区选择策略


随着新一代DNA测序技术出现,人们能够同时对多个DNA样本的宏基因组进行并行分析,尤其是以16S rRNA基因高变区为分子标记的测序已经成为微生物多样性研究最为简洁有效的方法. 目前二代高通量测序的读长不能覆盖16S rRNA基因的全长,需要选择一个有效的高变区进行测序.十多年来,对于16S rRNA基因高变区的选择策略没有统一的标准.本文分析了常用的高变区选择策略,指出不同环境条件是影响高变区选择的重要因素之一.在此基础上,提出了高变区选择的参考准则,同时建议应对选择的高变区进行有效评估.
 

The advent of next generation sequencing technology enables parallel analysis of the whole microbial community from multiple samples. Particularly, sequencing 16S rRNA hypervariable tags has become the most efficient and costeffective method for assessing microbial diversity. Due to its short read length of the 2ndgeneration sequencing methods that cannot cover the full 16S rRNA genomic region, specific hypervariable regions or V-regions must be selected to act as the proxy. Over the past decade, selection of V-regions has not been consistent in assessing microbial diversity. Here we evaluated the current strategies of selecting 16S rRNA hypervariable regions for surveying microbial diversity. The environmental condition was considered as one of the important factors for selection of 16S rRNA hypervariable regions. We suggested that a pilot study to test different V-regions is required in bacterial diversity studies based on 16S rRNA genes.


全 文 :基于分子标记的宏基因组 16S rRNA基因
高变区选择策略∗
张军毅1,2  朱冰川1  徐  超1  丁  啸2  李俊锋3  张学工3  陆祖宏2,4∗∗
( 1无锡市环境监测中心站, 江苏无锡 214121; 2东南大学生物科学与医学工程学院生物电子学国家重点实验室, 南京
210096; 3清华大学信息科学技术学院生物信息学教育部重点实验室 /合成与系统生物学研究中心, 北京 100083; 4北京大学
工学院, 北京 100871)
摘  要  随着新一代 DNA测序技术出现,人们能够同时对多个 DNA 样本的宏基因组进行并
行分析,尤其是以 16S rRNA基因高变区为分子标记的测序已经成为微生物多样性研究最为
简洁有效的方法. 目前二代高通量测序的读长不能覆盖 16S rRNA基因的全长,需要选择一个
有效的高变区进行测序.十多年来,对于 16S rRNA基因高变区的选择策略没有统一的标准.本
文分析了常用的高变区选择策略,指出不同环境条件是影响高变区选择的重要因素之一.在
此基础上,提出了高变区选择的参考准则,同时建议应对选择的高变区进行有效评估.
关键词  微生物多样性; 16S rRNA; 新一代测序技术; 选择策略
文章编号  1001-9332(2015)11-3545-09  中图分类号  Q93  文献标识码  A
Strategy of selecting 16S rRNA hypervariable regions for matagenome⁃phylogenetic marker
genes based analysis. ZHANG Jun⁃yi1,2, ZHU Bing⁃chuan1, XU Chao1, DING Xiao2, LI Jun⁃
feng3, ZHANG Xue⁃gong3,LU Zu⁃hong2,4 (1Wuxi Environmental Monitoring Centre, Wuxi 214121,
Jiangsu, China; 2State Key Laboratory for Bioelectronics, School of Biological Science and Medical
Engineering, Southeast University, Nanjing 210096, China; 3Ministry of Education Key Laboratory of
Bioinformatics, Bioinformatics Division / Center for Synthetic and Systems Biology, TNLIST and Depart⁃
ment of Automation, Tsinghua University, Beijing 100083, China; 4College of Engineering, Peking
University, Beijing 100871, China) . ⁃Chin. J. Appl. Ecol., 2015, 26(11): 3545-3553.
Abstract: The advent of next generation sequencing technology enables parallel analysis of the
whole microbial community from multiple samples. Particularly, sequencing 16S rRNA hypervari⁃
able tags has become the most efficient and cost⁃effective method for assessing microbial diversity.
Due to its short read length of the 2nd⁃generation sequencing methods that cannot cover the full 16S
rRNA genomic region, specific hypervariable regions or V⁃regions must be selected to act as the
proxy. Over the past decade, selection of V⁃regions has not been consistent in assessing microbial
diversity. Here we evaluated the current strategies of selecting 16S rRNA hypervariable regions for
surveying microbial diversity. The environmental condition was considered as one of the important
factors for selection of 16S rRNA hypervariable regions. We suggested that a pilot study to test dif⁃
ferent V⁃regions is required in bacterial diversity studies based on 16S rRNA genes.
Key words: microbial diversity; 16S rRNA; next generation sequencing (NGS); selection strategy.
∗江苏省环境监测科研基金项目(1320)、国家自然科学基金项目
(61227803)和 2013 年江苏省普通高校研究生科研创新计划项目
(CXLX13_114)资助.
∗∗通讯作者. E⁃mail: zhlu@ seu.edu.cn
2015⁃01⁃08收稿,2015⁃07⁃24接受.
    宏基因组(metagenome),也称环境微生物基因
组或元基因组,是指环境中全部微小生物 DNA的总
和[1] .自从 1998 年 Handelsman 等[1]提出宏基因组
学(metagenomics)这一概念,迄今为止,其研究对象
已从最初的土壤[1-3]发展到人体肠道[4-9]、口腔[10]、
皮肤[11-12]、阴道[13]等,水体[14-23]、沉积物[24-25]、空
气[26-27]、灰霾[28]、废水[29-30]、动植物体等载体中微
生物及其附生微生物[31-37] .利用宏基因组学方法,
人们对环境微生物的研究实现了从基因、基因组到
环境中全部基因组的跨越.由于微生物物种多样性
应 用 生 态 学 报  2015年 11月  第 26卷  第 11期                                                         
Chinese Journal of Applied Ecology, Nov. 2015, 26(11): 3545-3553
极其丰富,其中 99%以上无法在现有实验室条件下
进行培养[38-40],所以利用宏基因组技术来研究那些
大量未知的微生物基因序列,突破了微生物研究的
初始瓶颈,给环境微生物多样性的研究带来革命性
的发展[41-43] .微生物群落多样性的宏基因组研究主
要有系统进化分子标记测序(sequencing phylogene⁃
tic marker genes)和宏基因组 DNA 测序(sequencing
metagenomic DNA) [44] .近年来,基于系统进化分子
标记 16S rRNA[45-46]、 ITS 序列[47-48] 和 18S rRNA
等[49-52]的宏基因组技术在研究微生物群落多样性
方面得到了广泛运用.尤其是 2005 年之后,随着测
序技术的发展和条形码(sample⁃specific barcode se⁃
quences)技术的运用,使得对批量样本的平行、深度
测序成为可能[53-54] .随着测序准确度(accuracy)、读
长(reads length)和通量( throughput)等参数不断改
进,加之测序成本的持续下降,自然界大量的微生物
多样性及其与环境相互作用的规律将逐步得到揭
示,无疑将极大地促进环境科学和生态学的发展.
从目前的研究来看,以 16S rRNA 作为分子标
记的应用最为广泛.因此,这里我们主要探讨基于
16S rRNA基因扩增子(16S rRNA gene amplicons)的
宏基因组学研究.大肠杆菌(Escherichia coli)的 16S
rRNA全长约 1542 bp[55],但是目前主流的二代测序
技 术, 如 Illumina Miseq ( PE300 )、 Illumina
Hiseq2000 ( PE100)、 Illumina Hiseq2500 ( PE150)、
Roche 454 FLX+(PE600)和 Ion Torrent PGM(400)
等读长均不能覆盖 16S rRNA 全长,必须选择一个
或多个短的、有效的高变区( hypervariable regions)
作为替代.然而,扩增 16S rRNA 不同的 V 区会对原
核微生物群落结构的分析结果产生明显的影
响[56-58] .在 16S rRNA 高变区(V 区)的选择策略方
面还存在较大争议[8,32,59-60] .目前用于多样性分析的
V 区 主 要 分 为 两 类, 一 类 是 单 独 V 区, 如
V3[18,29-30,32,59-64]、 V4[7-8,12,32,35,60-62,65-69]、 V5[10,60,70]、
V6[8-9,16-18,30,59-60,62,71]和 V7[62]等,另一类是连续 V
区,如 V1 -V2[7,11,13,61-62,72-73]、V1 -V3[32,74-75]、V3 -
V4[36,76]、V3-V5[45,77]、V4-V5[78]、V4-V6[9,32]、V5-
V6[6,34,62]、V6 -V7[62]、V5 -V8[69]、V6 -V8[69]、V7 -
V8[62]、V1-V8[4,29,79],V5-V9[80-81]、V6-V9[24]和 NLF
(nearly full⁃length) [36,62]等.因此,对不同类型样本的
16S rRNA高变区(V区)进行有效性评估以及研究 V
区的选择策略对于原核微生物群落的研究非常必要.
1  16S rRNA的结构
在分析原核微生物多样性时,最为常用的基因
是核糖体 RNA(rRNA)基因.由于功能上高度保守,
序列上的不同位置具有不同的变异速率,核糖体
RNA (rRNA)是目前在微生物分子生态学上最为有
用以及应用最广泛的分子标记.一般认为,rRNA 基
因很少发生大规模的横向基因迁移,具有一系列由
非常保守到高变的区域,适合于原核微生物分类信
息的确定.通过 rRNA 序列比对,可以分析不同分类
水平的系统发育关系.对于 16S rRNA 基因序列,序
列之间有 97%以上的相似性可以认为是同种,95%
以上的相似性可以认为是同属,80%以上的相似性
则可认为是同门[82] .
16S rDNA 指的是基因组中与编码核糖体 16S
rRNA分子对应的 DNA 序列.一般进行系统进化分
析或是对某特定环境进行细菌群落结构分析时,所
分析的对象都是 16S rDNA.因为 DNA 提取容易,也
比较稳定,但研究者从习惯上往往还是以 16S rRNA
来进行描述.在基因组上,16S rRNA基因与 5S rRNA
和 23S rRNA的各自编码基因组成一个转录单元,
共同转录.大肠杆菌 16S rRNA 全长基因约为 1542
bp,由 9个可变区和 10 个保守区组成,其中保守区
反映了生物物种间的亲缘关系,而可变区则表明物
种间的差异[55,83],其位置和长度见图 1和表 1.
2  V区的选择
限于目前二代高通量测序读长并不能覆盖 16S
rRNA全长,必须选择一个或多个较短且有效的 V
区作为替代.而关于 V 区的选择策略目前还没有公
认的准则,不同研究者往往采用不同的 V 区,即使
相同类型的样本也往往运用不同的 V 区,这限制了
研究之间的连续性和可比性.表 2 列举了一些常用
的 V区和引物选择 .所以有必要建立一个V区的选
图 1  16S rRNA一级结构示意图
Fig.1  Primary structure of 16S rRNA.
6453 应  用  生  态  学  报                                      26卷
表 1  16S rRNA可变区在 16S rRNA基因上的位置
Table 1  Position of the 16S rRNA hypervariable regions in 16S rRNA gene
高变区 Hypervariable region V1 V2 V3 V4 V5 V6 V7 V8 V9
起点 Starting point (bp) 68 137 440 590 828 1000 1117 1243 1435
终点 End point (bp) 101 227 497 652 857 1037 1157 1295 1465
长度 Length (bp) 34 91 58 63 30 38 41 53 31
大肠杆菌 16S rRNA位点 Position in 16S rRNA of E. coli[55] .
表 2  16S rRNA可变区在生物多样性检测中的运用
Table 2  Application of V regions in microbial diversity investigations using 16S rRNA hypervariable regions
样本
Sample
V区
Hypervariable
regions
引物
Primer
参考文献
Reference
样本
Sample
V区
Hypervariable
regions
引物
Primer
参考文献
Reference
土壤 Soil V1⁃V2 8F;338R [62]
肠道 Human gut 8F;338R [7]
土壤 Soil 27F;338R [69]
阴道 Vagina 27F;338R [72]
皮肤 Hand surface 27F;338R [11]
废水 Wastewater 27F;355R [61]
阴道 Vagina 27F;355R [13]
人体病原菌 Human⁃pathogenic bacteria V3 334F;536R [59]
土壤 Soil 338F;530R [62]
肠道和深海 Hut and deep sea 338F;533R [18]
废水 Wastewater 338F;548R [61]
鸡肠道 Chicken gut 338F;533R [32]
废水 Wastewater 341F;518R [30]
土壤 Soil 341F;531R [60]
废水 Wastewater 341F;534R [29]
鼠肠道 Mice gut 341F;534R [63]
鼠肠道 Mice gut 341F;534R [64]
鸡肠道 Chicken gut V1⁃V3 8F;533R [32]
湖泊 Lake 27F;533R [75]
沉积物和岩石表面
Sediment and rock biofilm
27F;534R [74]
酸性矿排水 Acid mine drainage V4 515F;806R [68]
人肠道 Human gut 515F;806R [7]
沙漠 Desert 515F;806R [67]
土壤 Soil 515F;806R [11]
土壤 Soil 515F;806R [69]
室内环境 Indoor environment 515F;806R [12]
土壤 Soil 516F;798R [60]
人小肠 Human distal intestine 520F;802R [8]
土壤 Soil 530F;805R [62]
废水 Wastewater 530F;826R [61]
鸡肠道 Chicken gut 520F;802R [32]
鸡肠道 Chicken gut 520F;802R [35]
马肠道 Horse gut 520F;802R [66]
猪肠道 Pig gut V3⁃V4 341F;806R [36]
鼠肠道 Mice gut 338F;806R [76]
口腔 Oral cavity V5 785F;894R [10]
土壤 Soil 786F;926R [70]
土壤 Soil 788F;896R [60]
河水、生物反应器等
River, anaerobic bioreactor, etc.
V3⁃V5 338F;909R [77]
淡水 Freshwater 341F;907R [45]
沉积物 Sediment V4⁃V5 515F;907R [78]
土壤 Soil V6 921F;1068R [60]
人体病原菌 Human⁃pathogenic bacteria 960F;1085R [59]
深海 Deep sea 967F;1046R [17]
深海 Deep sea 967F;1046R [16]
肠道和深海 Human gut and deep sea 967F;1046R [18]
土壤 Soil 967F;1046R [62]
废水 Wastewater 967F;1046R [30]
猪肠道 Pig gut 967F;1046R [71]
人肠道 Human gut 967F;1062R [9]
人小肠 Human distal intestine 986F;1027R [8]
人肠道 Human gut V4⁃V6 515F;1062R [9]
鸡肠道 Chicken gut 515F;1114R [32]
人肠道 Human gut V5⁃V6 784F;1061R [6]
驴肠道 Donkey gut 789F;1068R [34]
土壤 Soil 805F;1046R [62]
土壤 Soil V7 1046F;1220R [62]
土壤 Soil V6⁃V7 967F;1220R [62]
土壤 Soil V5⁃V8 804F;1392R [69]
土壤 Soil V6⁃V8 926F;1392R [69]
土壤 Soil V7⁃V8 1046F;1392R [62]
人小肠 Human intestine V1⁃V8 8F;1391R [4]
废水 Wastewater 27F;1391R [29]
土壤 Soil 27F;1392R [79]
土壤 Soil 63F;1387R [79]
土壤 Soil V5⁃V9 787F;1492R [80]
马肠道 Horse gut 939F;1492R [81]
沉积物 Sediment V6⁃V9 926F;1392R [24]
土壤 Soil NFL 8F;1492R [62]
猪肠道 Pig gut 8F;1510R [36]
择标准,并推荐适于所在环境的目标扩增 V 区,比
如,V3、V4、V5、V6等 V区经常被应用.一般来说,可
变性和适中的保守性被认为是 V 区选择的标准.此
外,引物的选择也被认为和 V区的选择同样重要.引
物偏向性会使得某些物种的多样性被高估或低估,
一些群体甚至会整个缺失.融合引物往往用来解决
这一问题,但引物混合比例的差异也会影响分析结
果[17,84] .
宏基因组技术最先应用于土壤,目前已经在肠
道、深海、口腔、土壤、废水、湖泊等环境中得到了广
745311期                      张军毅等: 基于分子标记的宏基因组 16S rRNA基因高变区选择策略           
泛的运用(表 2).而在 V 区的选择上,以 V3、V4 和
V6居多.当然在此过程中,也有大量文章对单 V区、
以及单 V 区和连续 V 区之间进行了比较研
究[7-9,32,59-60,62,69,85-86] .
    由于各种环境中的细菌群落多样性差异明显,
以及试验的目的、条件和设计者的经验等原因使得
V区的选择复杂多变,但是考虑以下几个方面仍是
十分必要的.
2􀆰 1  试验目的
目的是评估环境中微生物丰富度(species rich⁃
ness)和均匀度(species evenness),还是更加关注精
确的分类,尤其是属和种水平上的精确分类和鉴定
等其他研究? 这对于 V 区和引物的选择意义重大.
例如,由于 V6 区长度偏短,可变性较高,虽然可以
用来进行多样性的评估和分析,以及表型( phylo⁃
type)研究,但是作为物种鉴定 ( taxonomy assign⁃
ment)却往往不是最合适的选择[8] . Wang 等[87]和
Liu等[88]通过对比研究,认为 V2 和 V4 在种类鉴定
时的错误率较低,同时也适用于群落聚类分析
(community clustering) [42] .有些研究需要对某些特
定门类进行筛选,有时需要鉴定到属和种的水平甚
至株系的水平时,则不同 V区的选择意义重大[7,59] .
2􀆰 2  测序平台、测序深度或经济成本
目前用于宏基因组生物多样性分析的高通量测
序平台主要有:Illumina、Roche 和 Ion Torrent(表 3).
但从发表论文数量来看,Illumina 是主流平台.同时
鉴于测序的长度,目前 V 区主要仍以单区为主. Ion
Torrent 平台的测序速度最快, Illumina 通量最高,
Roche单位碱基测序价格最高.从目前的研究来看,
大多测序深度在 1万 ~ 10 万条序列之间,当然也不
乏有研究者进行深度测序.作者运用 IlluminaHiseq
2000研究 2012年太湖水体的浮游细菌,以 V6 作为
目标扩增区,共 96 个样本,平均每个样本 320 万条
细菌序列和 70 万条古细菌序列,共获得 6605 个细
菌 OTU和 5114个古细菌 OTU.此外,经过计算机模
拟(in silico evaluation),引物在不同的平台 Illumi⁃
na、Roche、Ion Torrent 和 PacBio SMRT 上也有一定
的差 异 性. 例 如, S⁃D⁃Arch⁃0349⁃a⁃S⁃17 / S⁃D⁃Arch⁃
0519⁃a⁃A⁃16和 S⁃D⁃Bact⁃0341⁃b⁃S⁃17 / S⁃D⁃Bact⁃0785⁃
a⁃A⁃21被认为最适于 Illumina 和 Ion Torrent 平台的
古细菌和细菌调查[86] .
2􀆰 3  群落结构
不同的生境往往蕴含不同的生物群落. Ghai
等[20]通过宏基因组的方法研究了亚马逊河上游段
的细菌群落,发现 Actinobacteria 的 acI 和 aclV 家族
是明显的优势类群.其对比了可利用的其他水生态
系统的宏基因组数据,发现淡水系统之间具有很大
的相似性,但是与海洋生态系统相比差异较大.不同
生境群落结构之间的差别,不仅体现在门的水平上,
更体现在属的水平上,从而给 V 区的选择带来了挑
战.基于宏基因组研究的环境中细菌群落结构往往
是未知的,这显然给 V区的选择带来了困扰.尽管可
以根据以往的文献资料来了解研究对象的微生物群
落结构,但是进行一个 V 区评估的预试验仍然很有
必要.Peiffer 等[69]研究野外条件下的玉米根际微生
物群落,首先利用 Roche 454平台对多个 V区(27F⁃
338R, V1⁃V2; 515F⁃806R, V4; 804F⁃1392R, V5⁃
V8; 926F⁃1392R; V6⁃V8)进行了测序,对比研究了
4个 V区,结果表明,不同的引物对在门水平上形成
的细菌群落的测定结果略有不同,515F⁃806R 因在
域和门的水平上获得的多样性最好、得到的 reads
数量较多、能注释上的比例较高等原因而被选择进
行后续分析.
2􀆰 4  引物偏向性
引物的偏向性往往由引物、PCR 循环数和反应
的酶系统等引入[89] .偏向性在某些环境样本中的影
响会非常大,造成对某些种类过低或过高的估计,甚
至有些群体被完全遗漏.例如, 8F、 337F、 338R、
515F、915F、930R 和 1061R 等一些通用的引物在肠
道微生物群落的研究中,通过 RDP 数据库( riboso⁃
mal database project database)可以比对 95%以上的
主要门类( Firmicutes、Bacteroidetes、Actinobacteria、
表 3  宏基因组多样性分析的主要测序平台
Table 3  Technical specifications of NGS platforms in the sequence⁃based metagenome
平台
Platform
读长
Read length (bp)
运行时间
Time / run
通量
Output data / run
准确性
Accuracy
Illumina Miseq (PE300) 2×300 40 h 8 Gb Mostly>Q30
Illumina Hiseq 2000 (PE100) 2×100 >8 d >500 Gb Mostly>Q30
Ion Torrent PGM (400)∗ >400 3 h 1 Gb Mostly>Q20
Roche 454 FLX >700 24 h 700~1000 Mb Mostly>Q30
∗318芯片 318 chips.
8453 应  用  生  态  学  报                                      26卷
表 4  不同测序平台上的最佳引物选择
Table 4  Primers recommended for different sequencing platforms
普通命名
Common name
新命名
New name1)
平台
Platform
5′⁃3′序列
Sequence 5′⁃3′
位点
Position2)
长度
Length
(bp)
覆盖度 Coverage4)(%)
A B E
A519F S⁃D⁃Arch⁃0519⁃a⁃S⁃
15
Roche 454 CAGCMGCCGCGG⁃
TAA
519~535 15 76.6 0.0 0.0
Arch1017R S⁃D⁃Arch⁃1041⁃a⁃A⁃
18
GGCCATGCACCWC⁃
CTCTC
1041~1058 18
Bakt_341F S⁃D⁃Bact⁃0341⁃b⁃S⁃
17
CCTACGGGAG⁃
GCAGCAG
341~357 17 0.5 86.2 0.0
Bakt_805R S⁃D⁃Bact⁃0785⁃a⁃A⁃
21
GACTACHVGGG⁃
TATCTAATCC
785~805 21
Arch349F S⁃D⁃Arch⁃0349⁃a⁃S⁃
17
Illumina & Ion Tor⁃
rent
GYGCASCAGKCG⁃
MGAAW
349~365 17 76.8 0.0 0.0
Parch519R S⁃D⁃Arch⁃0519⁃a⁃A⁃16
GGTDTTACCGCG⁃
GCKGCTG
519~533 15
520F S⁃D⁃Bact⁃0564⁃a⁃S⁃
15
AYTGGGYDTA⁃
AAGNG
564~578 15 14.6 89.0 0.0
802R S⁃D⁃Bact⁃0785⁃b⁃A⁃
18
TACNVGGG⁃
TATCTAATCC
785~802 18
27F S⁃D⁃Bact⁃0008⁃c⁃S⁃
20
PacBio SMRT3)和经
典克隆库
AGRGTTYGATYMT⁃
GGCTCAG
8~27 20 0.1 78.0 0.0
1391R S⁃D⁃Bact⁃1391⁃a⁃A⁃
17
GACGGGCGGTGT⁃
GTRCA
1391~1407 17
1)引物的新命名参照 New primer name was given according to Alm et al.[91] ; 2)位点命名参照大肠杆菌命名系统 Position based on the Escherichia
coli system of nomenclature[92] ; 3)如果扩增子>1400 bp,推荐 S⁃D⁃Bact⁃0008⁃a⁃S⁃16 / S⁃D⁃Bact⁃1492⁃a⁃A⁃16 S⁃D⁃Bact⁃0008⁃a⁃S⁃16 / S⁃D⁃Bact⁃1492⁃a⁃
A⁃16 was recommended for nearly full⁃length sequences (>1400 bp); 4)根据 SILVA数据库 106进行评估 Evaluation was based on SILVA reference
database 106[93] ; A: 古细菌 Archaea; B: 细菌 Bacteria; E: 真核 Eukaryota.
Verrucomicrobia和 Proteobacteria)序列[42] .但是对于
某些门类的缺失也同样存在,如 784F 很难区分 Ver⁃
rucomicrobia的种类[6];967F 只能比对不足 5% 的
Bacteroidetes序列[16];1492R 只能比对 61%的 Acti⁃
nobacteria、54% 的 Proteobacteria序列和不到一半的
其他门类[90] .同时,也有通过优化引物设计来实现
对 98.0%的细菌和 94.6%的古细菌在 RDP 数据库
中同时分析的策略[36] .在玉米根际微生物群落的研
究中,可能是因为过长的原因,804F⁃1392R 产生的
序列数最少;27F⁃338R 对于 Verrucomicrobia 扩增的
效率不高;926F⁃1392R 扩增了大量的色素体(plas⁃
tid)16S rRNA 基因;515F⁃806R 在域和门的水平上
获得的多样性最好[69] .Klindworth 等[86]通过计算机
模拟在 SILVA数据库中研究了 175条引物和 512对
引物,结合引物的物种覆盖度( taxonomic coverage)
和门类覆盖度(phylum spectrum)认为仅有 10 条可
以被推荐为广谱性引物(broad range primers),推荐
扩增长度为 464 bp 的 S⁃D⁃Bact⁃0341⁃b⁃S⁃17 / S⁃D⁃
Bact⁃0785⁃a⁃A⁃21为最好的引物组合.不同测序平台
引物的表现有所不同,一些被认为是通用的引物(例
如 F515和 R806)表现也并非最为突出,详见表 4.
2􀆰 5  连续性和可比性
通过 DNA测序来分析微生物多样性的最大优
势是可以将来自不同研究的数据进行整合分析,这
是 DNA指纹图谱等方法所不具有的.但是,如果这
些数据出自各种不同的试验方法,将会给这种整合
分析带来麻烦.理论上来说,如果这些数据都来自同
样一个 V区,那么就可以直接进行操作单元(opera⁃
tional taxonomic units, OTUs ) 聚类和种类注解
(taxonomy assignment)的整合分析[9] .因此,连续性
和可比性对于 V区的选择也至关重要.
3  小结和展望
基于 16S rRNA基因序列分析细菌多样性的宏
基因组技术,突破了微生物研究纯培养的瓶颈,改变
了人们对微生物世界的传统认识.近年来,在农业、
工业、环境、食品和卫生等领域得到了广泛的运用.
然而限于目前主流测序平台的读长和成本,还很难
对 16S rRNA全长进行高通量测序.所以在未来一段
时间内,V区的选择对于基于序列分析的宏基因组
多样性研究者来说仍然是一项必须面对的困扰.本
文总结了如下 V区选择的参考准则:1) 结合研究目
的和意义,选择合适的测序平台和测序深度.2) 不
同生境条件所形成的特有群落结构是需要重点考虑
的因素,尤其是针对某种目的细菌门类的研究.3)
可变性和适中的保守性被认为是 V 区选择的标准.
V 区两边高保守的侧翼位点( conserved along flan⁃
king sites)有利于通过设计相应引物“抓取”更多的
945311期                      张军毅等: 基于分子标记的宏基因组 16S rRNA基因高变区选择策略           
种类.4) 连续性和可比性.
总之,从目前的研究结果来看,仍然没有一个统
一的 V区选择标准,研究者需要综合以上各种因素
进行 V区的选择.从引物的设计和选择来看,也没有
一个真正意义上的通用引物(universal primer)能够
覆盖所有的原核微生物.所以,建议在研究之前对选
择的 V区进行有效评估.此外,也有学者对现有的
16S rRNA数据库进行研究和改善[88,94-95]、对生物信
息学算法[96-98]进行改进,这些都是十分必要的.
高通量测序技术的快速发展,尤其是以长读长
为主要特点的第三代测序技术,如 Pacific Bio⁃
sciences (PacBio) RS SMRT 测序仪,其平均读长可
达 5000 bp以上,很容易覆盖 16S rRNA全长.Mosher
等[74]通过对比 PacBio RS SMRT 和 Roche 454 这两
个平台,认为在系统进化树的构建和种类的分类方
面第三代测序技术目前并没有表现出明显的优势,
其主要原因为第三代测序错误率高.随着测序技术
向低错误率、长读长和高通量方向发展,16S rRNA
全长的高通量测序将会成为现实,并将为原核微生
物群落结构的研究提供更加准确和全面的信息.此
外,随着测序技术的发展和成本的不断下降,利用鸟
枪法直接进行宏基因组 DNA 的测序 ( sequencing
metagenomic DNA)也将逐步成为主流.该方法不依
赖于 16S rRNA目标片段的大量扩增,减少 PCR 扩
增所带来的误差,进而提高了微生物多样性结果的
准确性和可靠性.
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作者简介  张军毅,男,1982年生,博士研究生. 主要从事高
通量测序和宏基因组学研究. E⁃mail: blocksharon@ 163.com
责任编辑  肖  红
355311期                      张军毅等: 基于分子标记的宏基因组 16S rRNA基因高变区选择策略