FASTQ文件详解【转】

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最新推荐文章于 2024-10-27 18:51:36 发布

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FASTQ是基于文本的，保存生物序列（通常是核酸序列）和其测序质量信息的标准格式。其序列以及质量信息都是使用一个ASCII字符标示，最初由Sanger开发，目的是将FASTA序列与质量数据放到一起，目前已经成为高通量测序结果的事实标准。

格式说明

FASTQ文件中每个序列通常有四行：

序列标识以及相关的描述信息，以‘@’开头；
第二行是序列
第三行以‘+’开头，后面是序列标示符、描述信息，或者什么也不加
第四行，是质量信息，和第二行的序列相对应，每一个序列都有一个质量评分，根据评分体系的不同，每个字符的含义表示的数字也不相同。

例如：

@SEQ_ID
GATTTGGGGTTCAAAGCAGTATCGATCAAATAGTAAATCCATTTGTTCAACTCACAGTTT

!''*((((***+))%%%++)(%%%%).1***-+*''))**55CCF>>>>>>CCCCCCC65

ILLUMINA SEQUENCE IDENTIFIERS

@HWUSI-EAS100R:6:73:941:1973#0/1

HWUSI-EAS100R	the unique instrument name
6	flowcell lane
73	tile number within the flowcell lane
941	‘x’-coordinate of the cluster within the tile
1973	‘y’-coordinate of the cluster within the tile
#0	index number for a multiplexed sample (0 for no indexing)
/1	the member of a pair, /1 or /2 (paired-endor mate-pair reads only)

@EAS139:136:FC706VJ:2:2104:15343:197393 1:Y:18:ATCACG

EAS139	the unique instrument name 唯一的仪器编号
136	the run id
FC706VJ	the flowcell id
2	flowcell lane
2104	tile number within the flowcell lane
15343	‘x’-coordinate of the cluster within the tile
197393	‘y’-coordinate of the cluster within the tile
1	the member of a pair, 1 or 2 (paired-endor mate-pair reads only)
Y	Y if the read fails filter (read is bad), N otherwise
18	0 when none of the control bits are on, otherwise it is an evennumber
ATCACG	index sequence

NCBI SEQUENCE READ ARCHIVE

@SRR001666.1 071112_SLXA-EAS1_s_7:5:1:817:345 length=36
GGGTGATGGCCGCTGCCGATGGCGTCAAATCCCACC

+SRR001666.1 071112_SLXA-EAS1_s_7:5:1:817:345 length=36
IIIIIIIIIIIIIIIIIIIIIIIIIIIIII9IG9IC

关于质量编码格式

质量评分指的是一个碱基的错误概率的对数值。其最初在Phred拼接软件中定义与使用，其后在许多软件中得到使用。其质量得分与错误概率的对应关系见下表：

Phred quality scores are logarithmically linked to errorprobabilities
PHRED QUALITY SCORE	PROBABILITY OF INCORRECT BASE CALL	BASE CALL ACCURACY
10	1 in 10	90 %
20	1 in 100	99 %
30	1 in 1000	99.9 %
40	1 in 10000	99.99 %
50	1 in 100000	99.999 %

Phred quality scores Q are defined as a property which is logarithmically related to the base-calling error probabilities P.

除了Phred质量得分换算标准，还有就是Solexa标准：

两种换算标准的比较：

Relationship between Q and p using the Sanger (red) and Solexa(black) equations (described above). The vertical dotted lineindicates p = 0.05, or equivalently, Q ≈ 13.

对于每个碱基的质量编码标示，不同的软件采用不同的方案，目前有5种方案：

Sanger，Phred quality score，值的范围从0到92，对应的ASCII码从33到126，但是对于测序数据（rawread data）质量得分通常小于60，序列拼接或者mapping可能用到更大的分数。
Solexa/Illumina 1.0, Solexa/Illumina qualityscore，值的范围从-5到63，对应的ASCII码从59到126，对于测序数据，得分一般在-5到40之间；
Illumina 1.3+，Phred qualityscore，值的范围从0到62对应的ASCII码从64到126，低于测序数据，得分在0到40之间；
Illumina 1.5+，Phred qualityscore，但是0到2作为另外的标示，详见http://solexaqa.sourceforge.net/questions.htm#illumina
Illumina 1.8+

下面是更为直观的表示：

SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS.....................................................
  ..........................XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX......................
  ...............................IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII......................
  .................................JJJJJJJJJJJJJJJJJJJJJJJJJJJJJJJJJJJJJJJ......................
  LLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLL....................................................
  !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~
  |                         |    |        |                              |                     |
 33                        59   64       73                            104                   126

 S - Sanger        Phred+33,  raw reads typically (0, 40)
 X - Solexa        Solexa+64, raw reads typically (-5, 40)
 I - Illumina 1.3+ Phred+64,  raw reads typically (0, 40)
 J - Illumina 1.5+ Phred+64,  raw reads typically (3, 40)
    with 0=unused, 1=unused, 2=Read Segment Quality Control Indicator (bold)
    (Note: See discussion above).
 L - Illumina 1.8+ Phred+33,  raw reads typically (0, 41)

文件后缀

没有特别的规定，通常使用.fq, .fastq, .txt等。

格式转换：

Biopython version1.51 onwards (interconverts Sanger, Solexa and Illumina 1.3+)
EMBOSS version6.1.0 patch 1 onwards (interconverts Sanger, Solexa and Illumina1.3+)
BioPerl version1.6.1 onwards (interconverts Sanger, Solexa and Illumina 1.3+)
BioRuby version1.4.0 onwards (interconverts Sanger, Solexa and Illumina 1.3+)
BioJava version1.7.1 to 1.8.x (interconverts Sanger, Solexa and Illumina1.3+)
MAQ canconvert from Solexa to Sanger (use this patch tosupport Illumina 1.3+ files).
fastx_toolkit Theincluded fastq_quality_converter program can convert Illumina toSanger

根据质量编码格式，fastq有多重变体，得到一个fastq文件，如何知道其编码格式（Detection of FASTQvariants）？确定其质量编码格式是许多分析处理的前提，有没有直接的简单的方法来判断？质量编码格式共有五种格式：

Sanger (Phred+33, 33 to 73)
Solexa (Phred+64, 59 to 104)
Illumina (1.3+) (Phred+64, 64 to 104)
Illumina (1.5+) (Phred+64, 66 to 104)
Illumina (1.8+) (Phred+33, 33 to 74)

具体的字母表示范围文章所示，通过扫描扫Fastq文件中的质量范围，从而推出其质量编码格式。QCToolkit就是采用这种方式对编码格式进行自动判断，perl代码如下：

my $file = $_[0];
my $isVariantIdntfcntOn = $_[1];
my $lines = 0;
open(F, "< $file") or die "Can not open file $file\n";
my $counter = 0;
my $minVal = 1000;
my $maxVal = 0;
while(my $line = ) {
        $lines++;
        $counter++;
        next if($line =~ /^\n$/);
        if($counter == 1 && $line !~ /^\@/) {
                prtErrorExit("Invalid FASTQ file format.\n\t\tFile: $file");
        }
        if($counter == 3 && $line !~ /^\+/) {
                prtErrorExit("Invalid FASTQ file format.\n\t\tFile: $file");
        }
        if($counter == 4 && $lines < 1000000) {
                chomp $line;
                my @ASCII = unpack("C*", $line);
                $minVal = min(min(@ASCII), $minVal);
                $maxVal = max(max(@ASCII), $maxVal);
        }
        if($counter == 4) {
                $counter = 0;
        }
}
close(F);
my $tseqFormat = 0;
if($minVal >= 33 && $minVal < = 73 && $maxVal >= 33 && $maxVal < = 73) {
        $tseqFormat = 1;
}
elsif($minVal >= 66 && $minVal < = 105 && $maxVal >= 66 && $maxVal < = 105) {
        $tseqFormat = 4;                        # Illumina 1.5+
}
elsif($minVal >= 64 && $minVal < = 105 && $maxVal >= 64 && $maxVal < = 105) {
        $tseqFormat = 3;                        # Illumina 1.3+
}
elsif($minVal >= 59 && $minVal < = 105 && $maxVal >= 59 && $maxVal < = 105) {
        $tseqFormat = 2;                        # Solexa
}
elsif($minVal >= 33 && $minVal < = 74 && $maxVal >= 33 && $maxVal < = 74) {
        $tseqFormat = 5;                        # Illumina 1.8+
}
if($isVariantIdntfcntOn) {
        $seqFormat = $tseqFormat;
}
else {
        if($tseqFormat != $seqFormat) {
                print STDERR "Warning: It seems the specified variant of FASTQ doesn't match the quality values in input FASTQ files.\n";
        }
}

参考以及相关链接

http://en.wikipedia.org/wiki/FASTQ_format
http://www.ncbi.nlm.nih.gov/books/NBK47537/
Cock et al (2009) The Sanger FASTQ file format for sequences withquality scores, and the Solexa/Illumina FASTQ variants. NucleicAcids Research,doi:10.1093/nar/gkp1137
MAQ webpagediscussing FASTQ variants
Galaxyfastq tools
Fastxtoolkit collection of command line tools forShort-Reads FASTA/FASTQ files preprocessing
Fastqc qualitycontrol tool for high throughput sequence data
http://www.illumina.com.cn/index.asp