HISAT2: a surprising read shows up in alignment results

[yip]

I was spot-checking the output of alignment results from HISAT2 and noticed one read (CATCTTGTTTCACACTAGTCACATTCTTGTTTTAAGTGCCTTTAGTTTTAACAGTTCACTTTTTACAGTACTATTT shown below) that does not exist in my input fastq files. Is this the result of a bug or am I missing anything?

$ hisat2 -p 40 -x hg19/genome -1 read1.fq.gz -2 read2.fq.gz 2>/dev/null | grep CATCTTGTTTCACACTAGTCACATTCTTGTTTTAAGTGCCTTTAGTTTTAACAGTTCACTTTTTACAGTACTATTT
HISEQ:287:CCK1AANXX:5:2307:14574:99798 409 chr19 14733987 0 76M = 14733987 0 CATCTTGTTTCACACTAGTCACATTCTTGTTTTAAGTGCCTTTAGTTTTAACAGTTCACTTTTTACAGTACTATTT GGGGGGGGCGGGEGG>[email protected] AS:i:-5 ZS:i:-5 XN:i:0 XM:i:1 XO:i:0 XG:i:0 NM:i:1 MD:Z:69G6 YT:Z:UP NH:i:2
$ zcat read1.fq.gz read2.fq.gz|grep CATCTTGTTTCACACTAGTCACATTCTTGTTTTAAGTGCCTTTAGTTTTAACAGTTCACTTTTTACAGTACTATTT
$ hisat2 -p 40 -x hg19/genome -1 read1.fq.gz -2 read2.fq.gz 2>/dev/null | grep HISEQ:287:CCK1AANXX:5:2307:14574:99798
HISEQ:287:CCK1AANXX:5:2307:14574:99798 137 chr10 33189363 0 76M = 33189363 0 AAATAGTACTGTAAAAAGTGAACTGTTAAAACTAAAGGCACTTAAAACAAGAATGTGACTAGTGTGAAACAAGATG [email protected]>GGEGGGCGGGGGGGG AS:i:-5 ZS:i:-5 XN:i:0 XM:i:1 XO:i:0 XG:i:0 NM:i:1 MD:Z:6C69 YT:Z:UP NH:i:2
HISEQ:287:CCK1AANXX:5:2307:14574:99798 409 chr19 14733987 0 76M = 14733987 0 CATCTTGTTTCACACTAGTCACATTCTTGTTTTAAGTGCCTTTAGTTTTAACAGTTCACTTTTTACAGTACTATTT GGGGGGGGCGGGEGG>[email protected] AS:i:-5 ZS:i:-5 XN:i:0 XM:i:1 XO:i:0 XG:i:0 NM:i:1 MD:Z:69G6 YT:Z:UP NH:i:2
HISEQ:287:CCK1AANXX:5:2307:14574:99798 69 chr10 33189363 0 * = 33189363 0 ATAAACATACAGTGGTCTGTTATGGCACTAACTCCCCGTACTCTGCGTTGGTATCAACGCAGAGTACGGGGAGTTA 33<::[email protected]GGEGGGG YT:Z:UP

The other two entries in the sam output with read name of HISEQ:287:CCK1AANXX:5:2307:14574:99798 are from my input fastq files as expected (a read pair).

I appreciate any comments. Thanks!

[yip]

Here's the answer (thanks to Daehwan from the HISAT2 team):
CATCTTGTTTCACACTAGTCACATTCTTGTTTTAAGTGCCTTTAGTTTTAACAGTTCACTTTTTACAGTACTATTT is the reverse complement of AAATAGTACTGTAAAAAGTGAACTGTTAAAACTAAAGGCACTTAAAACAAGAATGTGACTAGTGTGAAACAAGATG, which is read 1 of HISEQ:287:CCK1AANXX:5:2307:14574:99798.