This leads to more aggressive pairing - more properly paired reads. I have
found a few cases where, for example, read1 is umambiguously mapped to chr20
while its 100bp mate has a perfect match to another chr but has 3 mismatches
and 1 deletion when it is paired with read1 on chr20. With longer reads, it
seems that the chr20 hit is correct, although it is not obvious how this
happened in evolution.
In this case, bwa_fix_xref() will return insane coordinates. The old version
did not check the return status and write wrong CIGAR. This bug only happen to
very short assembly contigs.
The bug only happens when there is a 1bp del and 1bp ins which are close to the
end and there are no other substitutions or indels. In this case, bwa mem gave
a wrong band width.
If one end has a low quality tail that happens to have a score-20 hit,
the pair won't be flagged as properly paired because bwa-mem thought it has
multiple hits. By filtering with -T, we won't have this problem.
When backtracking, bwa-short does not keep the detailed alignment or the exact
start and end positions. To find the boundary and the CIGAR, the old code does
a global alignment with a small end-gap penalty. It then deals with a lot of
special cases to derive the right position and CIGAR, which are actually not
always right. It is a mess.
As the new ksw.{c,h} does not support a different end-gap penalty, the old
strategy does not work. But we get something better. The new code finds the
boundaries with ksw_extend(). It is cleaner and gives more accurate CIGAR in
most cases.
stdaln.{c,h} was written ten years ago. Its local and SW extension code are
actually buggy (though that rarely happens and usually does not affect the
results too much). ksw.{c,h} is more concise, potentially faster, less buggy,
and richer in features.
The changes after r317 aim to improve the performance and accuracy for very
long query alignment. The short-read alignment should not be affected. The
changes include:
1) Z-dropoff. This is a variant of blast's X-dropoff. I orginally thought this
heuristic only improves speed, but now I realize it also reduces poor
alignment with long good flanking alignments. The difference from blast's
X-dropoff is that Z-dropoff allows big gaps, but X-dropoff does not.
2) Band width doubling. When band width is too small, we will get a poor
alignment in the middle. Sometimes such alignments cannot be fully excluded
with Z-dropoff. Band width doubling is an alternative heuristic. It is based
on the observation that the existing of close-to-boundary high score
possibly implies inadequate band width. When we see such a signal, we double
the band width.
Heng Li