Creating a liftOver file is very similar to a whole-genome alignment. A liftOver file is a chain file, where for each region in the genome the alignments of the best/longest syntenic regions are used to translate features from one version of a genome to another.
With competing assemblies becoming more common, liftOver file is sometimes necessary for someone that sets up his/her own genome browser of a different assembly.
This page is based on Minimal_Steps_For_LiftOver, but is even more minimalistic.
Specifically, note the page Alignments_with_Blastz and understand the script, RunLastzChain sh.txt
- BLAT the new genome onto the old genome
- My genome is rather small, so I don't do any splitting or lifting steps anywhere, that makes it simpler. If you need to split your genome, see Minimal_Steps_For_LiftOver: The old genome is split in chunks and lifting file is generated. After the alignment of the chunks, the resulting chains are lifted back to the original coordinates.
- I have repeatmasked my new genome assembly. The masked fa files are in ../ci3/rm/masked
- The old assembly is called ci2.2bit, the new assembly is in the directory ../ci3. After repeatmasking it's in rm/masked
- I want the alignment .psl to be in the directory psl, so I did the alignment with
mkdir psl for i in ../ci3/rm/masked/*.masked; do blat ../ci2.2bit $i -tileSize=12 -fastMap -minIdentity=98 psl/`basename $i .fa.masked`.psl -noHead -minScore=100; done
- Translate psl files to chains in the directory chain:
mkdir chain for i in psl/*.psl; do axtChain -linearGap=medium -psl $i ../ci2.2bit ../ci3/ci3.2bit chain/`basename $i .psl`.chain; done
- Merge short chains into longer ones into the directory chainMerge:
mkdir chainMerge chainMergeSort chain/*.chain | chainSplit chainMerge stdin -lump=50
- concat and sort the chains:
cat chainMerge/*.chain > all.chain chainSort all.chain all.sorted.chain
- Need info about chromosome sizes for netting:
twoBitInfo ../ci3/ci3.2bit ci3.chromInfo twoBitInfo ../ci2.2bit ci2.chromInfo
- Netting: identify alignable regions from chains:
mkdir net chainNet all.sorted.chain ci2.chromInfo ci3.chromInfo net/all.net /dev/null
- Finally, select the right alignable regions using the nets, creating a "liftOver" file:
netChainSubset net/all.net all.chain ci2ToCi3.liftOver