MCscan (Python version)
Two external command line tools are needed:
-
LASTAL. Compile and then move
lastalandlastdbon yourPATH. - Optional SCIP. You'll need the executable
scipon yourPATH. You can also download a copy from here.
Let's assume that you want to perform synteny comparison between grape and peach.
For MCscan to work, we'll need sequences (FASTA format) and coordinates (BED format). For this example, let's get these data from Phytozome.
$ python -m jcvi.apps.fetch phytozome
...
Acoerulea Alyrata Athaliana
Bdistachyon Brapa Cclementina
Cpapaya Creinhardtii Crubella
Csativus Csinensis Csubellipsoidea_C-169
Egrandis Fvesca Gmax
Graimondii Lusitatissimum Mdomestica
Mesculenta Mguttatus Mpusilla_CCMP1545
Mpusilla_RCC299 Mtruncatula Olucimarinus
Osativa Ppatens Ppersica
Ptrichocarpa Pvirgatum Pvulgaris
Rcommunis Sbicolor Sitalica
Slycopersicum Smoellendorffii Stuberosum
Tcacao Thalophila Vcarteri
Vvinifera Zmays early_release
...
$ python -m jcvi.apps.fetch phytozome Vvinifera,Ppersi
...
$ ls
Ppersica_139_cds.fa.gz Ppersica_139_gene.gff3.gz Vvinifera_145_cds.fa.gz Vvinifera_145_gene.gff3.gz
Convert the GFF to BED file and rename them.
$ python -m jcvi.formats.gff bed --type=mRNA --key=Name Vvinifera_145_gene.gff3.gz -o grape.bed
$ python -m jcvi.formats.gff bed --type=mRNA --key=Name Ppersica_139_gene.gff3.gz -o peach.bed
I also like to clean headers to remove description fields from Phytozome FASTA files.
$ python -m jcvi.formats.fasta format --sep="|" Vvinifera_145_cds.fa.gz grape.cds
$ python -m jcvi.formats.fasta format --sep="|" Ppersica_139_cds.fa.gz peach.cds
Okay! Now we are ready for the next step.
We now have a clean set of files to proceed.
$ ls *.???
grape.cds peach.cds grape.bed peach.bed
$ python -m jcvi.compara.catalog ortholog grape peach
20:33:42 [base] lastdb peach peach.cds
20:34:13 [base] lastal -u 0 -P 64 -i3G -f BlastTab peach grape.cds >grape.peach.last
20:34:30 [synteny] Assuming --qbed=grape.bed --sbed=peach.bed
20:34:31 [blastfilter] Load BLAST file `grape.peach.last` (total 403868 lines)
20:34:31 [base] Load file `grape.peach.last`
20:34:38 [blastfilter] running the cscore filter (cscore>=0.70) ..
20:34:39 [blastfilter] after filter (294217->31229) ..
20:34:39 [blastfilter] running the local dups filter (tandem_Nmax=10) ..
20:34:39 [blastfilter] after filter (31229->21089) ..
...
A total of 30654 (NR:18661) anchors found in 678 clusters.
Stats: Min=4 Max=683 N=678 Mean=45.2123893805 SD=80.8407828815 Median=16.0 Sum=30654
NR stats: Min=4 Max=460 N=678 Mean=27.5235988201 SD=48.0461479616 Median=11.0 Sum=18661
That's it! This calls LAST to do the comparison, filter the LAST output to remove tandem duplications and weak hits. A single linkage clustering is performed on the LAST output to cluster anchors into synteny blocks. At the end of the run, you'll see summary statistics of the synteny blocks.
$ ls grape.peach.*
grape.peach.lifted.anchors grape.peach.anchors grape.peach.last.filtered grape.peach.last
The .last file is raw LAST output, .last.filtered is filtered LAST output, .anchors is the seed synteny blocks (high quality), .lifted.anchors recruits additional anchors to form the final synteny blocks.
The best visualization to see pairwise synteny is using dotplot. This is just one command.
$ python -m jcvi.graphics.dotplot grape.peach.anchors
We have the following dot plot in grape.peach.pdf.
To understand what the dot plot says, look at either horizontally or vertically and count up how many blocks we generally see. Horizontally, we have for each peach region, up to 3 synteny regions in grape. Also vertically, we have for each grape region, up to 3 syntenic regions in peach. It helps to understand that grape and peach shares a genome triplication event ("gamma") event, giving rise to this 3:3 pattern.
If you look closely, one of the 3 regions are often stronger, which corresponds to the orthologous regions between the two genomes. What if we just want to get the 1:1 orthologous region? We'll just repeat what we did but adding an option --cscore=.99. C-score is defined by the ratio of LAST hit to the best BLAST hits to either the query and hit. A C-score cutoff of .99 effectively filters the LAST hit to contain reciprocal best hit (RBH).
$ rm grape.peach.last.filtered
$ python -m jcvi.compara.catalog ortholog grape peach --cscore=.99
$ python -m jcvi.graphics.dotplot grape.peach.anchors
We could also quick test if the synteny pattern is indeed 1:1, by running:
$ python -m jcvi.compara.synteny depth --histogram grape.peach.anchors
Now let's move to a different kind of visualization using the same synteny output grape.peach.anchors. Aside from the BED files and synteny files we already have, we need to prepare two additional files.
First is the seqids file, which tells the plotter which set of chromosomes to include. Here, we've removed unplaced and small scaffolds. First line contains 19 grape chromosomes and second line contains 8 peach chromosomes.
chr1,chr2,chr3,chr4,chr5,chr6,chr7,chr8,chr9,chr10,chr11,chr12,chr13,chr14,chr15,chr16,chr17,chr18,chr19
scaffold_1,scaffold_2,scaffold_3,scaffold_4,scaffold_5,scaffold_6,scaffold_7,scaffold_8
Second is the layout file, which tells the plotter where to draw what. The whole canvas is 0-1 on x-axis and 0-1 on y-axis. First three columns specify the position of the track. Then rotation, color, label, vertical alignment (va), and then the genome BED file. Track 0 is now grape, track 1 is now peach. The next stanza specify what edges to draw between the tracks. e, 0, 1 asks to draw edges between track 0 and 1, using information from the .simple file.
# y, xstart, xend, rotation, color, label, va, bed
.6, .1, .8, 0, , Grape, top, grape.bed
.4, .1, .8, 0, , Peach, top, peach.bed
# edges
e, 0, 1, grape.peach.anchors.simple
Finally this command generates a .simple file which is a more succinct form than .anchors file.
$ python -m jcvi.compara.synteny screen --minspan=30 --simple grape.peach.anchors grape.peach.anchors.new
With all input files ready, let's plot.
$ python -m jcvi.graphics.karyotype seqids layout
This generates our karyotype figure.
Further customization is possible. For example, change the order of chromosomes in seqids could lead to a visually more appealing figure. Also, play with the positions, color, labels etc in the layout file.
What if we want to highlight a specific block? We should go into the .simple file, locate the relevant block. Please note that each line in the .simple file is a synteny blocks, with start and stop grape gene, then start and stop peach gene, final two columns are score and orientation.
$ vi grape.peach.anchors.simple
g*GSVIVT01012028001 GSVIVT01000604001 ppa011886m ppa008534m 392 +
GSVIVT01010441001 GSVIVT01000970001 ppa022891m ppa001358m 115 -
GSVIVT01000555001 GSVIVT01003228001 ppa002809m ppa010569m 359 +
...
(save the file)
$ python -m jcvi.graphics.karyotype seqids layout
We have then highlighted (with color 'g' green) a particular synteny block in the figure.
We can be really creative about karyotype plots! For a start, we can add as many genomes as we want. Let's add cacao. We just need to repeat the downloading and formatting on the cacao genome (extract cacao.cds and cacao.bed).
$ python -m jcvi.apps.fetch phytozome Tcacao
$ python -m jcvi.formats.fasta format --sep="|" Tcacao_233_cds.fa.gz cacao.cds
$ python -m jcvi.formats.gff bed --type=mRNA --key=Name Tcacao_233_gene.gff3.gz -o cacao.bed
Say we are interested in looking at peach and cacao comparisons.
$ python -m jcvi.compara.catalog ortholog peach cacao --cscore=.99
$ python -m jcvi.compara.synteny screen --minspan=30 --simple peach.cacao.anchors peach.cacao.anchors.new
Now that we have cacao.bed and peach.cacao.anchors.simple, we'll need to add them to the layout. Please note the two new lines - line 4 and 7. Section # edges says that we should connect track 0 (grape) with 1 (peach), track 1 (peach) with 2 (cacao).
# y, xstart, xend, rotation, color, label, va, bed
.7, .1, .8, 15, , Grape, top, grape.bed
.5, .1, .8, 0, , Peach, top, peach.bed
.3, .1, .8, -15, , Cacao, bottom, cacao.bed
# edges
e, 0, 1, grape.peach.anchors.simple
e, 1, 2, peach.cacao.anchors.simple
Remember to add the major cacao chromosomes (line 3) in seqids. Remember the order of the genomes in seqids need to match the order in layout.
chr1,chr2,chr3,chr4,chr5,chr6,chr7,chr8,chr9,chr10,chr11,chr12,chr13,chr14,chr15,chr16,chr17,chr18,chr19
scaffold_1,scaffold_2,scaffold_3,scaffold_4,scaffold_5,scaffold_6,scaffold_7,scaffold_8
scaffold_1,scaffold_2,scaffold_3,scaffold_4,scaffold_5,scaffold_6,scaffold_7,scaffold_8,scaffold_9,scaffold_10r
We can also customize the rotation (in degrees), so we are no longer limited to the boring horizontal arrangement of chromosomes. After the command:
$ python -m jcvi.graphics.karyotype seqids layout
We have the final product!
