Creating Sweepfinder2 input and Running by Chromosome and population in Plink files

run_sweepfinder2_byPop_byChrom.sh

#!/bin/bash
# Author: Yassine Souilmi <yassine.souilmi@adelaide.edu.au>

set -u

# module load BCFtools

bed=$1 # bed file name
ref = $2 # link to the reference fasta file

# Create a polarised VCF for each population:
awk '{print $1}' ${bed}.fam | sort -u | while read pop
do
	echo "${pop}" > pop.txt
	mkdir -pv ${bed}_popSplit

	plink --bfile ${bed} \
	--keep-fam pop.txt \
	--recode vcf-iid bgz \
	--keep-allele-order \
	--out ${bed}_popSplit/${pop}_${bed}

	tabix -p vcf ${bed}_popSplit/${pop}_${bed}.vcf.gz

	bcftools norm \
	-t $(seq 1 22 | tr -s '\n' ',' | sed 's/22,/22/') \
	-O z -o ${bed}_popSplit/${pop}_${bed}_fixedRefAllele.vcf.gz \
	-c s \
	-f ~/fastdir/Refs/gatk-bundle/b37/human_g1k_v37_decoy.fasta \
	${bed}_popSplit/${pop}_${bed}.vcf.gz

	tabix -p vcf ${bed}_popSplit/${pop}_${bed}_fixedRefAllele.vcf.gz

done

# Create SFS files
mkdir -pv ${bed}_popSplit_SF2_inputs

for chr in {1..22}
do
	python ~/bin/vcf2SF.py \
		-g \
		-v ${bed}_popSplit/${pop}_${bed}_fixedRefAllele.vcf.gz \
		-c ${chr} \
		-o ${bed}_popSplit_SF2_inputs/${pop}_${bed}_${chr}.sfs
done


# Running SweepFinder2
wd=$PWD

awk '{print $1}' ${bed}.fam | sort -u | while read pop
	for chr in {1..22}
	do
    	mkdir -pv ${pop}_sf2_wd/${chr}
    	cd ${pop}_sf2_wd/${chr}/ #Making sure all jobs run in a dedicated folder

    	SweepFinder2 -sg 1000 ${wd}/${bed}_popSplit_SF2_inputs/${pop}_${bed}_${chr}.sfs ./${pop}_${chr}.SF2out

	done # for loop (chromosomes)
done # while loop (populations)

vcf2SF.py

# File created by Arun Durvasula: https://gist.github.com/arundurvasula/30727385d4605fb26770
# Modified by Yassine Souilmi in May 2018




import gzip
import csv
import argparse
import sys

parser = argparse.ArgumentParser(description="script to convert an all sites vcf to sweepfinder format. FASTA description will be the sample name in the VCF header.Only does one chromosome/region at a time.")
parser.add_argument("-v", "--vcf", action="store", required=True, help="Input VCF file. Should be a multisample vcf, though it should theoretically work with a single sample.")
parser.add_argument("-o", "--out", action="store", required=True, help="Output filename")
parser.add_argument("-c", "--chromosome", action="store", required=True, help="Chromosome to output. Should be something in the first column of the vcf.")
parser.add_argument("-g", "--gzip", action="store_true", required=False, help="Set if the VCF is gzipped.")
parser.add_argument("-p", "--phased", action="store_true", required=False, help="Speficy if the genotypes are phased.")
args = parser.parse_args()

vcf_in = args.vcf
out_name = args.out
out_chr = args.chromosome

sample_names = []
sample_seqs = []

# Printing output information
print "Generating SFS"
print "**************"
print "From: ",vcf_in
print "For chrmosome: ",out_chr

if args.phased:
	sep = "|"
	print "The input VCF is phased"
else:
	sep =  "/"
	print "The input VCF is not phased"

print "Writing the output to: ",out_name
print "**************"

if args.gzip:
    opener = gzip.open
else:
    opener = open

sweep_out = open(out_name, 'w')
sweep_out.write("position\tx\tn\tfolded\n") # write header
folded = "1"

with opener(vcf_in, 'r') as tsvin:
    tsvin = csv.reader(tsvin, delimiter='\t')

    for row in tsvin:

        if any('##' in strings for strings in row):
            continue
        if any('#CHROM' in strings for strings in row):
            sample_names = row[9:]

        chrom,pos,id,ref,alt,qual,filter,info,format=row[0:9]
        haplotypes = row[9:]
        location = pos
    
    	if chrom == out_chr:
        	alt_list = alt.split(",")
        	x = 0
        	n = 0
        	for index,haplotype in enumerate(haplotypes):
            		if haplotype.split(sep)[0] != '.':
                		n = n + 1 # count up the non missing calls
            		if haplotype.split(sep)[0] != "1" and haplotype.split(sep)[0] != ".":
                		x = x + 1 # count reference alleles.
                
        	if x != 0 and x != n:    
           		sweep_out.write(location+"\t"+str(x)+"\t"+str(n)+"\t"+folded+"\n")
       	else:
             	continue

print "Done!"
print "**************"

Hi, there. I am new to population genetics and I am confused by the script vcf2SF.py.
I am confused about the 73th row, which count the derived allele. Because I read from the manual the "0" means the site is the same with column "REF", "1" means the site is the same with the first one of the column "ALT". So I feel like the expression show be if haplotype.split(sep)[0] != "0" and haplotype.split(sep)[0] != ".":.
I'll appreciate that, if you could answer my questions. Do please my rudeness if I said something wrong.

Thank you for your feedback. The value "1" in the script is contradictory to the comment, so you aren't wrong. However, the allele you are counting (reference or alt) shouldn't make a difference to the shape of your SFS if it is folded (i.e. not polarized by ancestral allele). So you can either use the script as-is or change it to "0" as you described.

Just for sake of demonstration, I ran a test on a small-ish dataset where I know I got some outliers with the version of the script above counting ref alleles, and one with a modified version where I'm counting the alt alleles. Here's how the values in general compare, the x-axis represent the former (counting ref) and the y-axis represents the modified version:

The plot above shows consistency in values, especially in high CLR values which is what determines your outliers. However, it isn't very informative about the genomic distribution of the values. So I also plotted the values in manhattan plot to show that the genomic distribution is the same, lightblue represents the default script and lightgrey the suggested modification:

Your question is very relevant and good for you for spotting my comment error. Hope this was informative.