Description of the workflow for constructing a genetic map using hsrecombi

Introduction

hsrecombi can approximate the genetic-map positions of genetic markers from SNP genotypes in half-sib families. It is referred to Hampel et al. (2018, Fron Gen) and Qanbari & Wittenburg (2020, Genet Sel Evol) for more information on the methodology. The workflow relies on paternal half-sib families but it can be adapted to maternal half-sib families with minor modifications.

In Part I, genotypic data from progeny and their common parents are simulated with the R package AlphaSimR and reshaped into the PLINK format. If genotypic data are already available in the required PLINK format, the workflow starts directly at Part II.

Required input files for Part II:

map files for each chromosome map_chr<chr>.map
data files for each chromosome hsphase_input_chr<chr>.raw

Note: A pipeline is available at GitHub.

library(hsrecombi)
library(AlphaSimR)
#> Loading required package: R6
#> Warning: package 'R6' was built under R version 4.0.5
library(doParallel)
#> Loading required package: foreach
#> Loading required package: iterators
#> Loading required package: parallel
library(ggplot2)
library(rlist)
Sys.time()
#> [1] "2022-11-30 12:46:26 CET"

Global variables

# number of chromosomes
nchr <- 2
# Number of simulated SNPs (e.g., p = 1500 resembles an average bovine chromosome)
p <- 150
# Number of simulated progeny in each half-sib family
n <- 1000
# Directory for (simulated) data
path.dat <- "data"
dir.create(path.dat, showWarnings = FALSE)
# Directory for output
path.res <- "results"
dir.create(path.res, showWarnings = FALSE)
# Number of computing clusters allocated
nclust <- 2

For instance, executing the following workflow with $p=1500$ and $n\geq 1000$ requires about 120 Mb storage space and 10 min computing time (2.6 GHz processor). With $p=150$, it takes about 7 Mb and 20 sec.

Part I: Data simulation and preparation

1: Simulation of genetic data with R package AlphaSimR

founderPop <- runMacs2(nInd = 1000, nChr = nchr, segSites = p)
SP <- SimParam$new(founderPop)
SP$setSexes("yes_sys")
# Enable tracing location of recombination events
SP$setTrackRec(TRUE)

pop <- newPop(founderPop)
N <- 10
ntotal <- N * n
my_pop <- selectCross(pop = pop, nFemale = 500, nMale = N, use = "rand", nCrosses = ntotal)

probRec <- list()
for (chr in 1:nchr) {
    co.pat <- matrix(0, ncol = p, nrow = ntotal)
    for (i in 1:ntotal) {
        if (nrow(SP$recHist[[1000 + i]][[chr]][[2]]) > 1) {
            # 1. line contains 1 1 by default
            loci <- SP$recHist[[1000 + i]][[chr]][[2]][-1, 2]
            co.pat[i, loci] <- 1
        }
    }
    probRec[[chr]] <- colMeans(co.pat)
}

save(list = c("SP", "founderPop", "pop", "my_pop", "ntotal", "probRec"), file = "data/pop.RData")

Two chromosomes, each with $p$ SNPs and 1 Morgan length, are simulated. The founder population consists of 1,000 animals with equal gender distribution. The follow-up generation consists of $N$ paternal-half sib families with equal number of progeny $n$. The study design with the equal number of progeny per family was preferred here for simplicity but this is not required in the subsequent steps.

2: Selection of sire haplotypes and progeny genotypes

PAT <- my_pop@father
rown <- paste(rep(unique(PAT), each = 2), c(1, 2), sep = "_")
H.pat <- pullSegSiteHaplo(pop)[rown, ]
X <- pullSegSiteGeno(my_pop)
# Physical position of markers in Mbp
map.snp <- lapply(founderPop@genMap, function(z) z * 100)

Note that founderPop@genMap includes genetic positions in Morgan units but physical positions are required in the next steps.

3: Reshaping data into PLINK format

map <- data.frame(Chr = rep(1:length(map.snp), unlist(lapply(map.snp, length))), Name = paste0("SNP", 
    1:length(unlist(map.snp))), locus_Mb = unlist(map.snp), locus_bp = unlist(map.snp) * 1e+06)

colnames(X) <- map$Name
FID <- "FAM001"
IID <- my_pop@id
MAT <- my_pop@mother
SEX <- 2
PHENOTYPE <- -9

for (chr in 1:nchr) {
    write.table(map[map$Chr == chr, ], file.path(path.dat, paste0("map_chr", chr, ".map")), col.names = F, 
        row.names = F, quote = F)
    write.table(cbind(FID, IID, PAT, MAT, SEX, PHENOTYPE, X[, map$Chr == chr]), file.path(path.dat, paste0("hsphase_input_chr", 
        chr, ".raw")), col.names = T, row.names = F, quote = F)
}

Part II: Main analysis

cl <- makeCluster(nclust)
registerDoParallel(cl)

Steps 1-5: Calculation of pairwise recombination rates

out <- foreach(chr = 1:nchr, .packages = "hsrecombi") %dopar% {
    
    # 1: Physical map
    map <- read.table(file.path(path.dat, paste0("map_chr", chr, ".map")), col.names = c("Chr", "Name", 
        "locus_Mb", "locus_bp"))
    map$SNP <- 1:nrow(map)
    locus_Mb <- map$locus_Mb
    
    # 2: Genotype matrix
    genomatrix <- data.table::fread(file.path(path.dat, paste0("hsphase_input_chr", chr, ".raw")))
    X <- as.matrix(genomatrix[, -c(1:6)])
    
    # 3: Assign daughters to sire IDs
    daughterSire <- genomatrix$PAT
    
    # 4: Estimate sire haplotypes and format data
    hap <- makehappm(unique(daughterSire), daughterSire, X)
    save("hap", file = file.path(path.res, paste0("hsphase_output_chr", chr, ".Rdata")))
    
    # Check order and dimension
    io <- sapply(1:nrow(map), function(z) {
        grepl(x = colnames(X)[z], pattern = map$Name[z])
    })
    if (sum(io) != nrow(map)) 
        stop("ERROR in dimension")
    
    # 5: Estimate recombination rates
    res <- hsrecombi(hap, X, map$SNP)
    final <- editraw(res, map)
    save(list = c("final", "locus_Mb"), file = file.path(path.res, paste0("Results_chr", chr, ".RData")))
    
    ifelse(nrow(final) > 0, "OK", "no result")
}
print(which(unlist(out) == "OK"))
#> [1] 1 2

In step 4, function makehap would be sufficient but makehappm allows comparison with a deterministic approach shown later. Note that sire haplotypes can also be obtained by some other (external) software. Then sire haplotypes and sire-to-daughter information need to be reshaped with makehaplist in step 4.

6: Check for candidates of misplacement

# 6a: Filter SNPs with unusually large recombination rate to neighbouring (30) SNPs
excl <- foreach(chr = 1:nchr, .packages = "hsrecombi") %dopar% {
    load(file.path(path.res, paste0("Results_chr", chr, ".RData")))
    checkCandidates(final)
}

# 6b: Heatmap plot of recombination rates for visual verification, e.g.:
chr <- 2
load(file.path(path.res, paste0("Results_chr", chr, ".RData")))
cand <- excl[[chr]][1]
win <- cand + (-100:100)
win <- win[(win >= 1) & (win <= max(final$SNP2))]

target <- final[(final$SNP1 %in% win) & (final$SNP2 %in% win), ]

ggplot(data = target, aes(SNP2, SNP1, fill = theta)) + geom_tile() + xlab("Locus 2") + ylab("Locus 1") + 
    coord_equal() + scale_y_continuous(trans = "reverse") + theme(panel.background = element_blank(), 
    panel.grid.major = element_line(colour = "grey", size = 0.1), panel.grid.minor = element_line(colour = "grey")) + 
    theme(text = element_text(size = 18)) + scale_fill_gradientn(colours = c("yellow", "red"), limits = c(0, 
    1 + 1e-10), na.value = "white")


# -> nothing conspicious
excl[[1]] <- excl[[2]] <- NA

Orange colour in the SNP neighbourhood implies an increased recombination rate. In case of a misplaced marker or a cluster of misplaced markers, the heatmap is characterised by an orange band.

7: Approximation of genetic positions

pos <- foreach(chr = 1:nchr, .packages = "hsrecombi") %dopar% {
    load(file.path(path.res, paste0("Results_chr", chr, ".RData")))
    out <- geneticPosition(final, exclude = excl[[chr]])
    list(pos.cM = c(out, rep(NA, length(locus_Mb) - length(out))), pos.Mb = locus_Mb)
}

Genetic position is reported as NA if the corresponding SNP has not been considered in the analysis (e.g., when no sire was heterozygous at the corresponding SNP).

stopCluster(cl)

8: Plot of physical versus genetic-map positions

data <- data.frame(Chr = rep(1:length(pos), times = unlist(list.select(pos, length(pos.Mb)))), Mbp = unlist(list.select(pos, 
    pos.Mb)), cM = unlist(list.select(pos, pos.cM)))
ggplot(data, aes(x = Mbp, y = cM)) + geom_point(na.rm = T) + facet_grid(Chr ~ .)

9: Plot of genetic mapping function

The genetic mapping function that fits best to the estimated data can be retrieved from a system of mapping functions (Rao et al. 1977, Human Hered); putatively misplaced markers need to be excluded. Given a parameter $p$, recombination rate $\theta \in (0,0.5)$ is mapped to Morgan units by \[ f(\theta; p) = \frac{1}{6}(p(2p - 1)(1 - 4p) log(1 - 2\theta) + 16p(p - 1)(2p - 1) \text{atan}(2\theta) + 2p(1 - p)(8p + 2)\text{atanh}(2\theta) + 6(1 - p)(1 - 2p)(1 - 4p)\theta) \]

for (chr in 1:nchr) {
    load(file.path(path.res, paste0("Results_chr", chr, ".RData")))
    final$theta[(final$SNP1 %in% excl[[chr]]) | (final$SNP2 %in% excl[[chr]])] <- NA
    final$dist_M <- (pos[[chr]]$pos.cM[final$SNP2] - pos[[chr]]$pos.cM[final$SNP1])/100
    out <- bestmapfun(theta = final$theta, dist_M = final$dist_M)
    plot(final$dist_M, final$theta, xlab = "genetic distance (Morgan)", ylab = "recombination rate", 
        col = 8)
    x <- seq(0, 0.5, 0.01)
    y <- rao(out$mixing, x)
    points(y, x, type = "l", lwd = 2)
    legend("bottomright", legend = paste0("map function (p=", round(out$mixing, 3), ")"), lwd = 2, lty = 1, 
        bty = "n")
}
#> Maximum recombination rate is set to 0.5
#> Maximum recombination rate is set to 0.5
#> Maximum recombination rate is set to 0.5
#> Maximum recombination rate is set to 0.5
#> Maximum recombination rate is set to 0.5
#> Maximum recombination rate is set to 0.5
#> Maximum recombination rate is set to 0.5
#> Maximum recombination rate is set to 0.5
#> Maximum recombination rate is set to 0.5
#> Maximum recombination rate is set to 0.5
#> Maximum recombination rate is set to 0.5
#> Maximum recombination rate is set to 0.5
#> Maximum recombination rate is set to 0.5
#> Maximum recombination rate is set to 0.5
#> Maximum recombination rate is set to 0.5
#> Maximum recombination rate is set to 0.5
#> Maximum recombination rate is set to 0.5
#> Maximum recombination rate is set to 0.5
#> Maximum recombination rate is set to 0.5
#> Maximum recombination rate is set to 0.5
#> Maximum recombination rate is set to 0.5
#> Maximum recombination rate is set to 0.5
#> Maximum recombination rate is set to 0.5
#> Maximum recombination rate is set to 0.5
#> Maximum recombination rate is set to 0.5
#> Maximum recombination rate is set to 0.5
#> Maximum recombination rate is set to 0.5
#> Maximum recombination rate is set to 0.5
#> Maximum recombination rate is set to 0.5

#> Maximum recombination rate is set to 0.5

The mixing parameter $p$ is estimated via quadratic optimisation in bestmapfun. A value of $p=0$ would match to Morgan’s mapping function, $p=0.25$ to Carter, $p=0.5$ to Kosambi and $p=1$ resembles Haldane’s mapping function.

Part III: Visual comparison with simulated data and deterministic approach

chr <- 2

1: Check with simulated data

load(file.path(path.dat, "pop.RData"))
sim.cM <- cumsum(probRec[[chr]]) * 100

The position of a recombination event is traced in SP$recHist which has a nested list structure (individual, chromosome, female/male haplotype). The recombination rate between adjacent SNPs is determined as the proportion of recombinant progeny and the positions are derived as cumulative sum over recombination rates.

2: Check with deterministic approach

load(file.path(path.res, paste0("hsphase_output_chr", chr, ".Rdata")))
hsphase.cM <- c(0, cumsum(hap$probRec)) * 100

The results are compared to the deterministic approach of Ferdosi et al. (2014) provided in the R package hsphase. Functions of that package are called in makehap and makehappm.

3: Comparative plot of physical versus genetic-map positions

par(mar = c(5.1, 4.1, 4.1, 9.1), xpd = TRUE)
plot(pos[[chr]]$pos.Mb, pos[[chr]]$pos.cM, xlab = "physical position (Mbp)", ylab = "genetic position (cM)", 
    ylim = range(c(sim.cM, hsphase.cM, pos[[chr]]$pos.cM), na.rm = T), pch = 20)
points(pos[[chr]]$pos.Mb, sim.cM, pch = 20, col = 4)
points(pos[[chr]]$pos.Mb, hsphase.cM, pch = 20, col = 8)
legend("topleft", inset = c(1.01, 0), legend = c("simulated", "likelihood-based", "deterministic"), pch = 20, 
    col = c(4, 1, 8), bty = "n")

The likelihood-based approach requires a sufficiently large sample size if SNP density is high. A verification of the bias of genetic-map positions can be found in the Supplemental Material of Qanbari & Wittenburg (2020, Genet Sel Evol).

unlink(path.dat, recursive = TRUE)
unlink(path.res, recursive = TRUE)
Sys.time()
#> [1] "2022-11-30 12:46:39 CET"