flexdog {updog}

Flexible genotyping for polyploids from next-generation sequencing data.

Description

Genotype polyploid individuals from next generation sequencing (NGS) data while assuming the genotype distribution is one of several forms. flexdog does this while accounting for allele bias, overdispersion, sequencing error. The method is described in detail in Gerard et. al. (2018) and Gerard and Ferrão (2020). See multidog() for running flexdog on multiple SNPs in parallel.

Usage

flexdog(
  refvec,
  sizevec,
  ploidy,
  model = c("norm", "hw", "bb", "s1", "s1pp", "f1", "f1pp", "flex", "uniform", "custom"),
  p1ref = NULL,
  p1size = NULL,
  p2ref = NULL,
  p2size = NULL,
  snpname = NULL,
  bias_init = exp(c(-1, -0.5, 0, 0.5, 1)),
  verbose = TRUE,
  prior_vec = NULL,
  ...
)

Arguments

refvec

A vector of counts of reads of the reference allele.

sizevec

A vector of total counts.

ploidy

The ploidy of the species. Assumed to be the same for each individual.

model

What form should the prior (genotype distribution) take? See Details for possible values.

p1ref

The reference counts for the first parent if model = "f1" or model = "f1pp", or for the only parent if model = "s1" or model = "s1pp".

p1size

The total counts for the first parent if model = "f1" or model = "f1pp", or for the only parent if model = "s1" or model = "s1pp".

p2ref

The reference counts for the second parent if model = "f1" or model = "f1pp".

p2size

The total counts for the second parent if model = "f1" or model = "f1pp".

snpname

A string. The name of the SNP under consideration. This is just returned in the input list for your reference.

bias_init

A vector of initial values for the bias parameter over the multiple runs of flexdog_full().

verbose

Should we output more (TRUE) or less (FALSE)?

prior_vec

The pre-specified genotype distribution. Only used if model = "custom" and must otherwise be NULL. If specified, then it should be a vector of length ploidy + 1 with non-negative elements that sum to 1.

...

Additional parameters to pass to flexdog_full().

Details

Possible values of the genotype distribution (values of model) are:

"norm"

A distribution whose genotype frequencies are proportional to the density value of a normal with some mean and some standard deviation. Unlike the "bb" and "hw" options, this will allow for distributions both more and less dispersed than a binomial. This seems to be the most robust to violations in modeling assumptions, and so is the default. This prior class was developed in Gerard and Ferrão (2020).

"hw"

A binomial distribution that results from assuming that the population is in Hardy-Weinberg equilibrium (HWE). This actually does pretty well even when there are minor to moderate deviations from HWE. Though it does not perform as well as the '"norm"' option when there are severe deviations from HWE.

"bb"

A beta-binomial distribution. This is an overdispersed version of "hw" and can be derived from a special case of the Balding-Nichols model.

"s1"

This prior assumes the individuals are all full-siblings resulting from one generation of selfing. I.e. there is only one parent. This model assumes a particular type of meiotic behavior: polysomic inheritance with bivalent, non-preferential pairing.

"f1"

This prior assumes the individuals are all full-siblings resulting from one generation of a bi-parental cross. This model assumes a particular type of meiotic behavior: polysomic inheritance with bivalent, non-preferential pairing.

"f1pp"

This prior allows for double reduction and preferential pairing in an F1 population of tretraploids.

"s1pp"

This prior allows for double reduction and preferential pairing in an S1 population of tretraploids.

"flex"

Generically any categorical distribution. Theoretically, this works well if you have a lot of individuals. In practice, it seems to be much less robust to violations in modeling assumptions.

"uniform"

A discrete uniform distribution. This should never be used in practice.

"custom"

A pre-specified prior distribution. You specify it using the prior_vec argument. You should almost never use this option in practice.

You might think a good default is model = "uniform" because it is somehow an "uninformative prior." But it is very informative and tends to work horribly in practice. The intuition is that it will estimate the allele bias and sequencing error rates so that the estimated genotypes are approximately uniform (since we are assuming that they are approximately uniform). This will usually result in unintuitive genotyping since most populations don't have a uniform genotype distribution. I include it as an option only for completeness. Please don't use it.

The value of prop_mis is a very intuitive measure for the quality of the SNP. prop_mis is the posterior proportion of individuals mis-genotyped. So if you want only SNPS that accurately genotype, say, 95% of the individuals, you could discard all SNPs with a prop_mis over 0.05.

The value of maxpostprob is a very intuitive measure for the quality of the genotype estimate of an individual. This is the posterior probability of correctly genotyping the individual when using geno (the posterior mode) as the genotype estimate. So if you want to correctly genotype, say, 95% of individuals, you could discard all individuals with a maxpostprob of under 0.95. However, if you are just going to impute missing genotypes later, you might consider not discarding any individuals as flexdog's genotype estimates will probably be more accurate than other more naive approaches, such as imputing using the grand mean.

In most datasets I've examined, allelic bias is a major issue. However, you may fit the model assuming no allelic bias by setting update_bias = FALSE and bias_init = 1.

Prior to using flexdog, during the read-mapping step, you could try to get rid of allelic bias by using WASP (doi:10.1101/011221). If you are successful in removing the allelic bias (because its only source was the read-mapping step), then setting update_bias = FALSE and bias_init = 1 would be reasonable. You can visually inspect SNPs for bias by using plot_geno().

flexdog(), like most methods, is invariant to which allele you label as the "reference" and which you label as the "alternative". That is, if you set refvec with the number of alternative read-counts, then the resulting genotype estimates will be the estimated allele dosage of the alternative allele.

Value

An object of class flexdog, which consists of a list with some or all of the following elements:

bias

The estimated bias parameter.

seq

The estimated sequencing error rate.

od

The estimated overdispersion parameter.

num_iter

The number of EM iterations ran. You should be wary if this equals itermax.

llike

The maximum marginal log-likelihood.

postmat

A matrix of posterior probabilities of each genotype for each individual. The rows index the individuals and the columns index the allele dosage.

genologlike

A matrix of genotype log-likelihoods of each genotype for each individual. The rows index the individuals and the columns index the allele dosage.

gene_dist

The estimated genotype distribution. The ith element is the proportion of individuals with genotype i-1.

par

A list of the final estimates of the parameters of the genotype distribution. The elements included in par depends on the value of model:

model = "norm":

mu:

The normal mean.

sigma:

The normal standard deviation (not variance).

model = "hw":

alpha:

The major allele frequency.

model = "bb":

alpha:

The major allele frequency.

tau:

The overdispersion parameter. See the description of rho in the Details of betabinom().

model = "s1":

pgeno:

The allele dosage of the parent.

alpha:

The mixture proportion of the discrete uniform (included and fixed at a small value mostly for numerical stability reasons). See the description of fs1_alpha in flexdog_full().

model = "f1":

p1geno:

The allele dosage of the first parent.

p2geno:

The allele dosage of the second parent.

alpha:

The mixture proportion of the discrete uniform (included and fixed at a small value mostly for numerical stability reasons). See the description of fs1_alpha in flexdog_full().

model = "s1pp":

ell1:

The estimated dosage of the parent.

tau1:

The estimated double reduction parameter of the parent. Available if ell1 is 1, 2, or 3. Identified if ell1 is 1 or 3.

gamma1:

The estimated preferential pairing parameter. Available if ell1 is 2. However, it is not returned in an identified form.

alpha:

The mixture proportion of the discrete uniform (included and fixed at a small value mostly for numerical stability reasons). See the description of fs1_alpha in flexdog_full().

model = "f1pp":

ell1:

The estimated dosage of parent 1.

ell2:

The estimated dosage of parent 2.

tau1:

The estimated double reduction parameter of parent 1. Available if ell1 is 1, 2, or 3. Identified if ell1 is 1 or 3.

tau2:

The estimated double reduction parameter of parent 2. Available if ell2 is 1, 2, or 3. Identified if ell2 is 1 or 3.

gamma1:

The estimated preferential pairing parameter of parent 1. Available if ell1 is 2. However, it is not returned in an identified form.

gamma2:

The estimated preferential pairing parameter of parent 2. Available if ell2 is 2. However, it is not returned in an identified form.

alpha:

The mixture proportion of the discrete uniform (included and fixed at a small value mostly for numerical stability reasons). See the description of fs1_alpha in flexdog_full().

model = "flex":

par is an empty list.

model = "uniform":

par is an empty list.

model = "custom":

par is an empty list.

geno

The posterior mode genotype. These are your genotype estimates.

maxpostprob

The maximum posterior probability. This is equivalent to the posterior probability of correctly genotyping each individual.

postmean

The posterior mean genotype. In downstream association studies, you might want to consider using these estimates.

input$refvec

The value of refvec provided by the user.

input$sizevec

The value of sizevec provided by the user.

input$ploidy

The value of ploidy provided by the user.

input$model

The value of model provided by the user.

input$p1ref

The value of p1ref provided by the user.

input$p1size

The value of p1size provided by the user.

input$p2ref

The value of p2ref provided by the user.

input$p2size

The value of p2size provided by the user.

input$snpname

The value of snpname provided by the user.

prop_mis

The posterior proportion of individuals genotyped incorrectly.

Author(s)

David Gerard

References

• Gerard, D., Ferrão, L. F. V., Garcia, A. A. F., & Stephens, M. (2018). Genotyping Polyploids from Messy Sequencing Data. Genetics, 210(3), 789-807. doi:10.1534/genetics.118.301468.

• Gerard, David, and Luís Felipe Ventorim Ferrão. "Priors for genotyping polyploids." Bioinformatics 36, no. 6 (2020): 1795-1800. doi:10.1093/bioinformatics/btz852.

See Also

Run browseVignettes(package = "updog") in R for example usage. Other useful functions include:

multidog()

For running flexdog() on multiple SNPs in parallel.

flexdog_full()

For additional parameter options when running flexdog().

rgeno()

For simulating genotypes under the allowable prior models in flexdog().

rflexdog()

For simulating read-counts under the assumed likelihood model in flexdog().

plot.flexdog()

For plotting the output of flexdog().

oracle_mis()

For calculating the oracle genotyping error rates. This is useful for read-depth calculations before collecting data. After you have data, using the value of prop_mis is better.

oracle_cor()

For calculating the correlation between the true genotypes and an oracle estimator (useful for read-depth calculations before collecting data).

Examples

## An S1 population where the first individual
## is the parent.
data("snpdat")
ploidy  <- 6
refvec  <- snpdat$counts[snpdat$snp == "SNP2"]
sizevec <- snpdat$size[snpdat$snp == "SNP2"]
fout    <- flexdog(refvec   = refvec[-1],
                   sizevec  = sizevec[-1],
                   ploidy   = ploidy,
                   model    = "s1",
                   p1ref    = refvec[1],
                   p1size   = sizevec[1])
plot(fout)



## A natural population. We will assume a
## normal prior since there are so few
## individuals.
data("uitdewilligen")
ploidy  <- 4
refvec  <- uitdewilligen$refmat[, 1]
sizevec <- uitdewilligen$sizemat[, 1]
fout    <- flexdog(refvec  = refvec,
                   sizevec = sizevec,
                   ploidy  = ploidy,
                   model   = "norm")
plot(fout)