# tidypopgen

This vignette gives a short introduction on how to use the `tidypopgen` package to perform basic data cleaning and a PCA on a dataset of European lobsters (Homarus gammarus). The original data are available at https://datadryad.org/dataset/doi:10.5061/dryad.2v1kr38, but for the purpose of this vignette we will use a subset of the data stored as a .bed file, available in the `inst/extdata/lobster` directory of the package. To install `tidypopgen` from r-universe, use:

    install.packages("tidypopgen",
      repos = c(
        "https://evolecolgroup.r-universe.dev",
        "https://cloud.r-project.org"
      )
    )

Next, load the `tidypopgen` package and the `ggplot2` package for plotting.

    library(tidypopgen)
    library(ggplot2)

# Creating a `gen_tibble`

In `tidypopgen`, we represent data as a `gen_tibble`, a subclass of `tibble` containing the columns `id` and `genotypes` for each individual. Genotypes are stored in a compressed format as a File-Backed Matrix that can be easily accessed by functions in `tidypopgen`. The `genotypes` column contains a vector of the row indices of each individual in the File-Backed Matrix, and when printed, the `genotypes` column shows the first two genotypes for each individual. For a visual representation of the `gen_tibble` object structure and a full discussion of how to manipulate `gen_tibble` objects, see vignette ‘The grammar of population genetics’.

Additionally, if data are loaded from a .bed file with information in the `FID` column, this is treated as population information and is automatically added to the `gen_tibble` as column `population`. As with a normal tibble, this information can be changed, updated, or removed from the `gen_tibble` if needed. `tidypopgen` can also read data form `packedancestry` files and `vcf`.

Let’s start by creating a `gen_tibble` from the `lobster.bed` file.

    lobsters <- gen_tibble(
      x = system.file("extdata/lobster/lobster.bed", package = "tidypopgen"),
      quiet = TRUE, backingfile = tempfile()
    )
    head(lobsters)

    ## # A gen_tibble: 79 loci
    ## # A tibble:     6 × 3
    ##   id    population  genotypes
    ##   <chr> <chr>      <vctr_SNP>
    ## 1 Ale04 Ale         [0,.,...]
    ## 2 Ale05 Ale         [1,0,...]
    ## 3 Ale06 Ale         [.,0,...]
    ## 4 Ale08 Ale         [.,2,...]
    ## 5 Ale13 Ale         [0,.,...]
    ## 6 Ale15 Ale         [1,0,...]

We can see the structure of the `gen_tibble` above, with the expected columns. In this case, our .bed file contains population information corresponding to the sampling site of each lobster in the dataset.

If we want to take a look at the genotypes of our lobsters, we can use the `show_genotypes` function to return a matrix of genotypes, where rows are individuals and columns are loci. This is a big table, so we will just look at the first ten loci for the first 5 individuals:

    lobsters %>% show_genotypes(indiv_indices = 1:5, loci_indices = 1:10)

    ##      [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
    ## [1,]    0   NA   NA   NA    2   NA   NA   NA    0    NA
    ## [2,]    1    0    1    2    1    1    0    1    1     1
    ## [3,]   NA    0    0   NA    2    2   NA   NA    0     2
    ## [4,]   NA    2    0    2   NA   NA   NA   NA    1    NA
    ## [5,]    0   NA    0   NA    2   NA    0   NA   NA    NA

And, similarly, if we want to see information about the loci we can use the `show_loci` function, which returns a tibble with information about each locus. Again this is a big table, so we will use `head()` to only look at the first few:

    head(lobsters %>% show_loci())

    ## # A tibble: 6 × 7
    ##   big_index name   chromosome position genetic_dist allele_ref allele_alt
    ##       <int> <chr>  <fct>         <int>        <dbl> <chr>      <chr>
    ## 1         1 rs3441 1                 1            1 G          A
    ## 2         2 rs4173 2                 2            2 C          T
    ## 3         3 rs6157 3                 3            3 G          C
    ## 4         4 rs7502 4                 4            4 C          T
    ## 5         5 rs7892 5                 5            5 A          T
    ## 6         6 rs9441 7                 7            7 A          G

Now we have a `gen_tibble` to work with, we can start to clean the data.

# Quality control

Let’s start by checking the quality of the data for each individual lobster in our dataset. We can use the `qc_report_indiv` function to generate a report which contains information about missingness and heterozygosity for each individual.

    indiv_qc_lobsters <- lobsters %>% qc_report_indiv()

We can take a look at this data using the `autoplot` function:

    autoplot(indiv_qc_lobsters, type = "scatter")

We can see that most individuals have low missingness and heterozygosity, but there are a few individuals missing >20% of their genotypes. We can remove these individuals using the `filter` function, specifying to keep only individuals with missingness under 20%.

    lobsters <- lobsters %>% filter(indiv_missingness(genotypes) < 0.2)

Now lets check our loci. We can use the `qc_report_loci` function to generate a report of the loci quality. This function will return another `qc_report` object, which contains information about missingness, minor allele frequency, and Hardy-Weinberg Equilibrium for each locus.

    loci_qc_lobsters <- lobsters %>% qc_report_loci()

    ## This gen_tibble is not grouped. For Hardy-Weinberg equilibrium, `qc_report_loci()` will assume individuals are part of the same population and HWE test p-values will be calculated across all individuals. If you wish to calculate HWE p-values within populations or groups, please use`group_by()` before calling `qc_report_loci()`.

Here, we get a message because the `qc_report_loci` function calculates Hardy-Weinberg equilibrium assuming that all individuals are part of a single population. As our dataset contains multiple lobster populations, we should group our data by population first:

    lobsters <- lobsters %>% group_by(population)
    loci_qc_lobsters <- lobsters %>% qc_report_loci()

That’s better. Now, lets take a look at minor allele frequency for all loci:

    autoplot(loci_qc_lobsters, type = "maf")

And we can see that we don’t have any monomorphic SNPs.

Now let’s look at missingness.

    autoplot(loci_qc_lobsters, type = "missing")

Our data mostly have low missingness, but we can see that some loci have > 5% missingness across individuals, and we want to remove these individuals.

In tidypopgen, there are two functions to subset the loci in a `gen_tibble` object: `select_loci` and `select_loci_if`. The function `select_loci` operates on information about the loci (e.g filtering by chromosome or by rsID), while `select_loci_if` operates on the genotypes at those loci (e.g filtering by minor allele frequency or missingness). In this case, we want to remove loci with >5% missingness, so we can use `select_loci_if` with the `loci_missingness` function, operating on the `genotypes` column of our `gen_tibble`.

    lobsters <- lobsters %>% select_loci_if(loci_missingness(genotypes) < 0.05)

Through our filtering, we have removed a few individuals and loci. We should now update the file backing matrix to reflect these changes, using the function `gt_update_backingfile`:

    lobsters <- gt_update_backingfile(lobsters, backingfile = tempfile())

    ##
    ## gen_backing files updated, now

    ## using FBM RDS: /tmp/Rtmpyi50Ar/file17f5362d612d1.rds

    ## with FBM backing file: /tmp/Rtmpyi50Ar/file17f5362d612d1.bk

    ## make sure that you do NOT delete those files!

    ## to reload the gen_tibble in another session, use:

    ## gt_load('/tmp/Rtmpyi50Ar/file17f5362d612d1.gt')

Now our data are clean and the backingfile is updated, we are ready to create a PCA.

# Impute

First, we need to impute any remaining missing data using the `gt_impute_simple` function.

    lobsters <- gt_impute_simple(lobsters, method = "random")

# PCA

Then we can run a PCA. There are a number of PCA algorithms, here, we will use the `gt_pca_partialSVD` function:

    partial_pca <- gt_pca_partialSVD(lobsters)

And we can create a simple plot using `autoplot`:

    autoplot(partial_pca, type = "scores")

That was easy! The `autoplot` gives us a quick idea of the explained variance and rough distribution of the samples, but we need to see the different populations within our dataset.

For a quick overview, we could add an aesthetic to our plot:

    autoplot(partial_pca, type = "scores") +
      aes(color = lobsters$population) +
      labs(color = "population")

However, if we want to fully customise our plot, we can wrangle the data directly and use `ggplot2`.

# Plot with ggplot2

For a customised plot, we can extract the information on the scores of each individual using the `augment` method for `gt_pca`.

    pcs <- augment(x = partial_pca, data = lobsters)

Then we can extract the eigenvalues for each principal component with the `tidy` function, using the “eigenvalues” argument:

    eigenvalues <- tidy(partial_pca, "eigenvalues")

    xlab <- paste("Axis 1 (", round(eigenvalues[1, 3], 1), " %)",
      sep = ""
    )
    ylab <- paste("Axis 2 (", round(eigenvalues[2, 3], 1), " %)",
      sep = ""
    )

And finally plot:

    ggplot(data = pcs, aes(x = .fittedPC1, y = .fittedPC2)) +
      geom_hline(yintercept = 0) +
      geom_vline(xintercept = 0) +
      geom_point(aes(fill = population),
        shape = 21, size = 3, show.legend = FALSE
      ) +
      scale_fill_distruct() +
      labs(x = xlab, y = ylab) +
      ggtitle("Lobster PCA")

Or we could create a labelled version of our PCA by determining the centre of each group to place the labels

    # Calculate centre for each population
    centroid <- aggregate(cbind(.fittedPC1, .fittedPC2, .fittedPC3) ~ population,
      data = pcs, FUN = mean
    )

    # Add these coordinates to our augmented pca object
    pcs <- left_join(pcs, centroid, by = "population", suffix = c("", ".cen"))

And then add labels to the plot:

    ggplot(data = pcs, aes(x = .fittedPC1, y = .fittedPC2)) +
      geom_hline(yintercept = 0) +
      geom_vline(xintercept = 0) +
      geom_segment(aes(xend = .fittedPC1.cen, yend = .fittedPC2.cen),
        show.legend = FALSE
      ) +
      geom_point(aes(fill = population),
        shape = 21, size = 3, show.legend = FALSE
      ) +
      geom_label(
        data = centroid,
        aes(label = population, fill = population),
        size = 4, show.legend = FALSE
      ) +
      scale_fill_distruct() +
      labs(x = xlab, y = ylab) +
      ggtitle("Lobster PCA")

That’s it! There are more functions to run different types of analyses (DAPC, ADMIXTURE, f statistics with admixtools2, etc.); each resulting object has `autoplot`, `tidy`, and `augment` to explore the results and integrated into the information from the `gen_tibble`.