Author: Vinicius Franceschi and Fabricia F. Nascimento
Date: 2025-09-27

# Node support values using treestructure

#### Vinicius Franceschi and Fabricia F. Nascimento

#### 2025-09-27

# Introductions

This tutorial uses the public data available for Ebola here to demonstrate the use of node support values (e.g. bootstrap and posterior probability) to avoid designating population structure in badly supported clades.

We will use their time-scaled phylogenetic tree estimated with BEAST.

First, we need to load the R package we will use in this tutorial.

    library(treeio)
    library(ggtree)
    library(treestructure)

Now, we will load the time-scaled phylogenetic tree with posterior probability support values:

    #get the dated tree by first downloading it from the URL below
    tree_url <- "https://raw.githubusercontent.com/ebov/space-time/master/Data/Makona_1610_cds_ig.GLM.MCC.tree"
    tmp_file <- tempfile(fileext = ".tree")

    #Download BEAST tree to a tmp file
    download.file(tree_url, tmp_file, mode = "wb")

    #read the downloaded tree
    beast_tree <- read.beast(tmp_file)

Then we need to convert the tree to a `phylo` object and add the posterior probability information from the Bayesian tree to the `phylo` object tree.

If we use the `ape:read.nexus` to read the tree that was estimated with BEAST, we won’t get the posterior probability associated to the clades.

We now convert the dated tree to `phylo` object.

    dated_tre <- as.phylo(beast_tree)

Now we add the posterior probability from the BEAST tree to the dated_tree object.

    # Get number of tips
    n_tips <- length(dated_tre$tip.label)

    # Get BEAST tree as tibble (it will include node numbers and posterior probabilities)
    tree_data <- as_tibble(beast_tree)

    #get posterior probability
    posterior <- as.numeric(tree_data$posterior[(n_tips + 1):nrow(tree_data)])

    #add the posterior probability information to the `phylo` object
    dated_tre$node.label <- posterior

## Assign clusters without using node support

Firstly, we will assign clusters without using node support values. Note that the parameter `nodeSupportValues` is set to FALSE.

    trestruct_res_nobt <- trestruct(dated_tre,
                                    minCladeSize = 30,
                                    nodeSupportValues = FALSE,
                                    level = 0.01)

Because `treestructure` will take several minutes to run, we can load the results:

    trestruct_res_nobt <- readRDS( system.file('trestruct_res_nobt.rds',
                                               package='treestructure') )

    plot(trestruct_res_nobt, use_ggtree = T) + ggtree::geom_tippoint()

The `treestructure` analyses resulted in 23 clusters.

## Assign clusters using branch support

Now, we will assign clusters using the information on branch support. As we are analyzing a dated tree estimated with BEAST, the branch support is the posterior probability.

We designate clusters that have at least 0.95 posterior probability. This is achieved by setting to 95 the parameter nodeSupportThreshold in the trestruct function.

Note that now the parameter `nodeSupportValues` is set to TRUE, which tells the algorithm that the node support values is provided with the `phylo` object as `node.label`.

You can also provide the `nodeSupportValues` as a vector with length equal to the number of internal nodes in the tree.

    trestruct_res <- trestruct(dated_tre,
                               minCladeSize = 30,
                               nodeSupportValues = TRUE,
                               nodeSupportThreshold = 95,
                               level = 0.01)

Because it will take a minute or so to run `treestructure`, we can load the result instead.

    trestruct_res <- readRDS( system.file('trestruct_res.rds',
                                          package='treestructure') )

    plot(trestruct_res, use_ggtree = T) + ggtree::geom_tippoint()

Now we have only 6 well-supported clusters with differences in coalescent patterns.

Note that this might change if you use a higher or lower value for the nodeSupportThreshold in the trestruct function.

## Using the CH-index

The CH-index will compare different values for the parameter `levels` in the `treestructure` algorithm. We will need to specify `level` = NULL and specify the lower and upper bound to optimize the parameter `level`.

Note that we will also take into consideration the use of branch support.

    trestruct_chindex <- trestruct(dated_tre,
                                   minCladeSize = 30,
                                   nodeSupportValues = TRUE,
                                   nodeSupportThreshold = 95,
                                   level = NULL,
                                   levellb = 0.0001,
                                   levelub = 0.001)

Because it will also take a minute or so to run `treestructure`, we can load the result instead.

    trestruct_chindex <- readRDS( system.file('trestruct_chindex.rds',
                                          package='treestructure') )

    plot(trestruct_chindex, use_ggtree = T) + ggtree::geom_tippoint()

Now we have only 2 well-supported clusters with differences in coalescent patterns after optimizing the significance level.

In this example, instead of using level = 0.01 (as in the default ), the optimization of level by using the CH-index identified level as 0.00073.

### Final considerations

The function `trestruct` has a parameter named `nsim` that refers to the number of simulations for computing the null distribution of the test statistics (Volz et al. 2020).

The `nsim` parameter in the `trestruct` function is is set to the default value of 10,000, which is a good value if the level parameter is set to 0.01 (which is the default value).

If decreasing the value of the level parameter, the value of `nsim` should be increased. In the above example using CH-index, the algorithm identified a significance level of 0.00073. However, the analyses were carried out using the default `nsim` of 10,000 simulations. We have future plans to automatically detect the adequate number of simulations (`nsim` value) given the sizes of clades being compared and the threshold p-value used in the test.

As treestructure is implemented in the moment, you should run a sensitive analyses on your own dataset, if you use a significance level smaller than 0.01. For example, if you use a significance `level` of 0.001 and a `nsim` > 10,000, then you can run your analysis twice and compare the results obtained in both runs, which should be very similar. If results are very different, you should increase the value of `nsim`.

# References

Volz, EM, W Carsten, YH Grad, SDW Frost, AM Dennis, and X Didelot. 2020. “Identification of Hidden Population Structure in Time-Scaled Phylogenies.” Systematic Biology 69: 884–96. https://doi.org/10.1093/sysbio/syaa009.