slendr (version 1.1.0)

ts_tracts: Extract ancestry tracts from a tree sequence (EXPERIMENTAL)

Description

Extract a data frame with coordinates of ancestry tracts from a given tree sequence.

Usage

ts_tracts(
  ts,
  census,
  squashed = TRUE,
  source = NULL,
  target = NULL,
  quiet = FALSE
)

Value

A data frame containing coordinates of ancestry tracts

Arguments

ts

Tree sequence object of the class slendr_ts

census

Census time. See the documentation linked in the Details for more information. If a slendr-specific tree sequence was provided as ts, the census time is expected to be given in slendr model-specific time units, and must correspond to some gene-flow event encoded by the model.

squashed

Should ancestry tracts be squashed (i.e., should continuous tracts that can be traced to different ancestral nodes be merged)? Default is TRUE. If FALSE, these effectively continuous ancestry tracts will be split into individual segments, each assigned to a specific ancestral node ID (recorded in a column ancestor_id).

source

From which source population to extract tracts for? if NULL (the default), ancestry tracts for all populations contributing gene flow at the census time will be reported. Otherwise, ancestry tracts from only specified source populations will be extracted. Note that this option is ignored for non-slendr tree sequences!

target

Similar purpose as source above, except that it filters for tracts discovered in the target population(s)

quiet

Should the default summary output of the tspop Python package be silenced? Default is FALSE.

Details

This function implements an R-friendly interface to an algorithm for extracting ancestry tracts provided by the Python module tspop https://tspop.readthedocs.io/en/latest/ and developed by Georgia Tsambos. Please make sure to cite the paper which describes the algorithm in detail: tools:::Rd_expr_doi("10.1093/bioadv/vbad163"). For more technical details, see also the tutorial at: https://tspop.readthedocs.io/en/latest/basicusage.html.

In general, when using this function on a slendr-generated tree sequence, please be aware that the output changes slightly to what you would get by running the pure tspop.get_pop_ancestry() in Python. First, ts_tracts() populates the output data frame with additional metadata (such as names of individuals or populations). Additionally, for slendr models, it is specifically designed to only return ancestry tracts originating to a an ancestral population which contributed its ancestry during a gene-flow event which started at a specific time (i.e., scheduled in a model via the gene_flow()) function. It does not return every single ancestry tracts present in the tree sequence for every single sample node (and every single potential ancestry population) as does the tspop.get_pop_ancestry() Python method.

That said, when run on a tree sequence which does not originate from a slendr simulation, the behavior of ts_tracts() is identical to that of the underlying tspop.get_pop_ancestry().

As of the current version of slendr, ts_tracts() only works for slendr/msprime sequences but not on slendr/SLiM tree sequences. Support for slendr-generated SLiM tree sequences is in development. Tracts from tree sequences originating from non-slendr msprime and SLiM simulations are not restricted in any way and, as mentioned in the previous paragraph, ts_tracts() in this situation effectively reduces to the standard tspop.get_pop_ancestry() call.

Examples

check_dependencies(python = TRUE, quit = TRUE) # dependencies must be present

init_env(quiet = TRUE)

# load an example model with an already simulated tree sequence
slendr_ts <- system.file("extdata/models/introgression_msprime.trees", package = "slendr")
model <- read_model(path = system.file("extdata/models/introgression", package = "slendr"))

# load the tree-sequence object from disk
ts <- ts_read(file = slendr_ts, model = model)

# extract Neanderthal ancestry tracts (i.e. those corresponding to the
# census event at the gene-flow time at 55000 kya as scheduled by
# the simulation which produced the tree sequence)
nea_tracts <- ts_tracts(ts, census = 55000, source = "NEA")
nea_tracts