Phylogenetics: HyPhy Lab
|EEB 349: Phylogenetics|
The goal of this lab exercise is to show you how to use the HyPhy program for data exploration and hypothesis testing within a maximum likelihood framework. Although much can be done with PAUP* and IQ-TREE, HyPhy lets you to do some interesting and useful things that these programs cannot, such as allowing the model of evolution to change across a tree.
Login to Xanadu
Login to Xanadu and request a machine as usual:
srun --pty -p mcbstudent --qos=mcbstudent bash
Loading modules needed
Load the module needed for this exercise:
module load hyphy/2.3.11 module load paup/4.0a-166
The goal of this lab
In this lab, we will recreate the very interesting parametric bootstrapping analysis performed in this paper:
RA Vandenbussche, RJ Baker, JP Huelsenbeck, and DM Hillis. 1998. Base compositional bias and phylogenetic analyses: a test of the "Flying DNA" hypothesis. Molecular Phylogenetics and Evolution 13(3): 408-416. [DOI:10.1006/mpev.1998.0531 https://doi.org/10.1006/mpev.1998.0531]
In short, this paper demonstrated that the "Flying DNA" hypothesis proposed earlier by Pettigrew (1994. Flying DNA. Curr. Biol. 4: 277–280) was not viable. The Flying DNA hypothesis proposed that microbats and megabats are actually unrelated, but appear as a monophyletic group in phylogenetic trees due to the fact that both have high AT bias in the genes used to reconstruct phylogeny. The idea is that this strong nucleotide composition bias makes convergence much more probable, as there are effectively only two states (A and T) rather than four (A, C, G, T), and parsimony is mistaking such convergence as historical relatedness.
The Vandenbussche et al. paper simulated data under the null hypothesis (micro- and mega-bats are unrelated) but added various amounts of AT bias when simulating the bat lineages. If Pettigrew was correct, trees reconstructed from such data should show bats monophyletic, even though they were not together in the true tree used for the simulation.
This type of simulation required ad hoc software in 1998 because most software that can carry out simulations on phylogenetic trees assumes that the model is the same across the tree. Fortunately, these days we have HyPhy, which offers a way to simulate (and analyze) under pretty much any model you can imagine.
Vandenbussche et al. used the K80 (kappa=4) model across most of the tree, but the lineage leading to Pteropus (the lone megabat in the analysis) and the lineages within the microbat clade (Tonatia bidens, Tonatia silvicola, and their stem lineage) used HKY85 with kappa=4 but nucleotide frequencies that are AT-biased (e.g. piA=0.4, piC=0.1, piG=0.1, piT=0.4). The question is: how much AT-bias does one need to put into the simulation in order to see the convergence that Pettigrew claimed was happening. Our goal will be to recreate the parsimony part of figure 4 from the Vandenbussche paper. We will use HyPhy to simulate the data, and PAUP* to do the parsimony analyses.
Creating the HyPhy batch file
HyPhy is its own scripting language. Like Python, a script written in HyPhy's batch language (HBL) can be run from the command line to carry out phylogenetic analyses. Start by creating a new file named bats.bf in a directory called bats (you can of course use any name you like, but these are the names I will be using). Also create a directory simdata inside your bats directory. So this is what your directory structure should look like at this point:
bats/ simdata/ bats.bf