Phylodynamics
TreeTime Basic Usage
from treetime import TreeTime
from Bio import Phylo
# Load tree and alignment
tree = Phylo.read('tree.nwk', 'newick')
# Create TreeTime object with dates
# dates_file: tab-separated with columns 'name' and 'date'
# Date formats: 2020.5, 2020-06-15, numeric (decimal year)
tt = TreeTime(
tree=tree,
aln='alignment.fasta',
dates='dates.tsv',
gtr='JC69' # Nucleotide model: JC69, HKY85, GTR
)
# Run molecular clock analysis
tt.run(
root='best', # Root optimization: 'best', 'least-squares', or clade name
Tc='skyline', # Coalescent prior: None, 'skyline', 'opt', or numeric
time_marginal='assign_ml' # Date estimation method
)
# Access results
print(f'Root date: {tt.tree.root.numdate:.2f}')
print(f'Clock rate: {tt.clock_rate:.2e} subs/site/year')
TreeTime CLI
# Install treetime
pip install phylo-treetime
# Basic time tree
treetime --tree tree.nwk --aln alignment.fasta --dates dates.tsv --outdir results/
# With coalescent prior (for population dynamics)
treetime --tree tree.nwk --aln alignment.fasta --dates dates.tsv \
--coalescent skyline --outdir results/
# Ancestral sequence reconstruction
treetime ancestral --tree tree.nwk --aln alignment.fasta --outdir results/
# Mugration (discrete trait analysis, e.g., geographic spread)
treetime mugration --tree tree.nwk --states locations.tsv \
--attribute location --outdir results/
Date File Format
def prepare_date_file(metadata, name_col, date_col, output_path):
'''Prepare dates file for TreeTime
Date formats accepted:
- Decimal year: 2020.5 (July 2020)
- ISO format: 2020-06-15
- Year only: 2020 (treated as midpoint)
For incomplete dates, use ranges:
- [2020.0:2020.5] for first half of 2020
'''
dates = metadata[[name_col, date_col]].copy()
dates.columns = ['name', 'date']
# Convert to decimal year if needed
dates['date'] = dates['date'].apply(convert_to_decimal_year)
dates.to_csv(output_path, sep='\t', index=False)
return output_path
def convert_to_decimal_year(date_str):
'''Convert date string to decimal year'''
from datetime import datetime
if isinstance(date_str, (int, float)):
return float(date_str)
try:
dt = datetime.strptime(str(date_str), '%Y-%m-%d')
year = dt.year
day_of_year = dt.timetuple().tm_yday
days_in_year = 366 if (year % 4 == 0) else 365
return year + (day_of_year - 1) / days_in_year
except:
return float(date_str) # Assume already decimal
Interpret Clock Results
def interpret_clock_rate(rate, genome_length):
'''Interpret molecular clock rate
Args:
rate: Substitutions per site per year
genome_length: Genome size in bp
Returns:
Estimated substitutions per year (genome-wide)
Typical rates:
- RNA viruses: 10^-3 to 10^-4 subs/site/year
- SARS-CoV-2: ~8×10^-4 (24 subs/year for 30kb genome)
- Bacteria: 10^-6 to 10^-7 subs/site/year
- E. coli: ~10^-7 (5 SNPs/genome/year)
'''
subs_per_year = rate * genome_length
print(f'Clock rate: {rate:.2e} subs/site/year')
print(f'Genome-wide: ~{subs_per_year:.1f} substitutions/year')
# Estimate time to MRCA from SNP distance
example_snps = 10
time_estimate = example_snps / (2 * subs_per_year)
print(f'{example_snps} SNPs = ~{time_estimate:.1f} years since MRCA')
Skyline Plot (Population Dynamics)
def extract_skyline(treetime_results_dir):
'''Extract coalescent skyline from TreeTime output
Skyline shows effective population size over time.
Useful for:
- Detecting population expansions (outbreak growth)
- Identifying bottlenecks
- Estimating R0 from growth rate
'''
import json
with open(f'{treetime_results_dir}/skyline.json') as f:
skyline = json.load(f)
times = skyline['times']
Ne = skyline['Ne'] # Effective population size
return times, Ne
BEAST2 Integration
def prepare_beast_xml(alignment, dates, template='strict_clock'):
'''Prepare BEAST2 XML configuration
BEAST2 templates:
- strict_clock: Single clock rate (simple outbreaks)
- relaxed_clock: Rate variation (diverse sampling)
- bdsky: Birth-death skyline (epidemic dynamics)
Note: BEAST2 requires more setup than TreeTime but provides
full Bayesian uncertainty estimates.
'''
# BEAST2 typically configured via BEAUti GUI
# For automation, use templates from beast2.org
print('BEAST2 setup:')
print('1. Install BEAST2: https://beast2.org')
print('2. Run BEAUti to configure analysis')
print('3. Execute: beast -threads 4 analysis.xml')
print('4. Summarize: treeannotator -burnin 10 trees.trees output.tree')
Related Skills
- epidemiological-genomics/transmission-inference - Infer who-infected-whom
- phylogenetics/modern-tree-inference - Build input phylogeny
- epidemiological-genomics/variant-surveillance - Track lineage dynamics
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