Bio Comparative Genomics Ancestral Reconstruction

Version Compatibility

Reference examples tested with: BioPython 1.83+, IQ-TREE 2.2+, PAML 4.10+

Before using code patterns, verify installed versions match. If versions differ:

• Python: pip show <package> then help(module.function) to check signatures

If code throws ImportError, AttributeError, or TypeError, introspect the installed package and adapt the example to match the actual API rather than retrying.

Ancestral Sequence Reconstruction

"Infer ancestral protein sequences at phylogenetic nodes" → Reconstruct ancient sequences using marginal or joint likelihood methods on a phylogeny for ancestral protein resurrection or evolutionary trajectory analysis.

• Python: PAML codeml with RateAncestor = 1 for ancestral reconstruction
• CLI: iqtree2 -s aln -m model --ancestral for marginal reconstruction

When ASR is Reliable vs Unreliable

ASR accuracy depends on several factors -- understanding these limits is critical before interpreting results:

Key limitation: ASR reconstructs the most likely ancestral sequence given the model, but the reconstructed protein may not be functional due to epistatic interactions not captured by site-independent models. Experimentally resurrected proteins should always be tested for activity.

Joint vs Marginal Reconstruction

• Marginal: Integrates over uncertainty at all other nodes; provides posterior probabilities per site. Better for identifying ambiguous positions and designing alternative constructs. Default in IQ-TREE.
• Joint: Finds the single most likely set of ancestral states across all nodes simultaneously. No per-site probabilities. Faster but less informative.
• Use marginal reconstruction for protein resurrection studies (need per-site confidence).

Alignment Recommendations

• Use PRANK for coding sequences -- correctly models insertions (other aligners overestimate deletions)
• Gaps in alignments represent indels; indel reconstruction is a separate problem from substitution reconstruction
• PAML treats gaps as missing data or removes gapped columns (cleandata=0 keeps them as ambiguity; cleandata=1 removes all columns with any gap)
• IQ-TREE handles gaps better than PAML for large datasets with extensive indel variation

PAML Ancestral Reconstruction

Goal: Reconstruct ancestral sequences at internal phylogenetic nodes using maximum likelihood.

Approach: Create a codeml/baseml control file with RateAncestor=1, run PAML, parse the RST file for ancestral sequences and site-wise posterior probabilities.

'''Ancestral sequence reconstruction with PAML codeml/baseml'''

import subprocess
import re
from Bio import SeqIO
from Bio.Seq import Seq


def create_asr_control(alignment, tree, output_dir, seq_type='protein'):
    '''Create control file for ancestral reconstruction

    RateAncestor = 1: Enable ancestral reconstruction
    Generates RST file with ancestral sequences

    IMPORTANT: Tree MUST be rooted -- ancestral states at internal nodes
    depend on the root position. Use outgroup rooting or midpoint rooting.

    For codons: Use codeml with seqtype = 1
    For amino acids: Use codeml with seqtype = 2
    For nucleotides: Use baseml

    Model selection:
    - Proteins: LG or WAG with +G (rate variation); use ModelFinder for data-driven choice
    - Codons: M0 (single omega) for ASR; site models are for selection testing, not ASR
    - Use the same model for tree inference and ASR for consistency
    '''
    if seq_type == 'protein':
        ctl = f'''
      seqfile = {alignment}
     treefile = {tree}
      outfile = {output_dir}/asr.mlc

      seqtype = 2
        model = 3
    aaRatefile = wag.dat

 RateAncestor = 1
    cleandata = 0
        '''
    else:  # codon
        ctl = f'''
      seqfile = {alignment}
     treefile = {tree}
      outfile = {output_dir}/asr.mlc

      seqtype = 1
    CodonFreq = 2
        model = 0
      NSsites = 0

 RateAncestor = 1
    cleandata = 0
        '''

    ctl_file = f'{output_dir}/asr.ctl'
    with open(ctl_file, 'w') as f:
        f.write(ctl)

    return ctl_file


def parse_rst_file(rst_file):
    '''Parse PAML RST file for ancestral sequences

    RST contains:
    - Tree with node numbers
    - Ancestral sequences at each node
    - Posterior probabilities for each site

    Node numbering: Extant sequences first, then internal nodes
    '''
    ancestors = {}
    current_node = None
    current_seq = []

    with open(rst_file) as f:
        content = f.read()

    # Find ancestral sequence section
    if 'Ancestral reconstruction by' in content:
        sections = content.split('Ancestral reconstruction by')
        for section in sections[1:]:
            lines = section.strip().split('\n')
            for line in lines:
                if line.startswith('node #'):
                    if current_node and current_seq:
                        ancestors[current_node] = ''.join(current_seq)
                    match = re.search(r'node #(\d+)', line)
                    if match:
                        current_node = f'Node_{match.group(1)}'
                        current_seq = []
                elif current_node and line.strip() and not line.startswith(' '):
                    # Sequence line
                    seq_part = ''.join(line.split()[1:]) if len(line.split()) > 1 else ''
                    current_seq.append(seq_part)

    if current_node and current_seq:
        ancestors[current_node] = ''.join(current_seq)

    return ancestors


def extract_marginal_probabilities(rst_file):
    '''Extract site-wise posterior probabilities

    High confidence: P > 0.95 (commonly used threshold)
    Moderate confidence: P > 0.80
    Low confidence: P < 0.80 (consider alternatives)

    Report ambiguous sites for experimental validation
    '''
    site_probs = []

    with open(rst_file) as f:
        in_probs = False
        for line in f:
            if 'Prob of best state' in line:
                in_probs = True
                continue
            if in_probs and line.strip():
                parts = line.split()
                if len(parts) >= 3:
                    try:
                        site = int(parts[0])
                        state = parts[1]
                        prob = float(parts[2])
                        site_probs.append({
                            'site': site,
                            'state': state,
                            'probability': prob,
                            'confidence': 'high' if prob > 0.95 else 'moderate' if prob > 0.8 else 'low'
                        })
                    except ValueError:
                        in_probs = False

    return site_probs

IQ-TREE Ancestral Reconstruction

Goal: Perform ancestral reconstruction using IQ-TREE's marginal likelihood method.

Approach: Run iqtree2 with --ancestral flag to produce a .state file containing per-node, per-site state probabilities, then parse into ancestral sequences.

def run_iqtree_asr(alignment, tree=None, model='LG+G4', output_prefix='asr'):
    '''Run IQ-TREE for ancestral sequence reconstruction

    IQ-TREE provides:
    - Marginal reconstruction (default)
    - Joint reconstruction (-asr-joint)
    - State file (.state) with probabilities

    Advantages over PAML:
    - Automatic model selection
    - Better handling of gaps
    - Faster for large datasets
    '''
    cmd = f'iqtree2 -s {alignment} -m {model} --ancestral -pre {output_prefix}'

    if tree:
        cmd += f' -te {tree}'

    subprocess.run(cmd, shell=True)

    return f'{output_prefix}.state'


def parse_iqtree_state(state_file):
    '''Parse IQ-TREE .state file

    Format: Node  Site  State  Probability  [other states and probs]
    '''
    ancestors = {}

    with open(state_file) as f:
        next(f)  # Skip header
        for line in f:
            parts = line.strip().split('\t')
            if len(parts) >= 4:
                node = parts[0]
                site = int(parts[1])
                state = parts[2]
                prob = float(parts[3])

                if node not in ancestors:
                    ancestors[node] = {'sequence': [], 'probabilities': []}
                ancestors[node]['sequence'].append(state)
                ancestors[node]['probabilities'].append(prob)

    # Convert to sequences
    for node in ancestors:
        ancestors[node]['sequence'] = ''.join(ancestors[node]['sequence'])

    return ancestors

Alternative State Analysis

def get_alternative_states(site_probs, threshold=0.1):
    '''Identify sites with plausible alternative ancestral states

    Alternative states with P > 0.1 should be considered
    for experimental validation (ancestral protein resurrection)

    These sites may:
    - Affect function differently
    - Represent true ancestral ambiguity
    - Be targets for directed evolution
    '''
    ambiguous_sites = []

    for site_data in site_probs:
        if 'alternatives' in site_data:
            significant_alts = [
                alt for alt in site_data['alternatives']
                if alt['probability'] > threshold
            ]
            if significant_alts:
                ambiguous_sites.append({
                    'site': site_data['site'],
                    'best_state': site_data['state'],
                    'best_prob': site_data['probability'],
                    'alternatives': significant_alts
                })

    return ambiguous_sites


def calculate_sequence_confidence(site_probs):
    '''Calculate overall confidence in ancestral sequence

    Metrics:
    - Mean posterior probability
    - Fraction of high-confidence sites (P > 0.95)
    - Number of ambiguous positions
    '''
    if not site_probs:
        return None

    probs = [s['probability'] for s in site_probs]
    high_conf = sum(1 for p in probs if p > 0.95) / len(probs)
    low_conf = sum(1 for p in probs if p < 0.8)

    return {
        'mean_probability': sum(probs) / len(probs),
        'high_confidence_fraction': high_conf,
        'low_confidence_sites': low_conf,
        'total_sites': len(probs),
        'overall_quality': 'high' if high_conf > 0.9 else 'moderate' if high_conf > 0.7 else 'low'
    }

Ancestral Protein Resurrection

Goal: Design protein constructs for experimental resurrection of ancestral proteins.

Approach: Use the maximum-likelihood ancestral sequence as the primary construct, then create alternative constructs at ambiguous sites (P < 0.95) for experimental validation.

def design_asr_construct(ancestral_seq, extant_reference, ambiguous_sites):
    '''Design constructs for ancestral protein resurrection

    Strategy:
    1. Use most probable (ML) state at each position as primary construct
    2. Create alternative constructs at ambiguous sites (P < 0.95)
    3. Consider codon optimization for expression host

    Epistasis warning: Site-independent models assume positions evolve
    independently, but intramolecular epistasis means certain residue
    combinations may be incompatible. The ML ancestral sequence may contain
    never-tested combinations of residues, potentially yielding non-functional
    proteins. Test multiple alternative constructs, not just the ML sequence.

    Validation:
    - Test activity of primary + alternative resurrected proteins
    - Compare activity to extant homologs as positive controls
    - Titrate ambiguous positions systematically (especially buried residues)
    '''
    constructs = [{'name': 'ASR_ML', 'sequence': ancestral_seq, 'description': 'Maximum likelihood ancestral'}]

    # Create alternative constructs for ambiguous sites
    for site in ambiguous_sites[:5]:  # Limit to top 5 ambiguous
        alt_seq = list(ancestral_seq)
        best_alt = site['alternatives'][0]
        alt_seq[site['site'] - 1] = best_alt['state']
        constructs.append({
            'name': f"ASR_alt_{site['site']}",
            'sequence': ''.join(alt_seq),
            'description': f"Alternative at position {site['site']}: {best_alt['state']}"
        })

    return constructs

Related Skills

• comparative-genomics/positive-selection - Selection analysis on ancestral branches
• comparative-genomics/ortholog-inference - Identify orthologs for reconstruction
• phylogenetics/modern-tree-inference - Generate rooted trees for ASR
• alignment/multiple-alignment - PRANK alignment for ASR (correctly handles indels)
• alignment/pairwise-alignment - Prepare MSA for reconstruction

Converted and distributed by TomeVault — claim your Tome and manage your conversions.