msa-parsing

name	msa-parsing
description	Parse and analyze multiple sequence alignments using Biopython. Extract sequences, identify conserved regions, analyze gaps, work with annotations, and manipulate alignment data for downstream analysis. Use when parsing or manipulating multiple sequence alignments.
tool_type	python
primary_tool	Bio.AlignIO

Version Compatibility

Reference examples tested with: BioPython 1.83+

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.

MSA Parsing and Analysis

Parse multiple sequence alignments to extract information, analyze content, and prepare for downstream analysis.

Required Import

Goal: Load modules for parsing, analyzing, and manipulating multiple sequence alignments.

Approach: Import AlignIO for reading, Counter for column analysis, and alignment classes for constructing modified alignments.

from Bio import AlignIO
from Bio.Align import MultipleSeqAlignment
from Bio.SeqRecord import SeqRecord
from Bio.Seq import Seq
from collections import Counter

Loading Alignments

Goal: Read an MSA file and inspect its dimensions.

Approach: Use AlignIO.read() specifying the file and format.

from Bio import AlignIO

alignment = AlignIO.read('alignment.fasta', 'fasta')
print(f'{len(alignment)} sequences, {alignment.get_alignment_length()} columns')

Extracting Sequence Information

Get All Sequence IDs

seq_ids = [record.id for record in alignment]

Get Sequences as Strings

sequences = [str(record.seq) for record in alignment]

Get Sequence by ID

def get_sequence_by_id(alignment, seq_id):
    for record in alignment:
        if record.id == seq_id:
            return record
    return None

target = get_sequence_by_id(alignment, 'species_A')

Access Descriptions and Annotations

for record in alignment:
    print(f'ID: {record.id}')
    print(f'Description: {record.description}')
    print(f'Annotations: {record.annotations}')

Column-wise Analysis

Goal: Analyze alignment content column by column to assess composition, conservation, and variability.

Approach: Use column indexing (alignment[:, idx]) and Counter to examine character frequencies at each position.

Get Single Column

column_5 = alignment[:, 5]  # Returns string of characters at position 5
print(column_5)  # e.g., 'AAAGA'

Iterate Over Columns

for col_idx in range(alignment.get_alignment_length()):
    column = alignment[:, col_idx]
    print(f'Column {col_idx}: {column}')

Count Characters in Column

from collections import Counter

def column_composition(alignment, col_idx):
    column = alignment[:, col_idx]
    return Counter(column)

counts = column_composition(alignment, 0)
print(counts)  # Counter({'A': 3, 'G': 1, '-': 1})

Find Conserved Positions

def find_conserved_positions(alignment, threshold=1.0):
    conserved = []
    for col_idx in range(alignment.get_alignment_length()):
        column = alignment[:, col_idx]
        counts = Counter(column)
        most_common_char, most_common_count = counts.most_common(1)[0]
        if most_common_char != '-':
            conservation = most_common_count / len(alignment)
            if conservation >= threshold:
                conserved.append((col_idx, most_common_char))
    return conserved

fully_conserved = find_conserved_positions(alignment, threshold=1.0)
mostly_conserved = find_conserved_positions(alignment, threshold=0.8)

Gap Analysis

Goal: Quantify gap distribution across sequences and columns to identify problematic regions or sequences.

Approach: Count gap characters per sequence and per column, then identify positions exceeding a gap fraction threshold.

Count Gaps Per Sequence

gap_counts = [(record.id, str(record.seq).count('-')) for record in alignment]
for seq_id, gaps in gap_counts:
    print(f'{seq_id}: {gaps} gaps')

Count Gaps Per Column

def gaps_per_column(alignment):
    return [alignment[:, i].count('-') for i in range(alignment.get_alignment_length())]

gap_profile = gaps_per_column(alignment)

Find Gappy Columns

def find_gappy_columns(alignment, threshold=0.5):
    gappy = []
    num_seqs = len(alignment)
    for col_idx in range(alignment.get_alignment_length()):
        column = alignment[:, col_idx]
        gap_fraction = column.count('-') / num_seqs
        if gap_fraction >= threshold:
            gappy.append(col_idx)
    return gappy

columns_to_remove = find_gappy_columns(alignment, threshold=0.5)

Remove Gappy Columns

def remove_gappy_columns(alignment, threshold=0.5):
    num_seqs = len(alignment)
    keep_columns = []
    for col_idx in range(alignment.get_alignment_length()):
        column = alignment[:, col_idx]
        gap_fraction = column.count('-') / num_seqs
        if gap_fraction < threshold:
            keep_columns.append(col_idx)

    new_records = []
    for record in alignment:
        new_seq = ''.join(str(record.seq)[i] for i in keep_columns)
        new_records.append(SeqRecord(Seq(new_seq), id=record.id, description=record.description))
    return MultipleSeqAlignment(new_records)

cleaned = remove_gappy_columns(alignment, threshold=0.5)

Alignment Trimming: Decision Framework

Trimming (removing unreliable columns before downstream analysis) is controversial. Research is split on whether it helps or hurts phylogenetic inference. The approach matters more than whether to trim.

Tool Recommendations (Best to Worst)

Tool	Approach	When to Use
ClipKIT	Retains parsimony-informative + constant sites	Default choice; kpic-smart-gap mode consistently outperforms others
trimAl -automated1	Gap + similarity scoring	When ClipKIT unavailable; equal or better than unfiltered in most tests
Gblocks (relaxed params)	Block-based removal	Only with relaxed parameters; default settings are too aggressive

Key insight: ClipKIT inverts the traditional approach. Instead of removing "bad" columns, it identifies and retains informative ones. This paradigm shift consistently produces better trees.

When NOT to trim: Single-gene phylogenetics with well-curated alignments. Aggressive trimming (>20-30% of sites removed) causes rapid tree deterioration.

# ClipKIT (recommended)
clipkit alignment.fasta -m kpic-smart-gap -o trimmed.fasta

# trimAl automated mode
trimal -in alignment.fasta -out trimmed.fasta -automated1

# trimAl with explicit gap threshold
trimal -in alignment.fasta -out trimmed.fasta -gt 0.5

Gap Handling for Phylogenetics

How gaps are treated in downstream phylogenetic analysis significantly affects tree topology:

Treatment	Method	Tradeoff
Missing data (default)	Gaps = unknown character	Most common; can be statistically inconsistent under ML
Fifth state	Gap = 5th nucleotide	Biologically problematic (gaps of different lengths treated equally)
Simple indel coding	Each unique indel coded as binary character	Most biologically realistic; adds phylogenetic signal

Indel coding and fifth-state outperform missing-data treatment ~90% of the time on empirical datasets. For important analyses, consider indel coding to capture the phylogenetic information in gap patterns.

Identifying Unreliable Alignment Regions

Columns exhibiting both high gap fraction AND low conservation are the strongest indicators of alignment uncertainty. These often reflect guide tree artifacts rather than true evolutionary events. Before phylogenetic analysis:

1. Flag columns with gap fraction >50%, which may be alignment artifacts
2. Check if gappy regions coincide with insertions in a single divergent sequence (remove that sequence and re-align)
3. For critical analyses, run GUIDANCE2 or MUSCLE5 ensemble to get per-column confidence scores; mask columns below the reliability threshold (default: 0.93 for GUIDANCE2)

Consensus Sequence

"Get consensus sequence" → Derive a single representative sequence from an MSA based on majority-rule voting at each column.

Goal: Generate a consensus sequence from the alignment using a frequency threshold.

Approach: At each column, select the most common non-gap character if it exceeds the threshold; otherwise mark as ambiguous.

Simple Majority Consensus

def consensus_sequence(alignment, threshold=0.5, gap_char='-', ambiguous='N'):
    consensus = []
    for col_idx in range(alignment.get_alignment_length()):
        column = alignment[:, col_idx]
        counts = Counter(column)
        most_common_char, most_common_count = counts.most_common(1)[0]
        if most_common_char == gap_char:
            counts.pop(gap_char, None)
            if counts:
                most_common_char, most_common_count = counts.most_common(1)[0]
            else:
                most_common_char = gap_char

        if most_common_count / len(alignment) >= threshold:
            consensus.append(most_common_char)
        else:
            consensus.append(ambiguous)
    return ''.join(consensus)

consensus = consensus_sequence(alignment, threshold=0.5)

Note on Bio.Align.AlignInfo

The AlignInfo.SummaryInfo class is deprecated in recent Biopython versions. The custom consensus_sequence() function above is the recommended approach. If you see deprecation warnings when using AlignInfo, use the custom implementation instead.

Extracting Regions

Slice by Column Range

region = alignment[:, 100:200]  # Columns 100-199

Slice by Sequence Range

subset = alignment[0:10]  # First 10 sequences

Extract Ungapped Regions from Reference

def extract_ungapped_regions(alignment, ref_idx=0):
    ref_seq = str(alignment[ref_idx].seq)
    ungapped_cols = [i for i, char in enumerate(ref_seq) if char != '-']

    new_records = []
    for record in alignment:
        new_seq = ''.join(str(record.seq)[i] for i in ungapped_cols)
        new_records.append(SeqRecord(Seq(new_seq), id=record.id, description=record.description))
    return MultipleSeqAlignment(new_records)

ungapped = extract_ungapped_regions(alignment, ref_idx=0)

Sequence Filtering

Goal: Subset an alignment to retain only sequences matching specific criteria (ID pattern, gap content, uniqueness).

Approach: Iterate over alignment records, apply filter conditions, and reconstruct a new MultipleSeqAlignment from matching records.

Filter by Sequence ID Pattern

import re

def filter_by_id(alignment, pattern):
    regex = re.compile(pattern)
    matching = [record for record in alignment if regex.search(record.id)]
    return MultipleSeqAlignment(matching)

bacteria_only = filter_by_id(alignment, r'^Bac_')

Filter by Gap Content

def filter_by_gap_content(alignment, max_gap_fraction=0.1):
    filtered = []
    for record in alignment:
        gap_fraction = str(record.seq).count('-') / len(record.seq)
        if gap_fraction <= max_gap_fraction:
            filtered.append(record)
    return MultipleSeqAlignment(filtered)

low_gap_seqs = filter_by_gap_content(alignment, max_gap_fraction=0.1)

Remove Duplicate Sequences

def remove_duplicates(alignment):
    seen_seqs = {}
    unique_records = []
    for record in alignment:
        seq_str = str(record.seq)
        if seq_str not in seen_seqs:
            seen_seqs[seq_str] = record.id
            unique_records.append(record)
    return MultipleSeqAlignment(unique_records)

unique_alignment = remove_duplicates(alignment)

Working with Annotations

Stockholm Format Annotations

alignment = AlignIO.read('pfam.sto', 'stockholm')

for record in alignment:
    if 'secondary_structure' in record.letter_annotations:
        ss = record.letter_annotations['secondary_structure']
        print(f'{record.id}: {ss}')

Add Annotations to Records

for record in alignment:
    record.annotations['source'] = 'my_analysis'
    record.annotations['quality'] = 'high'

Position Mapping

Goal: Convert between alignment column coordinates and ungapped sequence coordinates.

Approach: Walk through the sequence tracking gap characters to map between the two coordinate systems.

Map Alignment Position to Sequence Position

def alignment_to_sequence_position(record, align_pos):
    seq_pos = 0
    for i, char in enumerate(str(record.seq)):
        if i == align_pos:
            return seq_pos if char != '-' else None
        if char != '-':
            seq_pos += 1
    return None

Map Sequence Position to Alignment Position

def sequence_to_alignment_position(record, seq_pos):
    current_seq_pos = 0
    for i, char in enumerate(str(record.seq)):
        if char != '-':
            if current_seq_pos == seq_pos:
                return i
            current_seq_pos += 1
    return None

Quick Reference: Common Operations

Task	Code
Get column	alignment[:, col_idx]
Get sequence	alignment[seq_idx]
Column count	alignment.get_alignment_length()
Sequence count	len(alignment)
Find gaps	str(record.seq).count('-')
Consensus	Use custom consensus_sequence() function

Common Errors

Error	Cause	Solution
IndexError	Column index out of range	Check get_alignment_length()
Unequal sequence lengths	Invalid MSA	Ensure all sequences same length
Empty Counter	All gaps in column	Handle gap-only columns

Related Skills

• multiple-alignment - Run MSA tools (MAFFT, MUSCLE5, ClustalOmega) to generate alignments
• alignment-io - Read/write alignment files in various formats
• pairwise-alignment - Create pairwise alignments
• msa-statistics - Calculate conservation metrics
• phylogenetics/modern-tree-inference - Build trees from processed alignments