Bio Genome Annotation Eukaryotic Gene Prediction

Version Compatibility

Reference examples tested with: BUSCO 5.5+, HISAT2 2.2.1+, pandas 2.2+, samtools 1.19+

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

• Python: pip show <package> then help(module.function) to check signatures
• CLI: <tool> --version then <tool> --help to confirm flags

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

Eukaryotic Gene Prediction

"Predict genes in my eukaryotic genome" → Identify protein-coding gene structures (exons, introns, UTRs) using RNA-seq alignment evidence and/or protein homology to train ab initio predictors.

• CLI: braker.pl --genome=assembly.fa --bam=rnaseq.bam --prot_seq=proteins.fa (BRAKER3)

Predict protein-coding genes in eukaryotic genomes using evidence-based methods. BRAKER3 combines RNA-seq and protein homology evidence for the most accurate predictions. GALBA provides an alternative when only protein evidence is available.

Prerequisites

CRITICAL: The input assembly must be softmasked (repeats in lowercase). Run repeat-annotation first to softmask the genome. Unmasked assemblies produce many false positive gene predictions in repetitive regions.

BRAKER3 (RNA-seq + Protein Evidence)

BRAKER3 is the preferred pipeline when RNA-seq data exists. It combines GeneMark-ETP with Augustus training for high-accuracy gene models.

Evidence Preparation

# Align RNA-seq with HISAT2
hisat2-build assembly_softmasked.fasta genome_index
hisat2 -x genome_index -1 reads_R1.fastq.gz -2 reads_R2.fastq.gz \
    --dta -p 16 | samtools sort -@ 4 -o rnaseq_sorted.bam
samtools index rnaseq_sorted.bam

# Download OrthoDB proteins for the relevant clade
# Available: Metazoa, Viridiplantae, Fungi, Arthropoda, Vertebrata
wget https://bioinf.uni-greifswald.de/bioinf/partitioned_odb11/Viridiplantae.fa.gz
gunzip Viridiplantae.fa.gz

Run BRAKER3

braker.pl \
    --genome=assembly_softmasked.fasta \
    --bam=rnaseq_sorted.bam \
    --prot_seq=Viridiplantae.fa \
    --softmasking \
    --threads=16 \
    --species=my_species \
    --workingdir=braker3_out \
    --gff3

Key Options

Output Files

braker3_out/
├── braker.gtf           # Gene predictions (GTF)
├── braker.gff3          # Gene predictions (GFF3, if --gff3)
├── braker.codingseq     # CDS nucleotide sequences
├── braker.aa            # Protein sequences
├── hintsfile.gff        # All evidence hints
├── Augustus/            # Trained Augustus parameters
└── GeneMark-ETP/        # GeneMark intermediate files

GALBA (Protein-Only Evidence)

GALBA works best with closely related species proteins. Always prefer BRAKER3 when RNA-seq data is available, as intron evidence from RNA-seq substantially improves splice site accuracy.

galba.pl \
    --genome=assembly_softmasked.fasta \
    --prot_seq=closely_related_proteins.fa \
    --softmasking \
    --threads=16 \
    --species=my_species \
    --workingdir=galba_out \
    --gff3

When to Use GALBA vs BRAKER3

Augustus Standalone

For genomes with pre-trained parameters or when rerunning predictions with different hints.

# List available pre-trained species
augustus --species=help

# Run with pre-trained species
augustus \
    --species=arabidopsis \
    --softmasking=1 \
    --gff3=on \
    --UTR=off \
    assembly_softmasked.fasta > augustus_predictions.gff3

# Run with hints (evidence)
augustus \
    --species=my_species \
    --softmasking=1 \
    --gff3=on \
    --hintsfile=hints.gff \
    --extrinsicCfgFile=extrinsic.cfg \
    assembly_softmasked.fasta > augustus_hints.gff3

Evaluation with BUSCO

# Evaluate predicted proteins
busco -i braker3_out/braker.aa -m proteins -l embryophyta_odb10 -o busco_proteins -c 8

# Compare to genome-mode BUSCO
busco -i assembly_softmasked.fasta -m genome -l embryophyta_odb10 -o busco_genome -c 8

Interpretation

Python: Parse Gene Models

Goal: Compute summary statistics for predicted gene models to assess annotation quality and compare against known species benchmarks.

Approach: Load the GFF3 into a gffutils database, iterate through gene and transcript features to compute counts, exons per transcript, intron lengths, and single-exon gene fraction.

import gffutils
import pandas as pd

def load_gene_models(gff_file):
    '''Load eukaryotic gene models from GFF3/GTF.'''
    db = gffutils.create_db(str(gff_file), ':memory:', merge_strategy='merge')
    return db

def gene_model_stats(db):
    '''Compute statistics for predicted gene models.'''
    genes = list(db.features_of_type('gene'))
    transcripts = list(db.features_of_type(['mRNA', 'transcript']))

    exon_counts, intron_lengths, gene_lengths = [], [], []
    for gene in genes:
        gene_lengths.append(gene.end - gene.start + 1)
        for tx in db.children(gene, featuretype=['mRNA', 'transcript']):
            exons = list(db.children(tx, featuretype='exon'))
            exon_counts.append(len(exons))
            sorted_exons = sorted(exons, key=lambda e: e.start)
            for i in range(len(sorted_exons) - 1):
                intron_len = sorted_exons[i + 1].start - sorted_exons[i].end - 1
                intron_lengths.append(intron_len)

    stats = {
        'total_genes': len(genes),
        'total_transcripts': len(transcripts),
        'transcripts_per_gene': len(transcripts) / len(genes) if genes else 0,
        'median_exons_per_transcript': pd.Series(exon_counts).median() if exon_counts else 0,
        'median_gene_length': pd.Series(gene_lengths).median() if gene_lengths else 0,
        'median_intron_length': pd.Series(intron_lengths).median() if intron_lengths else 0,
        'single_exon_genes': sum(1 for e in exon_counts if e == 1),
    }
    return stats

db = load_gene_models('braker3_out/braker.gff3')
stats = gene_model_stats(db)
for key, val in stats.items():
    print(f'{key}: {val}')

Troubleshooting

Low Gene Count

• Verify softmasking (lowercase repeats present in assembly)
• Check RNA-seq alignment rate (>70% expected)
• Ensure OrthoDB proteins match the correct clade

Many Single-Exon Genes

• May indicate bacterial contamination
• Check if assembly was properly softmasked
• Consider filtering predictions by evidence support

BRAKER3 Fails

• Ensure all dependencies are in PATH (GeneMark, Augustus, DIAMOND)
• Check that genome headers have no special characters (pipes, spaces)
• Simplify FASTA headers: sed 's/ .*//' assembly.fasta > clean.fasta

Related Skills

• repeat-annotation - PREREQUISITE: softmask repeats before gene prediction
• functional-annotation - Add GO/KEGG/Pfam to predicted proteins
• read-alignment/star-alignment - Alternative RNA-seq aligner for evidence
• genome-assembly/assembly-qc - Verify assembly quality before prediction