Bio Comparative Genomics Ortholog Inference

Version Compatibility

Reference examples tested with: OrthoFinder 3.0+ (Emms et al 2026 Nat Methods 23:1327), SonicParanoid 2.0.8+ (Cosentino 2024), Broccoli 1.2+ (Derelle 2020), ProteinOrtho 6.3.0+ (Lechner 2011 + recent), OMA standalone 2.6.0+, FastOMA 0.3.5+ (Majidian 2025), eggNOG-mapper 2.1.12+, JustOrthologs 2.0+, DIAMOND 2.1.10+, MMseqs2 17-b804f+, IQ-TREE 2.3.6+, BUSCO 5.7+, Compleasm 0.2.7+, BioPython 1.84+, R 4.4+ for downstream tree-based reconciliation.

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

• CLI: orthofinder --help; sonicparanoid --help; oma --help
• Python: pip show eggnog-mapper; which fastoma

If code throws Diamond requires N more sequences than provided, KeyError on species tree taxa, STAG branch length 0, or HOG file format mismatch, the OrthoFinder v2 -> v3 file layout changed (Orthogroups/ -> Phylogenetic_Hierarchical_Orthogroups/; rooted gene trees are now per-HOG); update parsing accordingly.

Ortholog Inference

"Find the orthologs of my gene(s) across these species" -> Choose between graph-based (RBH / similarity-clustering: fast, lower recall) and tree-based (gene-tree reconciliation: higher accuracy, slower) frameworks; recognize that "orthology" splits into 1:1, 1:many, many:many, and the practical unit for most pipelines is the HOG (Hierarchical Orthologous Group) -- a maximal cluster of genes descended from a single ancestral gene at a defined taxonomic level (Altenhoff 2013 PLoS ONE 8:e53786). The "ortholog conjecture" (orthologs more functionally similar than paralogs) is supported but weakly (Altenhoff 2012 PLoS Comp Biol 8:e1002514); don't treat 1:1 ortholog labeling as automatic functional equivalence.

• CLI: orthofinder -f proteomes/ -t 16 -M msa -- HOG output in v3 layout
• CLI: sonicparanoid -i proteomes/ -o output --mode default -- ML predictor + protein language model
• CLI: broccoli.py -dir proteomes/ -threads 16 -- direct OG with chimeric handling
• CLI: oma standalone HOG inference at every taxonomic level
• CLI: proteinortho6.pl --project=run proteomes/*.faa -- graph clustering with optional synteny
• CLI: emapper.py -i proteins.faa --output project --cpu 16 -- eggNOG annotation transfer

Algorithmic Taxonomy

Methodology evolves; Quest-for-Orthologs benchmarks (Altenhoff 2024 NAR Genom Bioinform 6:lqae167) refresh annually. Verify the current QfO benchmark results before locking on a single tool for novel benchmarking-grade work.

Decision Tree by Experimental Scenario

Per-Method Failure Modes

Hidden paralogy from missing outgroup

Trigger: OrthoFinder run without a sufficiently distant outgroup; only ingroup taxa included.

Mechanism: OrthoFinder roots gene trees via STRIDE / STAG using outgroup-based duplication signals. Without an outgroup, the root inferred for each gene tree may be internal, mistakenly classifying a duplication-then-loss pattern as orthology. The "1:1 orthologs" returned may actually be hidden paralogs (Emms & Kelly 2017 MBE 34:3267 STRIDE).

Symptom: Phylogenomic concatenation produces low-bootstrap species tree; per-gene trees show inconsistent rooting; same-clade species show longer-than-expected branches in single-copy ortholog trees.

Fix: Include >= 1 outgroup taxon at the next-higher taxonomic level (sister phylum / class / order). For vertebrates, use cyclostomes (lamprey) as outgroup for jawed vertebrates; for plants, use a non-flowering plant for angiosperms. Re-run with outgroup; cross-check HOG file Phylogenetic_Hierarchical_Orthogroups/N0.tsv for 1:1 stability.

Splice isoforms inflating copy number

Trigger: Proteome FASTA contains multiple isoforms per gene (Ensembl, RefSeq with -NR flag).

Mechanism: OrthoFinder / SonicParanoid / OMA treat each protein sequence as a gene; isoforms are clustered together within orthogroups but inflate the apparent gene count per species, producing artifactual "co-orthologs" that are just isoforms of one gene.

Symptom: Per-species gene count > 2x what gene-annotation pipeline reported; orthogroups contain multiple proteins from same gene; gene names contain isoform suffixes (.1, -iso1, etc.).

Fix: Pre-filter proteomes to longest isoform per gene. Use agat_sp_keep_longest_isoform.pl (AGAT toolkit), Biopython snippet on Ensembl gene-isoform mapping, or Trinotate longest-ORF picker. OrthoFinder3 ships a wrapper tools/primary_transcript.py for Ensembl/UniProt format.

Annotation heterogeneity inflating lineage-specific OGs

Trigger: Genomes annotated by different pipelines (Augustus, MAKER, Funannotate, NCBI RefSeq) mixed in one analysis.

Mechanism: Pipelines differ in handling of intron predictions, gene boundaries, and small-gene filtering. A gene predicted by Augustus but missed by MAKER appears as an apparent lineage-specific gene in the MAKER-annotated species, producing spurious "species-specific" orthogroups.

Symptom: CAFE (gene family evolution) shows extreme expansions / contractions for species with different annotation pipelines; per-species "unique" gene count varies 5-10x between technically similar genomes.

Fix: Re-annotate all genomes with one pipeline (e.g. BRAKER3 or Funannotate) before orthology. When re-annotation isn't possible, normalize via BUSCO/Compleasm completeness (filter OGs absent from species with < 95% BUSCO complete). Document annotation pipeline per species in methods.

Ortholog conjecture violations

Trigger: Transferring GO/KEGG functional annotation from 1:1 ortholog without testing functional divergence.

Mechanism: Studies show orthologs are weakly more functionally similar than paralogs (Altenhoff 2012 PLoS Comp Biol 8:e1002514), though Stamboulian 2020 (Bioinformatics 36:i219) finds paralogs often comparable predictors; effect size is small and dependent on evolutionary distance. Subfunctionalization (Force 1999 Genetics 151:1531), neofunctionalization, and dosage subfunctionalization can rapidly differentiate 1:1 orthologs.

Symptom: Transferred annotation contradicts species-specific experimental data; ortholog has fold-change different expression patterns; rapid evolution (dN/dS > 0.3) on one branch only.

Fix: For high-confidence annotation transfer, require: (1) 1:1 orthology AND (2) low branch dN/dS (< 0.2 for both branches) AND (3) GO evidence code ISO with experimental support upstream. Tag annotation as "predicted from ortholog" rather than equating function. eggNOG ortholog-conjecture-aware mode provides confidence scoring.

RBH symmetric-but-wrong errors

Trigger: Reciprocal best hits (RBH) method on a gene that has been replaced by a paralog in one lineage.

Mechanism: Lineage A retains the original ortholog; Lineage B lost the ortholog and replaced its function with a paralog (xenologous replacement). RBH between A and B identifies the paralog as the "ortholog" because it's the best hit.

Symptom: dN/dS on RBH ortholog pair > 1 on the B branch (suggesting positive selection but actually paralog substitution); gene tree shows the B sequence is sister to other paralogs of the A gene, not to A.

Fix: Use tree-based methods (OrthoFinder3, OMA HOGs) for organisms with extensive paralog history. RBH is appropriate for orthogroup pre-screening only. JustOrthologs is explicitly an RBH method, so it's affected by this; reserve for closely related species.

Synteny ignored in WGD lineages

Trigger: Plant, yeast, fish, or salmonid analysis where one or more ancient WGD events occurred.

Mechanism: WGD doubles all genes; subsequent gene loss is biased toward certain functional categories (Dosage Balance Hypothesis; Birchler & Veitia 2007 Plant Cell 19:395). Sequence-only orthology cannot distinguish "ortholog" (single ancestral gene) from "homeolog" (paralog from WGD). In hexaploids (wheat, Brassica), this triples the confusion.

Symptom: Multiple co-orthologs per species in orthogroups for WGD-affected lineages; rate variation across "co-orthologs" suggests one is the true ortholog and the others are recent duplicates.

Fix: Use synteny-aware methods (GENESPACE -- Lovell 2022 eLife 11:e78526; ProteinOrtho -synteny; OrthoFinder3 + post-hoc synteny verification). Restrict 1:1 ortholog phylogenomics to genes in single-copy syntenic regions. For deep WGD ancestry (e.g. 2R vertebrate WGD), modern orthologs of post-2R paralogs (ohnologs) can no longer be unambiguously identified by sequence; use synteny + duplication-dating.

MAFFT-only alignment in OrthoFinder MSA mode

Trigger: orthofinder -M msa -A mafft default; very divergent orthogroups (deep taxonomy).

Mechanism: MAFFT default settings choose accurate-but-fast iterative refinement. For sequences with >50% divergence, MAFFT's auto choice may downgrade to FFT-NS-2, producing alignments with > 30% poorly aligned columns that distort gene-tree inference and downstream HOG construction.

Symptom: Per-OG MAFFT logs show "FFT-NS-2 selected"; gene trees have unstable rooting; bootstrap support < 60% for many internal branches.

Fix: Force mafft-linsi or specify -A mafft --thread -1 --localpair --maxiterate 1000. For deeply divergent OGs, run PRANK or MUSCLE5 post-hoc on critical OGs and re-build gene trees with IQ-TREE. PREQUAL or HmmCleaner segment-filtering improves resulting trees (Di Franco 2019 BMC Evol Biol 19:21).

Quantitative Thresholds