Notes on GFF3¶
AEGeAn’s preferred exchange format¶
The AEGeAn Toolkit, and the GenomeTools library upon which AEGeAn heavily relies, use the GFF3 format as the preferred encoding for genome annotations. GFF3 utilizes a tab-delimited plain text format that bears resemblance, and indeed owes its origins, to several related annotation encoding schemes. The GFF3 format is unique, however, in at least two critical ways. First, it leverages a controlled vocabulary, the Sequence Ontology, for describing genomic features and the relationships between them. Second, GFF3 enables greater flexibility and granularity in describing genomic features compared to alternative formats, such as the Gene Transfer Format (GTF) which focuses exclusively on protein-coding genes, or the Browser Extensible Data (BED) format which focuses primarily on feature visualization.Both GTF and BED support only a single level of feature decomposition, while GFF3 supports grouping features and subfeatures to arbitrary levels of granularity. Annotations encoded in GFF3 can be considered annotation graphs, directed acyclic graphs with nodes representing genomic features and edges representing relationships between the features (Gremme, 2013).
Ad hoc conventions¶
While the GFF3 specification requires the use of terms in the Sequence Ontology for describing genomic features, it does not dictate the level of detail to which genomic features must be described for a particular context. In many cases, one could use different sets of terms to describe the same genomic features. For example, features of a protein-coding gene include exons, introns, a coding sequence, untranslated regions, a start codon, and a stop codon. However, it is rare for all of these features to be declared explicitly in a GFF3 file. Intron features are rarely included since they can easily be inferred from exon structure. The coding sequence is sometimes encoded using a CDS feature (defined with multiple entries if it spans multiple exons), but other times is encoded using start and stop codons. Some GFF3 files list each mRNA as a subfeature of an explicitly declared gene feature, while other files do not explicitly declare gene features. The AEGeAn tools and API are designed to be flexible and should work correctly so long as the features of interest are described in sufficient detail.
- Some programs produce GFF3 that does not explicitly declare gene features for every transcript. The AEGeAn Toolkit includes code that can correct this issue by creating a gene parent with the same genomic location and attaching it as a parent to the transcript. However, this becomes problematic if, for example, the annotation includes alternatively spliced mRNAs without explicitly declared gene parents. Instead of inferring a single gene parent for all the alternative isoforms, AEGeAn will create a distinct gene feature for each individual isoform, leading to errors in subsequent analysis and reporting. If possible, users are much safer using GFF3 with gene features explicitly declared.
- Many whole-genome annotations, including those produced by NCBI’s GNOMON annotation pipeline, include an annotation of organellar genomes such as those in mitochondria or chloroplasts. Users are responsible to decide which annotations to include in a particular analysis and should pre-filter their data accordingly. It is also important to note that genes from organellar genomes are sometimes encoded differently than genes from the nuclear genomes (this is true for GFF3 files produced by NCBI) and may need additional pre-processing before they can be analyzed with AEGeAn.
- The Sequence Ontology includes the terms pseudogene (to describe a pseudogene’s location), pseudogenic_exon (to describe the pseudogene’s structure), and pseudogenic_transcript in cases where the pseudogene is transcribed. However, some GFF3 files do not correctly use these terms (I’m looking at you NCBI) and use terms like gene and exon instead, leading to confusion and issues with downstream analysis. Fortunately, feature attributes can often be used to correct this issue. For example, pseudogenes in GFF3 files produced by the annotwriter program from NCBI’s C++ Toolkit label pseudogenes as genes, but include a pseudo=true attribute in the 9th column of the record. AEGeAn can correct this issue, but sometimes the user must request this behavior.