Figure 1

Molecular models for transposable element co-option and domestication. (A) Transposable elements often insert into intergenic or intronic regions but are generally transcriptionally silenced through epigenetic modification. Over time mutations accumulate due to a lack of selective pressure to maintain sequences intact. If the TE contains sequences that drive transcription it may remain active if insertion takes place in a genomic location that favours expression over TE repression and silencing. (B) TE may insert into a region of the genome with inherent transcriptional activity or in close proximity to a cryptic CpG rich promoter (green box) that may result in TE avoiding epigenetic silencing to remain active. (C) Less frequently, TEs may insert downstream of the transcription start site of a host gene (i) either into an intron, or (ii) exon (iii) with the potential to create novel chimeric genes and/or proteins. (D) Insertion of a TE upstream of a host gene in an antisense direction. (i) Pre-insertion genomic environment displaying a typical multi-exon unidirectional gene. (ii) Incoming TE may insert within 1 kb of a CpG island gene promoter with transcription initiating within the TE itself. (iii) Alternatively the TE may insert further away from host promoter but will form a novel gene with a new upstream exon and an intervening intron that is spliced during mRNA formation.