retrogenes

A retrogene is a gene created when an mRNA transcript from a parent gene is reverse-transcribed and inserted back into the genome. The resulting copy is intronless and often lacks the regulatory regions of the original gene. While many retroposed copies become nonfunctional processed pseudogenes, a subset acquires the ability to be transcribed and translated, becoming functional retrogenes.

Mechanism: Retroposition is typically mediated by long interspersed element-1 (LINE-1) reverse transcriptase, which reverse-transcribes mature mRNA. The cDNA copy is integrated into the genome through DNA repair processes, often at sites near other genes. To become functional, retrogenes must gain regulatory sequences or be co-opted by nearby promoters; in some cases they become chimeric genes by fusing with adjacent exons or regulatory elements.

Evolutionary significance: Retrogenes contribute to genetic novelty and can show tissue-specific expression, especially in germline tissues. Some rapidly adapt and take on new functional roles, influencing phenotypes and evolution. A classic example in Drosophila is the jingwei retrogene, a functional retrogene that arose through retroposition and subsequent modification.

Detection and classification: Retrogenes are typically intronless copies with polyadenylation signals and target-site duplications at insertion. They are distinguished from processed pseudogenes by evidence of expression and functional conservation across species. Comparative genomics and transcript data are used to identify candidate retrogenes and infer parental genes.

Impact: Retrogenes illustrate how genomes repurpose existing genetic material. They are considered a source of evolutionary innovation and help explain gene family expansion and diversification.