Pollen-Specific Activation of Arabidopsis Retrogenes Is Associated with Global Transcriptional Reprogramming

Duplications allow for gene functional diversification and accelerate genome evolution. Occasionally, the transposon amplification machinery reverse transcribes the mRNA of a gene, integrates it into the genome, and forms an RNA-duplicated copy: the retrogene. Although retrogenes have been found in plants, their biology and evolution are poorly understood. Here, we identified 251 (216 novel) retrogenes in Arabidopsis thaliana, corresponding to 1% of protein-coding genes. Arabidopsis retrogenes are derived from ubiquitously transcribed parents and reside in gene-rich chromosomal regions. Approximately 25% of retrogenes are cotranscribed with their parents and 3% with head-to-head oriented neighbors. This suggests transcription by novel promoters for 72% of Arabidopsis retrogenes. Many retrogenes reach their transcription maximum in pollen, the tissue analogous to animal spermatocytes, where upregulation of retrogenes has been found previously. This implies an evolutionarily conserved mechanism leading to this transcription pattern of RNA-duplicated genes. During transcriptional repression, retrogenes are depleted of permissive chromatin marks without an obvious enrichment for repressive modifications. However, this pattern is common to many other pollen-transcribed genes independent of their evolutionary origin. Hence, retroposition plays a role in plant genome evolution, and the developmental transcription pattern of retrogenes suggests an analogous regulation of RNA-duplicated genes in plants and animals.