The Specific Requirements for CR1 Retrotransposition Explain the Scarcity of Retrogenes in Birds

Chicken repeat 1 (CR1) retroposons are the most abundant superfamily of transposable elements in the genomes of birds, crocodilians, and turtles. However, CR1 mobilization remains poorly understood. In this article, I document that the diverse CR1 lineages of land vertebrates share a highly conserved hairpin structure and an octamer microsatellite motif at their very 3' ends. Together with the presence of these same motifs in the tails of CR1-mobilized short interspersed elements, this suggests that the minimum requirement for CR1 transcript recognition and retrotransposition is a complex >50-nt structure. Such a highly specific recognition sequence readily explains why CR1-dominated genomes generally contain very few retrogenes. Conversely, the mammalian richness in retrogenes results from CR1 extinction in their early evolution and subsequent establishment of L1 dominance.