Developmental remodeling of ping-pong piRNA amplification in the vertebrate female germline

The piRNA pathway, responsible for silencing transposable elements (TEs) in the germline, features a distinctive amplification cycle known as ping-pong. While active in the male germline during spermatogenesis, the developmental profile of ping-pong in the vertebrate female germline remains less understood. Previous studies primarily examined adult ovaries and mature oocytes, periods when piRNA pathway components are reportedly scarce.

To gain a deeper understanding, researchers generated strand-specific RNA-seq and small RNA-seq data from pre-meiotic (E10.5) and meiotic entry (E16.5) chicken ovary samples. They also analyzed published single-cell data to track germ-cell composition during the same developmental window, extending the analysis to mature chicken ovaries and zebrafish across various stages.

The findings reveal that ping-pong amplification increases at the meiotic entry stage compared to the pre-meiotic stage across different TE classes. In mature ovaries, the signature weakens, with remaining ping-pong pairs predominantly associated with LTR/ERV retroelements. The study demonstrates that activating a meiotic entry transcriptional program in an in vitro chicken primordial germ cell model leads to an increased fraction of piRNA-sized reads with a partner showing a 10-nt 5' overlap, as well as an elevated 1U signature of piRNA-sized reads. This aligns with the notion that meiotic priming promotes piRNA biogenesis.

The zebrafish ovary exhibits a comparable meiosis-associated amplification and a preference for targeting LTR/ERV retroelements at maturity, despite carrying roughly 5.7 times more TE sequences than chicken ovaries. The striking similarities in these patterns across two lineages that diverged approximately 430 million years ago suggest that germline development plays a crucial role in shaping both the timing of ping-pong amplification and the TE classes most prominently associated with it.