Interplay Between Transposable Elements and PIWI Interactitng RNAs Among Laurasiatherian Mammals

Transposable elements (TEs) are DNA sequences that have the ability to relocate to novel regions of a host’s genome. Unrestricted TE proliferation can have negative, deleterious consequences on host genomes. Despite this, a high proportion of mammalian genomes are derived from retrotransposons. There is however wide variation in TE content and expression among even closely related species. PIWI proteins and PIWI interacting RNAs (piRNAs) are expressed in the male germ cells of most animals and act to suppress the potentially harmful proliferation of TEs. However, little is known about the evolutionary arms race between the piRNA/PIWI protein complex and TEs. To directly compare the interaction of TEs and piRNAs, we extracted small RNAs from the testes of an adult dog, cat, vesper bat, and horse; four Laurasiatherian mammals with distinct TE profiles. We found that piRNAs generally mapped to younger TE families compared to older families in all species, yet this targeting was variable among groups. This suggests piRNAs play a role in preventing the expression of active TEs in mature testis. We found little piRNA targeting of DNA transposons in the vesper bat, a class of TEs lacking an RNA intermediate that are rare in mammals. We also found high conservation of piRNA precursor gene size and content among the dog, cat, and horse. This work illustrates the complex dynamics between genomic parasites and defenses among mammalian genomes.