Promoter hypermethylation of PIWI/piRNA pathway genes associated with diminished pachytene piRNA production in bovine hybrid male sterility

Hybrid male sterility (HMS) is a postzygotic reproductive isolation mechanism that enforces speciation. A bovine example of HMS is the yattle (also called dzo), an interspecies hybrid of taurine cattle (Bos taurus) and yak (Bos grunniens). The molecular mechanisms underlying HMS of yattle are not well understood. Epigenetic modifications of DNA methylation and P-element induced wimpy testis (PIWI)-interacting RNA (piRNAs) are important regulators in spermatogenesis. In this study, we investigated DNA methylation patterns and piRNA expression in adult testes in hybrid infertile yattle bulls and fertile cattle and yak bulls using whole genome bisulphite-seq and small RNA-seq. Promoter hypermethylation in yattle were associated with DNA methylation involved in gamete generation, piRNA metabolic processes, spermatogenesis, and spermatid development (P