The piRNAs present in the developing testes of Chinese indigenous Xiang pigs

The piRNA pathway plays an essential role in defense against transposable elements in the germline tissues of animals and contributes to post-transcriptional regulation of genes. Xiang pigs present an earlier sexual maturation compared with most European pig breeds, but the role that the piRNA pathway plays in the development of Xiang pigs is currently not understood. In this study, we sequenced and analyzed piRNAs expressed in the testes of Xiang pigs at four different ages, and identified endogenous piRNAs which were highly abundant at each time point. The lengths of the identified piRNAs ranged from 24 to 34 nucleotides (nt), with the most abundant length being 29 nt. Additionally, there was a strong bias for uracil at the first position, a slight bias for adenine at position 10 and frequent 5'-10 nt complementary sequences, suggesting that ping-pong-mediated silencing is present in the Xiang pig germline. We observed that the piRNA composition changed from TE-associated piRNAs in two- and three-month-old testes to predominantly gene-derived and intergenic piRNAs in six- and twelve-month-old testes, with a gradual increase in the expression level of piRNAs over the course of testis development. And more than half of piRNA reads mapped to just a few of 473 predicted piRNA clusters. Additionally, we found that several genes were highly enriched by piRNA reads, including CYP19A1, PRMT8, SUZ12, WWOX, SGSM1 and MIF. The functions of these genes are primarily associated with steroidogenesis and histone modification. Changes in piRNA composition and widespread expression patterns during spermatid development indicate that these small ncRNAs may be responsible not only for transposon suppression but also for post-transcriptional regulation of several protein-coding genes essential for normal spermatogenesis.