Pinpointing the expression of piRNAs and function of the PIWI protein subfamily during spermatogenesis in the mouse.

PIWI proteins and piRNAs have been linked to transposon silencing in the primordial mouse testis, but their function in the adult testis remains elusive. Here we report the cytological characterization of piRNAs in the adult mouse testis and the phenotypic analysis of Miwi(-/-); Mili(-/-) mice. We show that piRNAs are specifically present in germ cells, especially abundant in spermatocytes and early round spermatids, regardless of the type of the genomic sequences to which they correspond. piRNAs and PIWI proteins are present in both the cytoplasm and nucleus. In the cytoplasm, they are enriched in the chromatoid body; whereas in the nucleus they are enriched in the dense body, a male-specific organelle associated with synapsis and the formation of the XY body during meiosis. Moreover, by generating Miwi(-/-); Mili(-/-) mice, which lack all PIWI proteins in the adult, we show that PIWI proteins and presumably piRNAs in the adult are required only for spermatogenesis. Spermatocytes without PIWI proteins are arrested at the pachytene stage, when the sex chromosomes undergo transcriptional silencing to form the XY body. These results pinpoint a function of the PIWI protein subfamily to meiosis during spermatogenesis.