Topoisomerase 1α is required for synchronous spermatogenesis in Physcomitrium patens

DNA topoisomerase 1 (TOP1) plays general roles in DNA replication and transcription by regulating DNA topology in land plants and metazoans. TOP1 is also involved in specific developmental events; however, whether TOP1 plays a conserved developmental role among multicellular organisms is unknown. Here, we investigated the developmental roles of TOP1 in the moss Physcomitrium (Physcomitrella) patens with gene targeting, microscopy, 3D image segmentation and crossing experiments. We discovered that the disruption of TOP1α, but not its paralogue TOP1β, leads to a defect in fertilisation and subsequent sporophyte formation in P. patens. In the top1α mutant, the egg cell was functional for fertilisation, while sperm cells were fewer and infertile with disordered structures. We observed that the nuclei volume of wild-type sperm cells synchronously decreases during antheridium development, indicating chromatin condensation towards the compact sperm head. By contrast, the top1α mutant exhibited attenuated cell divisions and asynchronous and defective contraction of the nuclei of sperm cells throughout spermatogenesis. These results indicate that TOP1α is involved in cell division and chromatin condensation during spermatogenesis in P. patens. Our results suggest that the regulation of DNA topology by TOP1 plays a key role in spermatogenesis in both land plants and metazoans.