Cytological observation of the fertilization of fern Lygodium japonicum

[Objective] The study aims to elucidate the characteristics and cytological mechanisms of fertilization in the evolutionary special group of fern Lygodium japonicum . [Methods] Using embedding and electron microscopy techniques, ultrastructural observation was performed on the stage of sperm entry into the egg, nuclear fusion, egg expansion, and zygote polarity establishment during the fertilization process of L. japonicum . [Results] L. japonicum didn􀆳t have the typical egg envelope and fertilization pore, and spermatozoa penetrated the egg through the pore region. The first spermatozoon entered the cytoplasm of the egg, and 2-6 spermatozoa could also pass through the pore region to enter the fertilization cavity below the pore region of the egg, but could not fuse with the egg cytoplasm. After sperm entered the egg, the fertilized egg shrinked markedly to prevent the entry of other sperms, and a membrane structure was formed at the pore region above the fertilization cavity, blocking the entry of other sperms into the fertilization cavity. Nuclear fusion was initiated 30 min after spermatozoa entered the egg cell. The nucleus of spermatozoa was initially concentrated at one end of the nucleus and gradually penetrated into the nucleus of the egg, and nuclear fusion continued as the fertilized egg were expanding. Around 10 h after fertilization, the zygote produced a large number of vesicles, which first established vertical polarity and then changed to horizontal polarity, and underwent the first division of the zygote. [Conclusion] When L. japonicum undergoes fertilization, spermatozoa enters from the pore region and produces a fertilization cavity, similar to the core leptosporangiatae.