Observations on Fertilization and a Novel Cytological Mechanism for Preventing Polyspermy in the FernOsmunda japonica

Abstract

Premise of research. In advanced ferns that possess an egg envelope and a fertilization pore, a sac that persistently blocks the fertilization pore is considered a barrier for preventing polyspermy. However, for those ferns with no egg envelope and fertilization pore, the cytological mechanism for preventing polyspermy is still unclear. In this investigation, the main cytological processes during fertilization and zygote formation of the primitive fern Osmunda japonica are observed, and a novel cytological mechanism for preventing polyspermy is described.Methodology. The fertilization process was observed under stereomicroscope, and the fertilization time was recorded. The selected plants were fixed with glutaraldehyde and osmic acid and embedded in pure Spurr’s resin. Specimens were first thick-sectioned and then thin-sectioned for LM and TEM observations.Pivotal results. The observations showed that only one spermatozoid penetrates into the egg at the center in the upper surface of the mature egg. The egg immediately shrinks as the spermatozoid enters into the cytoplasm of the egg. The volume of the fertilized egg decreases remarkably, and simultaneously, a large vesicle forms above the fertilized egg. This vesicle can block other spermatozoids approaching the egg. During nuclear fusion, the male cytoskeleton defines a spatial framework, within which the male chromatin progressively fuses with the egg nucleus. The male organelles, including the microtubular ribbon, multilayered structure, flagella, and male mitochondria, are finally digested in the zygotic cytoplasm. A functional zygote is then formed at the venter of the archegonium.Conclusions. In the primitive fern O. japonica, the large vesicle, formed during fertilization above the egg, can block other spermatozoids approaching the egg, which is considered to be a novel cytological mechanism for preventing polyspermy.