Polar bodies serve as a landmark for anteroposterior axis formation in spiders

The early embryogenesis of many spiders involves the formation of a radially symmetric germ disc. While the cells of the rim of this germ disc develops into anterior structures, the center of the disc will form the posteriorly located segment addition zone of the embryo. Therefore, germ disc formation sets the anterior-posterior (AP) body axis of spider embryos. The early spider egg is a spherical structure with no apparent asymmetry. So far it is unclear if the placement of the germ disc is a predetermined or a stochastic process. For this study, we have re-analysed early spider embryogenesis and found a link between the position of the polar bodies and the formation of the germ disc. Using cell tracking and time-lapse recordings of live embryos injected with the live cell nuclear stain SPY555-DNA, we show that the germ disc forms opposite to the polar bodies. Our results suggest that germ disc formation in the common house spider Parasteatoda tepidariorum is not a stochastic but a pre-determined process. By analyzing germ disc formation in a basally branching cobweb spider and a curtain-web spider, we provide evidence that the initial process of germ disc placement might be conserved between araneomorph and mygalomorph spider species.