Dynamic gene expression is required for anterior regionalization in a spider.

Patterning of a multicellular embryo requires precise spatiotemporal control of gene expression during development. The gradient of the morphogen bicoid regulates anterior regionalization in the syncytial blastoderm of Drosophila. However many arthropod embryos develop from a cellular blastoderm that does not allow the formation of transcription factor gradients. Here we show that correct anterior development of the cellularized embryo of the spider Achaearanea tepidariorum requires an anterior-to-posterior wave of dynamic gene expression for positioning the stripes of hairy, hedgehog, and orthodenticle expression. Surprisingly, this dynamic repositioning of the expression of these segmentation genes is blocked in orthodenticle(pRNAi) embryos and no anterior structures are specified in those embryos. Our data suggest that dynamic gene expression across a field of cells is required for anterior regionalization in spiders and provides an explanation for the problem of how positional values for anterior segmentation genes are specified via a morphogen-independent mechanism across a field of cells.