Secondary axis specification in sea urchin embryos

Secondary axis specification is a process that relies on asymmetric nuclearization of transcription factors in flies and vertebrates, such that the crucial factor is nuclear and therefore functional only in cells along one side of the embryo. In vertebrates, this transcription factor is β-catenin, which is canonically activated downstream from Wnt signals. However, the sea urchin uses asymmetric β-catenin nuclearization during specification of the primary animal-vegetal axis, rather than the secondary oral-aboral (OA) axis. OA specification relies instead on the asymmetric localization of p38 MAPK, a signaling kinase that directly modulates transcription factor activity. A number of genes are expressed in the oral territory downstream from p38, including Nodal and Goosecoid, both of which are associated with secondary axis specification in vertebrate embryos. Because the p38 asymmetry is the earliest known event in the specification of the OA axis, an outstanding question concerns identifying the apparatus upstream from p38 that regulates its asymmetrical activity. Intriguingly, this may be controlled by reactive oxygen species released from the mitochondria, which are asymmetrically distributed about the OA axis.