Sea urchin goosecoid function links fate specification along the animal-vegetal and oral-aboral embryonic axes

We have identified a single homolog of goosecoid, SpGsc, that regulates cel l fates along both the animal-vegetal and oral-aboral axes of sea urchin em bryos. SpGsc mRNA is expressed briefly in presumptive mesenchyme cells of t he similar to 200-cell blastula and, beginning at about the same time, accu mulates in the presumptive oral ectoderm through pluteus stage. Loss-of-fun ction assays with morpholine-substituted antisense oligonucleotides show th at SpGsc is required for endoderm and pigment cell differentiation and for gastrulation. These experiments and gain-of-function tests by mRNA injectio n show that SpGsc is a repressor that antagonizes aboral ectoderm fate spec ification and promotes oral ectoderm. differentiation. We show that SpGsc c ompetes for binding to specific cis elements with SpOtx, a ubiquitous trans cription activator that promotes aboral ectoderm differentiation. Moreover, SpGsc represses transcription in vivo from an artificial promoter driven b y SpOtx. As SpOtx appears long before SpGsc transcription is activated, we propose that SpGse diverts ectoderm towards oral fate by repressing SpOtx t arget genes. Based on the SpGse-SpOtx example and other available data, we propose that ectoderm is first specified as aboral by broadly expressed act ivators, including SpOtx, and that the oral region is subsequently respecif ied by the action of negative regulators, including SpGsc. Accumulation of SpGsc in oral ectoderm depends on cell-cell interactions initiated by nucle ar beta -catenin function, which is known to be required for specification of vegetal tissues, because transcripts are undetectable in dissociated or in cadherin mRNA-injected embryos. This is the first identified molecular m echanism underlying the known dependence of oral-aboral ectoderm polarity o n intercellular signaling.