BETA-CATENIN IS ESSENTIAL FOR PATTERNING THE MATERNALLY SPECIFIED ANIMAL-VEGETAL AXIS IN THE SEA-URCHIN EMBRYO

In sea urchin embryos, the animal-vegetal axis is specified during oog enesis, After fertilization, this axis is patterned to produce five di stinct territories by the 60 cell stage. Territorial specification is thought to occur by a signal transduction cascade that is initiated by the large micromeres located at the vegetal pole. The molecular mecha nisms that mediate the specification events along the animal-vegetal a xis in sea urchin embryos are largely unknown. Nuclear beta-catenin is seen in vegetal cells of the early embryo, suggesting that this prote in plays a role in specifying vegetal cell fates. Here, we test this h ypothesis and show that beta-catenin is necessary for vegetal plate sp ecification and is also sufficient for endoderm formation. In addition , we show that beta-catenin has pronounced effects on animal blastomer es and is critical for specification of aboral ectoderm and for ectode rm patterning, presumably via a noncell-autonomous mechanism. These re sults support a model in which a Wnt-like signal released by vegetal c ells patterns the early embryo along the animal-vegetal axis. Our resu lts also reveal similarities between the sea urchin animal-vegetal axi s and the vertebrate dorsal-ventral axis, suggesting that these axes s hare a common evolutionary origin.