b-catenin specifies the endomesoderm and defines the posterior organizer of the hemichordate XXXX .

@entity2270 @entity4409 @entity72862 @entity2472
The canonical Wnt/b-catenin pathway is a key regulator of body plan organization and @entity2270 formation in metazoans, being involved in germ layer specification, posterior growth and patterning of the anteroposterior @entity2270 . Results from animals spanning a wide phylogenetic range suggest that a unifying function of b-catenin in metazoans is to define the posterior/vegetal part of the embryo. Although the specification of vegetal territories (endoderm) by b-catenin has been demonstrated in distantly related animals (cnidarians, a protostome, echinoderms and ascidians), the definition of the posterior part of the embryo is well supported only for vertebrates and planarians. To gain insights into b-catenin functions during deuterostome evolution, we have studied the early development of the direct developing hemichordate @entity72862 . We show that the zygote is polarized after fertilization along the @entity4409 by cytoplasmic rearrangements resembling the @entity2472 . This early asymmetry is translated into nuclear accumulation of b-catenin at the vegetal pole, which is necessary and sufficient to specify endomesoderm. We show that endomesoderm specification is crucial for anteroposterior @entity2270 establishment in the ectoderm. The endomesoderm secretes as yet unidentified signals that posteriorize the ectoderm, which would otherwise adopt an anterior fate. Our results point to a conserved function at the base of deuterostomes for b-catenin in germ layer specification and to a causal link in the definition of the posterior part of the embryonic ectoderm by way of activating posteriorizing endomesodermal factors. Consequently, the definition of the vegetal and the posterior regions of the embryo by b-catenin should be distinguished and carefully re-examined.
