Figure 8. Conservation of expression patterns of orthologs of sea urchin genes that function in the development of ANE.

The territorial expression of orthologs in each embryo is shown to the left and right of each diagram. In cases where there is no information concerning the expression pattern of a particular ortholog, it is represented in light gray. Red asterisks designate that functional studies show these factors are involved in AP patterning of the presumptive neuroectoderm. The images of embryos are colored to indicate the expression patterns of foxq2, six3, wnt1, and wnt8. (A) ANE factors are initially expressed throughout the presumptive zebrafish neuroectoderm at late blastula/early gastrula stages (left-hand diagram). These ANE factors are progressively down-regulated in posterior regions of the presumptive neuroectoderm (dark gray) during gastrulation by a mechanism involving Fzl8a, Wnt8, Wnt1, and Dkk1, until they are confined to the anterior pole (middle diagram). Factors such as sFrp1 and Dkk3 subsequently pattern the forebrain. The right-hand diagram shows that, in the absence of Wnt/β-catenin and BMP signaling, ANE factors are expressed throughout most of the embryo. Embryos are oriented with their dorsal sides facing up from the page. Data taken from [17],[21],[29],[73]–[77]. (B) In amphioxus embryos, foxq2 is expressed throughout the anterior half, and wnt8 throughout the vegetal plate, of early gastrula embryos (left-hand diagram). By late gastrula, foxq2 and the putative ANE factors dkk1, six3, and dkk3 are expressed in the anterior-most ectoderm. wnt8 and wnt1 are expressed posterior to these putative ANE factors, consistent with a role in the restriction of foxq2, six3, and dkk3 expression to the anterior pole (middle diagram). Data taken from [78]–[82]. (C) Sea urchin embryo ANE factors are initially expressed throughout the presumptive ectoderm, and wnt1 and wnt8 are both expressed in the posterior half of blastula stage embryos (left-hand diagram). Then, ANE factors are progressively down-regulated from posterior ectoderm by a Wnt1, Wnt8, Fzl5/8, and Dkk1-dependent mechanism (middle diagram). In the absence of Wnt/β-catenin and TGF-β signaling, the entire sea urchin embryo expresses ANE factors (right-hand diagram). Data taken from this study and [22],[24]. (D) In blastula stage hemichordate embryos, foxq2 is expressed broadly in the anterior half of the embryo (data show that six3 and fzl5/8 also are expressed broadly by early gastrula stages) (left-hand diagram). By late blastula stages, putative ANE factors foxq2, six3, and sfrp1/5 are restricted to the anterior-most ectoderm, and functional data show that sfrp1/5 restriction involves Fzl5/8 (middle diagram). In the absence of Wnt/β-catenin signaling the entire hemichordate embryo expresses putative ANE factors six3 and sfrp1/5 (right-hand diagram). Data taken from [2],[19],[83],[84].