Asymmetrically expressed axin required for anterior development in Tribolium

Abstract

Canonical Wnt signaling has been implicated in an AP axis polarizing mechanism in most animals, despite limited evidence from arthropods. In the long-germ insect, Drosophila, Wnt signaling is not required for global AP patterning, but in short-germ insects including Tribolium castaneum, loss of Wnt signaling affects development of segments in the growth zone but not those defined in the blastoderm. To determine the effects of ectopic Wnt signaling, we analyzed the expression and function of axin, which encodes a highly conserved negative regulator of the pathway. We found Tc-axin transcripts maternally localized to the anterior pole in freshly laid eggs. Expression spread toward the posterior pole during the early cleavage stages, becoming ubiquitous by the time the germ rudiment formed. Tc-axin RNAi produced progeny phenotypes that ranged from mildly affected embryos with cuticles displaying a graded loss of anterior structures, to defective embryos that condensed at the posterior pole in the absence of serosa. Altered expression domains of several blastodermal markers indicated anterior expansion of posterior fates. Analysis of other canonical Wnt pathway components and the expansion of Tc-caudal expression, a Wnt target, suggest that the effects of Tc-axin depletion are mediated through this pathway and that Wnt signaling must be inhibited for proper anterior development in Tribolium. These studies provide unique evidence that canonical Wnt signaling must be carefully regulated along the AP axis in an arthropod, and support an ancestral role for Wnt activity in defining AP polarity and patterning in metazoan development.A-P axis patterning | short-germ segmentation | anterior patterning | maternal determination

Anterior and posterior centers jointly regulate Bombyx embryo body segmentation

“…Of note, however, is the recently identified negative regulator of the canonical Wnt pathway, the axin homolog from Tribolium (Tc-axin), with maternal transcripts that are localized in the anterior poles of freshly laid eggs. This regulator suppresses the anterior expression of the posterior organizing molecule Tc-cad (see below), a Wnt pathway target (Fu et al, 2012). Whether Tc-axin or other anteriorly localized molecules function as anterior instruction cues in short germ insects remains to be elucidated.…”

Speed regulation of genetic cascades allows for evolvability in the body plan specification of insects

“…If cad acts as the speed regulator of gap genes, then, according to the speed regulation model, changes affecting the cad gradient should also affect the dynamics of gap gene expression, including their sequential activation at the posterior end of the Tribolium embryo. To test this assumption, we analyzed the timing of gap gene regulation after knocking down legless (lgs, a positive regulator of cad through Wnt) (7,(45)(46)(47) Fig. 2 A and B; see ref.…”

Antero-posterior patterning of Drosophila ocelli requires an anti-repressor mechanism within the hh-pathway mediated by the Six3 gene Optix

“…This work has expanded our understanding of the general mechanisms by which the conserved Wnt and Hh signaling pathways interact (Royet and Finkelstein, 1997) and of the development and evolution of the eyes and dorsal head of arthropods (Posnien et al, 2010(Posnien et al, , 2011Samadi et al, 2015;Schomburg et al, 2015), and has helped in establishing parallels between head patterning across phyla. For instance, members of both the Wnt and Otx gene families are involved in anterior head/ neural tube patterning in both invertebrates and vertebrates (Lichtneckert and Reichert, 2005;Rhinn et al, 2005;Schinko et al, 2008;Steinmetz et al, 2011;Fu et al, 2012).…”