Axis specification in the spider embryo:dppis required for radial-to-axial symmetry transformation andsogfor ventral patterning

Abstract

The mechanism by which Decapentaplegic (Dpp) and its antagonist Short gastrulation (Sog) specify the dorsoventral pattern in Drosophila embryos has been proposed to have a common origin with the mechanism that organizes the body axis in the vertebrate embryo. However, Drosophila Sog makes only minor contributions to the development of ventral structures that hypothetically correspond to the vertebrate dorsum where the axial notochord forms. In this study, we isolated a homologue of the Drosophila sog gene in the spider Achaearanea tepidariorum, and characterized its expression and function. Expression of sog mRNA initially appeared in a radially symmetrical pattern and later became confined to the ventral midline area, which runs axially through the germ band. RNA interference-mediated depletion of the spider sog gene led to a nearly complete loss of ventral structures, including the axial ventral midline and the central nervous system. This defect appeared to be the consequence of dorsalization of the ventral region of the germ band. By contrast, the extra-embryonic area formed normally. Furthermore, we showed that embryos depleted for a spider homologue of dpp failed to break the radial symmetry, displaying evenly high levels of sog expression except in the posterior terminal area. These results suggest that dpp is required for radial-to-axial symmetry transformation of the spider embryo and sog is required for ventral patterning. We propose that the mechanism of spider ventral specification largely differs from that of the fly. Interestingly, ventral specification in the spider is similar to the process in vertebrates in which the antagonism of Dpp/BMP signaling plays a central role in dorsal specification.

Sog/Chordin is required for ventral-to-dorsal Dpp/BMP transport and head formation in a short germ insect

“…Because chordin͞sog is common to all systems and because Tc-Sog is the main BMP inhibitor in Tribolium, we suggest that a common ancestor of vertebrates and insects possessed a BMP-signaling system in which Sog͞Chordin is the sole BMP antagonist. Consistently, in a spider, sog also is the main BMP inhibitor, and its loss leads to the complete absence of the CNS (37).…”

Dynamic gene expression is required for anterior regionalization in a spider

“…Whole mount in situ hybridizations and TUNEL were performed with modifications for spider embryos as described previously (18,35,36). Parental RNAi was performed as described previously (18).…”

“…Whole mount in situ hybridizations and TUNEL were performed with modifications for spider embryos as described previously (18,35,36). Parental RNAi was performed as described previously (18). We used dsRNA against 3 different fragments of At-otd-1: A large 1042-bp fragment (nt 1-1042), covering almost the complete sequence, and 2 nonoverlapping fragments of 577 bp (nt 113-689) and 350 bp (nt 693-1042).…”

“…Because otd is an early anterior patterning gene in insects (7,8) and the early At-otd-1 expression is consistent with a role in anterior patterning, we next tested the function of At-otd-1 using parental RNAi (18). In embryos from mothers that had been injected with At-otd-1 dsRNA (otd pRNAi embryos) the anterior At-otd-1 expression is strongly reduced (Fig.…”

Phenotypic analysis of a novel chordin mutant in medaka

“…chordin is one of the well-conserved genes and is required for early dorsoventral (DV) pattering of embryos not only in vertebrates but also in invertebrates such as flies or spiders (Sasai et al, 1994;Francois et al, 1994;Akiyama-Oda and Oda, 2006). In vertebrates, Chordin is expressed in the organizer region and functions as an extracellular antagonist for bone morphogenetic proteins (BMPs; Sasai et al, 1995;Piccolo et al, 1996).…”