Expression of the Distalless-B gene in Ciona is regulated by a pan-ectodermal enhancer module

Abstract

The Ci-Dll-B gene is an early regulator of ectodermal development in the ascidian Ciona intestinalis (Imai et al., 2006). Ci-Dll-B is located in a convergently transcribed bigene cluster with a tandem duplicate, Ci-Dll-A. This clustered genomic arrangement is the same as those of the homologous vertebrate Dlx genes, which are also arranged in convergently transcribed bigene clusters. Sequence analysis of the C. intestinalis Dll-A-B cluster reveals a 378 bp region upstream of Ci-Dll-B, termed B1, which is highly conserved with the corresponding region from the congener Ciona savignyi. The B1 element is necessary and sufficient to drive expression of a lacZ reporter gene in a pattern mimicking the endogenous expression of Ci-Dll-B at gastrula stages. This expression pattern which is specific to the entire animal hemisphere is activated preferentially in posterior, or b-lineage, cells by a central portion of B1. Expression in anterior, or a-lineage cells, can be activated by this central portion in combination with the distal part of B1. Anterior expression can also be activated by the central part of B1 plus both the proximal part of B1 and non-conserved sequence upstream of B1. Thus, cis-regulation of early Ci-Dll-B expression is activated by a required submodule in the center of B1, driving posterior expression, which works in combination with redundant submodules that respond to differentially localized anterior factors to produce the total animal hemisphere expression pattern. Interestingly, the intergenic region of the cluster, which is important for expression of the Dlx genes in vertebrates, does not have a specific activating function in the reporter genes tested, but acts as an attenuator in combination with upstream sequences.

Vertebrate Cranial Placodes as Evolutionary Innovations—The Ancestor's Tale

“…While the neural ectoderm in Ciona expresses Zic and SoxB1, the Dlx2/3/5 homologue DllB, AP2, and GATA1/2/3 (but not FoxI) are widely expressed in nonneural ectoderm (Christiaen et al, 2002;Imai, Hino, Yagi, Satoh, & Satou, 2004;Imai, Levine, Satoh, & Satou, 2006;Irvine, Cangiano, Millette, & Gutter, 2007;Mazet et al, 2005;Miya & Nishida, 2003;Wada, Katsuyama, & Saiga, 1999;Wada & Saiga, 2002). DllB expression also covers nonneural rows V and VI of the "neural plate" and is required for activating epidermal and palp markers (Imai et al, 2006;Irvine, Vierra, Millette, Blanchette, & Holbert, 2011). It is still unclear how the oral siphon primordium (OSP), which has been fate-mapped to central row III/IV cells in Halocynthia (Nishida, 1987), escapes commitment to a neural fate.…”

Tfap2 and Sox1/2/3 cooperatively specify ectodermal fates in ascidian embryos

“…Because the upstream sequence of Dlx.b that drives a reporter in ectodermal cells contains Sox binding sites (Irvine et al, 2011), we tested the possibility that Sox1/2/3 directly regulates Dlx.b through these sites. A reporter construct containing this upstream region drove reporter gene expression at the late gastrula stage (Fig.…”

Heterogeneous Conservation of Dlx Paralog Co-Expression in Jawed Vertebrates

“…Outside the gnathostomes, no match for known CNEs could be identified in the P. marinus draft genome [32]. Outside the vertebrates, other regulatory sequences could be identified in Ciona , which were shown to be involved in driving expression in the non-neural ectoderm [49]. However the vertebrate CNEs could not be identified in Ciona and the Ciona element could not be isolated in vertebrates, suggesting that the conserved regulatory sequences obtained in bony fish were modified at some time between the “urochordates/vertebrates” divergence node and the “chondrichthyans/bony fish” divergence node.…”