Wnt8 Is Required for Growth-Zone Establishment and Development of Opisthosomal Segments in a Spider

Abstract

The Wnt genes encode secreted glycoprotein ligands that regulate many developmental processes from axis formation to tissue regeneration [1]. In bilaterians, there are at least 12 subfamilies of Wnt genes [2]. Wnt3 and Wnt8 are required for somitogenesis in vertebrates [3-7] and are thought to be involved in posterior specification in deuterostomes in general [8]. Although TCF and beta-catenin have been implicated in the posterior patterning of some short-germ insects [9, 10], the specific Wnt ligands required for posterior specification in insects and other protostomes remained unknown. Here we investigated the function of Wnt8 in a chelicerate, the common house spider Achaearanea tepidariorum[11]. Knockdown of Wnt8 in Achaearanea via parental RNAi caused misregulation of Delta, hairy, twist, and caudal and resulted in failure to properly establish a posterior growth zone and truncation of the opisthosoma (abdomen). In embryos with the most severe phenotypes, the entire opisthosoma was missing. Our results suggest that in the spider, Wnt8 is required for posterior development through the specification and maintenance of growth-zone cells. Furthermore, we propose that Wnt8, caudal, and Delta/Notch may be parts of an ancient genetic regulatory network that could have been required for posterior specification in the last common ancestor of protostomes and deuterostomes.

Dynamic gene expression is required for anterior regionalization in a spider

“…Anterior patterning in the spider, like posterior patterning (21-24), thus involves dynamic gene expression. While posterior patterning involves several waves of dynamic gene expression (21)(22)(23)(24), anterior pattering involves only a single wave of gene expression for each gene. We showed that At-otd-1 is required for controlling these dynamics during anterior patterning but it is unlikely that the transcription factor Otd is the only component controlling these dynamics.…”

“…Animals of the common house spider Achaearanea tepidariorum (Chelicerata, Aranida, Theridiidae) are kept in a colony in Cologne (24). Embryos are obtained as described (17).…”

The Evolution of the Wnt Pathway

“…In the spider Achaearanea sp. Wnt8 is required for posterior development and the maintenance of the growth zone (McGregor et al 2008). It is expressed in a solid circular domain at the posterior end of a radial symmetric embryo before its transition to axial symmetry (Fig.…”

“…It is expressed in a solid circular domain at the posterior end of a radial symmetric embryo before its transition to axial symmetry (Fig. 1A in McGregor et al 2008). In Drosophila, Wnt8 has a completely different function and acts as a feedback inhibitor of the NF-kB homolog Dorsal (the gene was therefore named Wnt inhibitor of Dorsal, WntD) in DV signaling and immunity (Gordon et al 2005;Gordon and Nusse 2006).…”

Asymmetrically expressed axin required for anterior development in Tribolium

“…Ectopic activation of Wnt targets by loss of axin function produces segmental defects (5) but does not result in posteriorization or loss of anterior identities. In Tribolium and other short-germ arthropods, where segments are added sequentially, posterior Wnt signals are required for germ-band elongation and formation of abdominal segments (6)(7)(8). Segments that form in the Tribolium blastoderm develop normally, suggesting Wnt signaling is not necessary for anterior development.…”