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Summary Background In insects, the gap gene hunchback ( hb ) is required for the formation of a set of adjacent segments through the regulation of downstream target genes of the pair rule and segment-polarity classes. In addition, hb is a major regulator of Hox genes and it has been suggested that this is the ancestral role of hb in insects or perhaps even arthropods. To date, however, hb function has been analyzed only in insects. Results Here we show that hb acts as a segmentation gene during anterior patterning of a noninsect arthropod, the spider Achaearanea tepidariorum . The leg-bearing segments L1, L2, and L4 are missing after downregulation of At-hb via RNAi. At-hb is required for the correct organization of target genes in this region of the embryo, suggesting that At-hb acts as a gap gene in the spider. In contrast to insects, hb does not control Hox gene expression in the spider. Furthermore, analysis of twist expression in At-hb knockdown embryos demonstrates that hb is not required for initiating the segmental organization of the mesoderm in the affected region, but only for its maintenance. Conclusions Our findings suggest that hb might have had a segmentation gene function in the arthropod ancestor and contradicts the suggestion that the control of Hox genes is the ancestral role of hb . Anterior spider segmentation thus utilizes a Drosophila -like genetic mode, whereas a vertebrate-like mechanism involving Wnt8 and Notch/Delta signaling is used to pattern posterior segments. These data support the hypothesis that short-germ arthropods employ two distinct mechanisms to segment their anterior and posterior body parts.

Summary 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 β-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.