Posterior patterning genes and the identification of a unique body region in the brine shrimpArtemia franciscana

Abstract

All arthropods share the same basic set of Hox genes, although the expression of these genes differs among divergent groups. In the brine shrimp Artemia franciscana, their expression is limited to the head,thoracic/trunk and genital segments, but is excluded from more posterior parts of the body which consist of six post-genital segments and the telson (bearing the anus). Nothing is currently known about the genes that specify the identity of these posterior structures. We examine the expression patterns of four candidate genes, Abdominal-B, caudal/Cdx, even-skipped/Evx and spalt, the homologues of which are known to play an important role in the specification of posterior structures in other animals. Abdominal-B is expressed in the genital segments of Artemia, but not in the post-genital segments at any developmental stage. The expression of caudal, even-skipped and spalt in the larval growth-zone suggests they may play a role in the generation of body segments (perhaps comparable with the role of gap and segmentation genes in insects), but not a direct role in defining the identity of post-genital segments. The expression of caudal at later stages suggests a role in the specification of anal structures. A PCR screen designed to isolate Hox genes expressed specifically in the posterior part of the body failed to identify any new Hox genes. We conclude that the post-genital segments of Artemia are not defined by any of the genes known to play a role in the specification of posterior segments in other arthropods. We argue that these segments constitute a unique body region that bears no obvious homology to previously characterised domains of Hox gene activity.

Ancestral role of caudal genes in axis elongation and segmentation

“…In Artemia, caudal is expressed in the growth zone starting from the earliest larval stages and persisting there until the last body segment is formed (20). The expression is restricted to the ectoderm and coincides with the elongation of the body axis and generation of all trunk segments from the growth zone ( Fig.…”

“…To further characterize the effects of caudal RNAi at the molecular level, we examined the expression of segmentation and Hox genes during the process of axis elongation and segmentation. In wild-type Tribolium and Artemia, Evenskipped is expressed in the growth zone and in stripes corresponding to newly formed segments (20,29,32). In Tribolium showing weak RNAi phenotypes (some elongation has taken place), Even-skipped expression in the region of the growth zone is weak and segmental expression is absent (Fig.…”

“…Immunochemical stainings in Artemia were carried out using specific polyclonal antibodies for Caudal and Eve (20), the monoclonal antibody 4F11 for En (26), and the monoclonal antibody FP6.87 for Ubx͞AbdA (27). Whole-mount immunochemical stainings were carried out according to standard protocols (28), with sonication of varying strengths, depending on the developmental stage.…”

“…The similarity of these phenotypes suggests that caudal genes have a common function in axis elongation and segmentation in diverse short-germ arthropods. Given that caudal is similarly expressed in short-germ insects, crustaceans, and myriapods (15)(16)(17)(18)(19)(20)(21)(22), this function of caudal most probably represents an ancestral function, deriving from the common ancestor of all arthropods.…”

“…1A), whereas more posterior segments are generated sequentially from a posteriorly located presegmental zone, usually referred to as the ''growth zone'' (12)(13)(14). caudal homologues have been cloned in some short-germ arthropods and found to be expressed consistently in this presegmental zone (15)(16)(17)(18)(19)(20)(21)(22), but until now their function in these species had not been studied.…”