caudal (cad͞Cdx) genes are essential for the formation of posterior structures in Drosophila, Caenorhabditis elegans, and vertebrates. In contrast to Drosophila, the majority of arthropods generate their segments sequentially from a posteriorly located growth zone, a process known as short-germ development. caudal homologues are expressed in the growth zone of diverse short-germ arthropods, but until now their functional role in these animals had not been studied. Here, we use RNA interference to examine the function of caudal genes in two short-germ arthropods, the crustacean Artemia franciscana and the beetle Tribolium castaneum. We show that, in both species, caudal is required for the formation of most body segments. In animals with reduced levels of caudal expression, axis elongation stops, resulting in severe truncations that remove most trunk segments. We also show that caudal function is required for the early phases of segmentation and Hox gene expression. The observed phenotypes suggest that in arthropods caudal had an ancestral role in axis elongation and segmentation, and was required for the formation of most body segments. Similarities to the function of vertebrate Cdx genes in the presomitic mesoderm, from which somites are generated, indicate that this role may also predate the origin of the Bilateria.caudal͞Cdx genes ͉ short-germ development ͉ Artemia ͉ Tribolium ͉ evolution T he caudal (cad͞Cdx) genes are homeobox genes involved in posterior patterning in diverse species (1-11). In early Drosophila embryos Caudal protein is distributed in a posterior to anterior concentration gradient that is needed for the activation of segmentation genes and segment formation in posterior parts of the animal. caudal mutant embryos have severe segmentation problems affecting posterior segments; in the most severely affected mutants, most abdominal segments are missing (1). This function of caudal is characteristic of long-germ development, found in Drosophila, where all of the body segments are molecularly determined during the blastoderm stage. This mode is thought to be evolutionarily derived within the higher insects and does not represent the ancestral mode of development (12). On the contrary, short-germ development is found in diverse and phylogenetically basal groups of insects and other arthropods, suggesting that this represents the ancestral mode for generating segments within the arthropods. In short-germ arthropods (which we take to include intermediate-germ species), only the most anterior segments are laid down in the blastoderm (e.g., Fig. 1A), whereas more posterior segments are generated sequentially from a posteriorly located presegmental zone, usually referred to as the ''growth zone '' (12-14). caudal homologues have been cloned in some short-germ arthropods and found to be expressed consistently in this presegmental zone (15-22), but until now their function in these species had not been studied.To explore the functional role of caudal genes in the growth zone of short-germ arthropods, we ...

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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.

Cytokine signaling through the JAK/STAT pathway regulates multiple cellular responses, including cell survival, differentiation, and motility. Although significant attention has been focused on the role of cytokines during inflammation and immunity, it has become clear that they are also implicated in normal brain function. However, because of the large number of different genes encoding cytokines and their receptors in mammals, the precise role of cytokines in brain physiology has been difficult to decipher. Here, we took advantage of Drosophila's being a genetically simpler model system to address the function of cytokines in memory formation. Expression analysis showed that the cytokine Upd is enriched in the Drosophila memory center, the mushroom bodies. Using tissue-and adult-specific expression of RNAi and dominantnegative proteins, we show that not only is Upd specifically required in the mushroom bodies for olfactory aversive long-term memory but the Upd receptor Dome, as well as the Drosophila JAK and STAT homologs Hop and Stat92E, are also required, while being dispensable for less stable memory forms.