Homeotic genes and the arthropod head: Expression patterns of the labial, proboscipedia, and Deformed genes in crustaceans and insects

cDNA fragments of the homologues of the Drosophila head homeotic genes labi al (lab), proboscipedia (pb), and Deformed (Dfd) have been isolated from th e crustacean Porcellio scaber. Because the accumulation domains of the head homeotic complex (Hox) genes had not been previously reported for crustace ans, we studied the expression patterns of these genes in P. scaber embryos by using in situ hybridization, The P, scaber lab homologue is expressed i n the developing second antennal segment and its appendages. This expressio n domain in crustaceans and in the homologous intercalary segment of insect s suggests that the lab gene specified this metamere in the last common anc estor of these two groups. The expression domain of the P. scaber pb gene i s in the posterior part of the second antennal segment. This domain, in con trast to that in insects, is colinear with the domains of other head genes in P. scaber, and it differs from the insect pb gene expression domain in t he posterior mouthparts, suggesting that the insect and crustacean patterns evolved independently from a broader ancestral domain similar to that foun d in modern chelicerates. P. scaber Dfd is expressed in the mandibular segm ent and paragnaths (a pair of ventral mouthpart structures associated with the stomodeum) and differs from insects, where expression is in the mandibu lar and maxillary segments. Thus, like pb, Dfd shows a divergent Hox gene d eployment. We conclude that homologous structures of the mandibulate head d isplay striking differences in their underlying developmental programs rela ted to Hox gene expression.