Crustacean (malacostracan) Hox genes and the evolution of the arthropod trunk

Representatives of the Insecta and the Malacostraca (higher crustaceans) ha ve highly derived body plans subdivided into several tagma, groups of segme nts united by a common function and/or morphology, The tagmatization of seg ments in the trunk, the part of the body between head and telson, in both l ineages is thought to have evolved independently from ancestors with a dist inct head but a homonomous, undifferentiated trunk, In the branchiopod crus tacean, Artemia franciscana, the trunk Hox genes are expressed in broad ove rlapping domains suggesting a conserved ancestral state (Averof, M, and Aka m, M, (1995) Nature 376, 420-423). In comparison, in insects, the Antennnpe dia-class genes of the homeotic clusters are more regionally deployed into distinct domains where they serve to control the morphology of the differen t trunk segments, Thus an originally Artemia-like pattern of homeotic gene expression has apparently been modified in the insect lineage associated wi th and perhaps facilitating the observed pattern of tagmatization, Since in sects are the only arthropods with a derived trunk tagmosis tested to date, we examined the expression patterns of the Hox genes Antp, Ubx and abd-A i n the malacostracan crustacean Porcellio scaber (Oniscidae, Isopoda), We fo und that, unlike the pattern seen in Artemia, these genes are expressed in well-defined discrete domains coinciding with tagmatic boundaries which are distinct from those of the insects, Our observations suggest that, during the independent tagmatization in insects and malacostracan crustaceans, the homologous 'trunk' genes evolved to perform different developmental functi ons, We also propose that, in each lineage, the changes in Hox gene express ion pattern may have been important in trunk tagmatization.