Chelicerate Hox genes and the homology of arthropod segments

Genes of the homeotic complex (HOM-C) in insects and vertebrates are requir ed for the specification of segments along the antero-posterior axis. Multi ple paralogues of the Hox genes in the horseshoe crab Limulus poliphemus ha ve been used as evidence for HOM-C duplications in the Chelicerata. We addr essed this possibility through a limited PCR survey to sample the homeoboxe s of two spider species, Steatoda triangulosa and Achaearanea tepidariorum. The survey did not provide evidence for multiple Hox clusters although we have found apparent duplicate copies of proboscipedia (pb) and Deformed (Df d). In addition, we have cloned larger cDNA fragments of pb, zerknullt (zen /Hox3) and Dfd. These fragments allowed the determination of mRNA distribut ion by in situ hybridization. Our results are similar to the previously pub lished expression patterns of Hox genes from another spider and an oribatid mite. Previous studies compared spider/mite Hox gene expression patterns w ith those of insects and argued for a pattern of segmental homology based o n the assumption that the co-linear anterior boundaries of the Hox domains can be used as markers. To test this assumption we performed a comparative analysis of the expression patterns for UBX/ABD-A in chelicerates, myriapod s, crustaceans, and insects. We conclude that the anterior boundary can be and is changed considerably during arthropod evolution and, therefore, Hox expression patterns should not be used as the sole criterion for identifyin g homology in different classes of arthropods.