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.