divergent patterning of the chelicerate opisthosoma

has only been investigated in representatives of two arachnid orders to date: Acari (mites and ticks) and Araneae (spiders). Limited data are available for the “primitive ” arachnid orders, such as Scorpiones (scorpions) and Opiliones (harvestmen). Here, we present the first data on Hox gene expression in the harvestman Phalangium opilio. Ten Hox genes of this species were obtained from a de novo assembled developmental tran-scriptome using the Illumina GAII platform. All 10 genes are expressed in characteristic Hox-like expression patterns, and the expression of the anterior and central Hox genes is sim-ilar to those of other chelicerates. However, intriguingly, the three posteriormost genes—Ultrabithorax, abdominal-A, and Abdominal-B—share an identical anterior expression bound-ary in the second opisthosomal segment, and their expres-sion domains extend through the opisthosoma to the posterior growth zone. The overlap in expression domains of the pos-terior Hox genes is correlated with the absence of opisthoso-mal organs posterior to the tubular tracheae, which occur on the second opisthosomal segment. Together with the stag-gered profile of posterior Hox genes in spiders, these data suggest the involvement of abdominal-A and Abdominal-B in the evolution of heteronomous patterning of the chelicerate opisthosoma, providing a mechanism that helps explain the morphological diversity of chelicerates