The ten Hox genes of the millipede Glomeris marginata

We have isolated the ten Hox genes from the pill millipede Glomeris marginata (Myriapoda:Diplopoda). All ten genes are expressed in characteristic Hox-gene-like expression patterns. The register of Hox gene expression borders is conserved and the expression profiles show that the anterior-most limb-bearing segment in arthropods (antennal/cheliceral segment) does not express any Hox gene, while the next segment (intercalary/second-antennal/premandibular/pedipalpal segment) does express Hox genes. The Hox expression patterns in this millipede thus support the conclusion that all arthropods possess a deuterocerebral segment. We find that there is an apparent posterior shift of Hox gene expression domains dorsally relative to their ventral patterns, indicating that the decoupling of dorsal and ventral segmentation is not restricted to the level of segment polarity genes but apparently includes the Hox genes. Although the mechanism for the decoupling of dorsal and ventral segmentation remains unsolved, the decoupling must be at a level higher in the hierarchy than that of the segment polarity and Hox genes. The expression patterns of Ultrabithorax and abdominal-A suggest a correlation between the function of these genes and the delayed outgrowth of posterior trunk appendages. This delay may be caused by an assumed repressor function of Ultrabithorax, which might partially repress the activation of the Distal-less gene. The Glomeris fushi tarazu gene is expressed in a Hox-like domain and in the developing central nervous system, but not in segmental stripes such as has been reported in another myriapod species, the centipede Lithobius. In contrast to the Lithobius fushi tarazu gene, there is no indication for a role in segment formation for the millipede fushi tarazu gene, suggesting that fushi tarazu first acquired its segmentation function in the lineage of the insects.