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In order to investigate the role of T-box genes in arthropod development two optomotor-blind gene fragments (Ek-omb1 and Ek-omb2) and one H15 gene fragment (Ek-H15) were isolated from the onychophoran Euperipatoides kanangrensis already before I started this project.

I carried out a PCR-screening approach for further paralogs of the Ek-H15 gene using various H15-specific primers. The screening approach repeatedly resulted in the finding of the known single Ek-H15 gene fragment. When I PCR-screened cDNA from another peripatopsid species, Cephalofovea clandestina with H15-specific primers, I obtained a H15 ortholog gene fragment that I named Cc-H15 and a brachyury ortholog gene fragment that I named Cc-byn. Ek-H15 and Cc-H15 are expressed in identical patterns in both onychophoran species. The H15-genes are expressed in the heart, in the CNS, in the jaw, the oral papillae and in the walking limbs of the developing onychophoran embryo. Cc-byn is exclusively expressed in the hindgut region in Cephalofovea clandestina embryos of all examined stages.

The results of my PCR screen indicate the presence of a single H15 copy in onychophorans in contrast to the two or more paralogs found in other arthropod groups like insects, myriapods and spiders. In arthropods the omb  and H15 genes pattern the dorsal and ventral side of the leg respectively. The onychophoran H15 gene is not expressed along the ventral side of developing limbs, but its expression instead resembles the expression pattern of the chelicherate genes Ta-H15-2 and Cs-H15-2 that may represent a hypothetical “ground expression pattern” for arthropod and onyhophoran H15 genes.

The expression analysis of the omb genes in Euperipatoides kanangrensis shows that the dorsal limb specific expression pattern is conserved in arthropods and onychophorans. I reevaluated the expression of the two paralogous T-box-genes Ek-omb1 and Ek-omb2 in Euperipatoides kanangrensis with a special focus on earlier unavailable older embryonic stages. Both genes are expressed in the brain, the jaw, the oral papillae and in the walking limbs in a very similar but not identical pattern.