Divergent functions of orthodenticle, empty spiracles and buttonhead in early head patterning of the beetle Tribolium castaneum (Coleoptera).

The head gap genes orthodenticle (otd), empty spiracles (ems) and buttonhead (btd) are required for metamerization and segment specification in Drosophila. We asked whether the function of their orthologs is conserved in the red flour beetle Tribolium castaneum which in contrast to Drosophila develops its larval head in a way typical for insects. We find that depending on dsRNA injection time, two functions of Tc-orthodenticle1 (Tc-otd1) can be identified. The early regionalization function affects all segments formed during the blastoderm stage while the later head patterning function is similar to Drosophila. In contrast, both expression and function of Tc-empty spiracles (Tc-ems) are restricted to the posterior part of the ocular and the anterior part of the antennal segment and Tc-buttonhead (Tc-btd) is not required for head cuticle formation at all. We conclude that the gap gene like roles of ems and btd are not conserved while at least the head patterning function of otd appears to be similar in fly and beetle. Hence, the ancestral mode of insect head segmentation remains to be discovered. With this work, we establish Tribolium as a model system for arthropod head development that does not suffer from the Drosophila specific problems like head involution and strongly reduced head structures.