Candidate Gene Screen in the Red Flour Beetle Tribolium Reveals Six3 as Ancient Regulator of Anterior Median Head and Central Complex Development

Abstract

Several highly conserved genes play a role in anterior neural plate patterning of vertebrates and in head and brain patterning of insects. However, head involution in Drosophila has impeded a systematic identification of genes required for insect head formation. Therefore, we use the red flour beetle Tribolium castaneum in order to comprehensively test the function of orthologs of vertebrate neural plate patterning genes for a function in insect head development. RNAi analysis reveals that most of these genes are indeed required for insect head capsule patterning, and we also identified several genes that had not been implicated in this process before. Furthermore, we show that Tc-six3/optix acts upstream of Tc-wingless, Tc-orthodenticle1, and Tc-eyeless to control anterior median development. Finally, we demonstrate that Tc-six3/optix is the first gene known to be required for the embryonic formation of the central complex, a midline-spanning brain part connected to the neuroendocrine pars intercerebralis. These functions are very likely conserved among bilaterians since vertebrate six3 is required for neuroendocrine and median brain development with certain mutations leading to holoprosencephaly.

Author Summary

All bilaterian animals evolved from one common ancestor. Previous gene function analyses have revealed that several genes play a role in the patterning of anterior regions in all bilaterian animals, suggesting similar mechanisms underlying anterior nervous system formation in humans and the patterning of the insect head and brain. In order to identify novel genes required for anterior development in insects, we have systematically tested genes known to be crucially involved in early nervous system development in vertebrates (e.g. mice and humans) for their activity in the head of the red flour beetle Tribolium casteneum. Indeed, all but one of these genes are required for head development. Intriguingly, we find that six3 is required for anterior median brain structures in insects just as it is in vertebrates, where six3 mutations lead to holoprosencephaly.