Sequence heterochrony led to a gain of functionality in an immature stage of the central complex: a fly-beetle insight

Animal behavior is guided by the brain. Therefore, adaptations of brain structure and function are essential for animal survival, and each species differs in such adaptations. The brain of one individual may even differ between life stages, for instance as adaptation to the divergent needs of larval and adult life of holometabolous insects. All such differences emerge during development but the cellular mechanisms behind the diversification of brains between taxa and life stages remain enigmatic. In this study, we investigated holometabolous insects, where larvae differ dramatically from the adult in both behavior and morphology. As consequence, the central complex, mainly responsible for spatial orientation, is conserved between species at the adult stage, but differs between larvae and adults as well as between larvae of different taxa. We used genome editing and established transgenic lines to visualize cells expressing the conserved transcription factor retinal homeobox, thereby marking homologous genetic neural lineages in both the fly Drosophila melanogaster and the beetle Tribolium castaneum. This approach allowed us for the first time to compare the development of homologous neural cells between taxa from embryo to the adult. We found complex heterochronic changes including shifts of developmental events between embryonic and pupal stages. Further, we provide, to our knowledge, the first example of sequence heterochrony in brain development, where certain developmental steps changed their position within the ontogenetic progression. We show that through this sequence heterochrony, an immature developmental stage of the central complex gains functionality in Tribolium larvae. We discuss the bearing of our results on the evolution of holometabolous larval central complexes by regression to a form present in an ancestor.