An atlas of the developing Tribolium castaneum brain reveals conservation in anatomy and divergence in timing to Drosophila melanogaster

Insect brains are formed by conserved sets of neural lineages whose fibers form cohesive bundles with characteristic projection patterns. Within the brain neuropil, these bundles establish a system of fascicles constituting the macrocircuitry of the brain. The overall architecture of the neuropils and the macrocircuitry appear to be conserved. However, variation is observed, for example, in size, shape, and timing of development. Unfortunately, the developmental and genetic basis of this variation is poorly understood, although the rise of new genetically tractable model organisms such as the red flour beetle Tribolium castaneum allows the possibility to gain mechanistic insights. To facilitate such work, we present an atlas of the developing brain of T. castaneum, covering the first larval instar, the prepupal stage, and the adult, by combining wholemount immunohistochemical labeling of fiber bundles (acetylated tubulin) and neuropils (synapsin) with digital 3D reconstruction using the TrakEM2 software package. Upon comparing this anatomical dataset with the published work in Drosophila melanogaster, we confirm an overall high degree of conservation. Fiber tracts and neuropil fascicles, which can be visualized by global neuronal antibodies like antiacetylated tubulin in all invertebrate brains, create a rich anatomical framework to which individual neurons or other regions of interest can be referred to. The framework of a largely conserved pattern allowed us to describe differences between the two species with respect to parameters such as timing of neuron proliferation and maturation. These features likely reflect adaptive changes in developmental timing that govern the change from larval to adult brain.

Automated summary

Insect brains are formed by conserved sets of neural lineages with cohesive bundles of fibers and characteristic projection patterns. The overall architecture of the neuropils and macrocircuitry is conserved, but variations in size, shape, and timing of development can occur. The developmental and genetic basis of this variation is poorly understood, but the use of genetically tractable model organisms like Tribolium castaneum allows for potential mechanistic insights. This study presents an atlas of the developing brain of T. castaneum, providing a comparison with Drosophila melanogaster and confirming a high degree of conservation. The findings highlight differences between the two species in terms of neuron proliferation and maturation timing, likely reflecting adaptive changes in brain development from larvae to adults.