A unique sense of smell: development and evolution of a sexually dimorphic antennal lobe - a review

Pheromones are pivotal to sexual communication in insects. These chemical signals are processed by sexually dimorphic circuitries in the antennal lobe (AL) of the insect brain. However, there is limited understanding of how these circuitries form during AL development. Our review addresses this issue by comparing how circuitries develop throughout the growth processes of peripheral and deutocerebral neurons in various insect orders. Olfactory sensory neurons (OSNs) expressing novel pheromone receptors are eligible candidates to initiate new sexually-dimorphic circuitries when these OSNs survive programmed cell death and match the physiological properties of pheromone-sensing sensilla. The probability of these OSNs forming new glomeruli is largely determined by the degree of glia-OSN interactions and projection neuron (PN) prepatterning. The relative contribution of either of these processes determines the degree of evolutionary neuroplasticity, which is particularly prevalent in those species with complex ALs lacking specific macroglomerular structures. The extent of sexual dimorphism is determined by sex-determination genes, such as Doublesex and Fruitless, that regulate factors inducing OSN programmed cell death. Currently, these mechanisms are largely unexplored. This review, therefore, aims to provide a solid foundation for ongoing research into the evolution of AL sexual dimorphism and formation of pheromone circuitries in the light of insect sex determination.