A Unique Renal Architecture in Tribolium castaneum Informs the Evolutionary Origins of Systemic Osmoregulation in Beetles
Maintaining internal salt and water balance in response to fluctuating external conditions is essential for animal survival. This is particularly true for insects as their high surface-to-volume ratio makes them highly susceptible to osmotic stress. However, the cellular and hormonal mechanisms that mediate the systemic control of osmotic homeostasis in beetles (Coleoptera), the largest group of insects, remain largely unidentified. Here, we demonstrate that eight neurons in the brain of the red flour beetle Tribolium castaneum respond to internal changes in osmolality by releasing diuretic hormone (DH) 37 and DH47 - homologues of vertebrate corticotropinreleasing factor (CRF) hormones - to control systemic water balance. Knockdown of the gene encoding the two hormones (Urinate, Urn8) reduces renal secretion and restricts organismal fluid loss, whereas injection of DH37 or DH47 reverses these phenotypes. We further identify a novel CRF-like receptor, Urinate Receptor (Urn8R), which is exclusively expressed in a unique secondary cell (SC) in the beetle renal organs, as underlying this response. Activation of Urn8R increases K+ secretion specifically through SCs, creating a lumen-positive transepithelial potential that drives fluid secretion. Together, these data show that beetle renal organs operate by fundamentally different mechanism than those of other insects. Finally, we adopt a fluorescent labelling strategy to identify the evolutionary origin of this unusual renal architecture within the large Order of Coleoptera. Our work thus uncovers an important homeostatic program that is key to maintaining osmotic control in beetles, which evolved in parallel to the radiation of the higher beetle families. Significance StatementBeetles are the most diverse animal group on the planet. Their evolutionary success suggests unique physiological adaptations in overcoming water stress, yet the mechanisms underlying this ability are unknown. Here we use molecular genetic, electrophysiology and behavioral studies to show that a group of brain neurons responds to osmotic disturbances by releasing diuretic hormones that regulate salt and water balance. These hormones bind to their receptor exclusively localized to a unique secondary cell in the renal organs to modulate fluid secretion and organismal water loss. This renal architecture, common to all higher beetle families, is novel within the insects, and provides an important clue to the evolutionary success of the beetles in colonizing an astounding range of habitats on Earth.