Irregular vomeronasal system of hedgehogs is intermediate between segregated and uniform: how was type-2 receptor (V2R)-pathway in Laurasiatheria lost during evolution?

This article focuses on the vomeronasal system (VNS) of hedgehogs (Atelerix albiventris) and its evolutionary implications within the Laurasiatheria clade. Unlike most Laurasiatherian species, which possess a uniform VNS characterized by only vomeronasal type-1 receptor (V1R)/Gαi2-neurons projecting throughout the accessory olfactory bulb (AOB), hedgehogs retain both V1R/Gαi2- and vomeronasal type-2 receptor (V2R)/Gαo-neurons. Immunohistochemical analyses revealed that hedgehogs exhibit an intermediate VNS projection pattern, with V1R/Gαi2-axons distributed across the entire AOB and V2R/Gαo-axons primarily localized to the posterior AOB, sometimes co-innervating the same glomeruli. These findings suggest that hedgehogs represent an evolutionary transitional state reflecting the gradual loss of the V2R/Gαo-mediated posterior pathway observed in other Laurasiatherian mammals.