How snakes smell with their tongue
The vomeronasal organ (VNO) is an olfactory structure that detects pheromones and environmental cues. It consists of sensory neurons that express evolutionary unrelated groups of transmembrane chemoreceptors. The predominant V1R and V2R receptor repertoires are believed to detect airborne and water-soluble molecules, respectively. It has been suggested that the shift in habitat of early tetrapods from water to land is reflected by an increase in the ratio of V1R/V2R genes. Snakes are missing from this analysis.
Snakes exhibit spectacular and exceptional adaptations that are readily recognizable. Deeply embedded in the popular psyche, snake tongue flicking (whose function is to deliver odorant molecules to the VNO) is for sure one of the characters most readily recognized by the public as "reptilian". It is likely that the evolutionary success of snakes is largely due to the exceptional development of their VNO. Indeed, the spectacular chemo-sensorial abilities of snakes are due to the exceptional development of their VNO associated with a sophisticated tongue delivery system. Blind snakes can survive in the wild because they rely much more on their VNO than on their visual system !
Using various molecular techniques, we now report in Genome Biology & Evolution on our analysis of the diversity, evolution, and expression pattern of the corn snake vomeronasal receptor repertoires. Our results indicate that snakes and lizards retain an extremely limited number of V1R genes but exhibit a large number of V2R genes, including multiple lineages of reptile-specific and snake-specific expansions. Hence, our analyses do not support the hypothesis that the shift to a vomeronasal receptor repertoire dominated by V1Rs in mammals reflects the evolutionary transition of early tetrapods from water to land.
This study sheds light on the evolutionary dynamics of the vomeronasal receptor families in vertebrates and reveals how mammals and squamate reptiles differentially adapted the same ancestral vomeronasal toolbox to succeed in a terrestrial environment.
Finally, we also show that the peculiar bigenic pattern of V2R vomeronasal receptor gene transcription observed in mammals is conserved in squamate reptiles, hinting at an important but unknown functional role played by this expression strategy.
This work was performed in Michel Milinkovitch’s lab in collaboration with Ivan Rodriguez, head of the Neurogenetics lab in the Dept of Genetics & Evolution of the University of Geneva.