Evidence suggesting three respiratory oscillators in frogs (Rana catasbeiana): implications for the Oscillator Homology Hypothesis and the evolution of breathing.

Recent data in mammals and frogs suggest at least two separate respiratory oscillators, one who's activity coincides with the powerstroke of lung inflation (the preBötzinger Complex and the lung oscillator, respectively) and a second priming oscillator, who's activity appears to precede the powerstroke (the PreI and buccal oscillators, respectively). According to the Oscillator Homology Hypothesis, the oscillators in mammals evolved from the same ancestors as those in lower vertebrates. However, the priming oscillator in mammals and frogs maybe located in different rhombomeric segments and their rostral‐caudal location relative to the powerstroke oscillator may be reversed. Using the “sheep‐dip” technique, a new method for systematically surveying respiratory rhythm promoting sites in lower vertebrates, we provide evidence suggesting that the lung oscillator in late‐stage metamorphic frogs can, itself, be subdivided into a powerstroke and a novel priming component. As in mammals, the novel lung‐priming component in frogs appears to extend more rostrally than the powerstroke component. These data suggest the Oscillator Homology Hypothesis has to be revised: the mammalian PreI and PreBötzinger Complex oscillators may have evolved from the same two components that gave rise to the frog lung oscillator and not from the buccal oscillator as previously thought. Work funded by NSERC.