Evolution of air-breathing and central CO2/H+ respiratory chemosensitivity: New insights from an old fish?

While little is known of the origin of air-breathing in vertebrates, primit ive air breathers can be found among extant lobe-finned (Sarcopterygii) and ray-finned (Actinopterygii) fish. The descendents of Sarcopterygii, the te trapods, generate lung ventilation using a central pattern generator, the a ctivity of which is modulated by central and peripheral CO2/H+ chemorecepti on, Air-breathing in Actinopterygii, in contrast, has been considered a 're flexive' behaviour with little evidence for central CO2/H+ respiratory chem oreceptors, Here, we describe experiments using an in vitro brainstem prepa ration of a primitive air-breathing actinopterygian, the longnose gas Lepis osteus osseus. Our data suggest (i) that gill and air-breathing motor patte rns can be produced autonomously by the isolated brainstem, and (ii) that t he frequency of the air-breathing motor pattern is increased by hypercarbia , These results are the first evidence consistent with the presence of an a ir-breathing central pattern generator with central CO2/H+ respiratory chem osensitivity in any primitive actinopterygian fish. We speculate that the o rigin of the central neuronal controller for air-breathing preceded the div ergence of the sarcopterygian and actinopterygian lineages and dates back t o a common air-breathing ancestor.