Immunohistochemical analysis of the distribution of molecules involved in ionic and pH regulation in the lancelet Branchiostoma floridae (Hubbs, 1922)

Evolution of the glomerulus in a marine environment and its implications for renal function in terrestrial vertebrates

Nearly a century ago, Homer Smith proposed that the glomerulus evolved to meet the challenge of excretion of water in freshwater vertebrates. This hypothesis has been repeatedly restated in the nephrology and renal physiology literature, even though we now know that vertebrates evolved and diversified in marine (saltwater) environments. A more likely explanation is that the vertebrate glomerulus evolved from the meta-nephridium of marine invertebrates, with the driving force for ultrafiltration being facilitated by the apposition of the filtration barrier to the vasculature (in vertebrates) rather than the coelom (in invertebrates) and the development of a true heart and the more complex vertebrate vascular system. In turn, glomerular filtration aided individual regulation of divalent ions like magnesium, calcium, and sulfate compatible with the function of cardiac and skeletal muscle required for mobile predators. The metabolic cost, imposed by reabsorption of the small amounts of sodium required to drive secretion of these over-abundant divalent ions, was small. This innovation, developed in a salt-water environment, provided a preadaptation for life in freshwater, in which the glomerulus was co-opted to facilitate water excretion, albeit with the additional metabolic demand imposed by the need to reabsorb the majority of filtered sodium. The evolution of the glomerulus in saltwater also provided preadaptation for terrestrial life, where the imperative is conservation of both water and electrolytes. The historical contingencies of this scenario may explain why the mammalian kidney is so metabolically inefficient, with ∼80% of oxygen consumption being used to drive reabsorption of filtered sodium.

Ion regulation at gills precedes gas exchange and the origin of vertebrates

Gas exchange and ion regulation at gills have key roles in the evolution of vertebrates^1-4. Gills are hypothesized to have first acquired these important homeostatic functions from the skin in stem vertebrates, facilitating the evolution of larger, more-active modes of life^2,3,5. However, this hypothesis lacks functional support in relevant taxa. Here we characterize the function of gills and skin in a vertebrate (lamprey ammocoete; Entosphenus tridentatus), a cephalochordate (amphioxus; Branchiostoma floridae) and a hemichordate (acorn worm; Saccoglossus kowalevskii) with the presumed burrowing, filter-feeding traits of vertebrate ancestors^6-9. We provide functional support for a vertebrate origin of gas exchange at the gills with increasing body size and activity, as direct measurements in vivo reveal that gills are the dominant site of gas exchange only in ammocoetes, and only with increasing body size or challenges to oxygen supply and demand. Conversely, gills of all three taxa are implicated in ion regulation. Ammocoete gills are responsible for all ion flux at all body sizes, whereas molecular markers for ion regulation are higher in the gills than in the skin of amphioxus and acorn worms. This suggests that ion regulation at gills has an earlier origin than gas exchange that is unrelated to vertebrate size and activity-perhaps at the very inception of pharyngeal pores in stem deuterostomes.