Master of Science (MSc)
Faculty of Science
Dr. Mike P. Wilkie
Lamprey (Petromyzontiformes) are a phylogenetically ancient group of jawless fishes that begin their lives as filter-feeding larvae (ammocoetes) before undergoing a complex metamorphosis into juvenile lamprey that involves major changes to their internal and external body plan. Some parasitic species, such as the sea lamprey (Petromyzon marinus), migrate to sea following metamorphosis, where they use their oral discs and rasping tongue to attach to and ingest vast quantities of blood from fishes. Thus, sea lamprey have to counter the simultaneous challenges of hyposmoregulation in sea water and the generation of large quantities of ammonia due to the catabolism of protein-rich blood. A goal of this study was to characterize how changes in the structure and function of the gills facilitated osmoregulation and nitrogenous waste (N-waste) excretion by sea lamprey following metamorphosis, particularly after acclimation to sea water and the ingestion of blood from teleost fishes. Accordingly, key features of the lamprey gill including the distribution and abundance of Na+/K+-ATPase (NKA) and H+-ATPase (V-ATPase) pumps involved in ion regulation, and ammonia transporting Rhesus glycoproteins and urea transporting proteins, were investigated using through immunohistochemical staining and Western blotting techniques.
In contrast to the sea lamprey, there are other species of lamprey that remain in fresh water following metamorphosis. Many of these species are non-parasitic including the northern brook lamprey (Ichthyomyzon fossor), but some such as the closely related silver lamprey (Ichthymyzon unicuspis) are parasitic. To learn more about how an exclusively FW existence affected ion transport and ammonia excretion by lampreys, the gills of post-metamorphic (juvenile) northern and silver lamprey were compared to those of larval and juvenile sea lamprey. As in sea lamprey, the gills of both species were characterized by the presence of Rhesus c-like glycoprotein (Rhcg-like) and urea transport (UT) protein but, the distribution of these proteins more closely resembled those of larval sea lamprey than juvenile sea lamprey. In both the silver and northern brook lamprey, Rhcg-like protein co-localized with V-ATPase, suggesting that H+ excretion was coupled with Rhcg-like protein mediated diffusion trapping of NH3. Similarly, UT abundance in both species was comparable to that of the larval sea lamprey. I conclude that in freshwater lampreys, NH3 extrusion via apical Rhcg-like proteins is coupled to V-ATPase mediated H+ excretion, which maintains favourable diffusion gradients by trapping NH3 as NH4+. Given that the lampreys and teleosts have evolved along separate lineages for at least 360 million years, I propose that this method of ammonia excretion is an ancient strategy used by aquatic organisms to facilitate ammonia excretion across the gills in fresh water. In contrast, the need for V-ATPase trapping of NH3 as NH4+ is not required in sea water, in which the Rhcg-like proteins were restricted to the basolateral membrane and co-localized with NKA in sea water mitochondrion-rich cells (SW MRCs). These findings suggest that Rhcg-like protein may mediate ammonia excretion by loading the SW MRC with ammonia, with the resulting NH4+ pumped out of the cell via substitution for H+ on an apical Na+/H+ exchanger, or via an outwardly directed NH4+ electrochemical gradient that favours excretion via paracellular junctions.
Sunga, Julia, "Gill Structure & Function in Parasitic and Non-parasitic Lampreys: The Effects of Metamorphosis and Freshwater-Seawater Transfer" (2018). Theses and Dissertations (Comprehensive). 2083.
Available for download on Tuesday, February 26, 2019