on several occasions, while losing the genetic factors responsible for hyperosmotic plasticity (Whitehead 2010). This goes according to the creation model which involves the devolutionary loss of genetic elements. It may be that while adapting to a new environment, concomitant gene loss barred species from reverse adaptation to their previous environment. Osmoconformers are fish species that do not expend energy to maintain the osmolarity of their internal medium, by minimizing the osmotic gradient. These include some species of stenohaline fish. Osmoregulators use energy-dependent mechanisms to maintain a higher external osmolarity. These include several estuarine and freshwater species of fish (Rivera-Ingraham and Lignot 2017). Ionocytes vary according to the contents of the first three types of ion channels. Type-I ionocytes occur in both freshwater and saltwater fish and contain only a basolateral NKA channel. Type-II and type-III ionocytes occur in freshwater species, where the type-II ionocyte has a basolateral NKA channel and an apical NCC channel, and the typeIII ionocyte has a basolateral NKA and NKCC1a channel, sometimes with an apical NH3 channel. This configuration of ion channels allows for ion absorption. Type-IV ionocytes are the same as type-III ionocytes, except that they have an apical CFTR channel instead of an NHE3 channel, and they also occur in saltwater fish species. This configuration allows for ion secretion (Hiroi and McCormick 2012), and indicates that the CFTR channel is mainly responsible for salt excretion. Figure 2 shows the location of the three main types of ion channels in a typical fish ionocyte. A second way of classifying ionocytes is whether their apical membrane binds a protein called peanut agglutinin (PNA), which was originally used to identify cells that secrete HCO3 -. In certain fish, such as freshwater rainbow trout (Oncorhynchus mykiss), PNA- and PNA+ ionocytes both have a basolateral NKA channel, whereas only PNA+ ionocytes have an apical NHE3 channel, which is restricted to the gills (Ivanis et al. 2008; Dymowska et al. 2012). In other fish species, such as zebrafish (Danio rerio), ionocytes can be classified in yet another manner. Zebrafish have an ionocyte repertoire that includes an NCC (Na+/Cl-) cotransporter cell, which corresponds to the type-II ionocyte described earlier, an NaR (NKA-rich) cell, that absorbs K+ into the cell, and secretes Na+. The third type of ionocyte in this ion channel repertoire is the H-ATPase (HA) ionocyte, which secretes H+ outwards to the lumen (Chang et al. 2009; Hwang et al. 2011). In the transition between freshwater and saltwater environments, euryhaline fish species, such as tilapia can switch between ion-absorbing type-III and ion-secreting type-IV ionocytes. In contrast, the stenohaline zebrafish has only type-III ionocytes. The NKA ion channel can also be differentially expressed in freshwater and saltwater. The NKA channel has an α and a β subunit, where the α subunit binds the ATP, Na+, and K+ substrates, and the β subunit is a structural element. The α subunit has two isoforms, of which NKAα1a is more abundant in freshwater, whereas NKAα1b is expressed in higher levels in saltwater (Pfeiler and Kirschner 1972). The NHE3 channel allows saltwater fish to excrete metabolic acids, by exploiting the Na+ gradient across the ionocyte membrane (Claiborne et al. 2002). Fluctuating salinity levels represent a form of oxidative stress, leadFigure 2. Schematic depiction of a fish ionocyte, showing the regular placement of the CFTR, NKA, and NKCC1 ion channels. CSERHATI Molecular baraminology of fish 2023 ICC 185
RkJQdWJsaXNoZXIy MTM4ODY=