Salinity-genes with contrasting ecotype expression
profiles
Genetic assimilation occurs when a plastic ancestral trait becomes
environmentally stable, resulting in a loss of plasticity (Lande 1976).
Many of the environmentally expressed genes were linked to known
osmoregulatory- and immune functions. Intracellular levels of calcium
play an important role in responses to osmotic stress and functions in
volume-regulation of the cells (Erickson et al. 2001), and the ability
to take up calcium at low Ca2+-concentration have
likely been selected for in freshwater. Genes that were differentially
expressed in salt- and freshwater linked to the uptake of
Ca2+, and to a certain degree also for
Na+ (Horng et al. 2007), included different ATP6Vs,
subunits of a V-type proton located at the basolateral membrane of
mitochondria rich cells (Figure 7). Consisting of two main parts, the
ATP6V1 comprise at least eight- and the ATP6V0 include at least five
different subunits (Sun-Wada et al. 2004), where five V1 and two V0
subunits had a higher expression in freshwater fish in this study
(contrast 3 and 4), clearly indicating that this gene has been important
for both freshwater and saltwater acclimation, as was also found for
killifish (Fundulus heteroclitus ) (Whitehead et al. 2012) . Other
genes related to calcium was ano1 and cacbn1 (Figure 6d), both with an
increased expression in saltwater fish.
Pathogen diversity in freshwater is often found to be higher than in
salt water (Wang et al. 2012), and in teleosts the skin, gills and gut
are continuously exposed to the external aquatic environment and are
therefore the main mucosal surfaces that represent potential entry ports
for pathogens (Gomez et al. 2013). Many of the mechanisms for antigen
sampling in the mucosal epithelium of teleost fish are mostly unknown,
as they lack many of the mammalian molecules for transporting pathogens
across the epithelia. Recent evidence suggests that two specific
antigen-sampling cell types exist in the gill, where one is expressing
protein tyrosine phosphatase receptor type C (PTPrc ) and IL-1β
(Kato et al. 2018). The PTPs catalyzes the dephosphorylation of protein
tyrosine kinases (PTKs ) directly or through their downstream
targets, and play key regulatory roles in multiple signal transduction
pathways, where most are expressed in immune cells (Mustelin et al.
2005). In this study, increasing expression of PTP’s were linked to high
salinity, as two members of the PTP-family were found to be
differentially upregulated in the saltwater ecotype (PTPN14 andPTPrfa ), and three were upregulated in comparison 2 (FwSw;PTPN2a, PTPN21, PTPrna ). In contrast, IL-1β had overall higher
expression in the freshwater ecotype. The second significant type of
teleost antigen-sampling cell types that were recently identified in
gills was a microfold cell (M-cell), expressing Anexin5 (ANXA5 ,
Figure 6d) (Kato et al. 2018), a gene that has been linked to apoptosis
by its ability to be recruited to the cell surface and co-localize with
phosphatidylserine; the “eat-me”-signal for macrophages (Lizarbe et
al. 2013). In the present study, ANXA5b was significantly
upregulated in the freshwater ecotype, which could indicate that the two
different antigen sampling cell types might have been under directional
selection in the opposite environments. ANXA5 has also been
linked to changes in calcium concentration, as they can bind around
twelve Ca2+ ions and exhibit calcium channel activity
in plasma membranes and in matrix vesicles (Lizarbe et al. 2013), so it
is unclear if ANXA5 has a immunological or osmoregulatory function in
freshwater fish (or both).