Introduction
How organisms are able to adapt to changing environmental circumstances
is still a central problem in biology (Chevin et al. 2010; Delgado and
Ruzzante 2020), as many fine-tuned mechanisms in one environment can be
selected against in slightly different ecological settings. If possible,
many organisms are able to use behavioural movements to avoid local
stressful conditions (Sih et al. 2011; Wong and Candolin 2015); fish can
for instance avoid warm surface water by moving to deeper and colder
areas, or evade fresher top layers in a fjord during snow melting in the
spring. If movement is restricted, adaptive physiological responses may
revert the organisms back to steady states within the new environment,
in response to the new environmental cues, a phenomenon referred to as
phenotypic plasticity (Pigliucci 2001; Sangiao-Alvarellos et al. 2005).
Although sounding like an optimal evolutionary force on the short scale,
there is an ongoing debate wether phenotypic plasticity is hindering or
facilitating genetic adaptation in the long term (Ghalambor et al.
2015); because plasticity may change the phenotypes available for
selection after exposure to a new environment, and consequetly influence
further genetic adaptations. Further, plastic responses can be either
adaptive or non-adaptive (Svensson et al. 2020) with respect to the
local phenotypic optimum, and is generally assumed to have an influence
on evolutionary trajectories through the altered distributions of
phenotypes (e.g expressional profiles) upon which selection can act.
Having a wide possibility for plastic responses is often viewed as
beneficial for adaption to new environments, as this gives a higher
chance of expressing a “new phenotypic optima” directly, which then
can be genetically assimilated in the new environment (Levis and Pfennig
2016; Waddington 1961), i.e., the induced expression pattern in the
novel environment will become “fixated”. Fixating
environmentally-induced plasticity through genetic assimilation should
hence reduce genetic- and plastic diversity in the derived population,
where the rate of stabilizing selection depend on the number of loci
that contribute to the additive genetic variance of the character(s)
(Lande 1976). Expression patterns of transcriptomes have been recognized
as being plastic (Evans 2015), implying that genetically similar
individuals can have different transcriptome profiles (phenotypes) as a
response to environmental cues (Mäkinen et al. 2016; Papakostas et al.
2014). Indeed, changes in gene expression can evolve very rapidly in
many species, including fish (Roberge et al. 2008) and could therefore
play an important role in the early steps of population divergence (Wolf
et al. 2010).
For aquatic organisms such as fish, the difference between saltwater and
freshwater represents considerably different selective forces. Most fish
cells maintain a constant ion concentration, and few species are able to
cross the salinity gradient between fresh and salt water (Delgado and
Ruzzante 2020). Most fish are therefore stenohaline, where the
osmoregulatory machinery only operates within relatively narrow salinity
boundaries (Hoar and Randall 1984). Only about 3-5% of all fish species
are euryhaline, meaning that they posess physiological mechanisms that
allow them to adjust to a wide range of salinities (McCormick et al.
2012). Shortly, in saltwater, a fish will have a lower concentration of
inorganic ions and hence a lower osmotic pressure compared to the
environment, and the fish will passively gain ions and loose water
(Evans et al. 2005; Rankin and Jensen 1993). The situation for a
freshwater fish is reversed, as the fish now has a higher concentration
of ions when compared to the surroundings, and the fish passively gain
water and loose inorganic ions. Consequently, to maintain homeostasis,
saltwater fish drink saltwater, where excessive salts are actively
secreted at the gills and water is absorbed in the intestine (Evans et
al. 2005; Hoar and Randall 1984), and freshwater fish actively absorb
ions at their gills, minimizes ion loss at their body surfaces, and
actively reabsorb ions in their kidney to minimize urinary ion loss
(Evans et al. 2005). Altogether, the cost
of osmoregulation is highly variable, depending on salinity, oxygen and
temperature (Ern et al. 2014), where the total cost range between a few
percent up to 30-50% of the total energy budget (Boeuf and Payan 2001;
Ern et al. 2014). In total, about 7% of the total energy budget can be
spent in the gill tissue alone (Mommsen 1984).
The threespine stickleback Gasterosteus aculeatus (hereafter
stickleback) is a small fish known to have a wide salinity tolerance
(Bell and Foster 1994) at a seemingly low osmoregulatory cost (Grøtan et
al. 2012). Originally of marine origin (Bell and Richkind 1981), the
stickleback has invaded and established populations in numerous
freshwater habitats since the last glaciation in the northern hemisphere
(Bell and Foster 1994). Thus, stickleback populations are found at a
wide osmotic range, spanning marine oceanic ecosystems, costal brackish
water systems, freshwater rivers and lakes. Many populations have become
landlocked after freshwater colonization, typically due to isostatic
uplifting of the land following deglaciation, often restricting the gene
flow between the founders and the derived populations. With reduced gene
flow, and with freshwater habitats having a stable salinity compared to
costal waters, one would expect strong directional selection on traits
that facilitate local adaptation to low salinity. Furthemore, one would
also expect traits promoting a broad salinity tolerance (being
euryhaline) to be selected against, due to the cost of sustaining
characters that has not been required for many generations, and the low
genetic variation typically found in derived populations (Schultz and
McCormick 2012). Genetic comparisons of marine and freshwater
stickleback populations show signs of strong selection, and several
outlier loci are identified by comparing whole genome sequences (Jones
et al. 2012b), SNPs (Guo et al. 2015; Hohenlohe et al. 2010; Jones et
al. 2012a) and microsatellite genotypes (DeFaveri et al. 2011; Taugbøl
et al. 2014b). Genetic studies further indicate that the frequency of
freshwater-linked alleles can increase rapidly in newly colonized
freshwater habitats (Lescak et al. 2015). However, with respect to gene
expression and potential gene-regulatory adaptations in response to
salinity, previous experiments have either tested candidate genes
(McCairns and Bernatchez 2010; Taugbøl et al. 2014a); compared
populations directly without exposure to non-native environments (Jones
et al. 2012b; Rastorguev et al. 2018); or tested for transcriptomic
expression differences after a longer period of acclimatization in the
non-native environments (30 days to 3 months; Gibbons et al. 2017; Wang
et al. 2014). However, less is known of the immediate expressional
patterns following acute exposure to contrasting salinities. The
objective of this study was to assess transciptomic expression and
compare regulatory changes in genes between marine and freshwater
sticklebacks subjected to abrupt salinity transfers.