4.2 Heat shock response between heat-tolerant and heat-sensitiveBrachionus species
Heat Shock Response, which involves the induction of heat shock protein
(hsp ) genes, is a well known and evolutionary conserved mechanism
present in both prokaryotes and eukaryotes (Feder & Hofmann, 1999).
Induction of hsps has been connected to several stress conditions
such as exposure to extreme temperatures, heavy metals, pathogens, and
osmotic stress (De Jong et al., 2008). In the present study, expression
of hsp related genes mirrored measured changes in the population
growth rate across a temperature gradient. Population growth rate was
low when hsp genes were up-regulated. This pattern was consistent
in both species, showing that hsps genes are indeed part of a
species’ stress response, when environmental conditions (here,
temperature) are outside the ‘comfort zone’ for optimal growth.
Closely related species differentially adapted to cold vs. warm
habitats have been found to express hsp genes differently also in
other aquatic organisms such as the amphipods Eulimnogammarus.
S pecies originated from a cold habitat when exposed to heat,
up-regulated hsp genes already at lower temperatures compared to
species from a warmer habitat (Bedulina et al. 2013). In the present
study, closely related Brachionus species with differences in
their ability to tolerate heat have been found to express hspgenes differently. More specifically, hsp genes were induced outside the
temperature of optimal growth: in the heat-sensitive B. fernandoi,
hsp genes were induced by heat, while in heat-tolerant B.
calyciflorus s.s. the majority of hsp genes were induced at the
lower end of temperature exposure (20 oC), indicating
that 20 oC may already be cold stress for the
heat-tolerant species.
Proteins of the hsp90 family serve to increase the available
chaperons in the cells in order to recover from cellular stress and
maintain structural integrity at high temperatures. In contrast to other
hsp genes, their expression patterns supported a specific involvement in
heat response, as genes encoding for hsp90 were up-regulated
towards the higher temperature regime in both species. The expression ofhsp90 gene has been found to be temperature dependent also in
other aquatic organisms, e.g. copepods, oysters (Schoville et al., 2012;
Kim et al., 2017; Lim et al., 2016). This suggests that induction ofhsp90 gene along with heat might be a common mechanism in aquatic
organisms. In B. calyciflorus s.s., genes encoding hsp20were also induced by high heat. Up-regulation of hsp20 genes has
been reported previously from other Brachionus species and
copepods as a response to elevated temperatures (Rhee et al., 2011; Seo,
Lee, Park, & Lee, 2006). Transformed bacteria (Escherichia coli )
expressing the Brachionus hsp20 (Br-hsps20 ) gene
had a 100 times increase in their survival compared to the non
transformed ones under high heat-stress, indicating that Br-hsp20specifically contributes to increased thermal tolerance (Rhee et al.,
2011). Up-regulation of hsp20 genes was found to also
increase resistance to oxidative stress (Rhee et al. 2011). Possibly, an
increase in expression of hsp20 reflects a cellular defense
mechanism in response to different stressors that might be common amongBrachionus species.
A reversed expression pattern among our two species was found in genes
encoding for hsp70 and hsp27 . In heat-tolerant B.
calyciflorus s.s., genes encoding for hsp70 were significantly
up-regulated towards the lower temperature, while in B. fernandoitowards the higher temperature. It seems that temperatures such as 20oC might constitute cold stress for warm adapted
species and a stress response might be initiated under these conditions.
In B. calyciflorus s.s., the same induction pattern withhsp70 genes followed genes encoding for hsp40 andhsp60 pointing towards a synergistic mechanism regulating the
expression of these three genes as reported also in B. manjavacas(Smith et al., 2012). Such a synergistic relationship has been reported
among hsp40 and hsp70 proteins, as hsp40 regulates
the ATPase activity of hsp70 (Cintron & Toft, 2006). These genes
were induced in B. manjavacas with increasing heat (Smith et al.,
2012). This indicates that these genes are induced under any condition
that constitutes temperature stress for a particular Brachionusspecies.