Page 17 - IJMS-2014v4n49

International Journal of Marine Science 2014, Vol.4, No.50, 1-22
http://ijms.biopublisher.ca
10
In contrast to incubation at a static, elevated
temperature for two days, exposure to a variable
temperature regime for seven days had a dramatic
influence on gene expression; 50% of the genes were
significantly affected by temperature in the VTE. This
was mainly driven by the Symbiodinium gene
expression profiles; although no Symbiodinium genes
were significantly influenced by a 2-day exposure to
30℃, four of the six targets (67%); rbcL, psI, pgpase
(assessed in Mayfield et al., 2012a), and hsp70
(assessed herein), were significantly influenced by
temperature regime in the VTE. This was a
significantly higher proportion, suggesting that
exposure to a variable temperature regime over seven
days had a more dramatic influence on the
Symbiodinium mRNA-level response than exposure to
a static, elevated one for two days. Mayfield et al.
(2012a) took the elevated FV/FM values in these same
samples to indicate that corals exposed to variable
temperature may have a high capacity for
photosynthesis, though an analysis of the degree of
carbon fixation (sensu Furla et al., 2000) will be
necessary to demonstrate whether such mRNA
concentration and FV/FM increases actually lead to
higher photosynthetic output of the resident clade C
Symbiodinium populations.
The reason why exposure to a familiar, variable
temperature regime had such a strong effect on
Symbiodinium gene expression may be due to the need
to rapidly adjust levels of protein translation in
unstable environments. It has recently been found that
corals may “front-load” gene mRNAs (Mayfield et al.,
2011; Barshis et al., 2013), whereby expression is
constitutively elevated at relatively high levels due to
potentially high protein translation demands stemming
from life in unstable environments. This strategy is
commonly employed by intertidal invertebrates
(Somero, 2010), such as limpets (Dong et al., 2008),
and could explain the high levels of photosynthesis
gene expression in the Symbiodinium populations
within samples exposed to a variable temperature
regime. mRNA front-loading theoretically allows for
faster rates of protein translation that may be
necessary when the standing pool of proteins becomes
denatured due to abrupt temperature changes, such as
the dramatic thermal shifts associated with upwelling
events (Hochachka and Somero, 2002). By
maintaining high levels of mRNA expression during
such periods, corals and their resident Symbiodinium
populations would have a higher scope for translation
of proteins that may have been compromised by
dramatic increases or decreases in temperature. This
front-loading phenomenon may also explain why 7 of
the 14 target genes were expressed at significantly
higher levels in corals from the UWS, Houbihu, while
only 3 were expressed at higher levels in those of the
NUWS, Houwan (Figure 6), a site characterized by a
considerably more stable temperature profile
(Mayfield et al., 2012a).
On the other hand, two of the three stress genes
targeted herein and in a prior work (Mayfield et al.,
2011), Symbiodinium and host coral hsp70, were
instead expressed at higher levels in corals from the
NUWS, suggesting that corals of the UWS do not
necessarily front-load stress gene expression to a
greater extent than conspecifics from the NUWS. As
such, the front-loading mRNA loading phenomenon
may be utilized for certain cellular response pathways,
such as photosynthesis, but not all. A more thorough
understanding of the behavior and function of
transcription factors involved in the thermal
acclimation response, such as heat shock factors
(HSFs, Akerfelt et al., 2010), will ultimately aid in the
development of an understanding of why the
mRNA-level response differed between corals
exposed to familiar or unfamiliar temperature regimes,
as well as between sites. Such an analysis may also
help to elucidate whether thermal history (e.g., prior
exposure to upwelling) or the temperature regime
itself is more important in driving the sub-cellular
response of S. hystrix to temperature changes.
It is currently unclear whether such differences in
gene expression between sites and temperature
regimes documented herein and in previous works
(e.g., Barshis et al., 2013) actually lead to an enhanced
capacity for protein translation, as expression of the
respective proteins was not measured in previous
coral-based studies, nor was their activity. As such, it
is hoped that both gene and protein expression, as well
as activity of the latter, will be simultaneously
measured in coral samples of future temperature
manipulation studies such that the understanding of
the molecular means by which corals respond to
differential temperature regimes can be better