Transcriptional counterbalance of parental acclimation in TGP
Transcriptional studies show that when WGP and TGP co-exist, these
plastic responses tend to follow different trajectories and are
influenced by different sets of genes and molecular functions (Shama et
al., 2016; Bell and Stein, 2017; Hales et al., 2017; Webster et al.,
2018; Bernal et al., 2022). However, this question has rarely been
considered when both plastic responses are adaptive. We address this
question in D. mojavensis (Diaz et al., 2020) , where we have
previously analyzed the level of parent-to-offspring environmental
predictability linked to TGP when comparing life stages. As expected
from previous thermal tolerance data, transcriptional TGP was only
detected in larva stages. Our results also support the hypothesis of
diverging transcriptional trajectories between plastic responses within
and across generations. However, in contrast to previous findings in
other organisms (Bell and Stein, 2017; Hales et al., 2017), we found
that such diverging trajectories involve many of the same genes normally
expressed in WGP. Approximately 56 % of DE genes associated with TGP
overlapped with WGP. Interestingly, most genes significantly associated
with TGP are up-regulated, as opposed to transcriptional WGP, where many
more genes are down-regulated in response to acclimation in the
offspring.
We suggest that this up-regulation caused by the parental acclimation in
TGP represents a transcriptional counterbalance of gene expression that
helps explain the molecular bases of the adaptive component previously
detected from heat tolerance data in TGP (Diaz et al., 2020). Thus, the
offspring might benefit from parental acclimation by restoring the
expression of genes that will be down-regulated by the heat-shock
response when they are themselves acclimated. To understand their role
in thermal tolerance, we identify different types of these genes
triggered by parental acclimation, which can be classified into two
groups. First, parental acclimation activates some genes clearly
associated with the heat shock response, such as proteolysis, and only
two Hsp genes (Sørensen et al., 2005; Mahat et al., 2016). One of these,
Hsp83, is down-regulated in WGP, suggesting a transcriptional constraint
in the activation of this gene in the larva stage, which is compensated
by parental acclimation. Interestingly, this gene has been associated
with maternal transfer to early embryos in D. melanogaster (Ding
et al., 1993), being expressed during oogenesis and embryogenesis.
We also identified genes activated by parental acclimation that might be
associated with heat tolerance but are not part of the heat-shock
response. Genes associated with GO categories of collagen modifications
are particularly interesting, such as procollagen-proline 4-dioxygenase
activity and peptidyl-proline hydroxylation to 4-hydroxyl-L-proline. We
identified four such genes, which encode enzymes that catalyze the
formation of hydroxyproline (Myllyharju, 2008; Gorres and Raines, 2010).
Collagens are extracellular matrix proteins that contribute to tissue
structure and remodeling (Myllyharju and Kivirikko, 2004). This
hydroxylation increases the melting temperature of helical collagen,
which allows these proteins to be stable at body temperatures in mammals
(Rappu et al., 2019). In addition, the down-regulation of these enzymes
has been previously associated with thermal sensitivity in low-tolerantD. melanogaster lines (Vermeulen et al., 2014). As a major
connective tissue, more thermally stable hydroxylated collagen in the
offspring larvae of acclimated parents might allow them to tolerate the
direct effect of heat stress without necessarily accessing the
energetically expensive heat-shock response (Vermeulen et al., 2014).
The fact that these pathways are not directly activated during WGP
suggests that such genes are constrained or negatively affected by the
massive up-regulation of chaperones during the heat-shock response in
WGP acclimation.
We provide compelling evidence demonstrating substantial and distinct
differences between adaptive WGP and TGP that contribute to explaining
how selection shapes their transcriptional evolution. The first
difference is evidenced in the role played by the mechanisms of
alternative splicing in transcriptional plasticity, which is linked to
acclimation within generations but not to parental acclimation.
Moreover, our results demonstrate the importance of exploring
alternative splicing in plasticity studies, as these mechanisms involve
distinct genes and functions from those detected in differential
expression analyses. The second difference between WGP and TGP was
detected in the direction of gene expression. Most differentially
expressed genes in response to thermal acclimation are common to both
types of plasticity but are primarily down-regulated in WGP and
up-regulated in TGP. We propose that this pattern might be a
transcriptional counterbalance, where parental acclimation compensates
for the negative effects of thermal stress on the expression level of
some genes despite their potential role in thermal tolerance. Instead of
enhancing the expression of the more energetically expensive molecular
chaperones that characterize acclimation in WGP, parental acclimation
counteracts the negative effects of the heat-shock response. How much of
this pattern can be extended to other organisms or traits remains to be
seen. However, we believe our findings help understand the molecular
bases of adaptive TGP and how the offspring benefit from parental
acclimation to increase their fitness compared to unacclimated parents.