4.5 Other molecular mechanisms of heat response in heat-tolerant
and heat-sensitive Brachionus species
Brachionus , as most monogonont rotifers, have two reproductive
modes, one asexual allowing for fast population growth and one sexual to
promote recombination under unfavorable environmental conditions
(Gilbert, 1974; Schröder, 2005). The sexual phase of reproduction is
generally induced due to environmental factors such as photoperiod,
population density and food composition (Gilbert, 1974; Pourriot &
Snell, 1983; Schröder, 2005). Rotifer species are capable of abandoning
either the sexual or the asexual phase. Abandoning asexuality is very
rare, however, abandoning sexuality is a common phenomenon in clones
that have been under laboratory cultivation over a long period of time
and it reliess on a recessive allele (Stelzer, 2008; Stelzer, Schmidt,
Wiedlroither, & Riss, 2010). In B. fernandoi , increase of
temperature resulted in significant up-regulation of genes related to
meiosis, indicating that temperature exposure above 23 °C triggered
sexual reproduction. In B. calyciflorus s.s., there was no
significant up-regulation of meiosis-related genes, neither at high nor
at low temperatures. Possible explanations are that this clone has lost
the ability of sexual reproduction or that sexual reproduction is
triggered by temperatures beyond the range tested here or stimuli other
than temperature.
Epigenetic control on transcription can be achieved by many mechanisms,
including DNA methylation or post-translational modifications to histone
tails, including histone methylation and acetylation. It is known from
genomic/transcriptomic studies on B. manjavacas and other
rotifers that rotifers lack DNA methyltransferases (Dnmt1, Dnmt3) for
epigenetic transcriptional regulation (Gribble, Mark Welch, 2017; Kim et
al., 2016). However, they do not lack the molecular machinery for
post-translational regulation to histone tails, which play an important
role in regulating gene expression. In B . fernandoi , we
found that exposure to high temperatures resulted in up-regulation of
histone related methyltransferase genes (H3K4, H3K79, H4K20).
Trimethylation of H3K4 and H3K79 are associated with activation of
transcription, while methylation of H4K20 has been related to silencing
chromatin (Hyun, Jeon, Park, & Kim, 2017). Silencing of chromatin might
be related to translation suppression that we found for the same species
under the same conditions of heat exposure. Transcriptional activation
via trimethylation of H3K79 and H3K4 might be associated with a numerous
environmental information processing pathways that were up-regulated in
this species under high heat exposure.
CONCLUSIONS
In conclusion, we found significantly different responses to heat
between heat-tolerant and heat-sensitive Brachionus species.
Transcriptomic responses were found to correlate with differences in
fitness and especially differences in population growth, indicating
underlying mechanisms of phenotypes’ responses to environmental change.
Generally, the respective species upregulated metabolism/translation
related genes under the temperature with highest growth rate, while
stress related (and – in one species – meiosis related) genes were
expressed beyond the temperature regime optimal for growth. What had
been historically considered the single species B. calyciflorusactually comprises several closely related rotifer species, which are
differentially adapted to different environmental conditions (here,
temperature) regarding their gene expression profiles and can hence
occur in sympatry. The genes found to be upregulated under heat stress
might be targets of selection potentially contributing to the ecological
divergence of the two species. Additionally, their expression profiles
might be used as biomarkers to assess species vulnerability to
environmental conditions and climate changes.