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.