The genome and comparative transcriptome of the euryhaline model ciliate Paramecium duboscqui reveal adaptations to environmental salinity.

Background: As a potential model organism for studies of environmental and cell biology, Paramecium duboscqui is a special euryhaline species of Paramecium that can be found in fresh, brackish, or marine water in natural salinity ranges between 0‰ and 33‰. However, the genome information as well as molecular mechanisms that account for its remarkable halotolerant traits remain extremely unknown. To characterize its genome feature, we combined PacBio and Illumina sequencing to assemble the first high-quality and near-complete macronuclear genome of P. duboscqui. Meanwhile, comparative transcriptomic profiles under different salinities gave underlying insight into the molecular mechanism of its adaptations to environmental salinity.

Results: The results showed that the MAC genome of P. duboscqui comprises 160 contigs, with 113 of them possessing telomere (~ 28.82 Mb haploid genome size). Through comparative genomic analyses with the other ciliate, we found that gene families encoding transmembrane transporter proteins have been expanded in P. duboscqui, showing enormous potential in salinity adaptation. Like other Paramecium, P. duboscqui utilizes TGA as its only termination codon and has reassigned TAA and TAG to encode glutamine. P. duboscqui showed different growth rates under different salinities, with an optimum growth rate in 5‰ salinity. A comparison of the transcriptomic profiles among P. duboscqui grown under different concentrations showed that genes involved in protein folding, oxygen respiration, and glutathione-dependent detoxification were upregulated in the high-salt group, whereas genes encoding DNA-binding proteins and transcription factors were upregulated in the low-salt group, suggesting distinct mechanisms for responding to low and high salinity. Weighted gene coexpression network analysis (WGCNA) linked the hub genes expressed at 30‰ salinity to cysteine-type peptidase activity, lipid transfer, sodium hydrogen exchange, and cell division, with the hub genes expressed at 0‰ salinity involved in transmembrane transport and protein localization.

Conclusions: This study characterizes a new euryhaline model Paramecium, provides novel insights into Paramecium evolution, and describes the molecular mechanisms that have allow P. duboscqui to adapt to different osmotic environments.