Identification of DNA (de)methylation-related genes and their transcriptional response to environmental challenges in an invasive model ascidian.

Marine invasive species are constantly challenged by acute or recurring environmental stresses during their range expansions. DNA methylation-mediated stress memory has been proposed to effectively affect species' response and enhance their overall performance in recurring environmental challenges. In order to further test this proposal in marine invasive species, we identified genes in the DNA methylation and demethylation processes in the highly invasive model species, Ciona robusta, and subsequently investigated the expression patterns of these genes under recurring salinity stresses. After a genome-wide comprehensive survey, we found a total of six genes, including two genes of DNA methyltransferase 3a (DNMT3a1 and DNMT3a2), and one gene of DNA methyltransferase 1 (DNMT1), methyl-CpG-binding domain protein 2 (MBD2), methyl-CpG-binding domain protein 4 (MBD4) and ten-eleven-translocation protein 1 (TET1). Phylogenetic reconstruction and domain arrangement analyses showed that the deduced proteins of the identified genes were evolutionarily conserved and functionally similar with their orthologs. All genes were constitutively expressed in all four tested tissues. Interestingly, we found time-dependent and stress-specific gene expression patterns under high and low salinity stresses. Under the recurring high salinity stresses, DNMT3a1 and TET1 conformed to the definition of memory genes, while under the recurring low salinity stresses, two DNMT3a paralogues were identified as the memory genes. Altogether, our results clearly showed that the transcriptional patterns of (de)methylation-related genes were significantly influenced by environmental stresses, and the transcriptional memory of some (de)methylation-related genes should play crucial roles in DNA methylation-mediated stress memory during the process of biological invasions.