Not just a Killing effect: Transcriptomic modulation of host paramecia in order to infection with the bacterial endosymbiont Caedibacter taeniospiralis.. Differential transcriptomics of Caedibacter infected and healed paramecia

Endosymbiosis is a widespread phenomenon. Hosts of bacterial endosymbionts can be found all over the eukaryotic tree of life. Likely, this evolutionary success is connected to the altered holobionts phenotype, the ‘new organism’ arising from a symbiotic association. The symbiont´s contribution to the novel phenotype is largely unknown.To get an insight in the host/symbiont relationship, we focused on an obligate bacterial endosymbiont (Caedibacter taeniospiralis), which confers a intraspecific “Killing” phenotype to its host Paramecium tetraurelia. In this study we investigated this symbiosis caused additional adaptive gene expression patterns in the host. Comparative transcriptomics of infected P. tetraurelia and genetically identical symbiont-free cells confirmed altered gene expression in the symbiont-bearing Paramecium line. Our results indicate the up-regulation of specific metabolic pathways genes whereas down-regulated genes were involved in signaling pathways and cell cycle regulation. Functional analyses to validate results from the transcriptomic approach revealed that the symbiont increases the maximal cell density of its host providing a fitness advantage. Likely, necessary resources are obtained from the up-regulation of host metabolic pathways mobilizing storage energy. Correlating functional and transcriptomic results, C. taeniospiralis apparently increases the density of its host not by providing essential nutrients but via manipulation of gene expression regulation. This is the first evidence for transcriptome modulation of a ciliate caused by a bacterial endosymbiont representing an important