Young to middle-aged symbionts in old opisthokonts. Three symbionts of Nuclearia thermophila

To shed light on symbiotic associations between eukaryotes and prokaryotes, we study symbioses of Nuclearia spp. (Opisthokonta, Nucleariidae) with bacteria. Together with animals and fungi these amoebae belong to the taxonomic supergroup Opisthokonta. In fact, among unicellular opisthokonts nucleariids are the only group known to be frequently associated with bacterial symbionts. Their close relatedness to the multicellular lineages animals and fungi and their frequent association with bacteria makes these amoebae ideal model organisms for the study of symbioses between ancient opisthokonts and bacteria. For this study we established a culture collection of 40 nucleariid isolates originating from four Swiss and 17 Austrian lakes. More than half of the isolated nucleariids were associated with bacterial symbionts. The bacterial species Paucibacter toxinivorans and the two Candidate species Endonucleariobacter rarus and Turbabacter delicatus were repeatedly found as symbionts. In fact, in two N. thermophila isolates these three symbionts were even co-occurring. Infection experiments and genome analyses were used for the characterisation of the symbionts and their association with the host species N. thermophila. We could show that for the three symbioses host specificities, impact on their host, physiological characteristics and genomic adaptions differed significantly. Different fitness effects on the host species along with comparison of symbionts’ genomes shed light on different states of adaption to the symbiotic life style. From an evolutionary point of view these three symbionts were acquired at different time points in the history of their host species. This is reflected by physiological as well as genomic adaption of the symbionts.