Fluorescence In Situ Hybridization Provides Evidence for the Presence of the Endosymbiotic Bacterial Genus Rickettsia in Tardigrades

Many ecdysozoans harbor endosymbiotic bacteria within their microbiota, and these endosymbionts can have a range of positive and negative effects on their hosts. Recent 16S rRNA gene amplicon sequencing studies have provided evidence for endosymbionts within the tardigrade microbiota. In a previous study amplicon study, we determined that sequences corresponding to the endosymbiotic genus Rickettsia were significantly more associated with tardigrades than with the substrate from which they were isolated. Here, we performed fluorescence in situ hybridization (FISH) using a Rickettsia-specific probe, RickB1, to determine if Rickettsia could be found in tardigrades. RickB1 and a probe targeting most bacteria, EUB338, colocalized within the tissues of tardigrades, indicating the presence of Rickettsia. We also performed FISH using RickB1 and a nonsense probe which allowed us to distinguish between false positives and true positives. This method revealed RickB1 signals in tardigrades that were not due to erroneous probe binding, providing further evidence that Rickettsia is present in tardigrades. Future research will be necessary to determine the effects, if any, of this endosymbiont on its tardigrade host.