Diversity and Universality of Endosymbiotic Rickettsia in the Fish Parasite Ichthyophthirius multifiliis

Abstract

Although the presence of endosymbiotic rickettsial bacteria, specifically <i>Candidatus</i> Megaira, has been reported in diverse habitats and a wide range of eukaryotic hosts, it remains unclear how broadly <i>Ca.</i> Megaira are distributed in a single host species. In this study we seek to address whether <i>Ca.</i> Megaira are present in most, if not all isolates, of the parasitic ciliate <i>Ichthyophthirius multifiliis</i>. Conserved regions of bacterial 16S rRNA genes were either PCR amplified, or assembled from deep sequencing data, from 18 isolates/populations of <i>I. multifiliis</i> sampled worldwide (Brazil, Taiwan, and USA). We found that rickettsial rRNA sequences belonging to three out of four <i>Ca.</i> Megaira subclades could be consistently detected in all <i>I. multifiliis</i> samples. <i>I. multifiliis</i> collected from local fish farms tend to be inhabited by the same subclade of <i>Ca.</i> Megaira, whereas those derived from pet fish are often inhabited by more than one subclade of <i>Ca.</i> Megaira. Distributions of <i>Ca.</i> Megaira in <i>I. multifiliis</i> thus better reflect the travel history, but not the phylogeny, of <i>I. multifiliis</i>. In summary, our results suggest that <i>I. multifiliis</i> may be dependent on this endosymbiotic relationship, and the association between <i>Ca.</i> Megaira and <i>I. multifiliis</i> is more diverse than previously thought.