| Description |
Many cicada species host two ancient bacterial obligate nutritional endosymbionts, "Candidatus Karelsulcia muelleri" and "Candidatus Hodgkinia cicadicola", which together synthesize 10 essential amino acids for their cicada hosts. In many of these cicada species, Hodgkinia splitting into different cell lineages that must cooperate to fulfill the functions of the original single lineage. Matsuura et al. (2018) identified the presence of yeast-like fungal symbionts (YLS) in cicadas that lacked Hodgkinia. Genome sequencing of the fungal symbiont has unveiled its remarkable metabolic adaptability, suggesting that it can potentially synthesize a comprehensive range of amino acids, vitamins, and other essential metabolites. However, they did not find any cicadas in which Karelsulcia, Hodgkinia, and YLS coexisted. Therefore, it is of great interest whether these three symbionts can coexist. If so, how do they interact? In addition, many cicada species have facultative symbionts whose nutritional contributions to their cicada hosts are also unknown. To shed light on such questions, we sequenced and analyzed the genomes of cicada endosymbionts to assess their nutritional contributions to the host and to explore their distribution characteristics across different cicada species. Additionally, we also analyzed their co-phylogeny with their host cicadas. We demonstrate, for the first time, a case of co-existence of Hodgkinia with Karelsulcia and a YLS, presenting a very late ongoing symbiont replacement process. In some populations of the cicada Chremistica ochracea, the Hodgkinia genome is extremely degenerated, which lacks distinct membrane structures but colonizes (instead of neighboring) its partner Karelsulcia. The physical fusion of bacterial endosymbionts yields a nested symbiosis when YLS is recruited, probably preserving essential metabolic pathways necessary for host nutrition and facilitating continued vertical symbiont transmission. Such fusion may have provided refuge for the degrading endosymbiont and delayed symbiont replacement. Our study sheds light on adaptive and non-adaptive evolutionary mechanisms involved in symbiont loss and replacement, offering fresh insights into endosymbiotic origins of cellular organelles. For more details, refer to our article titled: Genome degradation results in nested symbiosis and endosymbiont replacement in cicadas. |
