Cardinium endosymbionts are obligate intracellular bacteria found in a wide range of invertebrate hosts. In this study, we generated ten new Cardinium genomes from plant-parasitic nematodes of the genera Amplimerlinius, Bursaphelenchus, Cactodera, Ditylenchus, Globodera, Meloidoderita, and Rotylenchus, revealing their broad ecological and phylogenetic distribution. Using an expanded set of genes, we clarified the relationship between previously defined Cardinium groups B and F from nematodes, showing that they are closely related and likely share a single evolutionary origin within nematode-associated Cardinium. Among the newly assembled Cardinium genomes obtained in this study, two genomes originating from strains associated with wood-inhabiting Bursaphelenchus species exhibited remarkable genome reduction, with estimated sizes of approximately 695 kb. Functional annotation of Cardinium genomes indicated an absence of or a reduction in several central metabolic pathways, including the biotin biosynthetic pathway. A complete biotin pathway was found only in D. weischeri, and this pathway is only partially encoded in Cactodera sp. The polA gene, which encodes DNA polymerase I, showed partial loss in several Cardinium strains. Phylogenetic and comparative genomic analyses provided strong evidence that several carbohydrate, glycerophospholipid, and biotin metabolism genes in these endosymbionts have been acquired through horizontal gene transfer. Future research that integrates high-quality genome assemblies with functional analyses of host–symbiont interactions will be essential to elucidate how metabolic dependency, genome reduction, and horizontal gene transfer collectively shape the evolution and ecological diversification of Cardinium across nematode hosts.
Tarlachkov et al. (Tue,) studied this question.