# Diversity of Cardinium Endosymbiont Genomes from Plant-Parasitic Nematodes

## TL;DR

This study explores the diversity of Cardinium bacteria in plant-parasitic nematodes, revealing their evolutionary relationships and metabolic adaptations.

## Contribution

The paper provides ten new Cardinium genomes and clarifies the evolutionary origin of groups B and F within nematode-associated Cardinium.

## Key findings

- Cardinium genomes from nematodes show broad ecological and phylogenetic distribution.
- Two Bursaphelenchus-associated Cardinium genomes exhibit significant genome reduction.
- Horizontal gene transfer has contributed to key metabolic pathways in Cardinium.

## Abstract

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.