Genomic insights into nematode microbiomes reveal novel endosymbionts Rickettsiella

Bacterial endosymbionts are key drivers of invertebrate ecology and evolution. While the diversity and functional role of the nematode microbiome remain poorly explored.

We reconstructed and characterized 108 metagenome-assembled genomes from 10 published and 15 newly sequenced nematode genomes.

We report the first evidence of Rickettsiella in nematodes and discovered novel endosymbionts Cardinium and Wolbachia in plant-parasitic nematodes. The nematode microbiome is enriched with genes for carbohydrate metabolism and the biosynthesis of essential amino acids and vitamins, indicating a potential primary role in host nutrition. Notably, mobile genetic elements like prophages and insertion sequences (IS) are widespread and carry passenger genes involved in vitamin biosynthesis, suggesting horizontal gene transfer facilitates metabolic adaptation. Genomic reduction in the nematode Rickettsiella lineage, reveals extensive gene loss, particularly in amino acid biosynthesis. Crucially, we find no evidence of purifying selection on its residual nutritional pathways, and thus cannot clearly support a mutualistic role for this association.

Our findings expand the known host range of major endosymbiont groups and reveal a spectrum of symbiotic relationships in nematodes, from putative mutualism driven by nutritional supplementation to associations with neutral or parasitic traits, shaped by pervasive horizontal gene transfer and reductive genome evolution.