A highly divergent with a tiny genome in an insect-parasitic tylenchid nematode.

symbionts are the most successful host-associated microbes on the planet, infecting arthropods and nematodes. Their role in nematodes is particularly enigmatic, with filarial nematode species either 100% infected and dependent on symbionts for reproduction and development, or not at all infected. We have discovered a highly divergent strain of in an insect-parasitic tylenchid nematode, sp., in a nematode clade that has not previously been known to harbour . While this nematode is 100% infected with , we did not detect it in related species. We sequenced the symbiont (How) genome and found that it is highly reduced, comprising only 550 kilobase pairs of DNA, approximately 35% smaller than the smallest nematode symbiont genomes. The How genome is a subset of all other genomes and has not acquired any new genetic information. While it has lost many genes, including genes involved in cell wall synthesis and cell division, it has retained the entire haem biosynthesis pathway, suggesting that haem supplementation is critical. How provides key insights into our understanding of what are the lower limits of cells, as well as the role of symbionts in the biology and convergent evolution of diverse parasitic nematodes.