Multiple origins of obligate nematode and insect symbionts by a clade of bacteria closely related to plant pathogens

Significance Obligate symbioses are intimate associations between species in which neither partner can live without the other. It is challenging to study how obligate symbioses arise because they are often ancient and it is difficult to uncover early or intermediate stages. We have discovered a nascent obligate symbiosis involving Howardula aoronymphium , a well-studied nematode parasite of Drosophila flies, and a bacterium related to Pectobacterium , a lineage of plant pathogens. Moreover, this nematode symbiont is a member of a widespread group of invertebrate host-associated microbes that has independently given rise to at least four obligate symbioses in nematodes and insects, making it an exciting model to study transitions to obligate symbiosis.