Obligate symbioses involving intracellular bacteria have transformed eukaryotic life, from providing aerobic respiration and photosynthesis to enabling colonization of previously inaccessible niches, such as feeding on xylem and phloem, and surviving in deep-sea hydrothermal vents. A major challenge in the study of obligate symbioses is to understand how they arise. Because the best studied obligate symbioses are ancient, it is especially challenging to identify early or intermediate stages. Here we report the discovery of a nascent obligate symbiosis in Howardula aoronymphium, a well-studied nematode parasite of Drosophila flies. We have found that H. aoronymphium and its sister species harbor a maternally inherited intracellular bacterial symbiont. We never find the symbiont in nematode-free flies, and virtually all nematodes in the field and the laboratory are infected. Treating nematodes with antibiotics causes a severe reduction in fly infection success. The association is recent, as more distantly related insect-parasitic tylenchid nematodes do not host these endosymbionts. We also report that the Howardula nematode symbiont is a member of a widespread monophyletic group of invertebrate host-associated microbes that has independently given rise to at least four obligate symbioses, one in nematodes and three in insects, and that is sister to Pectobacterium, a lineage of plant pathogenic bacteria. Comparative genomic analysis of this group, which we name Candidatus Symbiopectobacterium, shows signatures of genome erosion characteristic of early stages of symbiosis, with the Howardula symbiont’s genome containing over a thousand predicted pseudogenes, comprising a third of its genome.

涉及细胞内细菌的专性共生体已经改变了真核生物的生活，从提供有氧呼吸和光合作用到使以前无法接近的生态位定居，例如以木质部和韧皮部为食，并在深海热液喷口中生存。研究专性共生的主要挑战是了解它们是如何产生的。由于研究最好的专性共生体是古老的，因此识别早期或中期阶段尤其具有挑战性。在这里，我们报道了在果蝇果蝇的线虫寄生虫方面经过深入研究的霍华德拉主动脉中发现了新生的专性共生。我们已经发现，^ h。or草其姊妹物种拥有母系遗传的细胞内细菌共生体。我们从未在无线虫的苍蝇中找到这种共生体，并且几乎所有野外和实验室中的线虫都被感染了。用抗生素治疗线虫会严重降低果蝇感染的成功率。协会是最近的，因为更遥远相关的昆虫寄生的tylenchid线虫没有宿主这些共生菌。我们还报告说，Howardula线虫共生体是无脊椎动物宿主相关微生物的广泛单系群成员，该微生物已独立产生至少四个专性共生体，其中一个在线虫中，一个在昆虫中，三个是共生菌，是共生细菌的姐妹。，一种植物病原菌。该组中，其中我们命名的比较基因组分析暂定Symbiopectobacterium，共生的早期阶段的基因组侵蚀特性的显示签名，与Howardula包含超过一千预测假基因的共生体的基因组中，包括其基因组中的第三个。