Researchers have discovered symbiotic bacteria inside planthopper insects with the smallest genomes recorded for any organism, measuring as little as 50,000 base pairs. These microbes, which have co-evolved with their hosts for about 263 million years, blur the line between independent bacteria and cellular organelles like mitochondria. The findings highlight extreme genome reduction in nutrient-providing symbionts.
Planthoppers, insects that feed exclusively on plant sap, rely on symbiotic bacteria housed in specialized abdominal cells to obtain essential nutrients absent from their sugary diet. Over millions of years, these bacteria have undergone significant genetic streamlining, becoming entirely dependent on their hosts.
A team led by Piotr Łukasik at Jagiellonian University in Kraków, Poland, examined 149 planthoppers from 19 families. By extracting and sequencing DNA from the insects' abdominal tissues, they reconstructed the genomes of two symbionts: Vidania and Sulcia. The results showed genomes under 181,000 base pairs in length, with some Vidania strains at just 50,000 base pairs—surpassing the previous record holder, Nasuia in leafhoppers, which exceeds 100,000 base pairs. These bacteria possess around 60 protein-coding genes, comparable to some viruses, such as the one causing covid-19 with about 30,000 base pairs.
The symbionts primarily synthesize the amino acid phenylalanine, crucial for insect exoskeleton formation. Łukasik's group suggests genome shrinkage may occur when host diet changes provide alternative nutrients or additional microbes assume former roles. This reduction echoes the evolution of mitochondria and chloroplasts, ancient bacterial descendants integrated into eukaryotic cells.
“Exactly where this highly integrated symbiont ends and an organelle starts, I think it’s very difficult to say,” Łukasik noted. “This is a very blurred boundary.” Nancy Moran from the University of Texas at Austin, who was not involved, agreed that labeling them organelles depends on definition, though differences persist: mitochondria, with 15,000-base-pair genomes originating over 1.5 billion years ago, pervade most cells, unlike these specialized symbionts. Łukasik views them on a “gradient of dependence,” anticipating even smaller examples ahead.
The study appears in Nature Communications (DOI: 10.1038/s41467-026-69238-x).