Scientists have identified a symbiotic bacterium with the smallest genome ever recorded, raising fresh questions about where a microbe ends and a cellular organelle begins.
The newly studied bacteria live inside specialized insect organs known as bacteriomes and have shed vast portions of their DNA over hundreds of millions of years. Their extreme genetic reduction mirrors the evolutionary path of ancient microbes that eventually became mitochondria the energy-producing organelles found in nearly all complex cells.
The research, led by Piotr Lukasik of Jagiellonian University, focuses on symbiotic bacteria inhabiting planthopper insects, including Callodictya krueperi. Fluorescent imaging revealed two key microbial partners Vidania and Sodalis living within the insect’s cells in tightly integrated relationships.
Over evolutionary time, these bacteria have lost much of the genetic material typically required for independent survival. By outsourcing essential biological functions to their insect hosts, they have streamlined their genomes to unprecedented levels.
Such genome reduction is a hallmark of long-term symbiosis. When microbes live exclusively within host cells, natural selection may favor the loss of genes that are no longer necessary for independent life. The result is a hyper-specialized organism that cannot survive outside its host.
Researchers say this pushes the conceptual boundaries of what constitutes a free-living organism. “Exactly where this highly integrated symbiont ends and an organelle starts is very difficult to say,” Łukasik noted, describing the boundary as increasingly blurred.
The discovery revives comparisons to mitochondria, which originated roughly two billion years ago from free-living bacteria that entered into symbiosis with ancestral eukaryotic cells. Over time, those microbes transferred most of their genes to the host cell nucleus, becoming permanent organelles.
The newly documented bacteria appear to represent an intermediate stage not fully independent organisms, yet not fully integrated organelles either. Their minimal genomes suggest that life can persist with far fewer genes than once believed possible.
The findings contribute to a growing body of research suggesting that the definition of life is more fluid than previously assumed. As scientists uncover increasingly streamlined symbiotic systems, they are forced to reconsider long-held distinctions between cellular components and standalone organisms.
In the microscopic world inside insect cells, evolution continues to challenge biology’s neat categories demonstrating that the transition from independent microbe to organelle may not be a single leap, but a gradual and ongoing process.