Symbiotic bacteria live inside specialized organs called bacteriomes in insects. This image shows a cross-section of the planthopper Callodictya kruperiwith fluorescent probes that label the microbes Vidania (red) and Sodalis (yellow)
Courtesy of Anna Michalik et al
Symbiotic bacteria that live inside insect cells have the smallest genomes known of any organism. The findings further differentiate between cellular organelles such as mitochondria and the most barebones microbes in nature.
“Exactly where this highly integrated symbiont ends and an organelle starts, I think it is very difficult to say,” says Piotr Łukasik of the Jagiellonian University in Kraków, Poland. “This is a very blurred line.”
Planthoppers are insects that live exclusively on plant sap, and supplement their nutrition thanks to an ancient relationship with symbiotic bacteria. Over many millions of years, these microbes have evolved to live inside specialized cells in the planthoppers’ abdomens, and produce nutrients that the planthoppers cannot get from the sugary diet. Many of these bacteria are completely dependent on their hosts and have allowed their genetic toolkits to degrade to a fraction of the size of their ancestors.
Łukasik and his colleagues were interested in the evolution of this relationship between bacteria and insects and how small these bacterial genomes could become. The team sampled 149 individual insects across 19 planthopper families, and extracted DNA from the insects’ abdominal tissue. The researchers analyzed and sequenced the DNA, and reconstructed the genomes of the symbiotic bacteria Vidania and Sulcia.
The bacterial genomes were unusually small. Genome length can be measured in the number of base pairs, the sequence of linked “letters” in the genetic code. The bacterial genomes were less than 181,000 base pairs long. By comparison, the human genome is billions of base pairs long.
Some of Vidania the genomes were only 50,000 base pairs long, the smallest known for any life form. Previously, it was the smallest Nasuiaa symbiotic bacterium in planthopper relatives called leafhoppers, measuring just over 100,000 base pairs.
At 50,000 base pairs is Vidania genomes are on the scale of those found in viruses, which are not considered to be living: the virus behind covid-19 has a genome around 30,000 base pairs long, for example. Some of Vidania has only about 60 protein-coding genes, among the lowest counts on record.
Planthoppers rely on symbiotic bacteria to supplement their specialized diets
Courtesy of Anna Michalik et al
The bacteria have co-evolved with their insect hosts for about 263 million years, independently evolving extremely small genome sizes in two different groups of planthoppers. One of the few things these bacteria do is produce the amino acid phenylalanine, which is a chemical precursor to making and strengthening insect exoskeletons.
Łukasik and his team think that the massive loss of genes could happen when the insects eat new food with nutrients that used to be supplied by the bacteria, or when more microbes move in and take over those roles.
The greatly reduced bacteria are reminiscent of mitochondria and chloroplasts – energy-producing organelles inside animal and plant cells that originate from ancient bacteria. The symbiotic bacteria reside in the same way in the host cells and are inherited between generations.
“‘Organelle’ is just a word, so I’m fine with calling these organelles if someone wants to include those in the definition,” says Nancy Moran of the University of Texas at Austin, who was not involved in the research. “But there remain differences from mitochondria or chloroplasts.”
Mitochondria are much older, having originated 1.5 billion years ago or more, and their genomes are even smaller—about 15,000 base pairs.
“These symbionts live only in specialized host cells, not in most cells throughout the organism, as seen in mitochondria and chloroplasts,” says Moran.
Łukasik considers these bacteria and mitochondria simply to be in different places on an evolutionary “gradient of dependence” on their hosts. He suspects that even smaller symbiote genomes have yet to be discovered.
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