Symbiotic bacteria live inside specialised organs called bacteriomes within insects. This image shows a cross-section of the planthopper Callodictya krueperi, with fluorescent probes labelling three microbes: Vidania (red), Sodalis (yellow) and Sulcia (green)
Courtesy Anna Michalik et al
Symbiotic bacteria living inside insect cells have the smallest genomes known for any organism. The findings further muddy the distinction between cellular organelles like mitochondria and the most barebones microbes in nature.
“Exactly where this highly integrated symbiont ends and an organelle starts, I think it’s very difficult to say,” says Piotr Łukasik at Jagiellonian University in Kraków, Poland. “This is a very blurred boundary.”
Planthoppers are insects that subsist entirely on plant sap, and supplement their nutrition thanks to an ancient relationship with symbiotic bacteria. Over many millions of years, these microbes evolved to live inside specialised cells in the planthoppers’ abdomens, producing nutrients that the planthoppers can’t get from their sugary diet. Many of these bacteria are totally dependent on their hosts and have let their genetic toolkits deteriorate to a fraction of their ancestral size.
Łukasik and his colleagues were interested in the evolution of this bacteria-bug relationship and just how small these bacterial genomes could get. The team sampled 149 individual insects across 19 planthopper families, extracting DNA from the insects’ abdominal tissues. The researchers analysed and sequenced the DNA, reconstructing the genomes of the symbiotic bacteria Vidania and Sulcia.
The bacterial genomes were exceptionally tiny. Genome length can be measured in numbers of base pairs, the sequence of paired “letters” in genetic code. The bacterial genomes were less than 181,000 base pairs long. For comparison, the human genome is billions of base pairs long.
Some of the Vidania genomes were just 50,000 base pairs long, the smallest known for any life form. Previously, the smallest was Nasuia, a symbiotic bacterium hosted by planthopper relatives called leafhoppers, measuring just over 100,000 base pairs.
At 50,000 base pairs, the Vidania genomes are on the scale of those found in viruses, which are not considered to be alive: the virus behind covid-19 has a genome around 30,000 base pairs long, for instance. Some of the Vidania have only about 60 protein-coding genes, among the lowest counts on record.
Planthoppers rely on symbiotic bacteria to supplement their specialised diets
Courtesy Anna Michalik et al
The bacteria have been evolving with their insect hosts for about 263 million years, independently evolving extremely small genome sizes inside two different groups of planthoppers. One of the few things these bacteria do is produce the amino acid phenylalanine, which is a chemical precursor for making and strengthening insect exoskeletons.
Łukasik and his team think that the massive loss of genes might happen when the insects eat new foods with nutrients that used to be supplied by the bacteria, or when more microbes move in and take over those roles.
The highly reduced bacteria are reminiscent of mitochondria and chloroplasts – energy-producing organelles inside animal and plant cells descended from ancient bacteria. The symbiotic bacteria similarly reside within the host cells and are passed down between generations.
“‘Organelle’ is just a word, so it’s fine with me to call these organelles if someone wants to include these in the definition,” says Nancy Moran at the University of Texas at Austin, who was not involved with the research. “But there remain differences from mitochondria or chloroplasts.”
Mitochondria are much older, having arisen 1.5 billion years ago or more, and their genomes are smaller still – about 15,000 base pairs.
“These symbionts live only in specialised host cells, not in most cells throughout the organism, as seen in mitochondria and chloroplasts,” says Moran.
Łukasik considers these bacteria and mitochondria as simply being at different places on an evolutionary “gradient of dependence” on their hosts. He suspects even tinier symbiote genomes have yet to be discovered.
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