Cockroach genomes are packed with DNA transferred by their endosymbiont bacterial partners

Genes aren't just transferred from parents to their offspring. Nature has found other ways to pass on genetic information, even between different species. And a new study, published in the journal Proceedings of the National Academy of Sciences , reports that cockroaches, in particular, are riddled with DNA transferred from another species.

Gene transfer between different species

The transfer of genes between different species, formally referred to as horizontal gene transfer (HGT), is common in bacteria and microorganisms. Bacteria frequently swap DNA this way, allowing them to rapidly spread advantageous traits across different species. A well-known example is the spread of bacterial resistance, which has allowed multiple bacterial species to become immune to antibiotics.

Yet HGT has been thought to be far less common in more complex organisms, like animals and plants. It has, however, been documented and seems to be somewhat more common in endosymbiont relationships, in which one organism lives inside the body or cells of another organism.

Some earlier studies have suggested that long-term bacterial partners in endosymbiont relationships rarely leave much DNA behind in host genomes, but research using modern long-read sequencing has been limited, and prior research has mostly focused on protein-coding genes. Short, noncoding DNA fragments from bacteria have been largely ignored, even though they can still affect genome evolution.

Putative ancestral insert evaluated by examination of the flanking region homology. Credit: Proceedings of the National Academy of Sciences (2026). DOI: 10.1073/pnas.2604240123

Cockroaches hosting Blattabacterium and their DNA

The authors of the new study hoped to get a clearer picture of HGT by measuring the true scale of bacterial DNA transfer from an obligate symbiont into an animal genome. They found that the relationship between cockroaches and the endosymbiont Blattabacterium cuenoti provided the perfect platform for their study. They say that the Blattabacterium genome is likely to come in close contact with host nuclear DNA during egg creation and development.

To find out just how much DNA had been transferred to cockroaches from Blattabacterium, the team chopped Blattabacterium genomes into short fragments and aligned them against 18 cockroach genomes to spot matching bacteria-like sequences. Only matches at least 50 base pairs long were counted, and multiple quality checks removed likely assembly errors or simple duplications.

The researchers found 40,485 Blattabacterium-derived inserts across the 18 different cockroach genomes, or anywhere from 93 to 4,900 per genome. They found that certain Australian burrowing cockroaches carry more than 3,000 bacterial DNA inserts each, more than 10 times the previous record for eukaryotes, aside from a microscopic freshwater animal called a rotifer.

The study authors write: "Inserts were clustered on some contigs but generally were broadly distributed across host genomes and were inferred to have been transferred across multiple time points, consistent with many independent HGT events."

The team found evidence that the inserts were being inherited vertically after HGT. A subset of them dated far back to cockroach evolution, persisting for at least about 29 million years in certain cockroach lineages.

Functional, neutral or somewhat deleterious?

The researchers were also curious whether these transferred genes were functional. Their persistence over many generations suggested that they were either functional, effectively neutral or only slightly deleterious, making it inefficient or impractical for evolution to weed them back out. To test whether any of the inserted bacterial fragments were being transcribed, the team used RNA sequencing from several cockroach species.

They found that 91.42% to 94.96% of inserts are not actively read into RNA, but a small fraction is transcribed and sometimes incorporated into exons, hinting at possible functional roles. They also say many inserts are "chimeric," or stitched together from multiple distant regions of the bacterial genome, suggesting complex DNA repair processes.

Although the study shows that inserts exist and can persist, it does not directly test their effects on cockroach biology. The study authors say that future research on cockroaches and other endosymbiotic relationships can help uncover functional effects of inserts and provide a better understanding of how HGT shapes genome evolution.

Written for you by our author Krystal Kasal , edited by Gaby Clark , and fact-checked and reviewed by Robert Egan —this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive.

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Publication details

Kyle M. Ewart et al, Uncovering thousands of endosymbiont DNA transfer events within single c…