Cockroach Genomes Reveal Unprecedented Horizontal Gene Transfer from Symbiotic Bacteria

Analysis of 18 cockroach and termite genomes uncovered 40,485 DNA fragments derived from the endosymbiont *Blattabacterium cuenoti*. The inserts, ranging from 93 to 4,900 base pairs, can contain up to nine short segments from different bacterial loci, a scale far beyond the <300 fragments previously reported in eukaryotes.

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*Blattabacterium cuenoti* lives inside cockroach cells, where it recycles nitrogen. Horizontal gene transfer is common among microbes but rare in complex eukaryotes; the study’s focus on non-coding fragments revealed a long-standing exchange that predates the split between cockroaches and termites.

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The survey covered 18 species, detecting 40,485 bacterial fragments. Individual inserts span 93–4,900 bp, and some have persisted for at least 28.7 million years, indicating long-term retention.

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Researchers suggest the acquired DNA could broaden cockroach genomic capabilities, potentially aiding adaptation and speciation. If functional, the inserts may provide molecular tools absent from the host genome, illustrating a possible route for rapid evolutionary change.

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The authors state that pervasive HGT from *B. cuenoti* to cockroaches reveals an underappreciated mechanism shaping eukaryotic genomes. They note the inserts may be functional, neutral, or mildly deleterious and call for targeted functional studies.

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The study acknowledges no direct evidence that the transferred fragments affect cockroach biology. The authors caution that the DNA could be slightly harmful yet persist due to weak selective pressure, emphasizing the need for experimental validation.

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All sources agree on the extent of DNA transfer, but they uniformly report the functional role of the inserts remains unknown, representing a major gap for future research.

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• “We identified numerous chimeric inserts comprising up to nine short segments from different locations in the B. cuenoti genome,” — University of Sydney researchers

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• “Our findings indicate pervasive HGT in eukaryote genomes, with potentially far-reaching implications for adaptation and speciation.” — University of Sydney researchers

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• “The persistence of numerous inserts over millions of years indicates that they may have assumed functional roles in both genes and intergenic regions, are effectively neutral, or are only slightly deleterious,” — University of Sydney researchers

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• “Future research on cockroaches and other species harboring obligate endosymbiotic prokaryotes will help to uncover any functional effects of inserts, providing a more comprehensive understanding of how HGT shapes genome evolution.” — University of Sydney researchers

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The team will test the functional impact of the bacterial inserts and expand surveys to other animals with obligate symbionts to assess whether similar HGT events are widespread.

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