A new PNAS study shows that horizontal gene transfer isn’t limited to microbes; cockroaches have been quietly stitching bacterial DNA into their genomes for millions of years. Researchers scanned the genomes of several roach species and found thousands of bacterial fragments, some stretching up to several thousand bases. The work overturns the assumption that multicellular animals rarely acquire foreign genes.

The team focused on roaches because they harbor Blattabacterium, endosymbiotic bacteria that recycle nitrogen from wood diets. By setting a 50‑base minimum, they recorded as few as 93 bacterial insertions in one species and as many as 4,900 in another, with a median length of just 160 bases. Over three‑quarters of these sequences lie outside coding regions, suggesting little functional impact.

Most of the bacterial fragments appear to be evolutionary accidents rather than adaptive imports, yet their persistence indicates that DNA uptake occurs regularly enough to leave a fossil record. This finding expands the known sources of genetic diversity in animals and may prompt re‑examination of other multicellular genomes for hidden microbial contributions. It underscores that genomes are more porous than textbook trees suggest.