Most animals straightforwardly inherit their genetic material. DNA passes from parent to offspring, generation after generation, creating the biological continuity that links families, populations and species. Yet evolution has never been entirely restricted to that route.
By examining cockroach genomes, scientists uncovered evidence that large amounts of DNA originating from bacteria have become embedded within the insects' genetic material. The scale of the discovery surprised even the researchers involved. Rather than finding a handful of isolated fragments, they identified thousands of bacterial DNA inserts scattered throughout individual cockroach genomes, suggesting that exchanges between species may have left a much larger imprint on animal evolution than previously appreciated.
The 150-Million-Year Partnership Behind Cockroach Evolution
According to the study published in Proceedings of the National Academy of Sciences, titled “Uncovering thousands of endosymbiont DNA transfer events within single cockroach genomes”, the source of these genetic fragments is a bacterium called Blattabacterium cuenoti, a long-term resident inside many cockroach species. Unlike free-living bacteria, it spends its entire existence within specialised cells of its host and is passed directly from one generation to the next through the egg.
That close association stretches back an extraordinary length of time. The relationship is thought to have existed for well over 150 million years, giving the bacterium countless opportunities to come into contact with the insect's own genetic machinery. Such intimacy matters. When organisms share a biological space over immense timescales, the possibility arises that pieces of DNA may occasionally move from one genome to another. The process, known as horizontal gene transfer, is common among bacteria but has traditionally been considered much less significant in animals. The new work suggests that the assumption may need revisiting.
Thousands of Bacterial DNA Fragments Discovered in Cockroach Genomes
To investigate the extent of bacterial DNA within cockroach genomes, researchers compared genetic material from numerous cockroach species with the genome of Blattabacterium. Rather than limiting the search to complete genes, they looked for much smaller stretches of matching DNA, including fragments that do not code for proteins.
As per the study, the broader approach revealed an unexpectedly crowded picture. Across the analysed genomes, more than 40,000 bacterial-derived inserts were identified. Individual species varied considerably, with some carrying only a few dozen detectable fragments while others contained several thousand. Certain Australian burrowing cockroaches stood out in particular, harbouring levels of bacterial DNA far beyond those previously reported for most complex organisms.
The inserts were not confined to one corner of the genome. They appeared across many different regions, indicating that transfers occurred repeatedly over evolutionary history rather than during a single ancient event.
Bacterial DNA Has Survived in Cockroach Genomes for Millions of Years
Finding foreign DNA is one thing. Explaining why it remains there is another. Genetic material that serves no purpose is often lost over time through mutation or natural selection. Yet some of the bacterial fragments identified in the study appear to have survived for tens of millions of years. By comparing related cockroach species, the researchers traced certain inserts back to ancestors that lived roughly 29 million years ago.
Their persistence raises intriguing questions. It does not necessarily mean these fragments perform important biological functions. Some may simply have little effect on the host and therefore escape removal. Others, however, could have acquired new roles after entering the cockroach genome. Evolution has a history of repurposing genetic material, and even DNA that begins as biological debris can occasionally become useful.
The study stops short of proving that any particular insert influences cockroach traits. What it does show is that many of these sequences have remained stable across immense stretches of evolutionary time.
Bacterial DNA May Have Become Part of the Genome
Most of the transferred DNA appears inactive. Analyses of RNA, which provides evidence that genetic sequences are being used by cells, suggested that the overwhelming majority of inserts are never read or expressed.
Even so, a small proportion showed signs of activity. Some were located within regions associated with genes, while a tiny number appeared within sequences that contribute to the final genetic instructions used by cells. According to the study, these cases are relatively rare, but they hint that at least some transferred fragments may have become integrated into the host's broader genetic landscape.
The researchers also encountered unusual hybrid inserts made up of pieces originating from several different locations in the bacterial genome. These genetic mosaics point towards complex DNA repair and insertion processes that are still not fully understood.
Taken together, the findings present a view of genomes that is less tidy than traditional textbook descriptions. Rather than acting solely as records of ancestry, genomes can also accumulate traces of long-term biological relationships. In cockroaches, one of those relationships appears to have left thousands of genetic signatures behind.
The work does not claim that bacterial DNA transformed cockroach evolution overnight. Instead, it highlights how gradual exchanges occurring over millions of years can quietly reshape genomes. As more high-quality genome sequences become available, similar hidden histories may emerge in other animals that have spent evolutionary lifetimes living alongside microbial partners.
