Cockroaches aren’t just cockroaches: their DNA contains traces of a 150-million-year-old bacterial partner |
Most animals inherit their genetic material directly. DNA is passed from parents to children, generation after generation, creating biological continuity that links families, populations, and species. However, evolution was by no means limited to this path.By examining cockroach genomes, scientists have discovered evidence that large amounts of DNA originating from bacteria have become embedded in the insects’ genetic material. The scale of the discovery surprised even the researchers involved. Instead of finding a handful of isolated fragments, they identified thousands of bacterial DNA inserted into the genomes of individual cockroaches, suggesting that interspecies exchanges may have left a much larger imprint on animal evolution than previously expected.
A 150-million-year-old partnership behind the evolution of cockroaches
According to the study published in the Proceedings of the National Academy of Sciences entitled “Detection of thousands of endogenous DNA transfer events within individual cockroach genomes. The source of these genetic fragments is a bacterium called Blattabacterium cuenoti, a long-term resident within many cockroach species. Unlike free-living bacteria, they spend their entire existence within the specialized cells of their host, and are transmitted directly from generation to generation via the egg.This close association extends over an extraordinary period of time. This relationship is believed to have existed for more than 150 million years, giving bacteria countless opportunities to communicate with the insect’s genetic machinery. Such intimacy is important. When organisms share biological space over huge timescales, the possibility arises that parts of DNA will occasionally be transferred from one genome to another. This process, known as horizontal gene transfer, is common among bacteria but has traditionally been considered much less important in animals. New work suggests that the assumption may need to be reconsidered.
Thousands of bacterial DNA fragments have been discovered in cockroach genomes
To investigate the extent of bacterial DNA within cockroach genomes, the researchers compared genetic material from several cockroach species with the Blattabacterium genome. Instead of limiting the search to complete genes, they looked for much smaller pieces of matching DNA, including parts that do not code for proteins.According to the study, the broader approach revealed an unexpectedly crowded picture. Across the genomes analysed, more than 40,000 bacterial-derived accessions were identified. Individual species vary greatly, with some carrying a few dozen detectable fragments while others contain several thousand. Some Australian burrowing cockroaches in particular stood out, as they contain levels of bacterial DNA far beyond those previously reported for most complex organisms.The insertions were not limited to one corner of the genome. They appear in many different regions, suggesting that transitions occurred repeatedly throughout evolutionary history rather than during a single ancient event.
Bacterial DNA has remained in cockroach genomes for millions of years
Finding foreign DNA is one thing. Explaining why he’s still there is another thing. Genetic material that serves no purpose is often lost over time through mutation or natural selection. However, 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 introductions back to ancestors who lived approximately 29 million years ago.Their persistence raises interesting questions. This does not necessarily mean that these parts perform important biological functions. Some may simply have little effect on the host and thus escape removal. However, others may have acquired new roles after entering the cockroach genome. Evolution has a history of reusing genetic material, and even DNA that starts out as biological debris can sometimes become useful.The study stopped short of proving that any particular input affects the cockroaches’ traits. What it shows is that many of these sequences have remained stable over huge periods of evolutionary time.
Bacterial DNA may have become part of the genome
Most of the transferred DNA appears to be inactive. RNA analyses, which provide evidence that cells use genetic sequences, indicate that the vast majority of input is never read or expressed.However, a small percentage showed signs of activity. Some were located within regions associated with genes, while a small 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 indicate that at least some of the transmitted parts may have been integrated into the broader genetic landscape of the host.The researchers also encountered unusual hybrid insertions made up of fragments originating from several different locations in the bacterial genome. This genetic mosaicism indicates complex DNA repair and insertion processes that are still not fully understood.Together, the results provide a view of genomes that is less ordered than traditional textbook descriptions. Rather than serving only as records of lineage, genomes can also accumulate traces of long-term biological relationships. In cockroaches, one such relationship appears to have left behind thousands of genetic signatures.The research does not claim that bacterial DNA transformed cockroach evolution overnight. Instead, it highlights how gradual exchanges that occur 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 spent evolutionary periods living alongside microbial partners.
