Cockroaches are generally seen as creatures capable of surviving in virtually any environment, partly due to their DNA, which can encode a variety of useful survival mechanisms—from neutralizing toxins to regenerating limbs.
Now, a study conducted by scientists at the University of Sydney has taken a fresh look at the cockroach genome and uncovered something quite unexpected. The findings were published in the journal PNAS.
It appears that these insects have borrowed a vast amount of DNA from an entirely different species.
This involves the bacterium Blattabacterium cuenoti, previously known for living inside cockroaches and aiding in nitrogen cycling, but it was not previously known to transfer tens of thousands of DNA fragments to its host.
The transfer of genes between species is technically referred to as horizontal gene transfer (HGT) and is frequently observed in bacteria and microorganisms.
Although there is evidence of this phenomenon in complex animals and plants (eukaryotes), it was previously thought to occur much less often.
“We identified numerous chimeric insertions consisting of nine short segments located in various positions within the genome of B. cuenoti,” the researchers write. “Our results indicate widespread horizontal gene movement in eukaryotic genomes, which could have far-reaching implications for adaptation and speciation.”
The researchers analyzed the complete genomes of 18 species of cockroaches and termites. Termites and cockroaches are closely related and share common ancestors, but when they diverged on the evolutionary tree, most termites lost the B. cuenoti species.
Unlike most similar studies conducted to date, the analysis involved searching for smaller non-coding DNA fragments that do not participate in protein synthesis—fragments that were overlooked in previous research focused on finding more complex DNA.
The search uncovered a total of 40,485 DNA fragments from B. cuenoti, ranging from 93 to 4,900 depending on the insect species. Until now, the maximum number of horizontal gene transfers found in eukaryotes was fewer than 300.
Moreover, some of these fragments appear to date back to the very early stages of cockroach evolution.
“Some insertions appear to have been retained in this group for ≥28.7 million years, which may reflect their functional role,” the researchers write.
While genes are typically passed down from generation to generation, the horizontal alternative occurs when cells from different species are in very close contact—in this case, cockroach cells absorb individual pieces of B. cuenoti.
For the eukaryote doing the absorbing, this could mean that its genome becomes more versatile and resilient over time, gaining molecular capabilities it would not otherwise have.
However, it is important to emphasize that the researchers do not yet know what impact these transferred DNA fragments have on cockroaches, if any—they might even cause minor harm, but not enough for evolutionary processes to eliminate them.
“The preservation of numerous insertions over millions of years suggests that they may have played functional roles in both genes and intergenic regions, been effectively neutral, or had only minor harmful effects,” the researchers write.
As analytical methods improve and scientific knowledge deepens, we are starting to observe quite a few examples of horizontal gene transfer beyond the usual suspects of bacteria and microorganisms.
This study shows that much remains to be uncovered. Many other animals have symbiotic relationships with bacteria, so this could be happening on a much larger scale than anyone previously imagined.
However, in addition to extending their research to other species, scientists also want to delve deeper into the cockroach genome to determine whether any of these 40,485 borrowed DNA fragments serve a useful function.
“Our results reveal extensive horizontal DNA transfer from prokaryotic symbionts to eukaryotes,” the researchers write. “Further studies of cockroaches and other species containing obligate endosymbiotic prokaryotes will help identify any functional effects of the insertions, providing a more complete understanding of how horizontal gene transfer influences genome evolution.”
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