United States: A recent study has revealed that some microscopic animals use genes of “stolen” bacteria to fight off infection. One such bacteria is bdelloid rotifers which is just a half a milimeter long and easy to miss but known to be among some of the toughest animals on the planet.
The tiny critters are found in freshwater and are composed solely of females. These are infamously known for “appropriating” genetic material from other organisms. This remarkable capability has enabled them to persist for 40 million years without sexual reproduction, prompting biologist John Maynard Smith to dub them an “evolutionary anomaly,” as per science.org.
Recent investigations shed light on how these peculiar creatures have thrived. A study in Nature Communications revealed that some bdelloids utilize chemical “formulations” acquired from bacteria to fend off infections. This finding exemplifies foreign genes functioning within animal genomes, according to Isobel Eyres, an evolutionary biologist at the University of Sheffield. It also offers insights into how these “ancient asexuals” have endured for such an extended period.
First documented over three centuries ago, bdelloid rotifers—pronounced “dell-oid”—are among the earliest organisms observed through a microscope. Resembling minuscule, translucent leeches, they inhabit virtually every aquatic environment worldwide. Scientists have identified more than 450 species, all exclusively female.
“They are the sole major animal class where males have never been recorded,” says Chris Wilson, an evolutionary biologist at the University of Oxford and co-author of the study. Each daughter rotifer is an exact replica of her mother.
This absence of sexual reproduction has puzzled evolutionary biologists, as it should render bdelloids highly susceptible to extinction. Sexual reproduction allows species to recombine genes and rapidly develop defenses against threats. Conversely, genetically identical crops can be eradicated by a single pathogen. However, bdelloids defy this vulnerability, surviving extreme conditions such as severe drought, intense radiation, and 24,000 years frozen in Siberian permafrost.
Wilson attributes bdelloids’ resilience to their “extraordinarily unique genome.” Researchers have long known that bdelloid rotifer DNA contains a substantial amount of genetic material “borrowed” from other species through horizontal gene transfer. Astonishingly, 10% of their active genes are derived from bacteria, fungi, and plants, according to reports by science.org.
“One-tenth of their genes are not even animal genes,” Wilson remarks. “They don’t belong there.” It’s akin to petting a cat and discovering grass growing from its skin instead of fur.
Many of these foreign genes prove beneficial, shielding bdelloids from harmful chemicals and other dangers. Wilson speculated that additional borrowed DNA might aid in disease resistance. His team observed that while many bdelloid species are vulnerable to the fungal pathogen Rotiferophthora globospora, which consumes them from within, some remain unaffected.
Wilson’s team exposed hundreds of specimens from two related bdelloid species to R. globospora to investigate. The fungus swiftly decimated Adineta vaga, but Adineta ricciae curled up and ceased movement. “After a few days,” Wilson recounted, “they revived and began crawling again.”
Intrigued, the researchers explored what enabled A. ricciae’s revival. They identified hundreds of genes acquired from bacteria, coding for large enzymes known as synthetases. These enzymes act as “formulas” for complex chemicals, including antimicrobial agents. While bacteria often use these chemicals to combat competitors, this is the first instance of such genes found in animals.
Despite their bacterial origin, these genes have evolved in bdelloids to produce compounds toxic to certain pathogens but harmless to bdelloid cells. This discovery could inform the development of more effective antibiotics, as many promising antimicrobial agents are toxic to animal cells.
Amanda Gibson, a biologist at the University of Virginia, notes that the findings raise further questions about bdelloid rotifers and their prolonged celibacy. Can horizontal gene transfer replace the genetic recombination of sexual reproduction, enabling bdelloids to keep up with their adversaries? Do their harsh environments offer escape mechanisms? “This work suggests it might be all of the above,” she concluded.