Bdelloid rotifers, a class of microscopic aquatic animals, have developed a unique way to combat fungal pathogens by using genes acquired from non-metazoan (plant, fungi, or bacterial) organisms. A study published in Nature Communications focuses on the role of horizontally acquired genes in bdelloid rotifers, particularly in their response to fungal pathogens.
Despite being recognized over 15 years ago, the functions of these genes are still not fully understood. This remarkable discovery highlights an unusual link between biological conflict and horizontal gene transfer (or HGT, a method of organisms passing genetic information to other organisms that are not their direct offspring), indicating that bdelloid rotifers possess an innovative means of resisting natural enemies through mechanisms that haven’t been seen in other animals, which could lead to the production of vital antimicrobial medications.
These rotifers reproduce asexually and face relentless attacks from various pathogens, including fungi. These attacks have driven the transfer and incorporation of genes from bacteria, fungi, and plants into their genomes. Unlike bacteria, which frequently exchange genes to adapt to new threats, eukaryotic (having a nucleus) organisms like bdelloid rotifers typically rely on sexual reproduction to shuffle their genetic material and pass it on to the next generation. However, bdelloid rotifers have managed to get around this limitation by integrating horizontally acquired genes into their genomes.
There are a few different ways that horizontal gene transfer occurs, namely: transformation, when bacterium picks up DNA from its surroundings; transduction, when a virus carries DNA from one bacterium to another; and conjugation, where bacteria directly transfer genetic material from another bacteria through a special tube-like structure called a pilus.
When exposed to a fungal pathogen, bdelloid rotifers were observed to upregulate (or use more actively) these horizontally acquired genes, more than twice as much as other genes, basically turning on an arsenal of weapons they didn’t make themselves, but “stolen” from other organisms.
Lead author Chris Wilson described the phenomenon by saying, “The main genes [used for defense] were instructions for chemicals that we didn’t think animals could make — they looked like recipes for antibiotics.” This response was stronger than those triggered by abiotic stressors such as desiccation. This distinction tells us that the horizontally acquired genes play a crucial role in the rotifers’ immune response against biological attacks. Among the upregulated genes, clusters resembling the enzymes bacterial polyketide and nonribosomal peptide synthetases, known to produce antibiotics, were particularly prominent. This suggests that bdelloid rotifers may use these acquired genes to produce antimicrobial compounds, providing an effective defense against fungal infections.
In experiments comparing two bdelloid rotifer species, Adineta ricciae and Adineta vaga, it was found that A. ricciae exhibited a much stronger upregulation of these antibiotic-producing gene clusters, nearly ten times higher than A. vaga. Consequently, A. ricciae showed significantly higher resistance to the fungal pathogen, with only 18% mortality compared to 71% in A. vaga. This stark difference shows the role of these horizontally acquired genes in providing a defense mechanism against fungal pathogens.
The study also identifies upregulation of carbohydrate-active enzymes of bacterial and fungal origin, which might degrade fungal cell walls or act as recognition receptors, contributing to pathogen defense. The authors suggest that this may reflect a complex balance between upregulating defensive genes and downregulating others in order to allow the organism to manage resources effectively.
An interesting observation is that HGT genes are often located in the areas of the organism’s genome where genetic changes take place, such as subtelomeric (near the ends of the chromosomes) areas rich in transposable elements (also known as “jumping genes,” which are sequences of DNA that can move around to different parts of the genome), which might facilitate gene duplication, mobilization, and recombination. This genomic dynamism could contribute to the variability of defensive mechanisms among different bdelloid rotifers. The study proposes that while bdelloids do not gain new antimicrobial pathways as quickly as bacteria, they have accumulated a diverse biochemical defense over time that enhances their abilities to resist attack.
The study provided significant insights into the evolutionary biology of bdelloid rotifers, demonstrating how horizontal gene transfer can drive adaptation and survival in the face of biological conflict. The ability to successfully acquire and express genes from non-animal sources offers bdelloid rotifers a powerful tool to resist natural enemies, contributing to their resilience, and offering a unique source of antimicrobial agents adapted for expression in animal cells, rather than the much more common bacteria and fungi. This could lead to the development of safer antibacterial medication, a “pressing need” according to the World Health Organization due to insufficient production of effective drugs to meet global demand.
