A genetic marvel, then a debate
Tardigrades are microscopic invertebrates famous for their ability to survive lethal conditions, from the vacuum of space to temperatures near absolute zero. In 2015, scientists studying the species Hypsibius dujardini (now known as Hypsibius exemplaris in lab cultures) published a startling finding. Their analysis of the tardigrade's genome concluded that approximately 17.5% of its genes—some 6,600 in total, did not come from its animal ancestors. They appeared to be acquired directly from bacteria, plants, fungi, and archaea.
This process is called horizontal gene transfer (HGT), the movement of genetic material between different species. While common in bacteria, HGT on this scale was unheard of in the animal kingdom. The researchers proposed that this massive influx of foreign DNA might be the secret to the tardigrade's legendary toughness, giving it a toolbox of survival genes stolen from hardier organisms. The discovery suggested that this tiny "water bear" was a genetic chimera.
Science corrects the record
The extraordinary claim immediately drew scrutiny. Within weeks, a different research group at the University of Edinburgh published their own sequencing of the same tardigrade species, ordered from the same supplier. Their results were dramatically different. They found at most 500 foreign genes, and concluded that the actual rate of HGT was only 1-2%, a level not unusual for animals.
The second team argued that the original study was skewed by bacterial contamination. When sequencing the DNA of a microscopic animal, it is difficult to completely separate the target organism from the bacteria living with it and on it. They concluded that the vast majority of the "foreign" genes were not part of the tardigrade genome at all, but were simply the DNA of contaminating microbes. Subsequent studies and re-analysis of the data confirmed this conclusion. The scientific community now understands that tardigrades do not have a massive amount of horizontally transferred DNA. The episode is an example of the self-correcting process of science, where remarkable claims are tested, debated, and refined by the global research community.