Rabbits, hares, kangaroos, and some small rodents usually hop around. In a rare breed of domestic rabbits, the Sauteur d’Alfort rabbits, the coordination of the limbs is so disturbed that they walk in handstands instead of hobbling. Using crossbreeding experiments, researchers have now identified which gene variant is responsible for this. As a result, a mutation of the so-called RORB gene is responsible for the peculiar mode of locomotion.
In order to run, you need finely tuned limb coordination. The legs must be moved in the correct order, and their position must be compared with sensory impressions. Normally, the corresponding signals are automatically processed correctly in the spinal cord, so that locomotion works without conscious control. Most animal species can easily switch between different gaits and, depending on the species, for example, trot, sneak, gallop or hop.
Running in a handstand
Sauteur d’Alfort rabbits are a rare exception. This is a special breed of domestic rabbits, which stands out above all for its unusual way of locomotion: As soon as they want to pick up speed, they raise their rear end in the air and walk in a handstand on their front legs. Walking slowly, however, they do not differ from other races of their own species. Researchers led by Miguel Carneiro from the Universidade do Porto in Portugal have now identified the cause of this locomotion disorder: “Using a combination of experimental crossings and genome sequencing, we have shown that a single mutation in the RORB gene explains the atypical gait of these rabbits,” they report .
For their study, Carneiro and his team first crossed Sateur d’Alfort rabbits with normal domestic rabbits. Of the 52 grandchildren of these rabbits, 40 were able to hop normally, while twelve did a handstand. Using blood samples, the researchers analyzed the complete genome of all animals. It was found that all handstand rabbits had a mutation in the RORB gene. This gene codes for a receptor that is mainly found in the brain and peripheral nerves. The mutation causes sateur rabbits to have less functional receptors in their spinal cord.
Gene for wiring the spinal cord
According to the researchers, this means that certain signals that are responsible for coordinating the legs when hobbling cannot be processed and passed on correctly. So instead of jumping off with both hind legs to move forward quickly, the sateur rabbits move their hind legs in an uncoordinated manner at the wrong time, which makes hobbling impossible. Within the first few months of life, they learn to compensate for this disadvantage by lifting their entire rear end and walking on their front legs alone.
As the genetic analyzes showed, this only applied to rabbits in which both alleles of the gene were affected, i.e. which had inherited the Sateur variant from both parents. In heterozygous individuals who had a mutated and a functional variant, the researchers were able to detect slightly less intact RORB receptors, but these animals were able to hop normally.
“This study provides a rare example of abnormal gait behavior that can be traced back to a single change in base,” the authors conclude. “At the same time, it is the first description of a gene that is required for jumping locomotion.” The RORB gene probably also plays an important role in motor coordination in other animal species. An earlier study found that mice with a mutation in this gene waddle in a waddling gait. “In line with previous studies in mice, our study underscores the importance of RORB in spinal cord wiring,” said Carneiro and colleagues. They suspect that a corresponding mutation could also prevent kangaroos and rabbits from jumping.
Source: Miguel Carneiro (Universidade do Porto, Portugal) et al., PLOS Genetics, doi: 0.1371 / journal.pgen.1009429