Selective breeding by humans has resulted in some incredibly strange and unfortunate pets over the years, and the sauteur d’Alfort rabbit is among the strangest of all.
This rare breed of rabbit does not jump or walk like any other existing rabbit or hare. When the highwayman is ready to go, he kicks his hind legs into the air and bounces forward on his front legs, like a human acrobat walking on his hands.
While this may seem like a fun trait, it sadly comes with other debilitating issues as well. Now the one bunny that can’t jump properly has helped us better understand the genetics of jumping in mammals.
By crossing a single robber male with a single New Zealand white female and then crossing the resulting offspring, the researchers raised 52 rabbits, 23 percent of which carried two copies of the mutant gene similar to the original parent. These numbers match the statistics expected when there is only one recessive gene involved in a mutation.
Combining the DNA of the young robbers and non-robbers, the researchers used whole genome sequencing to compare the two groups. In the end, as they anticipated, there was only one gene that stood out.
The cause of the malignanger hopping appears to lie in a mutation in an evolutionarily conserved site in a gene known as RORB, which provides instructions for mammalian cells to create certain proteins.
RORB proteins are generally found throughout the nervous system of the rabbit, where they help convert the genetic code into a protein building template. This particular mutation, however, causes a particularly sharp decrease in the number of neurons in the spinal cord that can actually produce this protein.
Two copies of the RORB mutation, in fact, did not result in proteins in the spinal cord, and this was related to the inability to jump. Other rabbits in the litter capable of jumping on their hind legs did not show such protein loss.
The RORB gene, the authors conclude, must be what allows rabbits to move. It could also be the key to jumping for other mammals.
Over the years, there has been much scientific interest in the special physiology and biomechanics that allow mammals, such as kangaroos, rabbits, hares, and some mice, to jump, but the underlying genetics of this feat has rarely been considered.
One of the few studies that have been done recently found that mice with the same RORB mutation as hopper rabbits are also unable to jump as usual. Instead, these rodents swing on their front legs like a duck, with their tail and hind legs raised in the air.
“I spent four years looking at these mice doing little handstands, and now I can see a rabbit doing the same handstand,” neuroscientist Stephanie Koch of University College London told Science News. “It’s amazing.”
Koch’s study in rabbits is the first to describe a specific gene required for jumping or hopping, and it aligns extremely well with what he has been observing in mutant mice.
Like mutant rodents, hopper rabbits also display other anatomical defects beyond their odd gait. Many are born blind and develop cataracts in their first year of life. RORB knockout mice also show retinal degeneration.
In mice, the RORB gene appears to play an essential role in cell differentiation in both the cerebral cortex and the retina. It could also do something similar in the spinal cord, which is involved in regulating sensory information and locomotion among mammals.
As such, this lack of protein could be what causes the hind legs of rabbits and mice to stand up instead of jumping. In skipping rabbits, for example, the RORB mutation appears to cause defects in the differentiation of spinal cord interneurons, although it is unclear whether this is actually causing the strange locomotion.
“In addition to its expression in the spinal cord, RORB is also expressed in many regions of the brain, such as the primary somatosensory, auditory, visual and motor cortex, in some nuclei of the thalamus and hypothalamus, in the pituitary gland and in the superior colliculus. “, the authors write.
“Therefore, we cannot exclude the possibility that an alteration of RORB function in the brain contributes to the characteristic locomotion phenotype of skipping rabbits.”
The effects of the RORB mutation will require further study, but it is obvious that it is involved in some way. This was the only variant identified in the entire rabbit genome sequence that had any impact on the jump.
While there could be more genes involved in hopping rabbits, it seems the poor hopper rabbits have certainly pointed us in the direction of one.
The study was published in PLOS Genetics.