In the past, many scientific discoveries have been made independently by two research teams from different countries. For example, in the 18th century, the scientists Joseph Priestley from England and Carl Wilhelm Scheele from Sweden discovered oxygen simultaneously.
Now, in a relatively rare academic event, a new understanding has been discovered almost at the same time by two independent research teams. Interestingly, they were separated by a full Earth circumference and were unaware of each other’s work.
This involves decoding the orange fur color of cats: Why are almost all orange cats male? And why can only female cats be calico?
In this photo, the orange cat is definitely a male. The other two calico and black cats are certainly female. But why is that?
The relationship between fur color and the sex of cats has been a mystery for 10,000 years since humans domesticated these animals. Sixty years ago, scientists thought they had found the answer.
However, it turned out to be only half the issue, and it wasn’t until now that two research teams in the United States and Japan worked independently to thoroughly solve this mystery.
They jointly announced their new findings on bioRxiv, a preprint scientific publishing platform. The Japanese research team published their findings a day earlier, likely simply because the sun rose earlier in Japan than in the United States.
This is an example of how fierce the scientific competition is among research groups around the world.
From the Dawn of Agriculture
About 10,000 years ago, during the dawn of agriculture, when our Homo sapiens ancestors began cultivating food crops. In a region known as Mesopotamia, located at the crossroads of Asia and Europe today, a chase akin to Tom and Jerry occurred.
A mouse, while fleeing from a cat, accidentally found its way into a grain storage facility created by humans, the first storage facilities existing after our ancestors had only foraged for survival.
Now they had learned to cultivate and store. And it was only when they started storing food that the granaries were built.
The mouse managed to escape from the cat—then still a wild creature living deep in the forests. When it returned to its nest, the mouse invited a host of its kind to raid the humans’ granary, a warm place where they could feast without fear of hunger or cold.
The invasion of mice infuriated humans. Since the mice were too small and quick, they couldn’t catch them. No matter how they secured their granaries, the mice always found a way to gnaw and enter.
So, reluctantly, humans had to share part of their hard-earned food with the mice. Until the cats appeared and roamed around human settlements.
They wondered: Why was this furry, alert creature that meowed showing up here? As it turned out, they came to catch mice. And thus, like finding treasure, humans began to bring cats into their homes to guard their grain stores.
They were grateful to the cats, venerating them in temples from then on.
Humans have been grateful to cats, venerating them in temples since then.
Whenever they moved, during nomadic migrations, transcontinental relocations, or sailing to conquer new lands across the seas, humans also brought cats along to catch mice, aiming to protect their food reserves.
Thus, over several thousand years, this furry creature began to invade the world, transforming from a tool into a friend, a lucky deity, and now our “master.”
The Breeding of Cats and the Mystery of Calico Cats
As cats followed humans around the world, they didn’t simply arrive in new lands, enjoy new sights, and eat new delicacies—mostly bird and mouse meat. They also had the opportunity to meet local cat populations, where they could find mates—which were mostly one-night stands.
So, you can imagine an Egyptian male cat with a striking orange coat “living it up,” being carried on merchant ships traveling along Mediterranean port cities, where it would continuously meet and mate with local female cats with black fur.
A few years after the merchant ship returned along its original route, the sailors noticed that the fur colors of the cats at the ports had changed.
Some have orange fur, some have black fur, and the rest have mottled fur between black, orange, and white. Strangely, all the orange cats were male, while any mix of orange and black cats would all be female.
It is extraordinarily rare to find a male calico cat, and conversely, a female cat with all orange fur. The ratio is about 1 in 3000.
“It is indeed a genetic mystery, a real puzzle,” said Professor Greg Barsh, a geneticist from Stanford University. It wasn’t until the 20th century that the mystery of fur color and sex in cats was partially explained when scientists discovered the X and Y chromosomes responsible for sex.
It turns out that a portion of the gene responsible for a cat’s fur color is found on the X chromosome. And a male cat, which only has one X chromosome in its XY pair, can only have a single solid color coat.
Conversely, female cats carry two X chromosomes, allowing them to have coats that blend black and orange. And when combined with basic white fur, which both black and orange cats may have, female calico cats are born.
But which specific gene on the X chromosome determines a cat’s fur color?
This remains half the mystery that scientists have been unable to solve for the past 60 years. The reason is that the X chromosome in cats contains 155 million base pairs and over 1,000 genes, making screening difficult.
However, in a recent study published on bioRxiv, Professor Greg Barsh and his colleagues at Stanford reported that they have finally identified the gene responsible for orange fur in male cats and calico patterns in female cats.
To achieve this, Professor Barsh visited cat sterilization clinics and acquired 8 feline fetuses, among which were 4 with orange fur. He then isolated the RNA present in their skin cells and discovered that RNA encoding a gene labeled Arhgap36 was present at a concentration 13 times higher in orange cats.
This gene is also located on the X chromosome of cats; thus, Professor Barsh suspected that this was what created orange male cats and calico female cats.
The Arhgap36 gene is responsible for the orange color of cat fur.
To test his hypothesis, the research team sequenced the genome of 188 cats, including 145 orange cats, 6 calico/tortoiseshell cats, and 37 solid-colored cats that were not orange.
The results confirmed that orange cats and calico cats carried a mutated version of the Arhgap36 gene, with a segment of DNA approximately 5 kilobases long, corresponding to 5,000 base pairs, having been deleted.
“Overall, these observations provide strong genetic evidence and a compelling gene set indicating that the loss of this 5 kb segment causes the orange color linked to sex in cats,” Professor Barsh stated.
Interestingly, just one day prior, a similar finding was also published by an independent research team from Kyushu University in Japan on bioRxiv. This paper indicated that Japanese scientists had also identified the Arhgap36 gene as the factor responsible for the orange color in cat fur.
It’s likely that when the two research teams learned about each other’s work, they immediately published their new findings to compete for the title of the first to decode the mystery behind the orange fur of cats.
In the past, there have been many similar scientific discoveries made independently by two research teams. For instance, in the 18th century, the scientists Joseph Priestley from England and Carl Wilhelm Scheele from Sweden discovered oxygen simultaneously.
In 1930, two American scientists, Clyde Tombaugh and Percival Lowell, nearly discovered Pluto at the same time when they pointed their telescopes to the sky.
The advantage of these coinciding independent studies is that they allow scientists to avoid spending additional time verifying their own findings, which is one of the fundamental principles of science.
Consequently, knowledge can be established immediately. Just as the existence of oxygen and Pluto has been confirmed, we can also be certain that the orange fur color of cats comes from the Arhgap36 gene located on their X chromosome.
This mystery has been completely unraveled.
