Research into the Causes of White Patches on Cat Fur Through the Lens of Genetics and Evolution

Historical Observations of Animals

Karl Franzovich Rulye, a professor at the Imperial Moscow University, despite his French origin, was a Russian man. He was characterized by a good-natured temperament, was of short stature and rather stout. In daily life, he preferred to wear boots and a kosovorotka, which sometimes caused confusion among visitors who might mistake him for a janitor.

Rulye began medical studies five years before Darwin, but unlike him, he completed his studies and wrote a doctoral dissertation on hemorrhoids. However, like Darwin, he experienced disappointment with medicine and switched to geology, paleontology, and zoology, achieving significant results in these fields. Parallel to Darwin, he arrived at the concept of evolution, although his approach had Lamarckian characteristics. He openly presented his views to both students and the general public, which led to criticism from the then-Minnauki, but brought him recognition from the foreign agent Alexander Ivanovich Herzen.

Although Karl Franzovich was not invited to publish in Darwin's 'Beagle', he did not feel deep despair, finding material for observations during travels through his native lands or simply observing nature from the window. He was a true naturalist, capable of finding objects for study everywhere. Rulye lived in Moscow on Tverskaya-Yamskaya, near inns, where he often sat on a bench, smoking a cigar and haggling over kalachi.

Statistical Conclusions on White Legs

One of Rulye's observations was made while ill, when he spent hours looking out the window. He noticed a large number of horses with white legs and then established that a horse with white legs more often had white hind legs than front legs. Far ahead of his time, he realized the importance of statistical analysis and conducted numerous observations. He noted that of hundreds of horses passing from a cavalry regiment, more than three-quarters conformed to his law: the whitening begins with the hind legs (most often the right hind leg), then moves to the front legs (most often the left front leg). He also derived patterns for cases with one or three white legs.

To test his law, Rulye organized an experiment with colleagues, making a bet: for every correctly guessed case where he was told the number of white legs on a horse, he received a penny, and for a mistake, he paid three pennies, which provided them with a constant profit.

Observations of Domestic Animals

After studying horses, Karl Franzovich turned his attention to other domestic animals, including cows, dogs, cats, and rabbits. He discovered that they have constant patterns in the appearance of white spots. He noted that the most susceptible areas to coloring in dogs and cats are the paws and chest. However, unlike horses, the onset of spotting in these animals begins on the chest, which, in his opinion, was a major inconsistency.

Rulye explained the appearance of spotting (white patches) in a Lamarckian manner: the parts of the body that undergo more contamination or friction (for example, from harnesses or collars) turn white. He acknowledged the incompleteness and artificiality of this explanation, justifying it by saying he was writing the article 'out of boredom'. To understand the mechanisms of white spot formation on cat fur, it is necessary to trace the path of pigment cell precursors and their migration to the destination.

Migration of Pigment Cells

On the 14th–15th day of embryonic development, the neural crest forms on the dorsal side, from which specialized cells—melanoblasts—are released. These cells subsequently transform into melanocytes, responsible for synthesizing and transporting pigments to the hair follicles. Since hair grows on the skin, not in the neural crest, melanoblasts must undertake a long migration to connect with the hair follicles and become melanocytes.

Long-distance movement of embryonic cells is characteristic not only of melanoblasts but also of other cell types, such as germ cells. Germ cells appear on the 10th–12th day of a cat's pregnancy, but initially are located not inside the embryo, but next to it on the posterior wall of the yolk sac. They then pass through the wall of the hindgut and advance along the mesentery supporting the intestines toward the gonadal primordia around the third or fourth week of gestation.

There are various interpretations of these cellular movements: some see in them confirmation of the old biogenetic law, according to which ontogeny repeats evolutionary changes, while another school considers it a process of selection for the quality of future germ cells. Melanoblasts migrate between skin cells using a standard set of internal motor proteins (actins and tubulins) and external signaling proteins that ensure interaction between skin cells.

Special proteins in the membranes of melanoblasts capture environmental signals and interact with extracellular matrix proteins and surface proteins of the skin, which initiates their movement from the place of origin to the target. The targets of migration are the skin, the retina of the eye, and the cochlea of the inner ear, and the distances from the neural crest are quite large.

Formation of Pigment Centers

Normal melanoblasts successfully cover these distances and reach their workplaces precisely when they are ready. Before populating the hair follicles throughout the body, melanoblasts first migrate to so-called pigment centers. In the cat embryo, these centers are localized on the crown, at the base of the tail, on the shoulder blades, and along the spine. Upon reaching the center, melanoblasts begin to divide and then spread evenly across the skin, embedding themselves in the hair follicles and completing differentiation into melanocytes ready to synthesize pigments.

Apparently, the probability of forming cranial and caudal pigment centers in a cat is higher than that of thoracic and spinal ones. Because of this, the head and tail are considered the most stable coloring zones according to Rulye's classification, while the least stable are areas distant from pigmentation centers, such as paws, abdomen, and chest. Even without genetic mutations, small white spots can appear in areas distant from pigment centers or in places difficult for melanoblasts to migrate to, such as the foreleg or the groin area.

Thus, white depigmented spots occur in those areas of the animal's covering where the precursors of pigment cells fail to arrive in time.