The “tiger eye”—featuring a bright yellow, amber, or orange iris—can give this gaited breed a dramatic look, apparently without side effects, said Rebecca Bellone, PhD, associate adjunct professor in the University of California, Davis, Department of Population Health and Reproduction and Veterinary Genetics Laboratory.
Bellone and two of her undergraduate students, Maura Mack and Elizabeth Kowalski, recently delved into the “fun study” of finding the genetic mutations responsible for this unusual eye color, she said. They investigated genomic information on more than 300 horses, including over 70,000 markers across the genome of 24 Puerto Rican Paso Finos. This revealed two specific mutations on the SLC24A5 gene that appear to be responsible for the tiger-eye coloring.
Mutations in this same gene in humans causes a form of albinism, Bellone said. But in humans, those mutations seem to cause reduced pigmentation not only in the iris but also in the hair and skin. The equine mutations, however, seem to affect iris color alone.
“We believe the SLC24A5 protein does not function normally in horses with the tiger eye variants, causing a reduction in the amount of pigment being produced, and that is why the iris is a lighter color,” she said. “We are still interested in knowing why, in horses, this gene only appears to influence pigmentation of the iris but not of the coat.”
Clinical evaluations did not reveal any vision issues in tiger-eye horses, Bellone said. While this is good news, allowing breeders to use genetics to select for an eye color that’s “quite beautiful,” more research is needed to confirm there are no ocular anomalies, she said.
The tiger eye allele shows up in different variations in the horse—namely, in two types (what they call the Tiger-Eye 1 and Tiger-Eye 2 variants). Tiger-eye horses can be either heterozygous (carrying only one copy of each tiger-eye allele) or homozygous (carrying both copies of the allele) for either of these variants. It’s possible that certain combinations of alleles could be related to eye issues and that the researchers just haven’t been able to determine that yet—mainly because more horses are needed for the research.
“At this point we haven’t detected any ocular differences attributed to the phenotype,” Bellone said. “However, we did not examine a horse that was homozygous for the Tiger-Eye 2 variant, as these appear to be rarer, so more work is needed to say for sure that there are no ocular anomalies associated with either variant. My research team studies the genetics of several ocular disorder and related pigmentation differences, so we are trying to better understand the biology behind the connections.”
The study, “Two Variants in SLC24A5 Are Associated with ‘Tiger-Eye’ Iris Pigmentation in Puerto Rican Paso Fino Horses,” was published in G3.