Sunshine (SUN): A Dilution Mutation in the SLC45A2 Gene in Horses

In 2019, Etalon’s team discovered the Sunshine (SUN) mutation in the SLC45A2 gene, in a Standardbred Stallion named Captain Sunshine. This newly identified mutation adds to the growing list of known color genes in horses and deepens our understanding of the SLC45A2 gene’s role in pigmentation.

What is Sunshine (SUN)?

The Sunshine gene is a recessive mutation in the SLC45A2 gene, the same gene responsible for other dilution genes (cream, pearl, and snowdrop). This variant produces a recessive dilution similar to pearl, and indicates changes or multiple alleles of SLC45A2 result in dilution phenotypes in horses.

How was Sunshine (SUN) discovered?

Sunshine (SUN) was first identified in a stallion named Captain Sunshine, who appeared as a double-dilute. When tested multiple times by various labs, his results came back positive for only one Cream gene (CR/n). This didn’t make sense to Captain Sunshine’s owners or Etalon’s team, since the genetics didn’t match his color or history! After extensive DNA sequencing, Etalon’s team identified a unique genetic mutation in the SLC45A2 gene that explained the unusual dilution.

Inheritance and Expression

Sunshine (SUN) is a recessive mutation. While a single Sunshine (SUN) allele does not affect the coat color, some horses may show skin lightening/freckling. A horse with one Cream (CR) and one Sunshine (SUN) allele is known as a compound heterozygote (CR/n + SUN/n), and appears similar to double cream or cream plus champagne dilutions.

Homozygous (sun/sun): Horses with two copies of Sunshine (SUN) are expected to show a golden or lightened coat with diluted red and black pigment, lighter skin, and amber or tan eyes.

Heterozygous (sun/n): Horses with just one copy usually appear normal, unless paired with another dilution gene such as Cream (CR), Pearl (PRL), or Snowdrop (SNO).

Compound Effect (CR + SUN): Horses with one Cream (CR) and one Sunshine (SUN) allele can mimic the appearance of a double dilute (i.e. cremello or perlino).

Health and Viability

No adverse health effects have been associated with the Sunshine (SUN) mutation. Sunshine (SUN) is not lethal.

Research Status

At this time, research for the Sunshine (SUN) mutation is ongoing to study the full effects. So far, this mutation has only been found in a small number of horses related to Captain Sunshine. The origin and allele frequency is unknown at this time, research is ongoing.

If you have a horse who you suspect may carry this gene we encourage you to test and join the research!

Conclusion

The Sunshine (SUN) variant is another exciting discovery showing that the world of horse coat colors is still full of surprises. Like Pearl (PRL), Cream (CR), and Snowdrop (SNO), Sunshine (SUN) changes the way pigment is expressed, creating striking and sometimes unexpected coats, pink skin, and light colored eyes. Since Sunshine (SUN) can mimic other dilutions and falsely present as a double dilute, genetic testing is the only way to confirm if a horse carries the Sunshine (SUN) variant! If you suspect your horse may carry this mutation, genetic testing will help you know for sure.

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Read The Sunshine (SUN) Paper Here