What is still to be discovered concerning appaloosa spotting?

This article is reproduced here with permission from the  Appaloosa Journal.   It was first featured in the November 2015 Appaloosa Journal’s Quick Genetic Question column.  This column is written every month by Dr. Rebecca Bellone as well as other members of the Appaloosa Project's research team.  For answers to some of the most frequently asked questions be sure to read the monthly column.  If you have a specific question you would like to see answered e-mail the editor of the Appaloosa Journal, Dana Russell,  at editor@appaloosajournal.com

In a few articles over this past year, we have discussed the LP gene, how it was discovered, and how to have your horses to DNA tested for it.

We also discussed that while all horses with LP have an appaloosa spotting pattern and associated characteristics, there is a tremendous amount of variability in terms of the amount of white spotting. In other words, there are appaloosa spotted horses that have a few white flecks on their rump and appaloosa spotted horses that are almost entirely white.

It is believed that other genes modify LP and are responsible for this variability in coat patterning. In the coming months we will describe some of our most current work detailing what is known about one of these modifying genes, namely PATN1.  However, relatively little is known about how this gene interacts with LP and what other modifiers might be involved.

While it is currently understood that LP mutation is responsible for the presence of appaloosa spotting, the precise biological mechanism of how LP causes white spotting (or disrupts pigmentation) is unclear. We still need to discover if these white patterned areas are the result of pigment cells not being present or if they are present and just not working as they do in non-LP spotted horses.

Now that Katherine (Katey) Hilburger has joined the Appaloosa Project research team, I have asked her to describe her passion for Appaloosas and discuss her thesis work related to this quick genetic question on “what is still to be discovered”.

Katey writes “I am currently an Animal Biology master’s student and the University of California, Davis. My interest in horses and more specifically, Appaloosas began when I was only five years old.

I took my first riding lessons with John Mullins, who was the breeder and owner of the ApHC Hall of Fame stallion, Little Navajo Joe. The first horse that I ever rode was Little Navajo Star, which is one of Joes’s many progeny.

Coincidentally, many offspring of Little Navajo Joe were utilized in the very first genetic studies on Appaloosa spotting that were conducted as part of Bellone’s Ph.D. thesis in 2000.

It is amazing to think that this connection that began when I was only five years old would eventually bring us together to work on this exciting project.

I have recently been very involved in the Appaloosa Horse Club and compete with my gelding, Heart Zips a Beat, in the all-around non-pro events.

I am a member of the board of the directors for my regional club, the Western New York Appaloosa Association. In 2013 and 2014, I was a top ten year-end finalist in the Novice Non-Pro Hunter Under Saddle. I also placed in the top ten in the Novice Non-Pro Hunt Seat Equitation and in the top five in the Novice Non-Pro Hunter Under Saddle at the 2014 National Appaloosa Show.

Since I have been an avid lover of Appaloosas for many years I now have the opportunity to combine my two passions of Appaloosa horses and biology to earn a master’s degree.

Now that I have joined this research team, I will be examining the cells in horses that produce pigmentation (these cells are called melanocytes). I will culture melanocytes from horses with different amounts of white spotting to investigate the shape of the cells and also what is happening at the molecular level.

Katherine Hilburger earned her B.S. in biology at the State University of New York (SUNY) at Oswego. She is currently a master’s student at the University of California, Davis and is studying equine genetics with Dr. Rebecca Bellone. Her research focuses primarily on how LP affects pigmentation cells (melanocytes) in horses. She is pictured her on her horse Hearts Zips A Beat just prior to leaving for DavisI hope that my work will help deepen our knowledge and provide insight as to how pigmentation is disrupted. The goal is to provide applicable information for appaloosa breeding programs.

Our aim is to fully understand how LP and other genes (some of which we hope to identify as part of this work) alter the function of pigment cells and thus produce the variety of beauty we see in appaloosa spotting.