Random events and environmental changes combined with a loss of genetic diversity have an enormous influence on the fecundity and survival of small populations.
Serious preservation breeders are faced with wrestling the pragmatic applications of two essential and apparently contradictory concepts .
Figuring out the minimum viable population that will sustain a healthy genetic diversity for your strain is essential.
In a closed breeding population, like a horse registry, the 50/500 rule is helpful. Ideally, a strain’s founding stock should consist of a minimum of 50 genetically distinct individuals. Then individual breeders can communicate with each to maintain a genetically sustainable population of at least 500 breeding individuals
So how do you select these first 50 founding individuals? You want horses that all belong to a distinct genetic cluster. Below is a map of equine genetic clusters found when comparing a Bronze Age horse skeleton’s DNA found in Anatolia with DNA of modern horses. Your real-life details will differ depending on the strain of Colonial Spanish horse you are working with.
Basically, you want your founding horses to cluster in a group like the little green triangles on the example. That is the genetic profile of your distinct strain. However, to get that degree of detailed information, you will need to work with a genetic researcher. And you will need to have enough individuals to be able to track patterns. If, like most Colonial Spanish horse breeders, you are working on a frighteningly small scale with a handful of horses you can still work towards the 50/500 minimum.
Pragmatically, you can use Texas A&M’s horse ancestry test along with the pedigree. Make sure your pedigree is accurate and you know where your founding stock originated
First, you look at the percentage of homo/heterozygosity in the genetic markers. If you have roughly 50% of the markers showing one variable and 50% showing two variables, you have a horse that is descended from a distinct group or strain of horses. I do recommend that you do a ‘clean genes’ genetic test before breeding any horse. But assuming your breeding stock is free form any detectable genetic defects, your foals should be of distinctive type with a healthy genetic diversity.
The most effective way to maintain that genetic diversity is to make sure you have a variety of sire’s lines as well as mare’s mtDNA. Personally, I like having at least three different sire’s lines of complementary type.
If your horse shows predominately heterozygous genetic markers, its ancestors are a mixed bag. Many horses registered in the CS registries are the result of breeders randomly mixing strains. Sometimes this was a purposeful act. More often poor documentation where the DNA results don’t actually match the paper pedigree left breeders ignorant of what they had.
Many of those breeders have become discouraged by inconsistent type in their foals. However, if you have a distinctively biomechanically Square Horse with predominately heterozygous pairs of genetic markers, you do have options for improving both the consistency of type and the genetic profile of future generations.
That option is tracking the mares mtDNA in your breeding stock’s profile. Arab and Barb horses have been interbred since at least 700ad. The fundamental genetic difference between Barb and Arab horses is that the Barb horses have been bred in distinct closed mare’s families for millennia.
So go to the tail line on your breeding stock’s pedigrees and track it back to the founding mares. Make sure that all your breeding stock, INCLUDING your stallion, descends from the same strain of founding mares. Again, assuming your breeding stock is free form any detectable genetic defects, your foals should be much more of your distinctive type with a healthy genetic diversity.
The second issue is called ‘genetic drift’ by geneticists and ‘setting type’ by horse breeders. As ‘purebred’ registries of all kinds are demonstrating, intensive selection for uniform type of any kind, whether color or height or facial profile also results in a loss of diversity in many unseen variable. Individuals in a closed population are selected by both environmental factors and by human choices.
If the genetic markers come back with predominately homozygous pairs of genetic markers, your strain has lost genetic diversity. Figuring out to maximize what genetic diversity you do have and increasing the degree of heterozygous pairs is a priority. And it might require bringing in individuals from more distantly related strains.
The most successful and historically accurate option for optimizing genetic diversity among inbred Square Horse strains is to increase the diversity of mare’s mtDNA of your herd. Barb and Iberian horses mtDNA generally fall into the ‘D’ clade. I don’t know of a lab currently offering mare’s mt DNA tests, but the variations on the ‘D’ clade are generally numbered.
Pragmatically, small scale breeders can go back to their horse ancestry results. Then check the breed profiles offered. See what breed profile pops up most consistently among your herd. Then reach out to other breeders and ask if they have any mares available that show that profile.
It may take persistence to find a fellow CS breeder willing to respond to your requests. Currently the Galcieno/Garrano breeders are the most likely to know the breed profile of their herd. But if you search out mares that share the phenotype that you like and then research their breeding, you can at least start the conversation.
Last but not least, appreciate and support your fellow CS breeders, especially when their breeding goals differ from yours. The most effective breeding long term program for the survival of a breed is a multitude of small sub-groups that differ just enough to maintain genetic diversity should one breeding population be threatened by disease or loss of fertility
sara annon 