By Kevin E. Noonan --
In retrospect, it was eerily prescient. The Wall Street Journal published an article Saturday, hours before the tragedy at Churchill Downs where Eight Belles, a two-year old filly was euthanized because she broke both front ankles after finishing second in the Kentucky Derby. That article was about the danger to thoroughbred horses stemming from severe inbreeding over the past 50 years.
The problems stem from the racing success of the stallion Native Dancer, who was an ancestor of every one of the twenty horses running in this year's Derby. And although this is the first time every horse in the race was related to this great progenitor horse, each of the last thirteen Derbys were won by one of his descendants. In all, the article estimates that 75% of all U.S. thoroughbreds are related to Native Dancer.
The reason for the success of Native Dancer's line comes from the propensity for these horses to be "precocious" and "speedy," particularly at the age when they are running prestigious races like the Kentucky Derby. But these positive attributes come with a "tragic flaw": these horses have difficulties with their feet, most recently and tragically (before last Saturday) illustrated by last year's Kentucky Derby winner, Barbaro. Barbaro won the Derby, but was injured during the running of the Preakness Stakes and ultimately euthanized after a valiant and public struggle to heal well enough to be put out to stud. Barbaro was Native Dancer's great-great-great grandson. Short racing careers (although rarely as tragic as Barbaro's) have been a characteristic of the Native Dancer line.
One reason for the excessive inbreeding to the Native Dancer line is that the overall number of stallions producing foals has dropped from 6,263 in 1992 to 3,083 in 2007, and the number of dams mated to each stallion last year was (on average) 60, up from 42 in 1998, according to the Journal. The reason is economic: using "established" or "proven" stallions increases the likelihood that the mating will produce a winner, and winners are what today's horse set wants. Part of this is due to the transition of the horse-owning population from essentially rural owners with generations of tradition, to today's amalgam of "billionaire sheiks," entertainment figures and others with less interest in tradition than in profits. This trend has been exacerbated by the increase in the prices paid for one-year olds, which last year was an average of $101,347.
The dangers of inbreeding have been known for many years with domestic animals, such as dogs and cats. For example, Irish setters are prone to early blindness and Bernese Mountain dogs, a particularly popular breed, have a shortened life expectancy. The problem threatens to become more pronounced as the products of genetic engineering are applied to animal breeding of farm animals. The technology used to produce the cloned sheep Dolly is being used to clone a prize bull, a holy grail of animal breeders since Clement Markert produced a triple allophenic mouse at Yale thirty years ago. (Markert's student, Jon Gordon, produced the first transgenic mouse a few years later in Frank Ruddle's lab.)
Yet such animals threaten to have an even greater, and much more rapid, effect on genetic diversity than Native Dancer. Although Native Dancer is an ancestor of a great many of today's thoroughbreds, each generation involved crosses of animals merely related to Native Dancer and thus carrying at least a portion of their genetic complement from other lines. A Native Dancer clone, on the other hand, would effect transfer of the same genes in every mating, and the risks of producing unexpected and deleterious mutations or variants, or combinations thereof, are incalculable. It is significant in this regard to recall that the cheetah population, one of the most endangered of the large cats, is particularly at risk because their population went through a "bottleneck" about 10,000 years ago, making all living cheetahs distant cousins (see "Tears of the Cheetah" by Steve O'Brien).
Ironically, Eight Belles was only distantly-related to Native Dancer: her father, Mr. Prospector, was the sire of her dam's dam with no other Native Dancer relatives in her immediate pedigree. It is impossible to know whether Eight Belles' fate was related to her genetic heritage. But it is also impossible to establish that it was not, raising the specter of some type of genetic deficiency (e.g., susceptibility to diseases) that may threaten food production in ways that could dwarf the fears attendant to the current transgenic animal and crops debate.
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