By Janelle Weaver

When it comes to racehorses, males get all of the attention. The founding stallions of the famed Thoroughbred breed are known to come from the Middle East, but historical records have neglected female pedigrees. Research now suggests that mares had more cosmopolitan origins than their male counterparts.

Information on the origins of mares is spotty because the importance of females to race performance was underplayed in the past, says Mim Bower, an archeogeneticist at the University of Cambridge in the UK. Seventy-four foundation mares have been recorded, but they often didn't have their own name, and several were simply called "A Royal Mare," Bower says.

In a new study, Bower and her team traced the maternal roots of the Thoroughbred by examining sequences of mitochondrial DNA, which offspring inherit only from the mother. By comparing genetic sequences across breeds, the scientists reconstructed the ancestry of the founding females and found that European populations made a significant genetic contribution. "We've taken a historical question and deconstructed it with science," says Bower, whose findings were published October 5 in the journal Biology Letters.

Motley mares

The researchers focused on a segment of mitochondrial DNA called the D-loop, which does not encode any proteins. Some sections of the D-loop mutate quickly and are highly variable across species and populations, so the segment is often used in evolutionary studies. They sampled DNA from the roots of horse hairs obtained from nearly 200 Thoroughbreds and more than 80 British native horses. They also analyzed more than 1,500 sequences from a publicly available database. Altogether, the data represented 30 maternal lineages of Thoroughbreds and more than 1,900 horses from the British Isles, Europe, Asia and the Middle East.

The team found that the genetic profile of Thoroughbreds shows the greatest overlap with those of Eurasian breeds, especially Connemara ponies from Ireland and Irish Draught horses. By contrast, Thoroughbreds are distantly related to Arabian breeds. When the authors sorted breeds by geographic region, they found that Thoroughbreds are more similar to populations from the British Isles and Europe than those from the Middle East or Asia. The racehorse breed received an estimated 60 percent of its genetic makeup from British and Irish horses, about 30 percent from Asian sources and only 8 percent from Arabs.

Horse sense

The finding that maternal lineages came from Europe is not surprising, says Samantha Brooks, a horse geneticist at Cornell University in Ithaca, N.Y. Shipping a horse from the Arabian Peninsula to the British Isles in the 1700s was no small task, and it was most economical to import stallions because they can produce many more offspring in their lifetime than females, she says.

Greger Larson, an evolutionary geneticist at Durham University in the UK argues that there's not enough data in the study to support robust conclusions about evolutionary descent. Instead of using short sequences of mitochondrial DNA, the authors should analyze complete mitochondrial genomes, he explains. In any case, he says that the general conclusions would probably not change much. "When people claim that a breed is absolutely from one place for prestige reasons, the odds are that they're wrong."

3 Comments

1. 1. jtdwyer 01:39 PM 10/6/10

   Didn't the entire 'native' horse population in the Americas come from European stock, primarily Spain, France and England?

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2. 2. daijoubuchan 06:59 AM 10/7/10

   The account given for the origin of Thoroughbred horses is that 3 stallions (and possibly more) of Arabian or Barbary descent (it depends on the source) were in various different ways brought to Europe and mated with local mares. This article is talking about the origin of this specific breed of horse.
   
   Yes it is true that the "native" horses as you called them were brought with settlers to the Americas, because horses had become extinct in the Americas thousands of years earlier. But horses did live in the Americas previously and in fact were thought to have evolved in the Americas and then spread to Asia and Europe.

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3. 3. jtdwyer in reply to daijoubuchan 02:31 PM 10/7/10

   Sorry I didn't explain: if horses now in the Americas essentially all originated from European stock, are they now indistinguishable from European horses?
   
   The possibility was not mentioned in the article, as if it had not been considered much less tested, but couldn't American horses have also contributed to Thoroughbred stock?

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