In mammals, an individual's sex is determined by the chromosomes it inherits from its parents. Two X chromosomes lead to a female, whereas one X and one Y lead to a male. Y chromosomes are only passed from fathers to sons, so each Y chromosome represents the male genealogy of the animal in question. In contrast, mitochondria are passed on by mothers to all their offspring. This means that an analysis of the genetic material or DNA of mitochondria can give information on the female ancestry. For the modern horse, it is well known that mitochondrial DNA is extremely diverse and this has been interpreted to mean that many ancestral female horses have passed their DNA on to modern horse breeds. Until recently, though, essentially no sequence diversity had been detected on the Y chromosome of the domestic horse. Not only does the lack of sequence markers on the Y chromosome make it impossible to trace male lineages with confidence, it also represents a scientific paradox. How can a species with so many female lines have so few male lines? The issue has now been addressed by Barbara Wallner and colleagues at the Institute of Animal Breeding and Genetics, University of Veterinary Medicine, Vienna (Vetmeduni Vienna).
Wallner initially selected seventeen horses from a range of European breeds. She pooled their DNAs and used modern sequencing technology to examine the level of diversity on a 200 kb portion of the Y chromosome she had previously sequenced. The Y chromosomes were found to be highly similar: only five positions turned out to be variable. As Wallner says, "the results confirmed what we had previously suspected: that the Y chromosomes of modern breeds of horse show far less variability than those of other domestic animals."
The five variable positions, or polymorphisms, were nevertheless sufficient to enable the researchers to derive a type of "family tree" for the various breeds of modern horse they investigated. An examinati
|Contact: Dr. Barbara Wallner|
University of Veterinary Medicine -- Vienna