After DNA tests on a skeleton found among the remains of an antique Roman house in 2004, Italian scientists assumed they had discovered a breed of horse which had been hitherto unknown or had died out.
An error occurred during these tests, is what a team of scientists from Cambridge University and the Institute of Forensic Genetics at Münster University now say. Their arguments have been published in a letter to the editor of the online edition of the Journal of Cellular Biochemistry. Evidently, donkey DNA became combined with horse DNA, producing an artificial hybrid DNA.
In the original study, analyses were made of five skeletons from equids, to which horses, donkeys and zebras belong. The skeletons had been excavated from the remains of a household in the antique city of Pompeii, in the stables of the Casa dei Casti Amanti (House of the Chaste Lovers). This well-known building is named after the wall frescoes depicting romantic scenes. The owner was probably Caius Iulius Polybius, a wealthy politician and baker. Archaeologists derive this latter occupation from the fact that an open baking oven and four millstones were found in the house. The horse skeletons had been conserved by means of a layer of volcanic ash which had buried Pompeii and the nearby settlement Herculaneum when Vesuvius erupted in 79 AD.
The team of researchers that carried out the original study examined the 2000-year-old mitochondrial DNA of the horses, i.e. DNA not from the cell nucleus but from the “energy powerhouses” (the mitochondria) of the cells. Four DNA types could be easily classified as they matched typical mitochondrial genetic material found in horses. The fifth horse, however, seemed to possess DNA similar to that of horses but otherwise unknown. The scientists came to the conclusion that the horse in question was of a breed hitherto unknown and presumably extinct.
Susan Gurney, a PhD student at Münster University’s Institute of Animal Physiology and a member of the Institute of Forensic Genetics at the University of Cambridge in the UK, took a closer look at the data. Gurney, an expert on the evolution of horses, concluded that an error had occurred in the initial tests. The mitochondrial DNA of a horse had evidently come into contact with that of a donkey, resulting in the formation of hybrid DNA which appeared to originate from an unknown breed of horse. Gurney demonstrated that the first 177 structural units (or nucleotides) of the DNA sequence matched the sequence of nucleotides for donkeys’ genetic material. The remaining 193 nucleotides match horse DNA. “It was easy to recognize that originally there must have been two separate DNA strands,” she says. The error might have occurred during the excavation work – perhaps DNA was transferred from one skeleton to another. Or possibly the error occurred inadvertently in the lab or afterwards during the data analysis at the computer.”
Although the scientists have not been able to confirm that a new breed of horse has been discovered, the result is still exciting, they say. If the donkey DNA really did come from the antique skeleton, this would demonstrate for the first time that the archetype of the domestic donkey typically found in Italy today was already being kept in ancient Pompeii. This lineage is descended from the Somali wild ass. In other European countries, by contrast, the donkeys kept are descended from the Nubian lineage. The ancient donkey DNA provides researchers with new insights into the history of donkey breeding.
The study involved not only Susan Gurney from Cambridge University, currently writing her PhD at the Institute of Animal Physiology headed by Prof. Wolf-Michael Weber, but also Dr. Peter Forster. Forster is a scientist at Münster’s Institute of Forensic Genetics, headed by Prof. Emeritus Bernd Brinkmann.
Susan M. R. Gurney (2010): Revisiting ancient mtDNA equid sequences from Pompeii. Journal of Cellular Biochemistry (Accepted manuscript online); DOI: 10.1002/jcb.22914
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