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Scandinavian Wolf Genetics: A Review

By Jody Haynes

This review article summarizes the genetic tests that were conducted on Scandinavian wolves and reported in a recent article by Vila et al. (2003). I would strongly encourage anyone who is interested in learning more about this important and interesting genetic testing to download and read the entire paper, at the following URL:

http://www.nature.com/cgi-taf/DynaPage.taf?file=/hdy/journal/v90/n1/full/6800175a.html&filetype=pdf

As far as I know, this study represents the first time that researchers have used a combination of genetic markers to try to determine (1) whether or not an animal was a wolf, a dog, or a wolfdog; and (2) the direction of hybridization, if the animal was found to be a hybrid. The study introduced a series of powerful analyses of molecular genetic data that allowed the researchers to make the determination. The specific objective of the study was to analyze the identity of a juvenile canid thought to have been a hybrid and to genetically characterize it as either a pure Scandinavian wolf, a migrant wolf from Finland or Russia, a domestic dog, or a first-generation hybrid between any of these groups.

The DNA samples studied were as follows: (1) Sample A came from the juvenile animal that was killed by a car; (2) Sample B came from a sample of snow that included urine and blood from a female Scandinavian wolf thought to have been in estrus [this sample was obtained during the estrus cycle that is thought to have produced the juvenile in Sample A]; (3) muscle tissue from wolves from Finland, northwest Russia, Latvia, and Estonia; (4) 44 domestic dogs from the US [pure-bred Huskies, Eskimo dogs, Akita, Elkhound, Wolfspritz, Great Pyrenees, Kubasz, and GSD]; and (5) 38 male, pure-bred Scandinavian dogs from diverse breeds [genotyped for Y chromosome markers].

The specific genetic markers analyzed included mitochondrial DNA (mtDNA, which examines maternal lineage), autosomal DNA (nDNA, which examines non-mitochondrial, non-sex-linked DNA), and Y chromosome DNA (which is sex-linked, and only occurs in males). The mtDNA marker used was a 350 base pair (bp) fragment of the mtDNA control region I, which had been used in previous studies of European wolves. The markers targeting nDNA and Y chromosome DNA were microsatellites, which are regions of repeated DNA sequences that occur randomly throughout the non-coding regions of DNA (in mammals, the non-coding regions make up the majority of the DNA). One highly polymorphic microsatellite marker was used to examine Y chromosome DNA and 18 microsatellite markers--originally developed for dogs--were used to analyze the nDNA.

The results of the mtDNA sequences indicated that both Sample A and Sample B were found to carry the H1 mtDNA haplotype. H1 is the only haplotype known to occur in Scandinavian wolves; however, it also occurs in 65% of North European wolves, but not in domestic dogs. The authors concluded from the mtDNA that the juvenile in Sample A was either a pure wolf or a hybrid with wolf ancestry in the maternal line. However, they were not able to determine the geographic origin of the wolf ancestor by mtDNA alone.

The results of the Y chromosome data showed that the juvenile (Sample A) was a male (because the Y chromosome marker was found) and that the female (Sample B) was, indeed, a female (the Y chromosome marker was not found). The particular allele (variant) of the Y chromosome marker found in Sample A was not found in any Scandinavian wolves, but has been seen in other North European wolf populations and in dogs. Although the results from the Y chromosome marker cannot discriminate between a wolf or a dog as the father of Sample A, they do suggest that the father was not a Scandinavian wolf.

The results of the nDNA analyses showed that Scandinavian wolves could be distinguished from domestic dogs. The data also suggested that Sample B falls within the range of expected microsatellite markers found in Scandinavian wolves, and that Sample A lies between the distributions of dogs and wolves, "a position that would be expected for a wolf-dog hybrid" (Vila, et al., 2003). Further analyses of nDNA microsatellite results showed that the genotype combination of Sample A was significantly different from that expected for pure dogs or pure wolves, but was inside the distribution expected for F1 hybrids. A similar analysis showed that neither Sample A nor Sample B could be identified as a wolf immigrant from Finland or Russia. In addition, the target samples were determined to be outside the expected distribution for an F1 hybrid between a dog and a Russian wolf immigrant. Finally, nDNA analyses supported Sample B as the female parent of Sample A.

Making the assumption that Sample B was, in fact, the mother of Sample A, the researchers determined that the likelihood of obtaining the haplotype of alleles found in Sample A assumed to have come from the father (i.e., the paternal contribution of nDNA) was extremely low for Scandinavian, Finnish, or Russian wolves, as well as hybrids between a Scandinavian wolf and a domestic dog. However, the likelihood that the paternal haplotype originated from a pure domestic dog was high.

A separate analysis using a model-based approach showed that all Scandinavian wolves have a probability of at least 0.95 of being classified as pure wolves. Similarly, all dogs but one had a probability higher than 0.95 of being genetically identified as pure dogs. Sample B had a probability of 0.998 corresponding to Scandinavian wolf. However, Sample A (the juvenile) only had a 0.264 probability of being a pure Scandinavian wolf. The probability of Sample A having one dog as a parent was 0.402 and the probability of having a dog as a grandparent was 0.334. The probability that Sample A could be a pure dog was 0.000. Consequently, the researchers determined that Sample A was likely to be of hybrid origin between a Scandinavian wolf and a pure dog (probability 0.736).

The authors stated the following in the discussion: "[t]he combined use of the markers allowed us to conclude that a hybridization event between dog and wolf had occurred in the endangered Scandinavian wolf population. The direction of hybridization was a male dog paired with a female wolf...." Although the authors warned that "[t]he strong genetic fragmentation of dogs into breeds may limit the power of hybridization tests like the one [presented]," they also suggest that "selection of local dogs belonging to breeds that could be most likely to hybridize could increase the resolution of the test, allowing for an increase in power that could enhance the likelihood of detecting F2 hybrids and backcrosses."

As a result of this study, government officials determined that the litter was of hybrid origin. Two of the litter were killed by government officials, another was believed to be killed illegally, and the last one is unaccounted for.

Citation

Vila, C., C. Walker, A-K. Sundqvist, Ø. Flagstad, Z. Andersone, A. Casulli, I. Kojola, H. Valdmann, J. Halverson & H. Ellegren. Combined use of maternal, paternal and bi-parental genetic markers for the identification of wolf–dog hybrids. Heredity 90:17–24.

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