Introduction

American mink Neovison vison (Schreber,1777) have been bred on fur farms in Europesince the 1920s when such ranches were estab-lished in Great Britain and Scandinavia (rev.

Dunstone 1993). Feral mink, which are nowfound over a wide geographic range in Westernand Northern Europe, are the descendants of es-capees from fur farms (Bonesi and Plazon 2007).However, in the former USSR, in addition tomink originating from farms, American mink

[1]

Acta Theriologica 54 (1): 1–10, 2009.

PL ISSN 0001–7051

Genetic variability of feral and ranch American mink Neovison

vison in Poland

Aleksandra MICHALSKA-PARDA, Marcin BRZEZIÑSKI*, Andrzej ZALEWSKI

and Micha³ KOZAKIEWICZ

Michalska-Parda A., Brzeziñski M., Zalewski A. and Kozakiewicz M. 2009.Genetic variability of feral and ranch American mink Neovison vison inPoland. Acta Theriologica 54: 1–10.

The diversity of 11 microsatellite loci was examined to estimate the geneticvariability of ranch and feral American mink Neovison vison (Schreber, 1777) inPoland. Samples were collected from 10 mink farms (182 individuals) and from5 areas in the north-eastern part of the country (87 individuals). At eachexamined locus the observed heterozygosity (Ho) was lower than the expectedheterozygosity (He). Feral mink showed lower genetic variability than ranchmink; however, in the former group the mean value of the inbreeding coefficient(FIS = 0.306) was higher than in the latter (0.242). These results demonstratedthat feral and ranch mink belong to two genetically close but separate groups.Genetic differences were identified between mink colour breeds but not betweenanimals from particular farms. The height of the modal values of �� indicatedthe presence of four genetic clusters: (1) farmed mink sapphire, (2) farmed minkstandard and pastel, (3) farmed mink pearl and (4) feral mink. Assignment ofmink individuals using assignment test, STRUCTURE and GeneClass 2.0.revealed that 12–16% of the feral mink group are likely to be ranch minkescapees. It may be concluded that approximately 30 years after the start of theexpansion of feral mink in north-eastern Poland, this wild-living populationexists without a major input of individuals bred on fur farms.

Department of Ecology, University of Warsaw, Banacha 2, 02-097 Warsaw, Poland (AM-P, MB, MK);Mammal Research Institute, Polish Academy of Sciences, 17-230 Bia³owie¿a, Poland (AZ, MK)

Key words: invasive species, fur farm, propagules, mink management, micro-satellites

* Corresponding author: marcinb@biol.uw.edu.pl

were released into the wild from the 1930s. InPoland, feral mink have been observed since1962; these individuals were probably escapeesfrom Polish fur farms but true colonization ofthe country started in the late 1970s when thefirst immigrants from Belarusian and Lithua-nian feral populations crossed the eastern bor-der (Ruprecht et al. 1983). Since that time, thegeographic range of feral mink in Poland has ex-tended west and south-west (Romanowski et al.1984, Ruprecht 1996, Brzeziñski and Marzec2003). In newly colonized areas, population den-sities usually increase rapidly due to the highproductivity of mink (Sidorovich 1993). Exam-ples of the rapid expansion of feral mink popula-tion have been recorded in many Europeancountries (Gerell 1967, Cuthbert 1973, Bevangerand Henriksen 1995, Kauhala 1996) and a simi-lar scenario was observed in Poland; by the endof the 1990s only southern regions of the countryhad not been colonized by mink (Brzeziñski andMarzec 2003).

The first mink farm was established in Po-land in 1953 (Lisiecki and S³awoñ 1980) andsince then fur farming has developed inten-sively, particularly in the western provinces.Currently about 50 mink farms are members ofthe Polish Association of Fur Animal Raisers(Ministry of Agriculture and Rural Development2002). It is well known that mink escape fromfur farms and it seems reasonable to assumethat to some extent the feral population is con-stantly supplemented by escapees. However, therelative importance of these new local additionsand the first generation immigrants in main-taining feral populations is unclear. Hammershojet al. (2005) found that individuals which had es-caped from Danish fur farms constitute the ma-jority of free-ranging mink in Denmark. InPoland we lack such information, apart for thereport of Golachowski (2002) who found that inthe Mazurian Lakeland (north-eastern Poland),mink escapees from local farms comprise about20% of the feral population.

The size of the feral population in a particu-lar area may depend on the local density of minkfarms and thus on the number of captive ani-mals that are likely to escape (Bowman et al.2007). The north-eastern provinces of Poland –

where feral mink became established nearly 30years ago – are still inhabited by the most thriv-ing populations, despite the rather low densityof mink farms (Brzeziñski and Marzec 2003). Es-timation of the influx of ranch mink to the feralpopulation by genetic comparisons could quan-tify the extent to which wild-living populationsare still dependent on immigrants from farms.As American mink is considered to be an inva-sive carnivore, which is responsible for the de-cline of many native bird and mammal species(rev. Bonesi and Palazon 2007), this informationis of great importance for nature conservation.

The aim of this study was to compare the ge-netic diversity of feral and ranch mink in Polandby the examination of 11 microsatellite loci andto estimate the proportion of farm escapees inwild-living populations.

Material and methods

Sample collection

Muscle tissue from 182 ranch mink was obtained from10 Polish mink farms distributed throughout the country(Fig. 1). Samples were taken from animals belonging to fourcolour breeds: standard (95 individuals from 7 farms), pas-tel (47 individuals from 5 farms), sapphire (6 individualsfrom 1 farm), pearl (23 individuals from 3 farms), and from11 individuals of unknown pelt colour from one farm. Tis-sue samples were also obtained from feral mink trappedlive by the authors in years 2003–2007 and from individu-als killed by hunters. A total of 87 samples from feral minkwere collected in five areas of north-eastern Poland: Mazu-rian Lakeland (19 individuals), Romincka Forest (7 individ-uals), Biebrza River (28 individuals), Narew River (13 indi-viduals) and Wis³a River (20 individuals) (Fig. 1). All tissuesamples were placed in concentrated alcohol and stored at–20�C prior to DNA extraction.

Microsatellite genotyping

DNA was extracted from tissue samples using an A&ABiotechnology DNA extraction kit according to the manu-facturer’s instructions. Eleven microsatellite loci developedfor Neovison vison were used to genotype individuals: Mvi087,Mvi075, Mvi020, Mvi054, Mvi586, Mvi111, Mvi219, Mvi072,Mvi099, Mvi027, Mvi192 (Belliveau et al. 1999, Fleming et

al. 1999). Microsatellites were amplified in three multiplexreactions prepared using a Multiplex PCR Kit (QIAGEN).Reaction mixtures contained approximately 2 �l of templateDNA in a total volume of 12.5 �l. The thermal cycle, per-formed in a Helena BioSciences HB cycler, consisted of aninitial denaturation step at 95°C for 15 min, followed by 35

2 A. Michalska-Parda et al.

cycles of 94°C for 30 sec, 60°C for 1 min 30 sec, and 72°C for1 min. The amplified fragments were resolved by electro-phoresis using a Beckman Coulter CEQ 8000 automatedDNA sequencer and analysed using the CEQ 8000 GeneticAnalysis System version 9.0.

Genetic variability analysis

The genetic variability of each locus within the studypopulations of feral and ranch mink was estimated by cal-culating the mean allele number (NA), observed hetero-zygosity (Ho) and expected heterozygosity (He) using FSTAT(Goudet 1995) and GenAlex version 6 (Peakall and Smouse2006). The mean number of alleles per locus is expected tobe sensitive to sample size, and estimates of the expectedallele number per locus and mink origin were corrected forunequal sample size (NE). The inbreeding coefficient (FIS)and potential deviations from the Hardy-Weinberg equilib-rium and linkage equilibrium were tested for each locusand mink origin (farm and feral) using GENEPOP 3.4 soft-ware (Raymond and Rousset 1995). Bonferroni’s correctionwas applied to multiple comparisons (Rice 1989).

Genetic differences between feral and ranch mink wereassessed using various methods. First, the assignment popu-lation test was performed for feral and ranch mink usingGenAlEx (Peakall and Smouse 2006). Additional Bayesianassignment testing (Rannala and Mountain 1997) and de-tection of first-generation migrants in the feral mink popu-

lation (Paetkau et al. 2004) were performed using GeneClass2.0 software (Piry et al. 2004). Cryptic genetic structurebetween feral and ranch mink was also assessed usingSTRUCTURE 2.2 software (Pritchard et al. 2000). Analysiswith STRUCTURE assumed no prior information about thepopulation and used the admixture model with correlatedalleles frequency parameters (� = 1), and a burn-in phase of50000 iterations followed by a run phase of 10000 iterations.Posterior probability values for the number of populations(K), ranging from 1 to 10, were calculated from 10 in-dependent runs to establish consistence. The greatest rateof change of the likelihood function with respect to K (�K)was used to find the most likely K (Evanno et al. 2005). As ameasure of differentiation between fur colour lines in ranchmink and feral mink we calculated pairwise FST usingFSTAT 2.9.3 software (Goudet 1995, Goudet 2002). Forvisualisation of differences based on FST, dendrograms wereconstructed using the program Mega 3.1 (Kumar et al. 2004).

Results

The examination of 11 microsatellite loci re-vealed that feral mink inhabiting north-easternPoland have lower genetic variability than cap-tive mink on Polish farms (Table 1). The average

Genetic variability of feral and ranch American mink 3

Romincka Forest

Mazurian Lakeland

Biebrza River

Choroszcz

Olsztyn

Narew River

Wis³a River

Chorzelów

Czarnow¹sy

PilawaDolna

NowyTomyœl

StaryKisielin

Ko³obrzeg

BuszkowyGórne

LeŸno

200 km

– ranch mink

– feral mink

52 No

19 Eo

Fig. 1. Location of 10 fur farms and 5 areas of Poland where samples from American mink Neovison vison were collected forgenetic studies.

number of alleles per locus (NA) in feral minkequalled 10, whereas in ranch mink this valuewas greater than 13. In both groups the effectivenumber of alleles (NE) was more than two-foldlower than the observed number of alleles andNE in feral mink was only slightly lower than inranch mink (4.55 vs 4.89). Ten loci in the ranchmink group (all except Mvi586) and six loci inferal mink were significantly deviating fromHardy-Weinberg equilibrium after correction formultiple comparisons. In all cases, the observedheterozygosity (Ho) was lower than the expectedheterozygosity (He) (Table 1). The mean He ofboth groups of mink was the same, but the Ho

was higher in feral individuals than in animalsfrom farms (pairwise t-test after arcsin squareroot transformation: t = 2.36, p < 0.05). Of 110pairs of loci, ten showed significant linkagedisequilibrium after correction for multiplecomparisons. Seven of these loci were identifiedin the ranch mink group and three in the feralgroup, but this pattern is unlikely to be causedby physical linkage since the pairs of loci in-volved in the two groups were different.

In the ranch mink group, the mean value ofthe inbreeding coefficient (FIS) was 0.306 ±0.0820 (SE), which is higher than in the feral

mink group (0.242 ± 0.0705 SE). Moreover,35.5% of alleles identified in ranch mink werenot observed in wild-living animals, whereas16.6% of alleles from feral mink were unique tothis group (Table 1).

To illustrate reciprocal relationships betweenthe groups of feral and ranch mink, the assign-ment test was performed using the GenAlExprogram. The results showed that feral andranch mink belong to two genetically close butseparate groups (Fig. 2). Assignment test showedthat 78 mink (88%) sampled from the feral popu-lation were assigned to the feral group, whereas10 mink (12%) were assigned to the ranch mink.The Bayesian method was used to separate ge-netically distinct groups. Calculation of �K fromthe output of STRUCTURE produced a modalvalue of this parameter at K = 4. While the larg-est value of �K was at K = 4, a second mode waspresent at K = 7. The height of the modal valuesof �K indicated clear substructuring of the com-plete dataset into 4 genetic groups, and less pro-nounced differentiation at K = 7. The fourclusters of genotypes corresponded to (1) ranchmink sapphire, (2) ranch mink standard and pas-tel, (3) ranch mink pearl, and (4) feral mink (Ta-ble 2 and Fig. 3). This result showed that there

4 A. Michalska-Parda et al.

Table 1. Genetic variability parameters for ranch American mink Neovison vison from 10 Polish fur farms and for feral minkfrom 5 trapping/hunting sites. n – number of animals, NA – number of alleles, NE – effective number of alleles, Ho – observedheterozygosity, He – expected heterozygosity, FIS – inbreeding coefficient, HWE – probability value (p) of deviation fromHardy-Weinberg equilibrium (* – significant, ns – not significant).

LocusRanch mink Feral mink

n NA NE Ho He FIS HWE n NA NE Ho He FIS HWE

Mvi 087 181 9 3.83 0.149 0.739 0.798 * 85 4 2.44 0.176 0.590 0.701 *Mvi 075 179 14 6.66 0.682 0.850 0.198 * 85 12 5.88 0.647 0.830 0.220 *Mvi 020 177 13 5.01 0.102 0.800 0.873 * 82 8 5.06 0.280 0.802 0.650 *Mvi 054 181 15 5.47 0.641 0.817 0.216 * 84 11 4.84 0.690 0.793 0.130 *Mvi 586 178 17 5.81 0.798 0.828 0.036 ns 83 11 5.45 0.819 0.817 –0.003 nsMvi 111 182 16 4.78 0.637 0.791 0.194 * 81 11 5.04 0.617 0.802 0.230 *Mvi 219 183 13 5.43 0.699 0.816 0.143 * 81 9 3.97 0.667 0.748 0.109 nsMvi 072 175 8 1.82 0.326 0.451 0.278 * 77 8 2.68 0.429 0.627 0.316 *Mvi 099 170 18 7.61 0.706 0.869 0.187 * 75 15 6.56 0.733 0.847 0.135 nsMvi 192 177 13 4.21 0.661 0.762 0.133 * 80 11 4.18 0.763 0.761 –0.002 nsMvi 027 120 8 3.1 0.467 0.686 0.319 * 72 10 4.08 0.625 0.755 0.172 ns

Mean 13.18 4.89 0.528 0.764 0.306 10.0 4.56 0.586 0.761 0.242

Genetic variability of feral and ranch American mink 5

-25

-20

-15

-10

-25 -20 -15 -10

Ranch mink

Fe

ralm

ink

Fig. 2. Assignment test for ranch and feral mink from Poland. The black squares – feral mink, white squares – ranch mink.Two ranch mink individuals and one wild individual were removed to improve the visual perception of the graph [X and Y val-ues of these samples: 1 (–31.47, –31.39); 2 (–26.78, –28.02); 3 (–6.26, –6.05)].

Table 2. Average proportional membership of the four most likely genetic clusters of mink individualsfrom sampled farms and regions determined with STRUCTURE software. The highest proportions foreach cluster is indicated in bold.

Origin Colour/Population n

Inferred clusters

1 2 3 4

Ranch mink Standard 95 0.410 0.194 0.282 0.114

Pastel 47 0.448 0.110 0.299 0.143

Sapphire 6 0.137 0.197 0.347 0.319

Pearl 23 0.193 0.544 0.175 0.088

Undetermined colour 11 0.306 0.362 0.110 0.222

Feral mink Romincka Forest 7 0.067 0.065 0.164 0.704

Mazurian Lakeland 19 0.153 0.073 0.095 0.678

Biebrza River 28 0.106 0.081 0.142 0.671

Narew River 13 0.150 0.073 0.185 0.591

Wis³a River 20 0.115 0.107 0.126 0.652

6 A. Michalska-Parda et al.

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are distinct groups of farm and feral mink, andthat for individual colour breeds there are nodifferences between animals at separate farms(Fig. 3). Similar results were obtained by analy-sis using GeneClass 2.0, which confirmed thatthe mink from all studied farms do not form dis-tinct genetic groups. In the case of the standardcolour breed, more than 30% of individuals wereassigned to an alien (not home) farm, and in caseof the pastel colour breed this value was nearly60%.

Assignment of mink individuals using STRUC-TURE revealed that feral mink and ranch minkform two distinct genetic lines; only 11 individu-als from the feral mink group (13%) were as-signed as ranch mink with threshold of q� 0.7. Toestimate the frequency at which local feral minkpopulations are supplemented with individualsfrom farms, the number of “first generation mi-grants” (ie farm escapees) in the group of feralmink was calculated using the program Gene-

Class 2.0. This analysis showed that farm escap-ees or their progeny comprise 16% of wild-livingmink. Furthermore, all individuals in the groupof farm escapees were of the standard (10%) orpastel (6%) colour breeds.

To examine the relationship between differ-ent colour breeds and feral mink, an FST matrixwas created (Table 3) and used to construct aneighbour-joining tree (Fig. 4). The arrange-ment of branches of the tree indicated closest ge-netic similarity between the standard and pastelcolour breeds, and between these two breeds andferal mink. The sapphire mink appeared to bethe most genetically distant from the other col-our breeds. Analysis of the level of genetic sepa-ration between different colour breeds (GeneClass2.0) showed that only among standard and pearlanimals did the majority of individuals repre-sent their own genetic line. Among the pasteland sapphire mink only about 50% of individu-als were genetically assigned to their own re-

Genetic variability of feral and ranch American mink 7

Table 3. Pairwise FST comparison between 4 colour breeds and feral mink from north--eastern Poland. * – p < 0.05

Colour ororigin Standard Pastel Sapphire Pearl Feral

Standard – * * * *Pastel 0.014 – ns * *Sapphire 0.088 0.089 – * *Pearl 0.045 0.064 0.089 – *Feral 0.042 0.036 0.084 0.060 –

I I

Standard

Pastel

Feral

Pearl

Sapphire

0.01

Fig. 4. Neighbour-joining tree illustrating relationships between feral mink from north-eastern Poland and the four farm col-our breeds.

spective colour breeds (Fig. 5). Analysis with theSTRUCTURE program also showed that farmedmink belonging to different colour breeds do notrepresent highly isolated genetic lines (Fig. 3).

Discussion

The finding that the genetic variability of fe-ral mink in north-eastern Poland is lower thanthat of ranch mink may be due to the fact thatall feral populations in Europe have developedfrom farm-born refugees (even those which orig-inated from individuals released to the wild).The lower genetic variability of feral mink andthe lack of Hardy-Weinberg equilibrium in theferal population could, therefore, be the result ofthe “founders’ effect” (eg Baker and Moeed 1987)and the relatively young age of the mink popula-tion in Poland (Brzeziñski and Marzec 2003). Inaddition, the genetic structure suggests that aspatial Wahlund effect is the most likely expla-nation for the significant homozygote excesspresent when data were pooled into two groups.The analyzed group of ranch mink came from 10farms and the feral mink originated from fivedifferent local populations. The identification ofa second mode at K = 7, also indicates that theferal mink population in north-eastern Poland isnot genetically homogenous.

The results of the assignment test show thatferal mink and ranch mink in Poland are geneti-

cally close to each other but form two clearlyseparate genetic lines. The microsatellite mark-ers that we used allow these two groups of ani-mals to be clearly distinguished. This methodalso made it possible to identify particular indi-viduals in the local feral populations that hadescaped from captivity, but it was not possible toname the particular home farms.

The low values for the effective number of al-leles (NE) and observed heterozygosity (Ho) sug-gest that gene flow within the group of ranchmink is limited. It has to be stressed, however,that the barrier preventing gene flow is not cre-ated by the geographic distance between partic-ular farms (isolation), but is due to the presenceof different colour lines, even within the samefarm. Farm owners often exchange animals forbreeding, but both intra- and inter-farm breed-ing is almost always within the same colourlines. This could explain the observed deviationfrom the Hardy-Weinberg equilibrium withinthe ranch mink, as indicated by the relativelyhigh FIS values within each colour line.

Our results confirm that mink of the stan-dard breed colour, which display the phenotypemost similar to the native North Americanwild-living animals, are the founder line for theother colour breeds and for feral animals. Fur-thermore, the majority of farm escapees belongto the standard and pastel colour breeds. Thismay be explained by the predominance of thesecolour breeds in Polish fur farms as well as the

8 A. Michalska-Parda et al.

0

20

40

60

80

100

Standard Pastel Sapphire Pearl

Standard

Pastel

Sapphire

Pearl

Pe

rce

nta

ge

ofm

ink

Fig. 5. Percentage of mink individuals assigned (using GeneClass 2.0) to given colour breeds from 10 Polish fur farms.

higher survival rate of standard and pastel minkin the wild. The arrangement of the branches onthe neighbour-joining tree confirms the close re-lationship between feral mink and both thestandard and pastel lines.

The role of mink escapees in supplying feralpopulations is unclear. Ranch mink have lostmany attributes of their wild-living ancestors(Gilligan and Frankham 2003) and thus theirability to survive in the wild should be ratherlow (especially individuals with a coat colourother than standard – these animals may havemore deleterious gene combinations due to in-tensive inbreeding). Mathematical models sug-gest that the permanent inflow of ranch minkdecreases the fitness of feral populations andmay lead to population decline (Hammershoj et

al. 2006). On the other hand, immigrants fromfarms have a lower survival rate only during thefirst two months after they escape to the wildand they quickly adapt to their new habitat(Hammershoj 2004). Bowman et al. (2007) founda positive relationship between the density ofmink farms and that of the feral mink popula-tion in Canada, indicating that the inflow ofranch mink may be important in sustainingwild-living populations. Despite their results,these authors suggest that mink populations inCanada have declined due to outbreeding de-pression following hybridization between ranchand wild mink and introgression of ranch allelesinto wild mink populations. In Denmark the pro-portion of farm escapees in the feral mink popu-lation is very high, reaching up to 86% (Ham-mershoj et al. 2005). However, the number ofmink farms in this country is extremely large;about 50% of European mink farms are locatedin Denmark (Bonesi and Palazon 2007). In Po-land, despite a larger average number of breed-ing females per farm – as compared to Denmark– the number of farms is 40-fold lower (Polishterritory is about 7-fold larger). These differ-ences may explain the lower proportion of ranchmink escapees in Polish feral populations. Ourresults show that individuals which have es-caped from farms represent 12–16% of feral pop-ulations. The frequencies of the studied allelesin the feral and ranch mink groups also showthat farm escapees probably do not permanently

supply feral populations. This conclusion seemsreasonable, especially considering that 35.5% ofalleles identified in farm animals are not foundin wild-living mink, and 16.6% of alleles foundin feral individuals do not occur in ranch ani-mals. In north-eastern Poland where feral minkbecame established about 30 years ago, this wildpopulation is still numerous and stable, despitethe low density of mink farms (Brzeziñski andMarzec 2003). We conclude that a constant in-flux of farm escapees is not necessary to supportthe existence of feral mink populations in north--eastern Poland.

Acknowledgements: We thank A. Buczyñski and M. Marzecfor help with trapping mink and local farms for providingmink tissue samples for our research. We are grateful to J.Gittins for correcting English. Trapping and handling pro-cedure were approved by Polish Ethical Commission for Re-search on Animal. The project was supported by MarieCurie European Reintegration Grant within the 7th Euro-pean Community Framework Programme to A. Zalewski.

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Received 16 July 2008, accepted 14 November 2008.

Associate editor was Krzysztof Schmidt.

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