Biological Journal of the Linnean Society (1994), 52: 273-286. With 3 figures
Genetic variability among root voles (Microtus
oeconomus) from different geographic regions:
populations can be distinguished by DNA
fingerprinting
JOHN ERIK STACY, UNN HILDE REFSETH, MARIANNE
THORESEN, ROLF ANKER IMS*, NILS CHR. STENSETH* AND
KJETILL S. JAKOBSEN
Division of General Genetics, Department of Biology, Uniuersity of Oslo, P.O. Box
1031, Blindern, N-0315 Oslo, Norway and *Division o f ~ o o l o g y Department
,
of
Biology, University o f Oslo, P.O. Box 1050, Blindern, N-0316 Oslo, Norway
Received 15 April 1993, accepted f o r publication 18 October 1993
Genetic variability among root voles (Microtus oeconomus [Pallas, 17761) originating from two
distantly separate regions of Norway (Valdres and Finnmark) was studied by DNA fingerprinting
using the probes 33.15, 33.6 and M13. All three probes revealed polymorphic, although relatively
simple, patterns. DNA fingerprint banding patterns were clearly diagnostic of the animals' region of
origin. Notably, Valdres animals display a high molecular-weight cluster of bands not round in
Finnmark, reflective of the isolation, and possibly an indication of separate colonization events, of
the two groups. O n the local level in Finnmark, bands associating with a specific trap site were
observed in trappings on consecutive years. Comparisons of Finnmark animals taken at three trap
sites at approximately 10 km intervals show a gradient of genetic similarity. Captive bred siblings
were also compared, yielding average values significantly higher than those seen from same-site
comparisons. We suggest that the sensitivity provided by DNA fingerprinting with multi-locus
minisatellite probes is appropriate for population genetic studies in M . oeconomus. Also, because
band-sharing correlates to spacing in M . oeconomus, we propose that DNA fingerprinting may be
used to study dispersal, recruitment and other population processes in this and possibly other rodent
species.
ADDITIONAL KEY WORDS:- population genetics - rodent cycles
CONTENTS
Introduction . . . . . . . . . . . . . . . . . . .
Material and Methods
. . . . . . . . . . . . . . . .
Root vole samples
. . . . . . . . . . . . . . . .
DNA extraction, digestion and Southern analysis . . . . . . . . .
Probe labelling . . . . . . . . . . . . . . . . .
Analysis of DNA fingerprints and statistics.
. . . . . . . . . .
Results . . . . . . . . . . . . . . . . . . . .
M. oeconomus DNA fingerprint patterns are simple and have features associated with
place of origin . . . . . . . . . . . . . . . . .
A high degree of band-sharing is observed within populations of M . aeconomus . .
274
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*Correspondence to K . S . Jakobsen.
0024-4066/94/070273
+ 14 $08.00/0
273
0 1994 The Linnean Society of London
2 74
J. E. STACY E T AL.
Discussion
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Possible causes of DNA fingerprint simplicity . .
Regional differences in patterns . . . . .
A gradient of genetic similarity on the local level .
Concluding remarks . . . . . . . .
Acknowledgements . . . . . . . . .
References . . . . . . . . . . .
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INTRODUCTION
It is often assumed that an understanding of the population dynamics of cyclic
microtines will require a description of the spatial-genetic structure of their
populations (e.g. Gaines, 1985; Cockburn, 1988; Gaines et al., 1991; Stenseth &
Lidicker, 1992). DNA fingerprinting (Jeffreys et al., 1985) may provide a
powerful method for discerning genetic variability in populations (Lynch, 1990,
1991). T h e degree of variation within and between populations can be inferred
from the average numbers of shared bands in DNA fingerprint patterns. TO
date, the method has provided insight into the genetic structure of populations in
some species (e.g. Gilbert et al., 1990; Reeve et al., 1990; Wayne et al., 1991;
Prodohl et al., 1992; Zeh et al., 1992; Menotti-Raymond & O’Brien, 1993).
Although the above mentioned studies, both empirical and theoretical, show
that DNA fingerprinting can be applicable at the population level, the method is
best known for investigations of familial relationships (see Lewin, 1989). Most
researchers exploiting DNA fingerprinting have been behavioural ecologists
working with systems where genetic data is correlated to observed behaviour. In
particular, DNA fingerprinting has been shown to be extremely effective in
quantifying parentage in birds, allowing the experimenter to correlate parentage
and parental investment (e.g. Burke et al., 1989). Small mammal ecologists have
been less successful in applying similar approaches to their study animals (a
notable exception is Ribble, 1991). Although a few authors have demonstrated
inheritance of parental DNA fingerprint bands in voles (Hoagland et al., 1991;
Ishibashi et al., 1992), none as yet have applied the technique to an experimental
system, or to the gathering of population genetic data.
I n this paper we investigate the applicability of DNA fingerprinting to studies
of population processes and genetics in the root vole Microtus oeconomus. We find
that the level of sensitivity provided by the method is appropriate for studies at
the population level in this species. We investigated animals from adjacent areas
and from two distant regions in Norway. Our results show that band-sharing in
DNA fingerprints of M . oeconomus is high, which is suggestive of high genetic
similarity between individuals (Lynch, 1991). High band-sharing in itself is
somewhat surprising considering the vole’s apparently large populations and
high mobility. We also see that the overall profile of DNA fingerprint patterns
differ between regions, providing region-specific characters. Furthermore,
because the proportion of band-sharing among animals from adjacent regions is
higher than expected, DNA fingerprinting can potentially be used to analyse
‘geographically related’ populations and subpopulations.
MATERIAL AND METHODS
Root vole samjdes
All M . oeconomus animals originated from one of two distantly separate regions
(1370 km apart) in Norway (Fig. 1): 0 v r e Pasvik (Finnmark County,
FINGERPRINTING O F M . OECONOMUS POPULATIONS
7
275
are
Figure I . Map of Norway showing the Valdres and Finnmark trapping sites. Enlarged region shows
the three Finnmark trapping sites with hatched areas and connecting lines representing lakes and
streams, respectively.
North-East Norway) and Valdres (Oppland County, South-Central Norway).
Animals were captured in 1990, 1991 and 1992. All trappings were performed in
optimal root vole habitat, i.e. sedge mires and swampy river banks. I n Valdres,
animals were caught in 1990 within a 500 m radius of Reinsj~bekken
(considered as one site), and in 1992 at H d e r a , a site 6 km from Reinsjrabekken.
In Finnmark, trappings were performed along a transect at three primary
locations; Bdevassbekken, Kjeldebekken and Vaggatem. Bdevassbekken and
Kjeldebekken are separated by 13 km, while the distance from Kjeldebekken to
Vaggatem is eight km. At these primary locations animals were caught within a
200 m radius. Home range size has been estimated to be within 50 m radii for
Canadian M . oeconomus (Lambin et al., 1992). The maximum home-range radius
measured in Norwegian M . oeconomus was within 35 m (K. Hertzberg,
unpublished data). Thus, the trappings in both Valdres and Finnmark were
designed to span several home-ranges per site. The studied families were from
laboratory stocks founded by animals caught wild in the two regions. Table 1
describes the number of specimens of various origins being examined here.
DNA extraction, digestion and Southern analysis
DNA was isolated from liver, kidney or in some cases limbs (adults and
juveniles). Fetuses found in pregnant females were separated from placental and
J. E. STACY E T AL.
216
TABLE1. Origin of M . oeconomus samples used in band-sharing
analysis. Familial specimens were from captive pairings
Numbers of animals
Valdres
1990
1992
15 (4)
6
Familial
Reinsjeb.
1991
Finnmark
~
Holera
1990
1990
1991
1991
1991
Familial
Kjeldeb.
Vaggatem
(ddevansb.
Kjeldeb.
Vaggatem
26 (5)
3
3
8
2
9
2
*Figures in parenthesis are the number of families.
uterine tissues and treated whole. T h e DNA isolation procedure was as described
by Westneat (1990).
Restriction digests of 40 pg DNA was performed in 100 pl volumes, 1 U AluI
(Promega, Madison, Wisconsin) per pg DNA. A positive control of restriction
was done for each tube by adding 1 pg plasmid DNA to a 5 pl aliquot of the
above mixture one hour into the incubation period. After overnight incubation,
these controls were run on a test gel to assay digestion. The primary restriction
samples were loaded as 20 pl aliquots of the restriction mixture (8 pg DNA) plus
5 pl glycerol: bromphenol blue gel loading buffer. DNA was separated by
electrophoresis through an 0.8% agarose gel (Seakem GTG; FMC BioProducts,
Rockland, Maine) at 1.4 V cm-' for 26 h at 4°C. Gels were blotted onto Gene
Screen membranes (New England Nuclear; DuPont, Boston, Massachusetts).
Prehybridization and hybridization were carried out in 1 x SSC-equivalent
buffer (Galau et al., 1986) in the absence of 'blocking' DNA at 55°C for two to
three hours and overnight respectively, T h e hybridization solution contained
10% dextran sulphate (Pharmacia, Uppsala, Sweden). Washing was done at the
same stringency (55"C, 1 x SSC-equivalent buffer) as for hybridizations. Judging
from the interpretable DNA fingerprints obtained after 2 h exposure with
intensifying screens at - 70°C, varying exposure times were chosen to produce
the autoradiograms shown here.
Probe labelling
The basic concept of MSPL (Magnetic Solid Phase Labelling) has been
described previously (Espelund et al., 1990; Stacy et al., 1991). T h e protocol for
obtaining an asymmetrically biotinylated PCR produce and subsequent
labelling using probes 33.6, 33.15 (Jeffreys et al., 1985) was as in Stacy et al.
(1991), except for replacing the washing buffer ( 1 x SSC, 0.1% SDS) with
Klenow buffer (50 m M Tris-HC1 pH 7.6, 10 m M MgCl,) and using a slightly
modified PCR program; 95.5"C/45 sec, 65"C/10 sec, 72"C/90 sec (Techne
Dri-Block PHC-2; Techne, Duxford, Cambridge, England). The M 13 probe
(Vassart et al., 1987) was derived from a 813 bp fragment (from position 1714 to
2527 in M13 mp18) containing the two tandem repeats (Refseth & Jakobsen, in
preparation) cloned in Bluescript ( Stratagene, La Jolla, California). T h e. M
13
.
FINGERPRINTING OF M . OECONOMUS POPULATIONS
277
insert was amplified and biotinylated as above, except using Taq-polymerase
(Promega, Madison, Wisconsin) instead of Vent (New England Biolabs, Beverly,
Massachusetts). Dynabeads M-280 Streptavidin (Dynal A/S, Oslo, Norway)
were used to immobilize probe templates. Using 200-300 ng double-stranded
PCR generated template, this protocol produces probes labelled to an activity of
5 x lo7 cpm/pg double stranded template.
Analysis of DNA Jingerprints and statistics
Autoradiograms of blots were scored such that each individual was compared
with every other individual. A ruler was used to align bands, using the marker
lanes as reference, and an acetate overlay to provide a record of the evaluation.
In many cases, monomorphic or near monomorphic bands provided across gel
‘references’. Differences in band intensities were generally ignored. If the identity
between two bands was in doubt, they were judged to be the same so bandsharing may be overestimated.
Band-sharing between lanes x and y (sxy) is defined as the fraction of shared
bands (Wetton et al., 1987); sxy = 2nxy/(nx+n,),where nxy is the number of
common bands scored and ( n x + n y ) is the total of all bands in lanes x and y
combined. Though this index is not easily interpreted in a population genetic
context, it is directly related to the average identity-in-state between randomly
chosen individuals (see Lynch, 1990). Lynch (1990) has also demonstrated that
traditional F-statistics (Wright, 1951 ) are statistically related to band-sharing
values (average s ) . However, we present all data in terms of band-sharing, since
this makes our results more directly comparable to those obtained in other DNA
fingerprinting studies.
Matrices derived from band-sharing evaluations (i.e. s,,-values) were tested
for correlation with the corresponding distances between trap sites (distance
matrices) or trapping times (years) using Mantel statistics (Mantel, 1967;
Manly, 1991). T h e correlation coefficients derived (Mantel correlation
coefficients), measure the association between the elements in the two matrices.
The statistical significance of the coefficients was determined by permutation
tests, i.e. the observed correlation coefficient was compared to the empirical
distribution of this statistic. This empirical distribution was derived from
permuting one of the matrices (e.g. the distance matrix ) 9999 times. For our
purposes, we used matrices of the normalized s,,-values and distance values. The
calculations were made by the computer package NTSYS (Applied Biostatistics
Inc.).
RESULTS
M. oeconomus DNA jingerprint patterns are simple and have features associated with
place of origin
O u r initial experiments were carried out to see whether DNA fingerprinting
could be used at the population level in M . oeconomus. As seen in Figure 2, DNAs
from 18 specimens, nine from each of the two geographically distant regions in
Norway (Finnmark and Valdres, see Table 1 & Fig. 1) were hybridized to the
DNA fingerprinting probes 33.6, 33.15 and M13. Upon visual inspection of
Figure 2, prior to performing any statistical analysis, it can be inferred that
278
J. E. STACY E T AL.
E I N G L R P R I N I ING O F M OECO 2OMC.C POPUL.4 I IONS
279
Figure 2. Examples of DNA lingerprints (AluI digcsts) from .U. o m m o m u individuals from districts
E’innmark (lanes 1-9) and L’aldrcs (lanes 10-18) hybridized to 33.6 (,4)
and 33.15 ( R I and 1113 (0.
Lanes 2, 5,6: Vaggatrrn, 1990. L a n e 7, 8, 9: I(jeldcbckkcn, 1990. Lanes I , 3: lab raised from stock
caught a ( Kjeldrbckkcn, 1988. Lane 4: offspring of individuals 2 and 3. Lancs 1 1 , 14-18:
Reiiisjc)bekkcn, 1990. Lanes 10. 12: lab raisrd from s t i r k caught at Rcinsjnhckktm, 1988. Lane 1 3 :
olt’spring of individuals 1 I and 12.
M . oeconomus patterns are unusual for apparently unrelated individuals. T h e
specific features that make these patterns unusual are: ( 1 ) There are relatively
few bands per lane; (2) bands are ‘clustered’ to portions of the gel; (3) the
patterns clearly fall into two groups corresponding to the regions of Finnmark
and Valdres.
T h e number of bands seen per lane in M . oeconomus DNA were fewer than
expected from previously published reports from several vertebrate species
(Burke & Bruford, 1987; Taggart & Ferguson, 1990; Jeffreys el al., 1987; Jeffreys
& Morton, 1987; Jeffreys et al., 1991; Ribble, 1991). This was true for all three
probes tried so far (probes 33.6, 33.15 & M13; see Table 2 & Fig. 2 ) . Using the
TABLE
2. Bands per lane* counted in AluI digests
of ‘$1,oeconornus DNA with the indicated probes
Probe
Average
~~
33.6
33.15
1113
St.dev.
8.1
9.0
9. I
n
~~
~
1.7
3.2
1.2
~~
12
33
12
~~
*Bands werc readable ( a n d tountcd) in the ranqe
10 2 k b fix ’3’1 G and 33 15 and t o 1 6 kh for 1113
doun
280
J. E. STACY E T A L .
same protocol and probe 33.15 on human DNA, band numbers per lane
averaged 27 ( fSD 2.2) (Stacy & Jakobsen, 1993) as compared with 9 ( fSD
3.2) in M . oeconomus (both on range above 2 kb).
Compared with probe 33.15, few additional bands were seen with 33.6 and
M13. Banding patterns derived from probes 33.6 and 33.15 overlapped to a
great extent, the main differences being in band intensity. Probe M13 gave even
fewer high molecular weight bands per lane and failed to yield any variable
bands not seen with the other two probes. Hybridization at lower stringency did
not increase (interpretable) band numbers for any of the probes. Experiments
with enzymes other than AluI (i.e. HaeIII, HinfI and Sau3AI) gave similar
band numbers (bot shown). Indeed, low band number when using G-rich
minisatellite probes appears to be a characteristic of M . oeconomus.
An additional aspect of M . oeconomus DNA fingerprints is the concentration of
bands to specific gel segments, i.e. some molecular weight ranges are practically
blank. Interestingly, at this level the patterns can be associated with the region of
origin (Finnmark or Valdres, see Fig. 1). In Figure 2, DNA fingerprint patterns
fall clearly into groups corresponding to Finnmark and Valdres. Notably,
Valdres individuals show a set of polymorphic high molecular weight bands
(Fig. 2A and B corresponding to probes 33.6 and 33.15 respectively) not seen in
Finnmark individuals. These ‘Valdres bands’ do not hybridize to M13, and the
use of this probe produces an apparently ‘opposite’ picture of the two groups
(Fig. 2C).
Considering all three probings together, we see that M . oeconomus DNA from
Finnmark and Valdres behave very differently from one another. Some of the
bands specific for the Valdres or Finnmark individuals behave as alleles of the
same loci clustered at different molecular weights. Other bands probably
represent loci that either go undetected in animals from one of the two regions
(due to low molecular weight or masking by other loci) or belong to loci present
only in animals from one of the two regions. To investigate this in further detail,
inter-regional crosses have to be performed.
O n inspection of DNA fingerprint profiles, it was also possible to distinguish
population genetic structure at the local level. In the Finnmark sample, we
discern ‘site-specific’ bands: one group displaying a band near 20 kb and the
other group missing this band but instead having one near 6.2 kb (Fig. 2A and
B, lanes 2, 4,5, 6 and 1, 3, 7, 8, 9 respectively). The bands are strong using 33.6
(Fig. 2A) and weak using 33.15 (Fig. 2B). The similar behaviour of these bands
suggests they derive from cognate loci. These putative alleles correlate with trap
locations Kjeldebekken (6.2 kb) and Vaggatem (20 kb, see Fig. 1 ) .
A high degree of band-sharing is observed within Populations o f M. oeconomus
The autoradiograms seen in Figure 2 show that M . oeconomus have similar
DNA fingerprint patterns over large areas (within the regions of Finnmark and
Valdres). Similarity above the family level is somewhat surprising, since this is
unusual for DNA fingerprint patterns (see Lynch, 1988). To see if a larger
sample from one of the regions also showed great similarity, animals were
collected in 1991 from three primary sites in Finnmark (see Fig. 1). Also, captive
crosses were set up in the lab such that a base statistic could be established for
first order relatives. Additional specimens were collected from Valdres in 1992,
FINGERPRINTING OF M. OECONOMUS POPULATIONS
28 1
B
A
0.8
0.8
n = 41
-
0.6-
0.6
M
,2
4
n = 33,
0.4
n = 33
a
9
0.4 -
n = 25
t
,
n = 25
c4
n = 19
0.2 -
0.2
n
1st order
relatives
=
12
different between
regions
sites
same
site
same
site
adjacent
(-10 km)
separats
by 21 km
Figure 3. Band-sharing values using probe 33.15 on a total of 72 individuals as described in Table 1.
Averages k S D are indicated by a circle on a vertical line. Median values are indicated by a short
horizontal line, and N indicates the number of individuals involved in the comparisons. A,
Band-sharing-values among first order relatives, same-site, different-site and between-region
(Valdres to Finnmark) comparisons. These values derive from six different gels. B, Values for
specimens taken on two consecutive years from a transect in Finnmark. Trap sites are
(ddevassbekken (n = 8 in 1991), Kjeldebekken ( n = 3 in 1990, n = 2 in 1991) and Vaggatem ( n = 3
in 1990, n = 9 in 1991). These values derive from one gel.
primarily to see if the ‘Valdres bands’ also occur at different trap sites. The total
sample is described in Table 1.
As in the 1990 sample collected at Reinsj~bekken,the high molecular weight
‘Valdres bands’ were seen in the sample taken in 1992 at Hdera, 6 km from
Reinsj~bekken.I n Finnmark, the 20 kb band associating with the Vaggatem
trap site in 1990 was identified in all nine of the specimens collected there in
1991. The 20 kb band was also identified in one of eight individuals sampled
20 km south of Vaggatem at (ddevassbekken. The suggestion of high genetic
similarity above the family level was borne out by the presence of these
individual markers (autoradiograms not shown).
Figure 3A shows the general trend in band-sharing between first order
relatives, same-site, different-site-same-region and different region specimens.
Average band-sharing for first order relatives was 0.66 ( + S D 0.17, median
0.71), which is higher than same-site comparisons (0.43 +SD 0.16, median
0.43). It should be noted that same-site samples are likely to contain some first
order relatives, but we did not see a clear first order component in the values (i.e.
the distribution was not bimodal). This again is consistent with the assertion that
relatively high similarity seen in overall (same-region) comparisons is a result of
similarity across family groupings. Averaged values for different-site (sameregion) comparisons was 0.36 ( + S D 0.14, median 0.38). In Finnmark to
Valdres comparisons 37 out of 81 were scored as having zero bands in common
(average band-sharing 0.08, & SD 0.08, median 0.09).
Figure 3B shows that band-sharing describes relatedness between sites in
Finnmark (see Fig. 1, enlarged area). Same-site comparison values are clearly
SD 0.17) than comparisons between adjacent sites
higher (average 0.41
+
J. E. STACY E T A L .
282
TABLE
3 . Mantel statistics from comparisons of band-sharing matrices to
distance matrices (9999 permutations). The abbreviations ’90, ’91 and ’92
indicate the year of trapping, whereas 0, k, g indicate Finnmark trap sites
Bdevassbekken, Kjeldebekken, Vaggatem and r, h indicate Valdres trap
sites Reinsjobekken and Halet-a. The expressions of the form ( 3 x 3 ) indicate
the sizes of the matrices compared
P
Matrices
k’90 (n = 3), g’90 ( n = 3): (3 x 3)
0’91 (n = 8)
k’90 (n = 3), k’91 (n = 2)
g’90 ( n = 3 ) , g’91 ( n = 9): (25 x 2 5 )
Correlation
[random 2 observed]
0.83
0.10
0.37
0.0001
-0.27*
k’90 (n = 3), k’91 ( n = 2): ( 5 x 5 )
0.10
g’90 (n = 3), g’91 (n = 9): (12 x 12)
0.15*
0.18
r’90 (n = 5 ) , h’92 ( n = 2): ( 7 x 7 )
0.36
0.05
*Same site, different year comparisons.
(average 0.32 f S D 0.13). Comparisons between the two extreme sites
(0devassbekken to Vaggatem) gave lower values (average 0.26 + S D 0.12)
suggesting a correlation between spacing and relatedness on a fairly large scale.
Mantel correlation coefficients between band-sharing matrices and distance
matrices were statistically significant (Table 3). Note also that Kjeldebekken
and Vaggatem samples contain individuals from both 1990 and 1991. Different
sampling year did not, however, introduce an additional element of
heterogeneity: correlations for same site matrices were not statistically significant
(Table 3 ) . This is consistent with the assertion that similarity at trap sites exists
above the level of the family, and is persistent through a period of at least one
year.
Band-sharing evaluations support the assertion of genetic similarity above the
family level. Although band-sharing values derived from other DNA
fingerprinting protocols are not necessarily comparable, our values for
M . oeconomus are suggestively higher than those reported for most other
vertebrate species (see Reeve et al., 1990, or Lynch, 1991). For example, for the
specimens evaluated in Figure 3B, (the most heterogenous sample from within a
single region) average band-sharing is 0.32 ( f SD 0.15, median 0.32).
DISCUSSION
DNA fingerprint similarity can be used as a n indicator of genetic variation at
the population level (see Lynch, 1990, 1991). Our data show three important
features: ( 1) M . oeconomus DNA fingerprints, though polymorphic, are relatively
simple. Similarities in banding pattern between (non-kin) sympatric individuals
are obvious. (2) Between Finnmark and Valdres, loci/alleles specific for each
region dominate the high molecular weight fraction of the gel such that banding
patterns are diagnostic of the region of origin. (3) At the local level, bandsharing correlates to geographic distance between groups of animals. The
immediate question that arises is: can the attributes observed in M . oeconornus
DNA fingerprints be best explained as the result of population processes?
FINGERPRINTING OF M . OECONOMUS POPULATIONS
283
Possible causes of D N A jingerprint simplicity
We suggest that the genetic similarity seen within regions is at least partially a
result of repeated founder effects due to multiannual density cycles. Microtus
oeconomus resembles several other microtine rodents in that it exhibits extensive
density cycles, with only disjunct, local survival during low phases (e.g.
Cockburn, 1988; Stenseth & Ims, 1993). These repeated bottle-necks, which
occur every 3 to 5 years and last for a year or more, may have the effect of fixing
alleles at otherwise hypervariable loci. The ‘clustering’ of bands on certain
ranges of molecular weight, and the differences between Finnmark and Valdres
DNA fingerprints can be accounted for under a model of repeated founder effect,
as will be discsussed below.
Isolation into small breeding groups could be cited as a possible alternative
explanation, or an additional factor contributing to the observed genetic
similarity. High band-sharing has been observed in geographically isolated
populations of fox (Urocyon littoralis [Baird, 18581 and wolves (Canis lupus,
Linnaeus, 1758) (Gilbert et al., 1990; Wayne et al., 1991), colonies of the naked
mole rate (Heteocephalus glaber Ruppel, 1842) (Reeve et al., 1990) and sympatric
populations of brown trout (Salmo trutta, Linnaeus, 1758) (Prodohl et al., 1992).
However, radiometric tracking shows that M . oeconomus frequently disperse as
much as 750 m per night (Steen, 1993). Also, in two DNA fingerprinting studies
of animals with apparently low dispersal it was not possible to distinguish
between localities based on band-sharing: (i.e. banner-tailed kangaroo rats
[Dipodomys spectabilis Merriam, 18901 Keane et al., 1991; and orange roughy
[Hoplostethus atlanticus Collette, 18891 Baker et al., 1992). It seems that without
actual physical isolation even low levels of dispersal are enough to cause local
population genetic structuring to deteriorate. Our data show that band-sharing
values within Finnmark samples correlate to distance (to be discussed below),
which is not consistent with a model based exclusively on solation into small
breeding groups.
Our DNA fingerprint results are consistent with the available alloenzyme
data, which show nearly complete monomorphism at 30 loci in samples from both
Finnmark and Valdres (K. Kandl et al., in preparation). Still, we cannot
completely rule out the possibility that genomic mechanisms are the cause of
simplicity in M . oeconomus DNA fingerprint patterns. Low mutation rates or a
high degree of linkage between detected loci would result in simple patterns. In
the event of either low mutation rate or linkage, overall genetic similarity in
M . oeconomus may not be as high as inferred from DNA fingerprinting. However,
until future genetic studies allow us to discern the specifics of M . oeconomus DNA
fingerprint genetics, it is not reasonable to assume that they are greatly different
from those in well studied rodent species; i.e. M u s musculus, Linnaeus, 1758
(Jeffreys et al., 1987; Kelly et al., 1989).
Regional dzferences in patterns
Why are Finnmark and Valdres DNA fingerprint patterns distinguishable? If
we interpret the observation in terms of allele fixation due to repeated founder
effect, then it is conceivable that different dominating sets of allele lengths are
maintained in each region. Such a situation could be maintained in the absence
284
J. E. STACY E’T AL.
of genetic exchange between these regions. That there is little or no exchange of
M . oeconomus between the two regions has been previously argued (Fredga et al.,
1980, 1986), and is supported by a lack of M . oeconomus skulls in an analysis of
cave deposits in an intervening area (E. Bstbye, personal communication). I t
has also been asserted that post-glacial colonization has occurred by separate
routes, Finnmark being colonized from the east while South Norway/Sweden has
colonized via a land bridge from Denmark (Fredga et al., 1980, 1986). I n the
event of separate colonization, dissimilarity may predate present day isolation.
One of the characteristics of the Valdres sample is the presence of a set of high
molecular weight bands. A phenomenon similar to the polymorphic, high
molecular weight ‘Valdres’ bands has been reported in populations of the
neotropical pseudoscorpion Cordylochernes scorpioides (Zeh et al., 1992). I n this
work specimens from two distant locations were DNA fingerprinted, and
banding intervals were associated with place of origin. The phenomenon of band
‘clusters’ near certain molecular weights is consistent with the model describing
changes in allele length as being non-random, i.e. when a minisatellite allele
changes length, the resulting length is at least partly dependent on the starting
length of the allele (Jeffreys et al., 1988). Recent results obtained from cloned
single locus probes from M . oeconomus are also in agreement with this model. In
the Finnmark sample, the single locus alleles detected thus far are ‘clustered’
between 950 and 1200 bp, whereas the Valdres sample has alleles between 1600
and 2000 bp (Thoresen, Refseth & Jakobsen, unpublished data). Perhaps initial
founder events are followed by incremental allelic divergence, resulting in sets of
allele lengths distributed near the length of the founder alleles?
A gradient of genetic similarity on the local level
The observation that comparisons made along the transect in Finnmark show
a gradient in band-sharing values, together with high overall similarity, indicates
that there is some level of genetic exchange between breeding groups. This
implies that at least some dispersers must be recruited from neighbouring areas,
or else we would expect to see band-sharing values to drop sharply instead of
gradually. Genetic exchange between groups is assumed to have the effect of
‘evening out’ population genetic structure, and it may seem a paradox that
differences can be maintained in spite of exchange. The paradox is resolved,
however, under the model of founder effect due to multiannual density cycles.
Groups lasting only a few years between population crashes would not have time
to come into genetic equilibrium with their neighbouring groups. In the absence
of equilibrium, gradual differences in population genetic structure are
conceivable.
Concluding remarks
Our data show that DNA fingerprinting can be a very useful tool in the study
of population processes in M . oeconomus. The results we obtained from our sample
suggest that M . oeconomus populations are genetically very similar. Considering
our results, DNA fingerprinting may in fact be one of the few adequately
sensitive methods available for studies of genetic differences and differentiation
in M . oeconomus at the local level.
FINGERPRINTING OF M . OECONOMUS P O P U L A l I O N S
285
We assert that DNA fingerprinting may reveal other important aspects of
population structure and dynamics. If the rates of de nouo allele generation at
M . oeconomus minisatellite loci are in fact similar to those in humans (e.g. 0.004
per locus per gamete, Jeffreys et al., 1988), then we may expect DNA fingerprint
pattern types associated with an area to be transitory. If this is the case, the
change from one DNA fingerprint type to another should correspond to crashes
in population density. O n the other hand, if minisatellite mutation rates in
M . oeconomus turn out to be unusually low, DNA fingerprint patterns will still
provide useful markers for studies at the population level. It should be possible to
study dispersal and recruitment using site-specific bands, either those occurring
at nearby sites or through the introduction of individuals from distant areas.
Another population parameter that can be estimated via DNA fingerprinting is
that of effective population size (Lynch, 199 1) .
At first glance it would seem that field studies would be the most direct
method for obtaining information on population structure and dynamics.
Although a great amount of information has been collected through marking
and trapping, many basic population parameters remain elusive (Gaines et al.,
1991). From our data we see that, applied properly, DNA fingerprinting should
yield a wealth of population information on cyclic species like M . oeconomus.
ACKNOWLEDGEMENTS
We would like to thank Harry Andreassen, Judith Ramdin and Tone
Duborgh Hoyland (University of Oslo) for keeping the breeding stocks. Ottar
Bjornstad (University of Oslo) and Nigel G. Yoccoz (University of Lyon) are
thanked for introducing us to the use of Mantel statistics. Harald Steen, Eivind
Bstbye, Karine Hertzberg (University of Oslo) and Karen Kandl (University of
Georgia, Savannah River Ecology Laboratory) are gratefully acknowledged for
sharing unpublished data and for helpful discussion. Arne Lovlie and Thomas
Hansen (University of Oslo) are thanked for constructive comments on the
manuscript. We are also grateful to Erik Hornes, Dynal A.S. (Oslo, Norway), for
providing Dynabeads and the oligonucleotide primers for PCR. This work was
supported by the Research Council of Norway (NRF).
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