Results of Long-term Genetic Monitoring of Animal Populations Chronically
Irradiated in the Radiocontaminated Areas
Rose GONCHAROVA and Nadezhda RIABOKON
Institute of Genetics and Cytology, National Academy of Sciences of Belarus,
27, F. Skorina st., 220072 Minsk, Republic of Belarus
Tel: 375 (017) 268 49 01. Fax: 375 (017) 268 49 17
E-mail: igc%bas32.basnet.minsk.by@demos.su
The γ-radiation dosage rates were measured atheight of 3-5 cm from the ground surface by thedosimeters SRP-68-01T and DBG-06T (in 1991-1996)
manufactured in the former Soviet Union.
The total γ-activity of the whole-body animals in
1986-1987 was analysed using 32 crystal spectrometryof gamma-gamma coincidences (scintillation NaI(Tl)detector, 150 x 100 mm2) at the Institute of Physics ofBelarus Academy of Sciences. The radiometricanalyses of laboratory mice and wild animals havebeen carried out since 1988 using γ-spectrometer
ADCAM-300 (ORTEC, detector GEM-30185) at theBelarus Meteorological Centre and the Institute ofRadiobiology (Belarus Academy of Sciences). The soil
samples selected in 1989 were analysed using the same
spectrometer.
The content of 90Sr in bank vole populations insites 3 and 4 were estimated by radiochemical analysis
in the Belarus State University (Minsk).
The radiation load in animals due to the external
γ-irradiation was estimated in term of the value ofradiation dose rate and due to the internal γ-activity by
the radionuclide concentration in the rodents body.
Evaluation of mutation process levels in bonemarrow cells of bank vole was carried out by standard
metaphase test [1]; structural (chromosome
aberrations) and genomic (polyploid cells) mutations
were analysed. The levels of germ cells mutability inbank vole were estimated by the frequency ofabnormal sperm head (ASH) in males [28]. Embryonal
lethality was analysed according to D. Anderson [2].
Bank vole have a short lifespan and every year 2-3
new generations appear in the populations. It is thus
believed that there were approximately 20-22
generations of these rodents for 1986-1996.

Introduction
The artificial geochemicalland where allorganisms lived and will live under the conditions ofincreased level of radiation was set up due to theChernobyl disaster in 1986. An urgent necessity forstudying the various biological effects of chronic
influence of low intensity radiation on both individualorganisms and populations arose.
Combined cytogenetic and radioecologicalinvestigations in wild populations of terrestrial small
mammals (bank vole = Clethrionomys glareolus,
Schreber and yellow-necked mouse = Apodemus
flavicollus, Melchior) and in laboratory mice havebeen carried out by our laboratory since 1986. Our test
organisms have contacted closely with low intensity
radiation in the radiocontaminated areas of Belarus andabsorbed low whole-body dose.
We study the following problems: dynamics ofradionuclide concentration in wild populations ofsmall rodents; dynamics of mutation process insomatic and germ cells over many generations as wellas embryonal lethality; dynamics of population
density, age and sex structure of mammalian
populations. The large part of results obtained arepresented here.
Materials and methods
Wild populations of small mammals (rodents)
Bank voles and yellow-necked mouse werecollected in summer-autumn period from four forest
populations in sites chosen for long-term monitoringwith limited people activities and differing 137Cs
contamination density of soil:
1. - Priluksky reserve (near Minsk, 330 km north-west
from the Chernobyl power station, 8 kBq/m2);
2. - Berezinsky Biosphere reserve (Vitebsk region,
400 km NNW, 18 kBq/m2);
3. - the suburbs of Majsk village (Bragin district ofGomel region, 60 km N, 90 kBq/m2);
4. - the suburbs of Babchin village (Khoiniki districtof Gomel region, 40 km NNW, 1526 kBq/m2).
According to the official data of the Belarus
Meteorological Centre, the density of soilradiocontamination for 90Sr ranges up to about 70
kBq/m2 in sites 3 and 4.

Laboratory population of mice
The males of hybrid mice (CBAxC57Bl/6j)F1 were
kept in the radiocontaminated settlements A and B
under the conditions of low intensity of external andinternal (radiocontaminated food of local production
has been used) irradiation during 4 months in 1989.
The radiocontamination densities of soil for 137Cs insettlement A (village Lomachi, Khoiniki district ofGomel region) and B (Bragin city of Gomel region)
were 2351 and 825 kBq/m2, respectively. The average194gamma dosage rates in cages with animals were 43.71
x 10-12 and 8.24 x 10-12 A/kg (or 610 and 115 µR/h) in
A and B, respectively. The control group of animals
was kept in vivarium of Institute of Genetics andCytology (γ-radiation dosage rate was about 0.86 x
10-12 A/kg or 12 µR/h).
The radiometric analyses of whole-body animals
were carried out using γ-spectrometer ADCAM-300(ORTEC, detector GEM-30185). The absorbed dose
due to the external γ-irradiation was estimated using
thermoluminecent dosimeters placed in the cages ofanimals.
The frequency of chromosome aberrations,polyploid cells in bone marrow as well as thefrequency of reciprocal translocations and othercytogenetic injuries in spermatopcytes of the firstmeiotic division and abnormal sperm head (ASH) werestudied by the conventional methods [1, 28].

The mean radionuclide concentration in animal
populations in all areas under investigation (Table 2, 3)
were positively
correlated with mean grounddeposition of the trapping sites (Spearman, r=1.00,
P<0.05 for data obtained in 1989). 137Cs and 134Cs
made the main contribution to the total γ-activity ofsoil and animals samples. Besides two caesiums, werecorded other γ-emitters such as 106Ru, 144Ce and241
Am (since 1991 for site 4). 90Sr mean concentration
estimated in 1991 in bank vole populations from sites 3
and 4 made up to 114 and 298 Bq/kg, respectively.
In rodents with fast generation succession weannually detected radionuclide concentrations in thecompletely renewed populations. Dynamics ofradionuclide content in consecutive generations ofbank vole (and yellow-necked mouse) over the periodof 1986-1996 (and 1986-1989) was characterised bythree phases: an increase, a maximum (peak) and adecrease [9, 12, 14]. The peaks of radionuclide
accumulation in the populations in the areas withdifferent radiocontamination density fell not at the first
but in the next years (1987-1989) following theChernobyl disaster, i.e. the peaks were observed insubsequent animal generations (Table 2, 3). Thus in
populations of two mammalian species we observed
shifts of the maximum radionuclide content within 1-3
years. The revealed regularity of the time shift in the
maximum of population average radionuclide
concentrations in animals as against the maximum of
their fallout in 1986 seems to be the result of increasein biological accessibility of radioisotopes to plants
and thus to the whole biota. The peculiarity of theradionuclide content dynamics in small mammal
populations after the Chernobyl accident is inagreement with the known data on the same retardationin the concentration maximum of 90Sr and 137Cs fromthe global fallout in hoofed animals [27] and arecorroborated by similar results obtained for fish fromthe Baltic Sea [8] and for various groups of people andanimals in Norway [26].

Dynamics of radiation loads on wild populations ofmammals in the radiocontaminated areas
Over the period under consideration the radiation
dosage rate was greatly reduced in all sites (Table 1).
The most fast reduction of dose rate was recorded insite 4 in May - August, 1986.
It was revealed that over the first five months
following the catastrophe (i.e. over the average
lifespan of animals under investigation) the absorbed
whole-body dose from the external gamma-irradiationin the rodents inhabiting sites 1 and 4 (with contrastinglevels of radiocontamination) made up 0,1 and 15 cGy,
respectively; and the absorbed dose in animals in site 4
over the five months of 1987 ran to 1,23 cGy [9]. So,
daily average dose from the external γ-component in
the individuals from the highly contaminated site 4
hardly exceeded 1 mGy even in the first time after theaccident. As a whole, it may be said that the animal
populations under investigation were exposed to
low-dose irradiation from the external and internal
components. The research of Cristaldi et al. [6]corroborates this conclusion.

Table 1. Gamma radiation dose rate 10-12 A/kg (µR/h or R x 10-6 / h in brackets) on the ground surface
in four sites in 1986-1996
Site
1986
1987
1988
1989
1991
1992
1996
1.79
0.86
0.86
0.86
0.86
0.86
1
(25)
(12)
(12)
(12)
(12)
(12)
2

-

-

-

-

0.86
(12)

0.86
(12)

0.72
(10)

3

42.99
(600)

4.66
(65)

4.66
(65)

4.66
(65)

2.29
(32)

-

1.93
(27)

4

1218.05*-177.69(17000*-2480)

46.57
(650)

46.57
(650)

43.71
(610)

10.75
(150)

-

11.68
(163)

*This level was measured in May 1986, the others were obtained as the average data over the periods of capture. In 1986
the first catchings were initiated in August (about 120 days after the disaster).

195

Table 2. Dynamics of radionuclide concentrations in wild populations of bank vole in 1986-1996
Site
Year
Number of
Total gamma-activity (Bq/kg)
animals
Minimum
Maximum
Mean
U-test
1986
27
41
525
187
1987
46
38
926
274
*
1988
24
5
750
245
n.s.(1)
1
1989
75
5
429
118
*
1991
15
5
625
140
n.s.
1996
30
4
20
6
**
1986-1996
217
4
926
160
1991
20
5
1524
565
1996
40
4
108
25
*
2
1991-1996
60
4
1524
205
1986
34
38
78070
9293
1988
91
111
215196
23623
**
1989
142
1237
41501
10591
**
3
1991
53
757
25293
5587
**
1996
18
85
344
162
**
1986-1996
338
38
215196
12629
1986
42
67
78070
17202
1987
65
3885
145410
26503
**
1988
174
58
950100
81966
**
1989
176
3636
463741
44407
**
4
1990
13
4724
22016
13272
**
1991
129
654
55132
11191
**
1996
49
148
4528
1204
**
1986-1996
648
58
95010040429(1)nonsignificant;* Ð< 0.05; ** Ð< 0.01 in comparison with data obtained in every site in 1986.

Reduction in radionuclide concentration in bankvole populations under investigation was recorded in
1989. However, a significant decrease in radionuclide
content as compared with the initial increase in 1986

has been observed only beginning from 1990 (10generations) in population of bank vole from thehighly radiocontaminated site 4. Since both theradiation dose rate and values of the population mean

Table 3. Dynamics of radionuclide concentrations in wild populations of yellow-necked mouse in
1986-1989
Site
Year
Number of
Total gamma-activity (Bq/kg)
animals
Minimum
Maximum
Mean
U-test
1986
27
42
348
237
1987
17
63
365
279
n.s.
1
1988
21
5
309
59
**
1989
27
5
265
78
**
1986-1989
92
5
365
157
1986
5
820
11111
4196
1988
7
1146
48799
11174
n.s.
3
1989
52
441
7235
2677
n.s.
1986-1989
64
441
48799
3725
1986
11
1539
46990
11493
1987
27
1894
38480
8411
n.s.
1988
14
1382
36472
9391
n.s.
4
XX
1989
24
1371
61204
15559
1986-1989
76
1371
61204
11295
n.s. nonsignificant;** P<0.01 in comparison with data obtained in every site in 1986 andÕÕ

in comparison with data obtained in site 4 in 1987, 1988.

196

body burden considerably decreased in all sites by
1991, it can be stated that there was a significant
reduction in radiation load starting from the 12th
generation of animals.

P<0.05).
In populations of animals living in the highly
contaminated sites 3 and 4, the aberrations ofchromosome type (paired fragments, Robertsoniantranslocations, pericentric inversions) formed the greatpart in total yield of aberrations in contrast to the
populations in sites 1 and 2 with less grounddeposition [13; 14]. It should be emphasised that in oneof voles captured in site 3 in 1991, more than 50% of
analysed metaphases contained pericentric inversion.
We believe that this stable aberration had emerged inbone marrow stem cell producing the clone of changed
cells.
Additionally, the cells with 2 and 3 aberrations
were found in animals from sites 3 and 4 while no cellswith above 1 aberration were observed in populations
in sites 1 and 2.
Unexpectedly high cytogenetic effects wererecorded in bone marrow cells of bank vole by the testof genomic mutations [13] as against the test ofchromosome aberrations (Table 4). It turned out that in1986 the frequencies of polyploid cells were increased
in animals at all sites in comparison with the
pre-Chernobyl data (0.04 % according to Yeliseeva et
al. [29]. The average frequency of polyploid cells
during the whole period 1986-1991 significantly
correlated with the soil contamination and theγ-radiation dose rate (Spearman, r=1, P<0.05). The
level of the genomic mutations rose significantly inpopulations under investigation from year to year
(X2-test, P<0.01) with one exception: population fromsite 2 in 1992. The frequency of polyploid cells
reached 9-12 % in animals from sites 3 and 4 in 1991
and was 200-300 times higher than the pre-accident

Genetic injuries in mammals
A. Dynamics of mutation processes in somatic cells ofconsecutive generations of bank vole
The mean frequency of aberrant cells in bank vole
population inhabiting site 1 (with the least soilradiocontamination density, 8 kBq/m2) in 1986 didn’t
differ from the historical pre-Chernobyl control thatmade up 0.41 % in site 2 [29], and had the tendency toincrease the next years (Table 4). It became
significantly greater in 1991, i.e. in the subsequent
generations of animals.
In populations from more contaminated sites 2-4(18-1526 kBq/m2) there were observed significant
2.8-6.4 fold increased levels of aberrant cells in bankvole over all period under consideration as against theresults for rodents in site 1 and the pre-accident data
(Table 4). The tendency towards increase in thechromosome mutation frequency was also noted inthese sites. In 1991 the level of aberrant cells in
population in site 3 was significant highly (X2-test,
P<0.05) than the data on the previous years (1986,
1988).
The analysis of frequencies of chromosome
aberrations and aberrant cells in highly
radiocontaminated sites 3 and 4 did not reveal
significant differences between these two populations
in each year studied. However, the average frequency
of aberrant cells during whole period 1986-1991 wassignificantly higher in site 3 than in site 4 (X2-test,

Table 4. Dynamics of the aberrant and polyploid cell frequencies (%) in bone marrow of bank vole in1986-1991
Number of
Number of
Aberrant
Polyploid
SiteYearanimals
cells
cells
cells scored
1986
10
997
0.40
0.50**
1988
3
310
0.65
0
1
1991
6
741
1.12*
3.51** xx
1986-1991
19
2048
0.69
1.51
1991
20
2164
1.11**
4.25**
1992
17
1995
1.22**
1.65** xx2
1991-1992
37
4159
1.17
3.01
1986
18
2011
1.71**
1.19**
1988
21
2380
1.75**
8.87** xx
3
1991
16
1824
2.54**
9.27**
1986-1991
55
6215
1.96
6.50
1986
16
1743
1.27**
0.23**
1987
363973
1.14**
7.50** xx
1988
27
2883
1.77**
5.86** xx
4
1991
30
4166
1.86**
12.31** xx
1986-1991
109
12765
1.53
7.71
* P<0.05; ** P<0.01 in comparison with data on site 2 in 1981-1983 [ 29];
xx

P< 0.01 in comparison with data obtained in every previous year (X2-test).

197

level.
The fact of annual significant rise in the genomic
mutation frequency in consecutive generations ofanimals living in areas with different grounddeposition deserves particular attention.
The hypothesis on the adaptive character ofcellular polyploidy at high levels of radiation is being
extended [4, 16, 23]. According to this hypothesis
polyploidy is considered as a mechanism forconcealment (hibernation) of genetic damages in cell:
polyploid formation process plays a protective rolesaving cells from unbalanced genome. In bonemarrow, at the same time, polypotent stem cells as wellas precursor-cells are target-cells in forming abnormal
myelopoiesis and acute forms of leukaemia [30],which are characterised by increased frequencies ofpolyploidy and other forms of aneuploidy in manycases. Since there are more questions than answers inhemopoiesis process, it can be assumed that, at someranges of the mutation effects, polyploidization ofbone marrow cells is of an adaptive character and, atother ranges, it goes to pre- and pathologic state. It
should be noted that there are data on a rise in thefrequency of infant leukaemia in children exposed to inutero radiation whose parents live in Greece withdifferent radiocontamination density due to theChernobyl fallout [18].
Thus, it can be only supposed that increased
frequencies (by 2-3 orders) of bone marrow polyploidcells recorded by us exceed the limits of normal
response of bank vole and are close to thepre-pathological state of hemopoiesis processes.
So, the study on dynamics of the mutation process
in bank vole populations inhabiting regions withdifferent soil radiocontamination density in 1986-1991
has shown that increased levels of chromosome

aberrations and genomic mutations emerging insomatic cells de novo in every generation wereobserved in bone marrow cells of animals within many
generations (from 1 to 14) after the accident.
Considering that cytogenetic effects inproliferating cells of bone marrow reflect the dose
received within one cell division cycle, increased
frequencies of chromosome aberrations and genomic
mutations recorded by us in 1986-1992 are considered
to have occurred at very low levels of absorbed doses.
Thus, according to calculations of Cristaldi et al. [6],the absorbed doses from the external γ and internal γ
and β irradiation from 134Cs and 137Cs in bank vole
populations in three regions of Sweden with different
soil contamination levels (22, 90 and 145 kBq/m2 for
137Cs) made up 8.8 x 10-6; 26.8 x 10-6 and 39.4 x 10-6
Gy per day in 1989, respectively. The levels of soil
contamination in our sites 2 and 3 were almost equal tothose of some Swedish sites. This suggests that theradiation load in animals in sites 2 and 3 were the sameorder of magnitude and it can be assumed that thevalues of the doses absorbed by the investigatedpopulations of animals within the cell division cycle
are of the order of a few tens or hundreds of mGy perday.
We believe that significantly greater meanfrequency of aberrant cells as well as existence of theindividual with high content of stable aberration can
indicate stronger structural injuries of chromosomes in
population from site 3 than from site 4. Since animals
in site 3 had lower range of dose loads than that inanimals from site 4, we believe that data obtained
resulted from abnormal dose relationship of structural
injuries of chromosomes at low doses.
B. Dynamics of mutation process in germ cells ofconsecutive generations of bank vole

Table 5. Dynamics of abnormal sperm head (ASH) frequency in males of wild small mammals
Bank vole
Yellow-necked mouse
Site
Year
NumberNumber
ASH
NumberNumberASHof animals
of cells(%)
of animals
of cells
(%)
1989
13
19500
0.149
10
10000
0.140
1991
7
10500
0.076
1
1996
13
65000
0.085*
1989-1996
33
95000
0.097
1991
22
33000
0.106
1996
43
2150000.091
2
1991-199665248000
0.093
1989
8
12000
0.175
23
23000
0.222x
1991
26
39000
0.126
3
1996
6
30000
0.123
1989-1996
40
81000
0.132
1989
34
51000
0.208
12
12000
0.300xx
xx
1991
39
58500
0.214
4
1996
7
35000
0.109**
1989-1996
80
1445000.186
xP<0.05; xx P< 0.01 in comparison with data on site 1 and within every year;* P<0.05; ** P<0.01 in comparison with data obtained in every site in 1989 (X2-test).
198

The frequencies of abnormal sperm head (ASH) in
both species of small mammals (bank vole andyellow-necked mouse) in 1989 were higher in sites 3
and 4 in comparison with the data on site 1 (Table 5).
The increased frequency of abnormal sperm remainedin bank vole population in site 4 in 1991. Thus, it canbe stated that many consecutive generations of animals
in site 4 with high radiocontamination density had thesame range of mutability in germ cells. The significant
decrease in the frequency of ASH (X2-test, P<0.05 andP<0.01) was observed in bank vole populations in
1996 (in sites 1 and 4 respectively).
Unfortunately, we have no control data (withabsolutely clear region) or pre-accident ones on thelevels of ASH in bank vole populations. And thus, we
believe that the frequency of abnormal sperm in highly
contaminated sites 3 and 4 in 1996 didn’t reach the
pre-accident level.
Comparative studies of the average frequencies of
ASH in bank vole populations during whole period
1989-1996 demonstrated no significant differences
between the least contaminated sites 1 and 2 (0.097
and 0.093 % ASH, respectively). Significant increase(X2-test, P<0.01) in the average frequency of ASH
were observed in population from more contaminatedsite 3 (0.132 %). And in males from the mostcontaminated site 4, the level of abnormal sperm(0.186 %) were significant higher (P<0.01) than in
animals from all other sites under investigation (1-3).
Thus, we can observe the following peculiarity: thehigher the contamination density of the site, the higher
frequency of ASH. But the frequency of ASH didn’t
statistically correlate with the soil contamination or thecontent of incorporated radionuclides. At the time, theaverage frequency of ASH significantly correlatedwith the average gamma radiation dose rate during
1989-1996 (Spearman, r=1.0, P<0.05). We believe thatthe lack of correlation between the level of abnormal
sperms and some radiation factors can indicatecomplex repair processes in germ cells of chronically
irradiated animals. This our speculation is inagreement with the data obtained in germ cells oflaboratory mice that is discussed below.
C. Embryonal lethality in wild populations of bankvole
The study on embryonal lethality was initiatedonly in 1988. During 1988-1996 we have caught a few
pregnant females in site 1. Thus we have no data onembryonal lethality in the site with the least grounddeposition. In bank vole populations from sites 2-4 atendency towards increase in the frequency ofembryonal lethality during the whole period underinvestigations was observed (Table 6). The averagefrequency of lethality throughout this period wassignificantly high only in site 4 as compared with theaverage data on site 2 (Table 6). But the averagefrequency of lethality didn’t correlate with the soilradiocontamination, gamma radiation dose rate or
average level of incorporated radionuclides(Spearman, r=0.0, P>0.05). In site 4 the frequency ofembryonal lethality positively correlated withpopulation density (Spearman, r=1.00, P<0.05).
D. Cytogenetic injuries in somatic and germ cells oflaboratory mice after protracted internal andexternal irradiation at low doses
It was revealed that animals kept in theradiocontaminated regions during 133 days
accumulated high concentrations of γ-emitting
radionuclides and took whole-body radiation loads inthe range of low doses (Table 6). Mutagenic effects oflong-term irradiation by tests of chromosome
aberrations and polyploid cells were observed in micebone marrow (Table 7). Significantly increased levelsof these two types of mutations were recorded inanimals kept in the highly contaminated region A(Table 7). There were 56% of chromosome typeaberrations from all number of aberrations recorded inTable 6. Dynamics of embryonal lethality in populations of bank vole
Number of
Pre-implantation
Dead implants (%)
Site
Year
femalesloss (%)
1991
12
0
5.36
1992
19
2.20
1.12
2
1996
7
12.50
0
1991-1996
38
3.74
2.27
1988
4
0
0
1989
30
5.80
0.77
1991
14
8.45
1.54
3
1996
3
20.00
8.33
1988-1996
51
7.32
1.33
1988
14
3.17
1.64
1989
40
4.48
1.56
1991
21
9.90
4.40
4
1996
11
13.73
9.09
1988-1996
86
6.73
3.19
2
* P<0.05 in comparison with data on site 2 (X -test).
199

Total embryonal
lethality (%)
5.36
3.30
12.50
5.88
0
6.52
9.86
26.67
8.54
4.76
5.97
13.86
21.57
9.62 *

Table 7. Frequency of aberrant and polyploid cells in bone marrow of laboratory mice (males) after
long-term internal and external irradiation in radiocontaminated regions of Belarus in 1989
137
Number Number Aberrant Polyploid
Whole-body doseCs
γ-activity
offrom external
Regiondeposition
cells
cells
of
of animals(kBq/m2)
(%)
(%)
cells
γ-irradiation (cGy) animals(Bq/kg)
0.31
0
0
6
0.03
3
646
Control
0.63
0.83
825
853
0.43
4
480
B
2351
1103
1.71
27
2970
1.16*
1.11*
A
* P<0.05 in comparison with control data (X2-test).
Table 8. Abnormal sperm head (ASH) in laboratory mice
RegionNumber of animals
Number of cells scored24
38208
Control
33
49985
B
52
79010
A
** P<0.01 in comparison with control data (X2-test).

ASH (%)
0.464
0.456
0.582**

Table 9. Frequencies of cytogenetic injuries in spermatocytes of laboratory mice (males)
Region Number of Number ofReciprocal
Fragments
Sex
Autosome
Polyploid
animals
cells
translocations(%)
univalents
univalents
cells (%)
scored
(%)
(%)
(%)
21
7114
0.06
0.33
13.27
5.56
5.74
Control
21
10615
0.18*
0.56*
11.77
2.92
10.76**
B
21
5257
0
0.23
14.23
3.87
9.53**
A* P<0.05; ** P<0.01 in comparison with control data (X2-test).
bone marrow cells of mice from group A, while theinput [20, 22], as well as under long-term external
animals from other two groups had no cells with these
γ+β-irradiation [5, 21].
type aberrations [10].
At the same time no reciprocal translocations were
The level of ASH (Table 8) was revealed torevealed in animals from the more contaminatedincrease in mice from group A when absorbed dose
station with higher radiation loads (radionuclide
from the external γ-irradiation was 1.71 cGy and theconcentration - 1103 Bq/kg and the dose taken fromcontent of incorporated radionuclides was 1103 Bq/kg
external γ-irradiation -1.71 cGy). Similar data were[10].
obtained on analysing another type of structural
Different type of cytogenetic injuries (structural
chromosome damages - fragments (Table 9).
and genomic) were observed in spermatocytes of males
Significant increase in chromosome fragment
studied (Table 9) [11]. The animals of all group were
frequency was recorded in animals from group B as
characterised by high individual variability for eachagainst the control and reduction in that parameter inparameter tested.
group A.
Reciprocal translocations among structuralThus, the increased frequencies of structural
chromosome injuries are of particular interest. It
chromosome injuries were observed in immature germshould be noted that radiation-induced reciprocalcells (spermatocytes) only at lower radiation loadtranslocations emerge with low frequency [1, 5, 20, 21,(group B). Absence of increased mutability, evaluated22, 25].
by chromosome aberration test, on increasing theTranslocations, consisting of two bivalents
absorbed dose seems to be the consequence of(quadrivalents) mainly in the form of chains and in rareinduction of repair system of the “adaptive response”
cases in the form of rings, occurred in the animals
type. There are abundant data on different inducible
analysed by us. One male from group B was
repair systems (adaptive response, SOS-repair, etc.) in
noteworthy for complicated configuration of 4
literature at the present. These repair systems can bebivalents (octovalent) in the form of chain. On the
induced with low doses of acute and chronicwhole, the mice from this group are characterised byirradiation. Discrepancy between dose-effect curves inthe significantly increased level of reciprocal
the chromosome aberration frequency in somatic (bonetranslocations (Table 9). The obtained level ofmarrow) and immature germ cells can be accounted forreciprocal translocations in group B under long-termby some reasons including dose differences incombined irradiation with low doses (0.43 cGy frominducing repair systems of germ and somatic cells.
external γ-irradiation and 853 Bq/kg of incorporated
It is known that an increase in radiation above theradionuclides) exceed the effects expected during
background level induces two contrary processes inhigher dose extrapolation by single or long-term 137Cs
living matter - injury and repair. There is a number ofmodels [24, 19] of abnormal dose dependence of200

cytogenic injury yield at low levels of absorbed doses.
According to one of them [19], repair processes canprevail over injury induction at some levels of low
doses. As a result, reduction in the number of injuries
even below the spontaneous level can be observed at a
certain site of dose-effect curve. However, the numberof induced injuries, which make a greater contribution
in comparison with repair to the dose-effect curve,
increases with a rise in the dose. We think that
abnormal dose dependence of chromosome aberration
yield in germ cells of the laboratory mice is inagreement with the above-mentioned model of low
dose radiation effect. The frequency of chromosome
aberrations, as well as reciprocal translocations can be
expected to start rising in animal germ cells at higher
radiation loads than at those we studied.
The analysis of sex and autosomal univalents(nonconjugated homologous chromosomes) did notexhibit radiation-related effects what is in agreementwith the literature data [5, 20, 21].
Genomic injuries in the animals tested (Table 9)
were represented by tetra-, hexa-, octo- and higher
level ploids. The high frequency of polyploid cells istypical for germ cells of mammal males and, inparticular, for mice [17]. Besides, the dependence ofthe yield of the polyploid germ cell level in mice on thevalue of absorbed doses is known in the literature [15,17]. On exposing spermatocytes to X-irradiation with
200 R dose, the frequency of polyploid cells increased
by a factor of 15 as against the control [17]. We haverevealed that, at much lower absorbed doses but at the
combined effects of external and internal irradiation,
the levels of polyploid cells in animals from theexperimental groups are twice as much as in thecontrol. Furthermore, the tendency towards increasing
the ploidy degree was observed in males at theradiocontaminated stations. The cells whose ploidy is
above 8n were revealed only in these animals.
Thus, the increased levels of various cytogenetic
injuries were detected in somatic and germ cells oflaboratory mice exposed to long-term combined
external and internal irradiation of low absorbed doses.

Our results obtained on the somatic and germ cellsof small mammals also indicate that the genomic test
(examination of polyploidy) has greater sensitivity toevaluate genetic effects of the increased radiation
background as compared with the test using
chromosome aberrations. The dose-response patterns
for genomic mutations in rodent somatic cells differ at
low doses from those for chromosome aberrations.
Our investigations show higher sensitivity of both
somatic and germ cells of animals to chronic
combined external and internal effect of radionuclides
in comparison with other irradiation conditions. All
cytogenetic effects obtained in wild and laboratory
animals at chronic low dose radiation exceeded the
expected ones based on extrapolation from the results
obtained in the range of higher doses by single or
long-term irradiation. Abnormal patterns of
dose-response are revealed for different type mutations
at low doses of long-term radionuclide influence.
There are data on high rates of genic mutations inanimal and human populations in the areas
radiocontaminated by the Chernobyl fallout. So, thefrequency of germline mutations at humanminisatellite loci in children born on heavily polluted
areas of Belarus was found to be twice as high as in thecontrol group [7]. In two species of voles (Microtus
arvalis and M. rossiaemeridionalis), collected near the
Chernobyl nuclear power plant, the range of genic
mutations was hundreds of times greater than thosetypically found in vertebrates [3].
Taking into account increased sensitivity of cellsof different organs and tissues of animals fromchronically irradiated populations to induction of all
types of mutations - genic [3, 7], genomic (polyploidcells) and structural (chromosome aberrations), it isnecessary to emphasise that the areas with a wide rangeof radiocontamination density due to the Chernobyl
fallout (8-2351 kBq/m2 in our tests) are zones withhigh genetic risk for animals and man.
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