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Genetic processes in
chronically irradiated
populations of small mammals

Genetic
processes

Nadezhda Ivanovna Ryabokon, Igor Ivanovich Smolich and
Rose Iosiphovna Goncharova

433

Institute of Genetics and Cytology of National Academy of Sciences of
Belarus, Minsk, Republic of Belarus
Keywords Population, Radiation, Genetics

Abstract Dynamics of population mutagenesis during 22 consecutive generations of animals,
as well as genetic radioadaptation were studied in natural populations of small mammals (bank
voles) under chronic low-intensive irradiation due to the Chernobyl accident. The data obtained
point to oppositely directed processes in irradiated populations: accumulation of mutations
(genetic load of populations) and formation of genetic radioadaptation. It is suggested that the
frequencies of genetic damages in populations could be higher in the absence of radioadaptation
process. A relationship between the frequencies of cytogenetic injuries and low doses of radiation
was revealed in animal generations studied. The non-linear dose-effect curves are most likely to be
defined by the complicated microevolutionary processes in populations. The results obtained
indicate the absence of genetic effect threshold of low dose radiation. Besides, they show that a
dependence of cytogenetic effects on radiation low doses in series of irradiated generations cannot
be revealed using linear equations.

Introduction
For predicting remote genetic consequences and working out the systems of
population protection against negative effects of chronic ionizing radiation, it is
necessary to know peculiarities of mutation process dynamics in a number of
chronically irradiated animal generations, as well as quantitative ``dose-effect''
dependencies for different types of mutations in these generations.
In classical radiation genetics, there are experimental data on distinctive

features of mutagenesis dynamics in chronically irradiated laboratory
populations of Drosophila (Wallace, 1956) and unicellular algae (Shevchenko
and Pomerantseva, 1985). Increased frequencies of mutations forming genetic
load of populations remain in the populations studied during dozens of
generations under long-term irradiation with high doses. Four stages were
The basic investigations represented in the given work were carried out within the framework
of the State Programme of the Republic of Belarus for Minimizing and Overcoming
Consequences of the Chernobyl Accident. Investigations of dynamics of the polyploid cell
frequency in bank vole populations were partially supported by the J.D. and K.T. MacArthur
Foundation (individual grant No. 95-31014 A-FSU).
The authors are grateful to Dr M.V. Malko (Institute of Physical and Chemical Radiation
Problems, National Academy of Sciences of Belarus) for his consultations in estimation of
absorbed doses, and to the staff of Antimutagenesis Laboratory (Institute of Genetics and
Cytology, National Academy of Sciences of Belarus) for skilled technical help and helpful
discussions.

Environmental Management and
Health, Vol. 11 No. 5, 2000,
pp. 433-446. # MCB University
Press, 0956-6163

EMH
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434

distinguished in dynamics of mutagenesis and genetic load: growth period
(accumulation of mutations), period of stabilisation (equilibrium between
induction of mutations and selection effects on them) and then reduction in the
mutation frequency (increase in radioresistance) and stabilisation at a new
lower level of mutagenesis (Wallace, 1956; Shevchenko and Pomerantseva,
1985; Dubinin, 1966). The UN Scientific Committee on the Effects of Atomic
Radiation recommended the mutagenesis level at the equilibrium state (the
second stage in dynamics of radiation mutagenesis) that was demonstrated in
the experiments with Drosophila and haploid algae (Wallace, 1956; Shevchenko
and Pomerantseva, 1985; Dubinin, 1966) for estimating genetic risk of
hereditary disease emergence in distant generations of chronically irradiated
human population (United Nations, 1986; Shevchenko, 1996).
As for dynamics of the mutation process in chronically irradiated animal
populations of higher taxonomic group, there were fragmentary data indicative

of an increased level of mutagenesis in somatic and germ cells of rodents
representing 20th-80th animal generations since the onset of additional chronic
radiation impact (Shevchenko and Pomerantseva, 1985; Bashlykova et al., 1987;
Maslova et al., 1984; Radioecology of Biocenosis, 1987; Grigorjev and Taskaev,
1985; Cristaldi et al., 1985, 1991; Taskaev, 1984; Gileva et al., 1996; etc.).
Long-term monitoring of natural populations of animals living in
radiocontaminated areas, in combination with a complex of radioecological and
genetic methods of investigations, and appropriate approaches to statistically
processed data is strongly required.
We have conducted such investigations in natural (free-living) populations
of the European bank vole (Clethrionomys glareolus, Schreber) which is an
indicator species of the environmental quality. The results obtained are
presented in this paper.
Methodology
Methodology of the investigation is based on studying the dynamics of
radiation loads and mutation process, as well as genetic radioadaptation in
populations of bank vole living under chronic low-intensity irradiation after the
Chernobyl accident during many generations.
Large forest areas for long-term monitoring were chosen in the territory of
the Republic of Belarus at different distances from the Chernobyl NPP, with

limited people activities and different radiocontamination densities of soil
surface. The levels of 137Cs contamination at monitoring sites (st.) 1, 2, 3 and 4
were 8, 18, 220 and 1,526kBq/m2 respectively. Summer-autumn trappings of
murine rodents were carried out in 1986-1996: the first one ± in AugustSeptember 1986, i.e. in the four months following the accident (at the st. 2 ± in
August 1991).
As with most murine rodents the bank vole has a short life span and a high
speed of reproduction. According to our estimates, in August-September 1986
we studied the first to second post-accidental generations of animals and every
subsequent year (or trapping season) the monitoring populations quit

completely consisted of no less than two new generations. Thus, from 1986 to
1996 we carried out monitoring work during approximately 22 irradiated
generations of voles.
-irradiation dosage rates at monitoring sites were measured by dosimeters
SRP-68-01-T and DBG-06T. The radiometric analysis of soil samples and
whole-body frozen animals were mostly performed using
-spectrometry
ADCAM-300 (ORTEC, detector GEM-30185). Specific activity of 90Sr and
transuranic elements was determined by radiochemical methods. Conventional
formulas and coefficients were used when calculating absorbed doses (Moiseev

and Ivanov, 1990). The individual absorbed doses were calculated in about
1,200 bank voles captured from four sites in 1986-1996.
The levels of mutation process were estimated using conventional methods:
in somatic cells, metaphase analysis of bone marrow cells (Adler, 1984) and
micronuclei assay of polychromatic (immature) erythrocytes (PCE) of bone
marrow (Schmid, 1975; Adler, 1984) and normochromatic (mature) erythrocytes
(NCE) of peripheral blood; in germ cells, abnormal sperm head (ASH) assay
(Wyrobek et al., 1984). The total number of animals analysed exceeds 500.
Assessment of the embryonal mortality level before and after implantation
was performed in 175 pregnant bank vole females by the conventional
approaches also (Anderson, 1984).
A regression analysis was used to estimate peculiarities of radiation load
dynamics and of mutation process dynamics, as well as to study relationships
between individual dose loads and individual genetic effects.
To study genetic radioresistance of chronically irradiated populations, the
captured animals were additionally exposed to acute 10, 50, 100 and 400 cGy
dose of
-rays from a 137Cs source at exposure dose rate 5.2R/min and the
frequencies of micronucleated polychromatic erythrocytes were recorded at
24h after treatment.

Dynamics of radiation loads and mutation process in chronically
irradiated populations
External and internal irradiation induced by caesium isotopes (137Cs, 134Cs)
made a basic contribution to the total absorbed dose in the animals studied.
Radiation load on the populations was the highest in the year of the accident
and decreased greatly in the posterior ten years. Thus, the average rate of the
total absorbed dose in voles at the st. 1, 3 and 4 decreased 2-15 times (Figure 1).
``Time-absorbed dose rate'' relationships were approximated by exponential
functions.
During the whole period of observations, irradiation due to external
- and
internal
‡ -irradiation with incorporated isotopes was low-intensive (up to
0.3cGy/day per animal). In the majority of individuals (> 50 per cent) in all
monitoring populations the absorbed doses were low, i.e. did not exceed 5 cGy.
Monitoring sites from the first to the fourth represented the gradient of
radiation loads in the range of low doses (Figure 1). Individual total absorbed

Genetic
processes

435

EMH
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436

Figure 1.
Dynamics of the average
rate of external
- and
internal
‡ -radiation
dose (Gy/day)

doses reached initially 1Gy only in some individuals at the st. 4. However, for
first two years the radiation loads in animals at the st. 3 and 4 were
underestimated because we did not have data on external
‡ -irradiation.

Using the metaphase method for counting the frequencies of chromosome
aberrations (CA) and polyploidy in bone marrow cells, as well as micronucleus
assay of PCE of bone marrow, it was revealed that an increased level of the
somatic cell mutability remained during the whole period of observations in the
animals in monitoring regions (st. 14). Thus, the CA frequencies were 3-7 times
and polyploid cell (Pp-cell) ones were up to 300 times higher than the
background (pre-accident) levels. The frequencies of micronucleated PCE (MNPCE) reached double excess of background levels.
After the Chernobyl accident there were accumulated data on increased CA
frequencies in somatic cells of the first generations of fish reared in
radiocontaminated areas (Goncharova et al., 1996), in some generations of

murine rodents (Gecen, 1987; Eliseeva et al., 1996; Rakin and Bashlykova, 1996)
and amphibians (Eliseeva et al., 1996) inhabiting the Chernobyl
radiocontaminated trace in 1986-1995.
Only not long ago was published information on remaining increased
frequencies of cytogenetic injuries in somatic cells (peripheral blood) of the first
to third generations of people in Altai region after long-term irradiation
induced by explosions at Semipalatinsk testing ground (Suskov et al., 1997)
and in the first to second generations of people living in the East-Uralian
radioactive trace contaminated with 90Sr-90Y due to the accident in ``Mayak''

(Suskov et al., 1997).
However, the investigations (Shevchenko and Pomerantseva, 1985; Gecen,
1987; Eliseeva et al., 1996; Rakin and Bashlykova, 1996; Suskov et al., 1997;
Suskov et al., 1997) in both pre- and post-Chernobyl periods had scanty
material available or had no necessary statistical approaches to revealing
peculiarities of the mutation process dynamics in chronically irradiated
populations of animals and humans.
As a result of our monitoring, a large body of information on CA and
polyploidy was amassed for estimating the dynamics of the mutation process
in somatic cells over a series of bank vole generations. A regression analysis
has revealed the relationship, common for the populations studied (st. 1, 3 and
4), between the mutagenesis level in somatic cells and the number of irradiated
generations. The relationship for CA and Pp-cells was approximated by the
exponential function and the second order parabola respectively (Figure 2).

Genetic
processes

437

Figure 2.
Dynamics of CA and
Pp-cells in bank vole
populations at three
sites under chronic low
dose irradiation

EMH
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438

This way, distinctive features of dynamics of radiation mutagenesis in
populations of mammals were revealed. In particular, a gradual rise in the
frequency of CA in somatic cells was observed up to the 22nd generation of
animals since the onset of radiation impact. The number of Pp-cells increased
in the 1st-12th generations and decreased by the 22nd one.
An increase in the frequencies of structural (CA) and genome (Pp-cells)
mutations during many generations of animals at decreasing radiation doses
points to higher sensitivity of somatic cell genomes of subsequent animal
generations to low-intensive irradiation as against sensitivity of the previous
generations. It could be caused by accumulation of genetic load in the
populations and by rise in genomic instability.
Dynamics of the mutation process in somatic cells of a number of
generations of chronically irradiated animals are in agreement with the abovementioned literature data on dynamics of mutagenesis in germ cells of
Drosophila and in unicellular algae (Wallace, 1956; Shevchenko and
Pomerantseva, 1985; Dubinin, 1966). However, the earlier revealed regularities
in algae (Wallace, 1956; Shevchenko and Pomerantseva, 1985; Dubinin, 1966)
are defined for the cases when the analysed mutations can be passed from one
generation to another: in Drosophila, through germ cells; in algae, from
maternal cell to daughter ones. We have studied the dynamics of somatic
mutations emerging in each animal generation de novo. Nevertheless,
accumulation of genetic load in germ cells of irradiated animals could give rise
to instability of somatic cell genomes that result in increase in the frequencies
of somatic mutations in consecutive generations of animals.
Genome response of bone marrow cells to irradiation was many times higher
(up to 300-fold excess over background frequencies) than CA induction (up to
seven-fold excess over the background level). In this way, the occurrence rate of
individuals with the high content of Pp-cells exceeded that of individuals with a
high percentage of aberrant cells. Animals with very high sensitivity of bone
marrow cells to polyploid induction were most probably to eliminate earlier from
the populations since a high individual frequencies of Pp-cells (up to 47 per cent)
in voles from the radiocontaminated sites can indicate pre-leukosis state or, in
fact, leukosis in these animals. As a result, the period of increasing the Pp-cell
frequency lasted within the lesser number of consecutive generations than for CA.
The mutagenesis dynamics in germ cells of bank voles were studied in 19891996 or approximately from the 7th-22nd irradiated generations of animals. The
total analysis (Cochran's test) of the data all over the sites has shown a
considerable (p < 0.01) reduction in the relative quantity of ASH during
observations. The mutations responsible for ASH emergence were most likely to
be quickly eliminated from the populations, so morphologically atypical spermia
exhibit decreased fertilizing capacity. Thus, there is no considerable
accumulation of this type of mutations in the populations. However, a great
number of other gene mutations can be induced by radiation, pass through the
sieve of negative selection at the level of germ cells and then inherit by progenies
and make a basic contribution in future to genetic load of the populations.

Similar data were obtained by Pomerantseva et al. (1996). The authors
revealed that in 1987-1993 there was no additional increase in initially elevated
frequencies of ASH, reciprocal translocations and recessive lethal mutations in
wild house mouse populations living in areas contaminated with radionuclides
after the Chernobyl disaster (maximal total absorbed doses in male gonads did
not exceed 34Gy/month in 1986-1987 and gradually decreased). Pomerantseva
et al. assumed that radiation-induced mutations may lead to elimination of
germ cells and of mice heterozygous with decreased viability. These processes
result in removing excess mutations from populations. Only genetic damages
that do not influence the viability of germ cells and early embryonal
development stay in populations. And these mutations consist of genetic load
of populations (Pomerantseva et al., 1996).
We consider that statistically considerable rise in the frequency of embryonal
losses up to the 22nd post-accident generation of bank vole is also caused by
accumulation of genetic load in the populations studied (Figure 3). Some
similarity between the dynamics of the relative quantity of embryonal lethals in
females and that of the CA frequency in somatic cells of adult animals could be
caused by the fact that most of embryonal lethals are due to CA in germ and
zygotic cells. As the genetic load is accumulated in chronically irradiated
populations, genome sensitivity of both somatic and germ cells to radiation
increases that results in a rise in the CA frequency and then in embryonal
mortality.
It should be noted that relationship between the dynamics of population
density and that of embryonal lethality was not revealed. At the same time,
very high frequencies of embryonal losses in monitoring populations of bank
voles after the Chernobyl accident (Figure 3) point to radiation causality of the
observed effects. These frequencies had a multiple excess over both the preaccident levels in population at the st. 2 (Razhdestvenskaya, 1984) and the
known background frequencies of embryonal lethals, typical for the whole
habitat area of bank vole under different ecological conditions (Bashenina,
1981).
Significantly higher losses before implantation as compared with late
embryonal lethality attract attention. They increased over the period of
observations more than four times, being 30-50 times the pre-accident levels.

Genetic
processes

439

Figure 3.
Dynamics of the
frequency of the total
embryonal lethality in
bank vole populations
(the 0th generation
according to
Razhdestvenskaya,
1984)

EMH
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440

This is in agreement with the known data on high sensitivity of postovulatory
ova and pre-implanted embryos to irradiation (United Nations, 1986; Molls et
al., 1980; Nomura, 1984; etc.) and with the data on increased losses prior to
implantation in root voles (Microtus oeconomus) inhabiting for above 20 years
(approximately over 40 generations of rodents) uranium-radium contaminated
sites at low radiation doses (about 2cGy/year) (Maslova et al., 1974).
It was noted earlier that the moment of implantation is a functional test of the
zygote state (Lyaginskaya and Smirnova, 1963). There could be underestimation
of radiation effects for embryons when pre-implantation losses are not analysed.
Subsequent stages of embryonal development proved to be more radioresistant
(United Nations, 1986). Nevertheless, increased embryonal lethality after
implantation was also observed in our populations of bank vole and populations
of other species of murine rodents (Amvrosjev et al., 1993) at low doses of
external and internal irradiation in the Chernobyl radioactive trace. Besides,
there is information on greatly increased (up to 48 per cent) frequencies of
embryonic malformations in bank and root voles representing approximately the
11th-18th post-accident generations of animals (1991-1994) inhabiting Bryansk
Region (Russian Federation) radiocontaminated by Chernobyl fallout (Krylova,
1998). The high frequencies (by 40-83 per cent) of congenital developmental
malformations were in children born in 1987-1995 in the regions of Belarus
affected by the Chernobyl accident (Lazjuk et al., 1998).
Main features of the mutation process dynamics revealed in chronically
irradiated populations of small mammals within 22 animal generations could
point to high genetic risk for existing and subsequent generations of humans
and animals living in contaminated areas.
The data obtained can be used and were partially used (Goncharova, 1997;
1998) in predicting remote genetic consequences for human populations under
chronic low dose irradiation. At the same time they display the necessity for
long-term comprehensive and combined investigations of populations of
humans and animals under chronic or prolonged low-intensive irradiation.
Formation of genetic radioadaptation in populations under lowintensive irradiation
The latter two stages in dynamics of radiation mutagenesis (reduction in the
mutation frequency and stabilization at a new lower level of mutagenesis) that
were revealed in laboratory populations of haploid algae (Shevchenko and
Pomerantseva, 1985) and Drosophila (Wallace, 1956) define formation of genetic
radioadaptation in populations. Adaptation of populations to mutagenic
radiation impact is a long-term process. Thus, radioresistant clones of
unicellular algae appeared in experimental populations after chronic
irradiation of several tens of generations (Shevchenko and Pomerantseva,
1985). Radiosensitive individuals are gradually eliminated from the
populations during that time and in the posterior period.
The following peculiarities of radioadaptation formation in populations
were revealed when studying radioresistance in natural populations of haploid

algae (Shevchenko and Pomerantseva, 1985), higher plants (Shevchenko and
Pomerantseva, 1985; Dubinin et al., 1980), gastropods (Shevchenko and
Pomerantseva, 1985; Dubinin et al., 1980) as well as of murine rodents
(Shevchenko and Pomerantseva, 1985; Iljenko and Krapivko, 1989) inhabiting
for some years the East-Uralian radioactive trace. Radioresistant individuals
were shown to have more active repair systems. In the 25th-30th irradiated
generations of murine rodents (northern redbacked vole ± Clethrionomys rutilis,
and long-tailed field mouse ± Apodemus silvaticus) (Shevchenko and
Pomerantseva, 1985) under additional administration of 90Sr-90Y were revealed,
on the one hand, increased total radiosensitivity (Shevchenko and
Pomerantseva, 1985) and, on the other hand, high genetic radioresistance
analysed for the frequency of CA in bone marrow cells (Shevchenko and
Pomerantseva, 1985) and spleen (Iljenko et al., 1980). The total radioresistance
that was determined from the viability of animals following additional external
-radiation increased by the 30th-40th generations of rodents (Iljenko et al.,
1989).
Based on these data, the process of population adaptation to chronic
irradiation was noted to be very complicated and to proceed in different ways.
This can be selection of radioresistant forms induced by chronic irradiation or
existing earlier, as well as temporary activation of repair systems (Shevchenko
and Pomerantseva, 1985; Iljenko and Krapivko, 1989).
Clearly defined features of the dynamics of Pp-cells in bank vole populations
in Chernobyl radioactive trace (Figure 2) could indicate the processes of genetic
adaptation in series of irradiated generations of animals.
We have compared genetic radioresistance of bank vole populations living
in areas with different levels of radiation loads. Under additional acute
exposure of the 21st-22nd irradiated bank vole generations and application of
micronucleus assay for PCE of bone marrow, the population inhabiting the st. 4
with high radiocontamination density was revealed to differ in higher
radioresistance than rodents from the less contaminated st. 2 (Figure 4). So, the
formation of genetic radioadaptation has begun by the 21st-22nd animal
generations in the populations with higher radiation load.

Genetic
processes

441

Figure 4.
Cytogenic effects in
bone marrow
erythrocytes of bank
voles after additional
acute
-irradiation from
137
Cs source

EMH
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442

Thus, genetic radioadaptation was revealed to be formed rather earlier in
murine rodent populations at our monitoring sites than it was shown in papers
(Shevchenko and Pomerantseva, 1985; Iljenko et al., 1980), under conditions
when absorbed doses were in the range from 1 to 100Gy (Gileva et al., 1996).
Formation of genetic radioresistance in chronically irradiated populations is
associated with the changes in functioning of complex system ``adaptive
response'' and mechanisms of biological protection of tissues.
In particular, we have revealed functioning of system ``adaptive response'' in
the 21st-22nd irradiated bank vole generations at the st. 4. Thus, under
successive
-irradiation of voles with an adapting 10cGy and then damaging
(challenging) dose 100cGy the cytogenetic effect (frequencies of MN-PCE of
bone marrow) was considerably lower than the effect of single irradiation with
100cGy. Besides, a sharp reduction in the frequencies of micronucleated mature
(normochromatic) erythrocytes (MN-NCE) of peripheral blood in comparison
with increased frequencies of immature erythrocytes with cytogenetic injuries
(MN-PCE) in hematopoetic tissue (bone marrow) was revealed in the 21st-22nd
vole generations (st. 14). This fact could be fully explained by elimination of
cells with cytogenetic damages to protect peripheral blood of chronically
irradiated animals against defective cells.
So, the pursued investigations have shown that the mutagenesis levels
(frequencies of CA, Pp-cells and micronuclei) observed in the 21st-22nd
generations of animals resulted from oppositely directed processes in irradiated
populations: accumulation of mutations (genetic load of populations) and
formation of genetic radioadaptation. The recorded frequencies of genetic
damages in populations could be higher in the absence of radioadaptation.
However, it should be kept in mind that for adaptation, populations pay by
elimination of the least adapted and the least viable individuals.
Dose-effect relationships for cytogenetic injuries in somatic cells of chronically
irradiated animals
The relationship between individual frequencies of cytogenetic injuries (CA,
Pp-cells and MN-PCE in bone marrow as well as MN-NCE in peripheral blood)
and low levels of individual radiation loads in animals (st. 14) was revealed by
using a regression analysis of the data. So, the relationship between the
frequencies of cytogenetic injuries and concentration of incorporated
radionuclides in the range of 4-145,410Bq/kg, dosage rate from 2 to 730Gy/
day and the total absorbed dose in the range of 0.02-7.3cGy was shown
(Table I). The animals inhabiting the st. 1-4 were pooled in one sample within
every year of investigations. In that way, analysed groups of animals represent
individuals of approximately the same post-accident generations in the
gradient of radiation loads (Table I, Figure 5).
It should be noted that causality of the observed cytogenetic injuries due to
low radiation doses is followed over 22 generations of animals.
The form of the relationship between cytogenetic effects and radiation load
might be described by linear equations in some cases. However, an

Year (animal
generations)

Radionuclide
concentration
Range of
concenNumber of tration,
animals
Bq/kg
R2

Chromosome aberrations
1986 (1-2)
42
1987 (3-4)
35-36
1988 (5-6)
1991 (11-12)
1996 (21-22)
Polyploid cells
1986 (1-2)
1987 (3-4)

38-43
32-41
37

1988 (5-6)
1991 (11-12)
1996 (21-22)

38-43
32-41
37

42
35-36

Absorbed dose rate
Range of
dose rate,
Gy/day

R2

Absorbed dose
Range of
dose, mGy

Genetic
processes

R2

443
38-24,844
3,959145,410
58-385,810
5-20,736
4-2,911

0.13*
0.17*

6-670
205-615

0.07
3-730
0.48** 3-132
0.27** 2-46

0.12* 0.2-267
0.23** 0.2-11
0.15* 0.3-23

0.03
0.31**
0.21**

38-24,844
3,959145,410
58-385,810
5-20,736
4-2,911

0.06
0.16*

0.12*
0.16*

0.16**
0.02

6-670
205-615

0.17** 3-730
0.32** 3-132
0.06
2-46

0.13*
0.17*

0.4-73
3-30

0.4-73
3-30

0.12* 0.2-267
0.21** 0.2-11
0.11
0.3-23

0.22*
0.23**

0.10
0.41**
0.06

Notes: R2 Coefficient of determination; * p < 0.05 and ** p < 0.01

Table I.
Relationships between
the frequencies of
chromosome
aberration, polyploid
cells in bone marrow
and radiation loads in
bank voles (polynomial
approximation)

Figure 5.
Dose-effect curves
(polynominal of degree
2) for the frequencies of
MN-PCE of bone
marrow and MN-NCE of
peripheral blood in bank
voles (the data of 1996)

overwhelming majority of the data (for separately considered populations or at
their pooling) was better approximated by a polynomial function (Table I,
Figure 5). The non-linear ``dose-effect'' relationships could be explained by
peculiarities of low dose effects. But a different radiation history of populations
and complicated microevolutionary processes in each irradiated population
were most likely to increase population variability in individual
radiosensitivity and in efficiency of biological system protection against

EMH
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444

injuries (including repair systems). This could lead to complicated forms of
dose-effect curves for cytogenetic injuries in somatic cells of a number of
animal generations.
The results obtained point to the absence of genetic effect threshold of low
doses of combined external and internal irradiation. Besides, they show that a
dependence of genetic effects on radiation low doses in series of irradiated
generations could not be revealed by using linear equations.
Concusions
Combined genetic and radioecological methods of investigation as well as
application of a regression analysis for describing the mutation process dynamics
and for estimating the relationship between individual frequencies of cytogenetic
injuries and individual dose loads made it possible to obtain new knowledge on
peculiarities of the genetic process dynamics in chronically irradiated natural
populations of mammals and to determine the quantitative relationships between
the frequencies of cytogenetic injuries and low doses of irradiation.
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