Genetic effects of static magnetic fields. Body size increase and lethal mutations induced in populations of Drosophila melanogaster after chronic exposure

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Genetic effects of static magnetic fields. Body size

increase and lethal mutations induced in populations of

Drosophila melanogaster after chronic exposure

G Giorgi, D Guerra, C Pezzoli, S Cavicchi, F Bersani

To cite this version:

Original

article

Genetic

effects of static

_magnetic

fields.

Body

size increase and

lethal mutations

induced in

_populations

of

_Drosophila

melanogaster

after chronic

_exposure

G

_Giorgi

D

Guerra

C Pezzoli

S Cavicchi

F Bersani

1

Department of Evolutionary

and

Experimental Biology, Bologna;

2

Department of Physics, University of Bologna,

40126

Bologna, Italy

(Received

9

_August

_1991;

_accepted

5 June

₁₉₉₂₎

.

Summary - The effect of static

_magnetic

fields on

_body

size of

Drosophila melanogaster

was

_analyzed

on 3

_laboratory

stocks reared under chronic exposure to a

_magnetic

field

10-12-fold _greater than the earth’s. A

_significant

increase in

_body

size was observed which

persisted

even when the flies were returned to control environmental conditions after a few

generations

of _exposure. The

_genetic

basis of the differences observed between treated and

control lines was assessed

_analyzing

2

_fitness

_components and 4 dimensional characters.

The increase in

_body

size was

_mainly

associated with cell

_{number, suggesting}

that the

magnetic

field effect on size

depends

on _genes which control cell

_{proliferation.}

The evolution

of the

fitness

components

during

the

_generations

of exposure

gives

some evidence of the

underlying genetic

mechanisms involved. Lines made

_isogenic

for the 3

_major

chromosomes

were tested to establish whether the size _response obtained was

dependent

on the

_genetic

variability

or not. These lines behaved in a similar fashion to outbred lines,

suggesting

that the _response in size

_depends

on an increased mutation rate. A

_mutagenic

test confirmed that one

_generation

of _exposure induces X-linked lethal mutations. These events could reflect the fact that the _magnetic field acts as a

_{physical mutagenic}

_agent at some _stage

during development.

Drosophila melanogaster

/

magnetic field / lethal mutation

_/

_body size / cell

prolif-eration

Résumé - Effet génétique des champs magnétiques statiques : augmentation de la taille _corporelle et mutations létales induites dans des populations de Drosophila melanogaster après exposition chronique.

L’effet

des

_{champs magnétiques}

_statiques

*

Correspondence

and reprints: G

Giorgi, Dipartimento

di

Biologia

Evoluzionistica

chez

Drosophila melanogaster

a été

_analysé

dans 3 souches de laboratoire

chronique-ment soumises à des valeurs du

_{champ magnétique}

10-12

_{fois plus}

élevées que le

champ

magnétique

terrestre.

_{Après quelques générations}

on observe une _augmentation

significa-tive de la taille

_corporelle,

qui se maintient

lorsque

les souches sont

replacées

dans les

con-ditions standard. Pour établir la base

_génétique

des

différences

observées entre les

_lignées

traitées et non

_traitées,

_{2 composantes} de la valeur

_adaptative

et la

_{longueur des}

4 nervures

longitudinales

de l’aile ont été

analysées

sur une des 3 souches. Le croisement entre la

lignée

traitée et la

lignée

témoin de cette souche montre que la

_{différenciation génétique}

entre les

_{2 lignées}

est très

_{forte. L’augmentation}

de la taille de l’aile est surtout associée

au nombre des cellules. Cela

_suggère

_que

l’effet

du

champ magnétique

est

dépendant

des

gènes

qui contrôlent la

_{prolifération}

cellulaire. L’évolution des _composantes de la fitness

pendant l’exposition

donne un _aperçu des mécanismes

génétiques impliqués.

Au cours des

premières générations,

une réduction

marquée

de la

fécondité

et un

_{affaiblissement}

du

développement

de

l’ceuf

chez l’adulte sont observés dans toutes les

_lignées

étudiées. Pour

établir si la

réponse

obtenue

dépend

de la variabilité

génétique

des souches

considérées,

nous avons testé des

lignées isogéniques

_pour les 3 chromosomes majeurs. Ces dernières

répondent

de la même

_façon

_que les

lignées d’origine,

ce _qui

_indique

_que

_{l’augmentation}

de

la taille

dépend

d’une

_fréquence

de mutation

_plus

élevée. Un essai de

_{mutagenèse confirme}

que

l’exposition

à des

champs magnétiques

10 à 12

fois plus

élevés que la normale conduit à des taux de mutations.létales liées au sexe

_{10 fois plus}

élevés _que chez les témoins.

Drosophila

melanogaster

/ champ magnétique

/ taille corporelle

/ mutation létale

/

prolifération cellulaire

INTRODUCTION

Studies on the effect of

magnetic

fields on

_{living organisms}

are

_interesting

from

both theoretical and

practical points

of view.

The

_geomagnetic

field is a natural environmental factor variable in _space and

time so that all

_{living organisms}

are affected

_differently.

This is due to the fact that the

_intensity

of the

geomagnetic

field follows a

_{geographic slope}

from the

_magnetic

equator

to the

_{geomagnetic poles, approximately}

from 0.25 to 0.70

_Gauss,

and the

dipole

field

_intensity

has

_decayed

_by

7%

in the last 100 _years

_(Bloxham

and

Gubbins,

1985).

A

_fascinating

theory

for a

_significant

influence of

_geomagnetism

_upon terrestrial

life is the correlation between the time of extinction of certain

_living

_species

and the

occurrence of

geomagnetic polarity

reversals which may

trigger

the loss of

_magnetic

shielding during

the

_possible

zero

_dipole

shield condition

_(Watkins

and

_Goodell,

1967).

The

_practical

interest in the effect of static

magnetic

fields involves _many

_topics

in

_physiology

and

_medicine,

_including

nuclear

_magnetic

resonance

(NMR)

_imaging,

magnetic

separation

of

biological

materials and orientation of cell

_fragments

in

suspension.

Over _{the last 20 years the}

_biological

effects of

_magnetic

fields studied in

Mutagenic

effects have been

investigated. Exposure

of adults for a short

period

to

_{high magnetic}

fields failed to reveal

significant

differences between

exposed

and

sham-exposed

groups of

Drosophila

(Mittler,

1971;

Kale and

Baum, 1979, 1982;

Mulay

and

_Mulay,

_1961,

_1964),

Salmonella

_(Anderstam

et

_{al, 1983;}

Juutilainen and

_Liimatainen,

₁₉₈₆₎

or mice

(Mahlum

et

_al,

_1979).

Morphogenetic

anomalies and altered

_development

times were

_apparent

when

Drosophila melanogaster

pupae were

_subjected

to

_magnetic

fields

_(Levengood,

1966,

1967)

and when

Drosophila melanogaster

flies remained in a

_gradient

of low

magnetic

field until the _appearance of the first

_offspring

_{(Tegenkamp, 1969).}

Physiological

and

developmental

effects have also been

_{investigated.}

The

regula-tion of

_growth

and differentiation of

Drosophila

(Goodman,

1976;

Goodman et

_al,

1979),

Escherichia coli

_(Ramon

et

_al,

₁₉₈₁₎

and mammalian cells

_(Malinin

et

_al,

1976;

Frazier et

_al,

₁₉₇₉₎

are affected

by

_strong

_magnetic

fields.

Little information is available on

_permanent

_genetic

effects. A reduction in

spawn rate and

gestation period

was found when

_guppies

(Lebistes reticulatus)

were

chronically

exposed

to a

homogeneous magnetic field,

but the effect is concealed

when the fish are removed from the

_magnetic

field

(Brewer, 1979).

Delays

in the mitotic

cycle

of the

_myxomycetes,

_Physarum

_{polycephalum,}

were

noted after continuous exposure to

_{electromagnetic}

fields. This effect also

disap-pears,

though

not

immediately,

when the culture is removed from the field simulator

(Marron

et

_al,

_1975).

More recent molecular studies have demonstrated further effects. In the presence

of

_{varying magnetic fields,}

the

_{transcription autoradiogram}

of

dipteran salivary

gland

cells

_markedly

increased the

_{specific activity}

of messenger RNA

(Goodman

et

_al,

_1983,

_1987;

Weisbrot et

al,

1988)

and enhanced DNA

_synthesis

in human fibroblasts was also

_{reported following}

_exposure

(Liboff

et

_al,

_1984).

All these

findings

suggest

a raised mitotic rate which could lead to an increase in size.

The

_relationship

between environmental stresses and

_{genetic reorganization}

has been examined in a number of

_instances,

but the _{ways in} which

_{organisms perceive}

the stress and

_respond

are unknown.

Changes

in

_plant

_{DNA, environmentally}

induced,

have

_already

been shown in certain flax varieties. In a

_single

_generation

of

stress,

there was an alteration which was heritable and could not be reversed

_by

restoration of the

_original

conditions

_(Durrant,

1971; Cullis, 1986,

1990).

A _range of

_phenomena

can be

responsible

for these

rapid genomic

changes,

including

the

_activity

of

_transposable

_{elements, amplification}

and deletion events. Some

experimental results,

controversially,

have been

_interpreted

as evidence for a

form of directed mutation

_(Cairns

et

_al,

_1988),

but further

experimental

data will be

required

to

_study

the

_possibility

of

_{environmentally}

induced mutation in the

genome.

This

_study

aimed to

_approach

this

topic by

means of formal

_genetic

_analysis

using

Drosophila melanogaster,

an

_organism

suitable for this kind of

_study.

Here we

determine the effect of a chronic static

magnetic

field 10-12-fold

_greater

than the

earth’s on

_body

size and relate the effect to an increased mutational rate.

As differences in

_body

size seem related to variations in cell size and number

MATERIALS AND METHODS

Exposure

system

The exposure

system

consisted of a function

_generator

and a _power

amplifier

(50

Hz) (generator

stabilized

_by

a continuous

_current)

connected to several coils for simultaneous exposure to

_magnetic

field

_strengths.

The coils used for exposure were

constructed of aluminium tube

_(inside

diameter 2r = 4.6 _cm;

length

I = 10.3 _cm;

turns of 0.2 mm _{copper wire} No =

3 500).

The

_magnetic

flux

_density

or

_magnetic

induction B at the center of the coils was

calculated from:

where I = electric current

(A)

and p

o =

4!r10-3

H/m (Henry

_per

meter)

is the

magnetic

permeability

of free space

expressed

in Gauss.

An active uniform horizontal

_magnetic

field was calculated from the center of the

coils of 7.0

_Gauss,

to the external section of 4.0 Gauss. This

_intensity

_(10-12-fold

greater

than the

geomagnetic

field)

was measured

_by

a

_gaussmeter.

The coils were custom-built to house 2 vials

_(outsite

diameter 2r = 3.2

cm)

in

the center of the horizontal beam of

_{magnetic strength.}

The vials were inserted one

against

the other so as to ensure that all individuals were confined to the most uniform

_point

of maximum

_magnetic

field

_intensity

_throughout

their

_development.

No rise in

_temperature

was noted within the coils which were stored in thermostated

chambers at 25°C

_alongside

control lines where

only

the local

_geomagnetic

field was

measured.

Stocks

Three different stocks of

Drosophila melanogaster

were used:

1)

Oregon-R

(0)

maintained for over 20 yr in our

laboratory

under standard

breeding conditions;

and 2 wild stocks collected in 2 different Italian

localities;

2)

near

_Bologna

_(C)

and

3)

near Rieti

(R)

and

kept

for 5 yr and 3 _yr

respectively

in the

laboratory

under

controlled conditions of

temperature

(25°C)

and

_humidity

_(70%).

The 3 stocks showed a

good degree

of additive

genetic

variability

(h

2

)

for

wing length

(0.29

in O

_strain;

0.33 in C

_strain;

0.34 in R

_strain)

calculated

_by

the sib

_analysis

method and as

_{parent-offspring}

_regression

(Falconer, 1970).

The

_experiment

of _exposure

Thirty

random

_pairs

from each of the 3 stocks were left to

_lay

_eggs

_overnight

in 6

different vials. Twelve

_samples

of _{80 eggs} each were collected the

following morning:

6 made _up the control line

_subjected

to the effect of the

_geomagnetic

field alone while the other 6 were

exposed

to the induced

_magnetic

field.

Control lines were maintained within coils the same size as those

_adopted

for the

On

reaching

sexual

_maturity,

adults were counted and measured and 30

pairs

were

randomly

left to

_lay

within the coils in a new set of

_vials;

_breeding

took

_place

at a constant

_temperature

of 25°C.

A fixed number of eggs per vial avoided

competition

for the medium which may

affect larval survival and

_body

size.

This

_{experimental protocol}

was started with stock C and continued for 59

generations.

The same

_procedure

was

_repeated

to create 2 further treatment lines

(C

2

and

_C

₃

_),

maintained for

only

4

_generations.

The

experiment

was further

repeated

with the other 2

_stocks,

0 and

_R,

both of which were followed for 43

generations.

Many

releases were set _up from all the treated lines at different

experimental

times for a

_varying

number of

_generations.

In

_particular,

C line releases were set _up

at the

_3rd,

_5th,

10th and 27th

_generation

while releases from

C

3

were

_only

made at the 4th

_generation;

2 release lines were made at the 5th

_generation

for the 0 and R

_populations.

Wing

length

was used to determine

_changes

in

_body

size. The

_length

of

_L

₄

vein

was taken as

_wing

_length

and measured under a

_microscope

at

_{magnification}

x

_25.2,

with an ocular micrometer of 100 divisions. In all tested lines

(treated,

control and

released)

for some

_generations

(from

1 to

_10,

from 16 to

_27,

the 34th and the

_59th)

the

_wings

were dissected and mounted on slides and a variable number of

_right

female

_wings

of each

_experimental

line were measured

(see results).

To check whether variations in

_wing

size were reflected in other

_body

_dimensions,

thorax and head size were measured at the 34th

_generation

for stock C

_(including

2 release

_{lines) and}

at the 18th

_generation

for stocks R and

_Oregon

_(together

with

1 release

_line).

Genetic test

For stock C

alone,

a scheme of

_reciprocal

crosses between treated and control lines was made after 9

_generations

to assess

_possible

_genetic

differences. Parents and

_F

₁

s S were raised

_{simultaneously}

with

F

2

s,

by

crossing

parental

strains twice in successive

generations.

Ten

_single

_pairs

for 3

_replicates

per

reciprocal

cross were left to

_lay

and the

_development

time and number of

_offspring

recorded. At each

generation,

the

_right

_wings

of 6 females of each progeny were

dissected,

mounted on slides and

measured.

Variations in cell size and number were evaluated

considering

a

_wing

surface

delimited

_by

the

_triangle

whose sides are

_{represented by}

the

L

2

and

L

4

veins and the

distance between them at the

_margin

of the

wing.

The

_triangle

area was

_computed

by

Erone’s formula:

where p is half the

perimeter

and _a, b and c are the

_lengths

of the sides of the

triangle.

Cell area was estimated under a

_microscope

at a total

magnification

x 375

by

counting

the number of

_{bristles/cells}

_present

on a dorsal surface of 84.05 x

10-4

mm’

limited

by

a reticle

placed

in the

_eyepiece.

In this

_work,

the reticle was

_placed

at first between

L

2

and

_L

₃

_veins,

then between

L

are

regularly

_arranged

on the

_wing

surface and counts in different

_regions

are

quite

well correlated so that variations in one

_{wing region}

reflect variations in the whole

wing

(Robertson,

1959a;

Delcour and

_Lints,

_1966).

The _average cell area was estimated

by dividing

the area of the reticle

by

the

number of cells counted. Cell number was obtained

_by

_dividing

_wing

size

_by

cell

area; the measurements were converted to natural

_logarithms.

In this

_{form, wing}

area is the sum of cell area and number.

Isogenic

lines

In order to establish whether the effect of

_magnetic

field

depends

on

_pre-existing

genetic

variability

in a

_{given population}

or _not, a short time test on

_isogenic

lines was made. These lines were made

isogenic

for the 3

major

chromosomes as there is

no evidence in the literature of 4th chromosomes _genes involved in size variations.

Virgin

females of the natural stock C were mated with males of a

_multiple

balanced stock:

_Binsc;

_SM

_5/

_bw&dquo;

₁

_;

_TM

_3/

_Sb

_(Lindsley

and

Grell,

1968).

Female

heterozygotes

Binsc/

+

;

SM

S/

+

;

TM

3/

+

were backcrossed in

_single

_pairs

with males

from the balanced stock to obtain

replications

of the same chromosomes in males

and females.

_Isogenic

males and females from all 3 chromosomes were obtained in

the

_{subsequent generation.}

Sex-linked recessive lethal test

Two vials with 30 random

_pairs

from the 2

_wild-type

stocks

_(C

and

_R)

were

kept

within the coils

_generating

the

_magnetic

field. The

_emerging

flies were removed and

each male _progeny was mated with a

_virgin

female from the

_FM

_6/

_FM

₇

balancer stock

_(Lindsley

and

_Grell,

_1968).

Each

F

1

progeny was examined and

pairs

mated in vials. The

F

Z

_progeny which

hatched in these vials

were

examined for the presence or absence of males.

Complete

absence of such males indicated a lethal mutation. All

suspect

lethals were confirmed

by testing

for one more

_generation.

The

_experiment

was

repeated

several times and a total of 4 197 X-chromosomes were

_analyzed.

RESULTS

Wing length

Wing length

of control and treated stocks

_during

different

_generations

is

_given

in

table I and variation coefficients in table II.

_{Only replicate}

_(a)

for the C stock is

shown.

The effect of chronic

_magnetic

field is also

_given

as

_percentage

deviation from

controls in

_figures

1 and 2

_(a

= line

C;

b = lines

C

2

and

C

3

;

c = line

O ;

d =

line

_R).

The results show that the

_magnetic

field

_always

increases

_wing

size. A

_sharp

significant

response to

_magnetic

field is observed in the first

_{generations, becoming}

slower from the 20th

_{generation. Responses}

were similar for all stocks and stabilized

A variable number of flies were measured each

generation: only

24 flies in control and treated lines of the C stock at the 1st

_generation,

and a number

_ranging

from 30 to 119

flies in the

remaining

stocks and

generations.

Measures are

_given

in micrometric units.

One unit

_corresponds

to 3.8 x

10-

2

mm.

Only

1

_replicate

(a)

of the C stock is

_reported.

Table III lists the rate of increase in

_wing

size for the lines tested. Results are

expressed

in terms of

_regression

_(b

t

_SE)

of the standardized response over the first

10

generations.

Table III also

_gives

the rate of increase for line C

_following

artificial selection in the

_plus

direction with a selection

_intensity

of

20%.

No

significant

treatment lines

_presented

a rate of increase similar to that obtained

_following

artificial selection.

Phenotypic variability

in all the studied lines was

_reported

as coefficients of

variation. The results are

presented

in table II and in

figure

2 as the ratio between values obtained in the treated lines and values in controls.

Only replicate

(a)

of the C stock is shown. The treated lines are characterized

_by

a

relatively

lower

variability

than the

controls,

which is maintained in the released lines. This effect is

_present

from the first

generations.

Other

_body

size traits

The whole

_organism

was measured for size _response at

the

34th

generation

for stock

C and at the 18th

_generation

for 0 and R.

Table IV summarizes the differences between the treated and control lines in

thorax

_length

and

_width,

head width and

_wing

_length.

There was a

_significant

difference in whole

_body

size

_although

an allometric _response was

_present

as a

Fitness

_components

Fecundity

and

_viability

of control and treated lines are

_given

in table V and as

percentages

of controls in

_figure

3a and 3b. There is evidence of a marked reduction

in egg

laying

in all the treated lines in the first

_generations.

This decrease

_disappears

from the 5th-6th

_{generation, returning}

to values similar to controls.

The differences in the

_percentage

of _eggs

_yielding

adult flies

_(viability)

between treated and control lines are

_given

in

_figure

3b.

_Magnetic

field reduces the

_viability

of all lines studied

_compared

with controls. The trend is less

_regular

than that of

fecundity;

however,

there are

_{highly significant}

differences

₍

_X

₂

test

based on 2 x 2

Response of isogenic

lines

To establish whether the _{size response} obtained is

_dependent

on

_pre-existing

genetic

variability,

lines made

_isogenic

for the 3

_major

chromosomes were bred

for 7

_generations

at the same

_magnetic

field

_intensity

as that used

previously.

Table VI shows the

_wing

_{size response} of 2

_{replicated isogenic}

lines from the C stock. The _{response is} similar to that obtained at the same

_generation

in the

3 stocks

_previously

considered.

Genetic

_analysis

The

_genetic

basis of the differences observed between the 2

_lines,

treated and

control,

was detected on C stock at the 9th

_generation

_{by crossing}

treated and untreated

flies,

after one

_generation

of transfer out of the

_magnetic

field.

_Wing

size and 2 fitness _components are taken into account:

_development

time measured as the

average number of

days

required

from

_deposition

to _emergence of the _progeny of 1

female after 1

_{day’s laying,}

and

_{productivity,}

measured as the number of

_emerged

flies which combined to the different

_components:

female

_fecundity,

egg

viability,

male

_{mating ability}

and

_fertility.

As

_{regards wing}

_length,

differences between

_F

₁

_reciprocal

crosses were tested in

order to check maternal

_and/or

X-linked effects. Since

reciprocal

crosses did not

differ,

the

_comparisons

between

_mid-parent

_F

_I

and

_F

₂

_progenies

were

_performed

on

averaged

means

_{(table VII).}

The results show that the differences between

_magnetic

field and control lines have a

_genetic

basis. The _genes involved seem to act

_additively,

since no differences between the means were detected in the

_following

_generations.

Table VII also reports variance estimates. The variance contains both

_genetic

and environmental components, but the differences among

parental

lines and

_hybrid

generations

should be

_{largely genetic.}

The test on variance shows a

_constancy

in

F

I

variances and a

_high

level of

_segregation

variance in the

F

2

generation.

The results

_concerning

fitness

_components

are

_given

in table VIII. The

differ-ences between

_reciprocal

crosses were

always

not

_significant

and mean values and

parents.

This shows evidence of heterosis in the crosses between

_magnetic

field and

control lines.

Cell number and cell area

The

_{developmental}

reasons for the size differences observed between the 2 lines were

investigated by

studying

cell size and number variation. Both parameters seem to be under

_genetic

control

_(Robertson,

_1959a;

Cavicchi et

_al,

_1985).

The

_{relationships}

between

_wing

_size,

cell size and number are

_given

in table IX for

magnetic

field and control lines. In the

_control,

both cell area

_and,

more

_strongly,

cell number are

_positively

correlated with

_wing

_size,

but

_they

are not related to each other. In the

_magnetic

field

_{line, only}

cell number is

_positively

correlated with

_wing

area, while cell area and number show an inverse correlation. It _seems,

therefore,

that both cell

_parameters

are involved in

_wing

surface determination in

the control

line,

but

_only

cell number is involved in the

_magnetic

field line. So the

size differences between

_magnetic

field and control line

_depend

either on cell size

or

_number,

but cell area seems to

_compensate

for cell number variations.

Similar behaviour of cell number and

_wing

area is also evident in the

_genetic

analysis

reported

in table X. The results are less clear-cut than those obtained on

_wing

_length,

_owing

to the decrease in cell number and

_wing

size means in the

F

2

and the non

_significant

increase of

F

2

variances

_compared

with those of

mid-parent.

The

F

1

crosses exhibit a

_significant

and

_inexplicable

decrease in

_wing

size

and cell number variances when

_compared

with the

_parental

ones and a

significant

increase in

_F

_Z

variances

_compared

with the

_F

₁

ones. On the

_contrary,

both means

and variances of cell area remain constant

_during

cross

_generations.

On the

_whole,

the results indicate

_segregation

of _genes

_controlling

cell number and

_wing

area but not of _genes which

_regulate

cell size.

These results confirm that cell area and number are 2

_independent

_parameters,

genetically

correlated in

_{determining wing}

size even

_though

cell number rather than

cell area is the

_parameter

most affected

_{by magnetic}

field.

Mutagenesis

test

Table XI summarizes the numbers of sex-linked recessive lethal _genes obtained in

obtained in 4 successive

_experiments

and refer to a total of 2 230 X-chromosomes

(treated

+

_control)

for stock C and 1967 X-chromosomes for stock R.

The _percentages of

_lethality

obtained in the control lines are within the _range of

values found when similar tests were

_performed

on wild

populations

of

Drosophila

(Wagner

and

_Mitchell,

_1964).

Much

_higher

_(10-fold)

_lethality

estimates were found

similar to the results of tests made on wild

_populations

of

_Drosophila

treated with

X-rays

at an

_intensity

of 1000

_roentgen

(Spencer

and

Stern, 1948; Uphoff

and

Stern,

1949).

DISCUSSION

Mutagenicity

tests

_performed

on several

_systems

(Mittler,

_1971;

Kale and

Baum,

1979, 1982;

Mileva et

_{al, 1985;}

Juutilainen and

_Liimatainen,

₁₉₈₆₎

have failed to demonstrate _any

_mutagenic

effect of

_strong

_magnetic

fields for short

periods,

but

a

_{statistically significant}

increase of chromosomal aberrations has been observed in

human

_lymphocytes

in an

_experiment

of _exposure to

_{pulsed electro-magnetic}

fields of

_{amplitudes ranging}

from 10-40 Gauss

_{(Garcia-Sagredo}

and

_Monteagudo,

_1991).

Morphological

modifications

_triggered

by biomagnetism

were

reported by

Brewer

(1979).

A

_significant

increase in

_body

size in

_comparison

with both treated and control lines and in the size of the progeny examined in 3

subsequent generations

was found in Lebistes reticulatus

_subjected

to a continuous treatment of a 500

Gauss

_{homogeneous magnetic}

field.

_However,

these effects are not

_permanent:

in 2

_generations

after removal from the

_magnetic

_field,

brood size was

nearly

normal

for a

_laboratory

stock.

Our results offer evidence that chronic _exposure to a

_magnetic

field 10-12

times

_greater

than the earth’s increases

_body

size in

_populations

of

_Drosophila

melanogaster

and this increase

_persists

even when flies are returned to control environmental conditions after a few

_generations

of _exposure.

This variation is non-random since the

_change

in

_always

in the

_plus

direction in

all the treated lines. Similar results are found with colchicine _treatment, in different lines of Loli!cm _perenne, where variations of

_{agronomic quantitative characters,}

stable over _{many years} and transmitted

_through

a selfed-seed

_generation,

are

_always

in the

plus

direction

_(Francis

and

_Jones,

_1989).

_Generally

_speaking,

_laboratory

populations

of

_Drosophila

_melanogaster

_subjected

to directional artificial selection

pressure for

body

size resume the control size if selection is relaxed after a few

generations

(Robertson,

1957; Tantawy

and

El-Helw,

1966).

In our

_experiment,

the size increase

_{depends, therefore,}

on _genes that are selected

_early

on

and/or

induced after very few

generations

of exposure as also revealed

by

the

genetic

test

performed

after

_only

9

_generations

of exposure.

Moreover,

it is known that in

_Drosophila

the environmental effect on

_body

size

(eg temperature)

is

_mainly

focussed on the _genes which control cell size

_(Robertson,

1959b;

Cavicchi et

_al,

_1985).

On the other

hand,

our results

emphasize

that the

increase in

_body

size is

_mainly

associated with cell

_{number, suggesting}

that the

magnetic

field effect on size

_depends

on _genes which control cell

_{proliferation.}

The

_{significantly longer}

duration of the larval

_period

exhibited

_by

flies

main-tained in a

_{higher magnetic}

field could be correlated with the

_magnetic

field-induced

increase in

_body

size since there is a

high

correlation between

length

of

development

and

body

size under favourable conditions

_{(Robertson, 1957).}

Although

body

size is known to be controlled

_by

several _genes located on different

chromosomes in

_{Drosophila rnelanogaster}

_(Kearsey

and

Kojima, 1967;

Cavicchi et

_al,

_1989),

our

_findings

do not indicate which _genes are

mainly

involved in

The

segregational

pattern of the crosses does not establish whether one or several

genes are involved. More

_{specific genetic}

_analyses

are

_planned

in this

_regard.

In

any case,

wing

size variations noted in the

generations

bred in the

_magnetic

field

are _very similar to those obtained

_following

artificial selection. This

_suggests

that the

_magnetic

field affects several _genes. The evolution of the fitness

_components,

fecundity

and

_viability,

in the

_generations

of lines

_subjected

to continuous treatment

gives

some evidence of the

_{underlying genetic}

mechanism involved. The sudden

drop

in fitness values in a

_{population subjected}

to _any treatment _may occur for 2

genetic

reasons:

₁₎

the treatment induces lethal or sublethal

_mutations;

2)

the treatment constitutes environmental conditions unfavourable to the

_population

which

_undergoes

an increase in selection _pressure.

In the first

_instance,

the

_mutagenic

effect is

_expected

to

_persist

_through

the treated

_generations,

unless

_only

some _genes are the treatment

_targets.

In the second case, it is

plausible

to assume that if the conditions are

compatible

with

_continuing

vital

_functions,

the

_population

will

_adapt

to the new conditions with a

_consequent

increase in fitness. In our _case, the lower

_percentage

of

_emerged

flies in relation to _egg number found in the line bred in the

_magnetic

field is a clear indication

of

_{embryonic lethality}

or

_mortality

at some larval

_stage.

This event could reflect the fact that the

_magnetic

field acts as a

physical mutagenic

_agent

at some

_stage

during embryonic

or larval

_development.

The

_mutagenicity

test carried out in this

study

confirms that one

_generation

of _exposure induces lethal mutations 10-fold

greater

than the natural rate observed in the control _group. The response in terms of increased size would therefore appear to be due to mutations of _genes involved

in cell

_{proliferation.}

In this

_connection,

it is

_interesting

to note the size variations obtained in

isogenic

lines

_following

treatment. As each

_isogenic

line

_presents

each of the 3

major

chromosomes

duplicated

and identical both within an individual and in

all

_individuals,

its

_genetic

_variability

is zero. No form of selection can act on a

population

without

_{genetic variability.}

_Hence,

the same size differences observed

following

treatment in the

_isogenic

lines as in the outbred stocks can

_only

be mutational.

However, phenotypic

variation of our lines

_subjected

to

_magnetic

field treatment

is not

_random,

since the

_change

is

_always

in the

_plus

direction. On this

_basis,

the different

_hypotheses

could not be

_mutually

exclusive. In

_fact,

some mutations

should be deleterious and can decrease fitness but a

_lucky

few should be beneficial

and also

_help

the

_adaptation

of the

_population

to the new environmental conditions.

Furthermore,

some

_organisms

could withstand a

high

mutation rate and still be able to _compete. If this

happens

we must think that there is

_genetic

variation in the rate of mutation or that individuals whith different rates differ in fitness.

Some results of the

dose-response

relation for

_X-ray

induced mutations in

Drosophila melanogaster

confirmed a

_genetic

_response to chronic radiation

_dosage

that lowered the rate of mutation.

_Although

X-irradiation causes an initial reduction

of

_fertility,

after several

_generations

an

_adaptation

of irradiated

_populations

was

shown. At least 2 mechanisms are

_suggested

to

_{explain adaptation:}

an increased

oviposition

rate

_and/or

a decreased

_{radiosensitivity}

_(Nothel,

1970,

1987).

In our case the second mechanism seems at

_work,

since

_oviposition

follows the

The evolution of fitness characters

_generations

could reflect the _presence of clusters of

_target

_genes, but this can be verified

directly only by

a

mutagenicity

test after _many

_generations

of treatment.

Though

further research will answer all the

_questions

raised in this

work,

we can

conclude at

_present

that chronic _exposure to a

_permanent

static

magnetic

field has

mutagenic

effects on

_living

_organisms.

The low

_magnetic

field

intensity adopted

in

this

_experiment

_implies

that

_geomagnetic

field variations in time and _{space may} be involved in

_{evolutionary phenomena.}

ACKNOWLEDGMENTS

We thank R Barale and G

_Luigi

Dalla Pozza for valuable

_suggestions.

This research was

supported by

a _grant from the Ministero della Universita e Ricerca

_Scientifica,

Rome,

Italy.

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Weisbrot

_DR,

Goodman

_R,

Henderson A

₍₁₉₈₈₎

Identification

_{by transcription}

au-toradiography

of

specific

chromosome bands in

_{Drosophila melanogaster, responding}