Closely related Drosophila melanogaster strains with altered fitness also show changes in their hobo element properties

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Closely related Drosophila melanogaster strains with

altered fitness also show changes in their hobo element

properties

Vn Bolshakov, Ap Galkin, Lz Kaidanov, Va Gvozdev, C Louis

To cite this version:

Original

article

Closely

related

_{Drosophila melanogaster}

strains

with altered

fitness

also show

changes

in

their hobo element

_properties

VN Bolshakov

AP Galkin

2

LZ

Kaidanov

2

VA Gvozdev

3

C

Louis

1

Institute

_{of Molecular Biology}

and

Biotechnology, FORTH,

PO Box

_1527,

711.10 Heraklion,

Crete, Greece;

2

Saint

Petersburg

State

_{University, Department of Genetics,}

Saint

_{Petersburg, Russia;}

3

Russian

_{Academy of Sciences,}

Institute

of Molecular Genetics, Moscow, Russia;

University

of Crete, Department of Biology, Heraklion, Crete,

Greece

(Received

4

_{August 1993; accepted}

5

_January

₁₉₉₄₎

Summary - A set of

_{Drosophila melanogaster}

strains of common

_origin,

but with

different fitness

characteristics,

established either

_{spontaneously}

or after selection for the

specific

fitness parameters, appear to induce non-P-element-mediated

gonadal atrophy

in

_appropriate

crosses to tester strains

_containing

or

_lacking

hobo elements

_(H

and E

strains).

Using

the

_{gonadal dystrophy}

_(GD)

_sterility

_assay, as well as

_genomic

Southern

blot

_{hybridization}

and in situ

_{hybridization}

on

_{polytene chromosomes,}

it was found that

all these _{strains possess} hobo elements whose

_{dysgenic activity, composition,}

_copy number and

_cytogenetic

locations

_appeared

to be variable. In

_general,

low fitness strains have moderate

_{hobo-activity}

and

_{hobo-repression potentials,}

while

_high

fitness strains show no

hobo-activity

but also

_quite

_high

_{hobo-repression potentials.}

_Although

distinct differences in the

_composition,

_copy number and location of the hobo elements in the _genome were

observed between these 2 groups of strains, these variations did not show any

profound

correlation with either hobo-related

_{dysgenic potential}

or the fitness of the strains.

Drosophila melanogaster

/

selection

_/

fitness

_/

transposable element

_/

hobo

Résumé - Des souches étroitement _apparentées de Drosophila melanogaster avec

des _{aptitudes reproductives} modifiées manifestent aussi des changements dans les

propriétés de leurs éléments hobo.

Différentes lignées

de

Drosophila melanogaster,

issues d’une

_population

naturelle russe, ont été sélectionnées sur des

caractéristiques

de fitness. Ces

_lignées,

comme les

_{lignées témoins, présentent}

des activités

_dysgéniques

variables non

reliées à l’élément

_transposable

P. Nous montrons que la stérilité GD

_(gonadal

_dystrophy)

qu’elles

induisent est due à l’élément hobo et, de

plus,

que ces

_lignées

ont des

potentiels

*

dysgéniques

différents.

Les

_analyses

moléculaires réalisées par

buvardage

de Southern et

par

hybridation

in situ sur les chromosomes

_{polytènes font apparaître}

que ces

_lignées

possèdent

un nombre variable d’éléments

hobo,

qui ont des structures et des localisations

cytogénétiques différentes.

En

_général,

les

_lignées

_ayant une fitness basse

_présentent

une

activité hobo et un

_potentiel

de

_{répression modérés,}

alors que les

lignées

ayant une fitness

plus forte

n’ont pas d’activité

hobo,

mais un

_potentiel

de

_répression

assez élevé. _{Bien que}

ces

_{2 groupes diffèrent}

par la structure, le nombre et la localisation

_{cytogénétique}

de leurs éléments

_hobo,

les

_différences

mises en évidence ne _{montrent pas} de corrélation

_directe,

que ce soit avec le

potentiel dysgénique

ou avec la fitness des

_{lignées analysées.}

Une relation

entre les sélections réalisées pour établir les

lignées

et l’évolution de leurs éléments hobo est discutée.

Drosophila melanogaster

/

sélection

_/

fitness / élément transposable / hobo

INTRODUCTION

The evaluation of the

_genetic

consequences of selection is one of the

key

_purposes

of

_population

_genetics,

_especially

in the context of the

_theory

of

_breeding

_(Hill

and

Caballero,

1992).

To address this

_problem,

a set of

Drosophila melanogaster strains,

originating

from a natural

_population

from Yessentuki

(Russia),

was established

after close

_inbreeding

and

_long-term

selection for differences in male

_{mating activity.}

Selection for low male

_{mating activity}

also led to correlated

_changes

in a number of

morphological, physiological,

behavioural,

biochemical and

_genetic

features

_which,

altogether, greatly

reduced the overall fitness of these

_strains,

and thus made it

possible

to characterize them as low fitness or

_high

fitness strains

_(reviewed

in

Kaidanov, 1980,

1990).

In

_spite

of the close

_inbreeding,

these strains

_appeared

to _possess a

_significant

genetic

load and a

_high

rate of

_spontaneous

_{mutability, including}

the occurrence

of chromosomal

_{rearrangements}

_(Kaidanov,

_1980,

_1990;

Kaidanov et

_al,

_1991).

Experiments

directed to

_investigate

the sources of this

_genetic

_instability

showed

that in the course of

isogenization

of the strains’ 2nd chromosomes in order to evaluate their

_genetic

loads and the rates of

_spontaneous

_mutability,

the female

progeny exhibited a

high

rate of

_{gonadal dystrophy}

_(GD).

In these crosses, males

from one of the low fitness

_strains,

LA,

were crossed with the females of a

_laboratory

strain

containing

a balancer for the 2nd chromosome. This

_finding

was

highly

reminiscent of

_{hybrid dysgenesis,}

a

_syndrome

attributed to the activation of the

transposable

elements P or hobo

_(Louis

and

Yannopoulos, 1988;

Blackman and

Gelbart,

1989;

Engels,

1989).

Since it was known that LA does not contain _{any P} elements in its _genome

_(Pasyukova

et

_al,

_1987),

the

_probability

that hobo elements

were

responsible

for the GD

sterility

detected was checked

by

crosses to

_appropriate

tester

_strains,

_containing

or

_lacking

hobo elements. These

_experiments

revealed

the presence of active hobo elements in LA

(Kaidanov

et

al,

1991)

thus

raising

the

_possibility

that hobo _may be the causative factor for the

_genetic

_instability

observed. We extended our

analysis

to several strains

which,

though

related,

exhibited different fitness characteristics.

Here,

we

_report

results on

_composition,

MATERIALS AND METHODS

The

_following

D

_melanogaster

strains

_(kept

on standard corn-meal food at

_25°C)

were used for

_experiments:

LA: low

_activity

_strain,

obtained from a natural

_population

in Yessentuki

(Russia)

in 1965 as a result of

inbreeding

and

long-term

selection for low male

mating activity.

By

the time of the

_beginning

of the

_experiments

described

_here,

it had

passed -

600

_generations

of selection.

HA and

LA

+

:

_high

_{activity strains,}

obtained

_{independently}

from LA at its 70th and 163rd

_generation

of

_maintenance,

_{respectively, by}

selection for

_high

male

_mating

activity

and close

_inbreeding.

These 3 strains

_(LA,

HA and

_LA

₊

₎

were

_kept

in the collection as families obtained from individual brother-sister

_matings

and were

described in detail

_by

Kaidanov

_{(1980, 1990).}

LA6- and

LA6

+

:

low

_activity

and

_high

_activity

_strains,

_{respectively,}

selected

by

close

_inbreeding

from a

_high

fitness strain

(LA6)

that arose

_{spontaneously}

from one of the families of

_LA,

and was

_subsequently

lost from the collection. After the

initial selection and close

_inbreeding,

LA6- and

LA6‘

+

are

kept

in the collection

by

mass

_mating

without further selection.

LApas:

low

_{activity strain,}

established from one of the families of LA and reared

by

mass

_mating

without further selection.

23.5MRF/CyL

4

:

a strain

_containing

active P and hobo elements

(PH-strain)

and

capable

of weak induction of P-M

_hybrid

_dysgenesis

and

_strong

induction of H-E

hybrid dysgenesis

(Yannopoulos

et

al, 1987;

Stamatis et

_al,

_1989).

CyL/Pm:

an ME strain

_containing

a 2nd chromosome balancer and unable to suppress H-E

hybrid dysgenesis.

Female progeny

resulting

from crosses between

this strain’s females to

_23.5MRFICyL

males

exhibit

_high

levels

_{of gonadal}

atrophy

(Kaidanov

et

al,

_1991).

The test for male

_{mating activity}

was

_performed

as described

_by

Kaidanov

_(1980,

1990).

Single

mature males from the strain to be tested were

_put

in the same vial

with 2-3

_virgin-wild

_type

females and the _process of

_copulation

was observed. Male

mating activity

was determined as the

_percentage

of males

succeeding

in

copulating

with

_virgin

females within 30 min. For each

_strain,

55-90 males were tested.

The GD

_sterility

assay was

performed according

to the conditions described

_by

Yannopoulos

(1978).

To determine the

_{hobo-activity}

_potential,

the males of the strain under

_{investigation}

were crossed with the

CyL/Pm

tester

_females,

while to determine the

_{hobo-repression potential,}

the females of the strain were crossed with

23.5MRFICyL’

males. As a

_control,

the GD

_sterility

assay was also

_performed

both

within-strain and for the

_reciprocal

crosses. All crosses were

_performed

at 25°C

to maximize the manifestation of H-E

_hybrid

_dysgenesis

_(Stamatis

et

_al,

_1989).

Female progeny were collected _every

day

and transferred to fresh vials for 3-4 d for maturation of the

_gonads,

which were then dissected to

_analyze

their

_morphology.

The induction of

_GD-sterility

was measured as the

_frequency

of

atrophic

_gonads.

For each

_strain,

1-4

_replicates

of each cross were

_performed

and 50-500 females were scored.

Genomic DNA for Southern-blot

_{hybridization}

was extracted from 200

_flies/

strain as described

by

Ashburner

(1989).

Approximately

2 pg DNA was

digested

gels

and blotted onto

_nylon

membrane filters

_following

standard

_protocols

(Sam-brook et

_al,

_1989).

_{Hybridization}

was

_{performed according}

to Church and Gilbert

(1984)

using

as a

_probe

32P-labelled

_{plasmid pHFLl}

which contains a

_complete

hobo element

_(Blackman

et

_al,

_1989).

In situ

_{hybridization}

to

_{salivary gland polytene}

chromosomes was

_performed

as

described

_by

Ashburner

_(1989),

_using

biotin-labelled

_{plasmid pHcSac,}

_containing

a

complete

hobo element as a

_probe

_(Stamatis

et

_al,

_1989).

The t-test

_(Sokal

and

_Rohlf,

₁₉₆₉₎

was used to _compare the means in our

experiments.

RESULTS

Male

_{mating activity}

As some of the strains under

_{investigation}

are

_kept

without _any further selection

with

respect

to their male

mating activity,

we first

_performed

a series of tests for all

LA derivatives

_analyzed

here. The data on their male

_{mating activity}

are

_presented

in table I.

They

confirm the status of LA and

LApas

as low

activity

strains

(no

matings

within the first 30

_min).

The

_remaining

strains showed a

gradual

increase in

male

_mating

_activity,

from 25.0 and

52.0%

for the more

_recently

established strains

LA6- and

LA6‘!

to 76.6 and

94.0%

for HA and

LA

+

which had been established earlier.

Activity

and

_repression

abilities of hobo elements

hobo-mediated

_genetic

_instability.

The

hobo-activity potential

of the strains was

determined

_by

the GD

_sterility

assay

using

the females from the E strain

_CyL/Pm

as tester

_strains,

a

_high

_percentage

of

_{dystrophic gonads}

in the _progeny of these crosses

being

indicative of a

higher

hobo-mediated

dysgenic

induction. The

hobo-repression

potential

was determined in a similar _way, this time

_crossing

females

from the LA derivatives with males of the H-strain

_23.5MRF/CyL

₄

_.

In this _case, the

_higher

the

_proportion

of

_{atrophic gonads}

among female progeny, the lower

the

_potential

of the strain under

investigation

to repress the action of hobo. In

a test cross of

CyL/Prrz

females to

_23.MRF/CyL

₄

_males,

the GD

_sterility

among

150 female _progeny reached

_{90%, confirming}

that the latter still retained its

_strong

hobo-activity

potential.

As a

control,

intrastrain GD

sterility

_assays were

performed

and,

with the

_{only exception}

of

_LA,

where intrastrain

_sterility

reached a moderate

rate of about

_12%,

the

_proportion

of

_{atrophic gondas}

in all other strains did not exceed the

_background

levels

_(0-2.5%,

table

_I).

For both

_experiments,

in control

reciprocal

crosses, the

proportion

of

_atrophic

gonads

was _very low if _any

(0-1.6%,

table

I),

thus

_implying

that the hobo elements are the most

_probable

causative factors of the GD-

_sterility

in

_experimental

crosses.

The data

_presented

in table I show that low

_activity

strains LA and

_LApas

_appeared

to have similar moderate

_{hobo-activity potential}

_(32.6

and

36.5%

of

_{atrophic gonads,}

respectively;

t =

_0.66,

_p >

_0.05),

while the intermediate strains LA6‘ and

LA6

+

and the

_high

_activity

strains HA and

LA

+

_apparently

lost their induction

_potential

sometime after. their establishment

_(GD

_sterility

=

0-0.8%).

The

hobo-repression

potential

was

_lowest,

_though

_different,

for LA and

LApas

(47.0

and

38.0%

of

atrophic

gonads, respectively;

t =

2.18,

p <

_0.05).

Strains LA6- and

LA6

+

showed

similar

_{hobo-repression}

potentials

(GD

sterility

= 25.0 and

23.0%,

respectively;

t =

0.25,

p >

_0.05),

which were

_{significantly higher}

than that of

_LApas

_(t

=

2.06,

p < 0.05 and t =

2.19,

p <

0.05,

respectively).

The

strongest

hobo-repression

potentials

were observed in

LA

+

and HA

(19.8

and

14.0%

of

_{atrophic gonads,}

respectively;

t =

_1.63,

_p >

_0.05),

values that are not

_{significantly}

different from that observed for

LA6‘!

_(t

=

0.63,

p > 0.05 and t =

_1.44,

p >

_0.05,

_{respectively).}

Composition

of hobo elements

Genomic DNAs of the strains were

digested

with Xho I restriction

endonuclease,

which has

_recognition

sites close to both ends of the

_complete

hobo

_element,

_yielding

a characteristic

_fragment

of 2.6 kb after Southern-blot

hybridization

with a hobo

probe.

In

_addition,

Xho I

_digests

also

_{produce fragments}

of smaller molecular

weight, corresponding

to different internal deletion derivatives of hobo

_(Streck

et

_al,

1986).

As a

_control,

23.5MRF/CyL

4

flies

were also

_analyzed

(fig lA),

_showing

their

characteristic

_pattern

of

_{hybridization}

_(Stamatis

et

_al,

_1989),

while as

_expected,

no

hybridization corresponding

to either

_complete

or deleted hobo elements was seen

in the

corresponding digests

of the

_CyL/Prn,

strain

_{(fig lA),}

_confirming

that it is a

bona

_fide

E strain

_(Kaidanov

et

_al,

_1991).

All tested LA derivatives

_appeared

to _possess the 2.6 kb

_{fragment indicating}

the presence of

full-length

hobo elements in their genome. On the other

hand,

the

pattern

of hobo-deletion derivatives was

_{qualitatively}

and

_{quantitatively}

different

2.2 kb

long.

In the LA6‘ the most

_prominent

band also

corresponds

to a

_length

of 1.7 kb and there is a _very weak band of 1 kb. The most abundant

_fragment

in the 2

_high

_activity

strains

_(HA

and

_LA

₊

₎

has a

length

of 1.15 kb with an additional

intense band of 1.4 kb

_appearing

in

_digest

of HA as well as numerous weaker bands.

Finally,

the

_only

intense band observed in

LA6

+

is 0.9 kb

_long.

hobo elements copy number and their distribution on the

_polytene

chromosomes

To determine the copy number of the hobo elements and their location in the

genome of the strains under

investigation

we

_performed

an in situ

_{hybridization}

of a labelled hobo element to

_polytene

chromosomes.

_Squashes

were

_prepared

from

mass

_matings,

or from 2 vials

(families)

for each of the strains

_kept

in families.

The _copy number of hobo elements and the sites of their

_cytological

location were

analysed

for each individual larva. The data from this

_experiment

are

presented

in table II. Both intrastrain and interstrain variation in _{copy number} and their

location are characteristic for all strains. In low

_activity

strains LA and

LApas

the

number of hobo elements were between 12 to 15 and 10 to 22

_{respectively,}

while in the intermediate

_activity

strains LA6‘ and

LA6

+

_they

were from 8 to 11 and

from 13 to

_15,

_{respectively.}

Both

_high

_{activity strains,}

HA and

_LA

₊

_,

showed a

considerably higher

copy number

(40-42

and

36-37,

respectively).

The

_proportion

of

_polymorphic

sites

_(expressed

as the ratio of the number of sites

varying

among individuals from one

_strain,

to the total number of sites revealed within the same

_strain)

also

_appeared

to be different between the strains

_(table

_II).

Quite

low

_polymorphism

was observed in strains

_kept

as individual families with

selection. HA had

12%

polymorphic

sites,

the

_{corresponding}

number for LA+ was

24%,

and a

higher

value

(47%)

was observed for LA-. On the other

hand,

the

strains

_{kept by}

mass

_matings

without selection

(LApas,

LA6- and

LA6

+

)

had even

higher

values

_(69,

56 and

63%

of

_polymorphic

sites,

respectively).

Analysis

of

_cytological

locations of hobo elements revealed

that,

in the course of selection and

_subsequent

maintenance of the

_strains,

their hobo elements underwent

massive

transpositions.

Only

LA and

_LApas

share 9 common sites among a total

of 46 sites for both strains

_(or

_20%),

while all other

_{pair-wise comparisons}

revealed

only

2-5 common sites _among a total of 33-84

_sites,

or

3-12%

_{(table III).}

Another

strain with decreased

_activity,

_LA6-,

shared

_only

2

_(6.5%)

and 4

_(8.9%)

sites with LA and

_{LApas, respectively. Only}

2 sites at 17E and 96E were common to all 3. The

_high

_activity

strains

_HA,

_LA

₊

_,

and

LA6‘!

had 2-5 sites in common

(3.0-7.7%),

and there were no sites that were shared

_by

all 3. The most

_frequent

sites _among all strains

_analyzed

were

_{17A, 38C,}

67A and 96E.

DISCUSSION

In a

_previous

paper

(Kaidanov

et

al,

1991)

we showed that the D

_{melanogasterstrain}

LA,

established from a natural

_population

_by

close

_inbreeding

and selection for low

male

_{mating activity,}

harboured active hobo elements in its _genome. We extended this

_analysis

to encompass LA-derivative strains with altered male

_{mating activity,}

and in this

_report

we

_present

evidence that the hobo elements in these strains

underwent considerable

_changes

in their

_dysgenic

_{properties, composition}

and _copy

number,

accompanied by

extensive

_{transpositions.}

Both low

_{activity strains,}

LA and

_LApas,

_preserved

_dysgenically

active hobo ele-ments

_resulting

in similar intermediate

_{hobo-activity}

and

_{hobo-repression}

_potentials,

as revealed

_by

the GD

_sterility

_assays.

_They

also _appear to _possess

_quite

similar

copy numbers of hobo

elements,

and the differences observed in the

composition

of

hobo elements

_(notably

the _presence of the

_prominent

band of 1.7 kb in

_LA)

and in

their locations in the _genomes seem to have no direct effect on

_dysgenic

_properties

of both strains.

In the strains with intermediate

_(LA6-

and

_L!46!)

and

_high

male

_{mating activity}

(HA.

and

_LA

₊

₎

we have found the full absence of

_{hobo-activity}

and a considerable

strains

_LA6‘!,

HA and

_LA

₊

_,

were

_{accompanied by}

a drastic

_{recomposition}

of

hobo-deletion

derivatives,

which

_appeared

to be

_completely

different from the

_complement

presence or absence of a deletion derivative and the

_{dysgenic properties}

and fitness

of the strains under

_{investigation.}

For

_example,

LA and LA- have

_quite

similar

composition

of hobo

_elements,

_yet

_they

differ

_markedly

in both characteristics.

Similarly,

LA

+

and

LA6‘!

have similar

_dysgenic

_properties

and fitness but exhibit

strong

differences in the restriction

_patterns

of the hobo elements. More

_experiments

are

_clearly

needed to assess the

_impact

of different members of the hobo element

family

in the

_dysgenic

_properties

of a

_given

strain.

While LA6- and

LA 6

+

did not differ

_considerably

from LA and

LApas

in terms of the copy number of hobo

elements,

in HA and

LA

+

we observed an

amplification

of

the members of this

_family.

Not

_{unexpectedly,}

all these _processes were

_accompanied

by

massive

_{transpositions}

of hobo elements.

_Although

the in situ

_{hybridization}

data

are

_limited,

we could not detect _any

_{specific transpositions}

of hobo elements that would be

coupled

to

_changes

in the

_dysgenic

properties

or the fitness of the strains.

selection for

_high

and low fitness

_(Pasyukova

et

_al,

_1986),

we observed an almost

complete

reshuffling

in the

_cytological

localization of hobo elements. Our data may

imply

that the process of

change

in male

mating

activity

of the strains also

_changed

some

_genetic

mechanisms

_regulating

the

_activity

of hobo elements.

In

_spite

of the fact that hobo elements have been studied for almost a

_decade,

little is known about the mechanisms

_regulating

their

activity

in the genome. Unlike

active P

_elements,

which are

fully

_suppressed

_by

a P

_cytotype,

and are inactive in

the

_germline

and can be mobilized under normal conditions

_(ie

without

_considering

strains

_{containing engineered}

P

_elements)

_only

in

_dysgenic

crosses

(see

Engels,

1989),

hobo elements can

_transpose

within the strains known to contain active hobo elements without a need for outcrosses

_(Blackmann

et

al, 1987;

Hatzopoulos

et

_{al, 1987; Lim,}

₁₉₈₈₎

and

_they

seem to also be active in somatic cells

_(Kim

and

_Belyaeva,

_1991).

The fact that hobo may

play a significant

role in

population

biology

is also

_exemplified

_by

the

_finding

that these elements have been found to be located on the

_breakpoints

of

_naturally

occurring

inversions

_(Lyttle

and

_Haymer,

1992).

It is not

_surprising

that most of the D

_medanogaster

natural

_populations,

although

possessing

both

_complete

and deleted hobo elements in their genome,

have

_completely

lost the

_ability

to activate them in

_dysgenic

crosses

(no

hobo-activity

potential)

while

_strongly

_suppressing

the

_activity

of

_foreign

hobo elements

(high

hobo-repression

potential) (Pascual

and

_P6riquet,

_1991).

This _may

_imply

that some

_genetic

mechanisms

_develope

_{to suppress} the

_activity

of hobo elements

and thus

_prevent

deleterious _consequences of their

_{transposition}

_(lethal

mutations and chromosomal

_{rearrangements).}

These mechanisms may involve the effects of

some

_specific

hobo-deletion derivatives that are wide

_spread

in natural

_populations

(P6riquet

et

al, 1989;

1990)

but also other

_{genetic factors, acting}

in both maternal

and

_zygotic

fashion

_(Ho

et

_al,

_1993).

Some recent

_findings

also

_imply

that these mechanisms can be

_flexible,

as the

_{hobo-activity}

and

_{hobo-repression potentials}

in natural

_populations

can _vary

_seasonally

and

_{change rapidly}

when wild strains are

brought

into a

_laboratory

environment

_(Zabalou

et

_al,

_1991).

Considering

this,

we can assume that the selection for low fitness _may have

somehow

_prevented

the full

_development

of the mechanisms

_suppressing

the

_activity

of hobo

elements,

as is the case in LA. This would have resulted in the

_{transposition}

of hobo elements within the

_strains,

revealed as

_{heterogeneity}

in their locations in the _genome of several individuals

_(as

a

_possible

source of

_genetic

variability

under

the conditions of

_highly

unfavourable direction of

_selection)

and their activation

in

_dysgenic

crosses. Selection for the

high

fitness

(or

its

_spontaneous

increase)

eventually

derepressed

the formation of

_genomic

mechanisms

_controlling

the hobo elements’

_activity.

The fact that all LA-derived strains with increased fitness have

_completely

different

_composition,

_copy number and

_genomic

distribution of hobo elements and at the same time exhibit an absence of

hobo-activity

and

increased

_{hobo-repression potentials}

_point

to the

_predominance

of these

non-hobo-mediated

_regulatory

mechanisms over the element’s own contribution. The

considerable increase in copy

number,

classes of deletion derivatives and intrastrain site

_polymorphism

of hobo elements that

accompanied

the formation of

repression

sterility

assay

(AP

Galkin and LZ

Kaidanov,

unpublished

observations).

This may

have created the

_diversity

in the content of hobo elements observed in the HA and

LA

+

strains.

_Experiments

are in _progress to monitor the

_changes

in hobo

_properties

during

the course of selection of LA from low to

_high

male

_{mating activity}

and the

impact

of different chromosomes in this process.

ACKNOWLEDGMENTS

We are indebted to V Yu

_Sergienko

and OV Iovleva for their technical assistance as well as to G

_Yannopoulos

for

_helpful

discussions. This work was

_{supported by}

and EMBO East

European Fellowship

to

VNB, by

grants from the Frontiers in Genetics programme of the Russian

Academy

of Sciences to

_APG,

VAG and LZK and

_by

_grants from SCIENCE programme of the

European

Communities

(Sci 0171-C)

and the Hellenic Secretariat General for Research and

_Technology

to CL.

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