Analysis of larval behaviours underlying the pupation height phenotype in Drosophila simulans and D melanogaster

HAL Id: hal-00894189

https://hal.archives-ouvertes.fr/hal-00894189

Submitted on 1 Jan 1997

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of

sci-entific research documents, whether they are

pub-lished or not. The documents may come from

teaching and research institutions in France or

abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est

destinée au dépôt et à la diffusion de documents

scientifiques de niveau recherche, publiés ou non,

émanant des établissements d’enseignement et de

recherche français ou étrangers, des laboratoires

publics ou privés.

Analysis of larval behaviours underlying the pupation

height phenotype in Drosophila simulans and D

melanogaster

P Casares, Mc Carracedo, L García-Florez

To cite this version:

Original

article

Analysis

of

larval

behaviours

_underlying

the

_pupation

_height

_phenotype

in

Drosophila

simulans and D

_melanogaster

P

Casares MC Carracedo

L

García-Florez

Departamento

de

Genetica,

Facultad de

Medicina,

Julian Claveria

_s/n,

33071

Oviedo, Spain

(Received

21 October 1996;

accepted

27

_August

₁₉₉₇₎

Summary - Lines of

_{Drosophila melanogaster}

and D

_{simulans, previously}

selected for increased and decreased

_{pupation height,}

have been

_investigated

for larval correlated

responses to selection. Larvae of all lines and

_species

showed

_{higher pupation}

sites when

humidity

increased from 40 to 100% RH.

_{Pupation height}

measured in

_light

and in dark revealed that

only

one out of the nine lines

_changed

its

_phototactic

behaviour

during

selection. In both

species,

the

high pupation

lines showed greater

mobility

than the

corresponding

controls. Selection for

_{high pupation}

sites diminished the

_digging

behaviour

in D simulans but not in D

_{melanogaster,}

whereas selection for low sites

_augmented

the

percentage of

_digging

in D

_{melanogaster.}

A different

_{correspondence}

between

_pupation

height

and

_gravity

_response was found in each species. In D

_simulans,

low lines were

geopositive

and

_high

lines

_neutral,

while low lines were neutral and

_high

lines

_geonegative

in D

_{melanogaster.}

The results indicate that

_pupation

_height

is a

complex

trait determined

by

other

_{simpler behaviours,}

so that a

_{given phenotype}

can be

_{produced by}

different

genetic

systems. This

_question

stresses the

_difficulty

of

_deducing,

on

_{biological grounds,}

the

_meaning

of the

_genetic

architecture of a

complex

behavioural trait whose

underlying

basic mechanisms are unknown.

artificial selection

_/

_complex behavioural trait

/

correlated response

/

larval pupation

height

/

Drosophila

Résumé - _Analyse des comportements larvaires pour la hauteur de pupaison chez

Drosophila simulans et D melanogaster. Les

réponses

larvaires liées à la sélection ont été

analysées

dans des

lignées

de

Drosophila melanogaster

et de D simulans

_{préalablement}

sélectionnées _pour

_{l’augmentation}

ou la diminution de la hauteur de _pupaison. Les larves de toutes les

lignées

et

_espèces

ont eu des sites

_plus

élevés de

_{pupaison quand}

l’humidité relative a

_augmenté

de

₄₀

à 100 %. La hauteur de pupaison mesurée à la lumière ou

à l’obscurité a montré _que seulement une des

_{neuf lignées}

a

_changé

son _comportement

phototactique pendant

la sélection. Dans les deux

_espèces,

les sites de

_pupaison

_plus

élevés

ont

_correspondu

à une

_{plus grande}

mobilité des larves. La sélection _pour les sites élevés de pupaison a diminué le _comportement de creusement chez D simulans mais _pas chez

D

_{melanogaster,}

tandis

que la

sélection pour les sites bas a

augmenté

ce _comportement

réponse

à la

_gravité

chez les deux

_espèces.

Chez D

_simulans,

les

_lignées

basses ont été

géopositives

et les

_lignées

hautes ont été neutres, tandis que, chez D

melanogaster,

les

lignées

basses ont été neutres et les

_lignées

hautes

_{géonégatives.}

Les résultats

_indiquent

que

la hauteur de pupaison est un caractère

_complexe

déterminé _par d’autres conduites

_plus

simples,

de sorte

_{qu’un phénotype}

donné peut résulter de

_{différents systèmes génétiques.}

On doit insister sur ce _point si l’on veut

obtenir,

à _partir de considérations

_biologiques,

l’analyse

de l’architecture

_génétique

d’un caractère de comportement

complexe, quand

les mécanismes de bases ne _{sont pas} connus.

sélection artificielle

_/

caractère complexe de comportement

/

réponse corrélée

_/

hauteur de _pupaison larvaire

_/

_drosophile

INTRODUCTION

There exists

_today

a lot of information

_describing

various

_aspects

of

_Drosophila

activities in both larvae and adults

_(see

_{Grossfield, 1978; Spieth}

and

_Ringo,

_1983).

Most of the activities can be described as behaviours and in this sense have been studied from a

_{genetic perspective.}

In some _cases,

_however,

the studies are unable

to reveal the

_genetic

basis

_underlying

a

_{specific behaviour, particularly}

when a trait

is the result of several

_single

behaviours

_{(eg, taxes)}

_acting

in response to a set of stimuli

_{simultaneously}

_{perceived by}

the

_organism.

We have studied in

_depth

a

_pre-adult

behaviour observed in

_Drosophila

melanogaster

and D simulans: the

_pupation

behaviour. Once

_developing

larvae reach the third

_stage,

_they

leave the humid food

_searching

for a

dry

site in which

to

_pupate.

In the

_laboratory,

_{drosophila develop}

in

_glass

vials with food at the

bottom,

and

_pupation

occurs

_mostly

on the vertical walls at

_varying

_heights.

Pupa-tion

_height

is

_strongly

affected

_by

biotic and environmental

_variables,

such as larval

density

(Sokal

et

_al,

_1960;

_{Barker, 1971;}

Casares and

_{Rubio, 1984;}

Casares and

Car-racedo,

1984a),

light

(Rizki

and

_{Davies, 1953; Markow, 1979; Manning}

and

Markow,

1981),

humidity

(Sameoto

and

_Miller,

_1968;

Casares and

_Carracedo,

_1984b),

_etc,

and also

_by

_genetic

determinants

_(Markow,

_1979;

_Ringo

and

_{Wood, 1983;}

Casares and

Carracedo, 1986a, b;

Garcia-Florez et

_al,

_1989).

The interest of

_studying

pupation

behaviour in the

_laboratory

is

_{supported by}

the

_parallelism

between the

pupation

sites observed in an

_orchard,

and the

_{corresponding height}

attained in the

laboratory

(Sokolowski,

1985;

Sokolowski et

_al,

_1985).

The choice of a

_pupation

site in the

_laboratory

has

_important

fitness

repercus-sions,

since

_mortality

_{is very}

_high

for _pupae located in the humid food

_(Sameoto

and

Miller,

1968;

Wallace, 1974;

Casares and

_Rubio,

_1984).

In this sense, we have found

that

_populations

of D

_{melanogaster prefer}

to

_pupate

on the vial walls and rather

higher

than

_populations

of the

_sibling,

_{sympatric species}

D

_simulans,

which

_prefer

to

_pupate

in the food or

_neighbouring

sites. This

_{species difference,}

which

_greatly

affects their

_pre-adult

fitnesses

_(Casares

and

_Rubio,

_1984),

must be maintained in nature

_by

some kind of natural

_selection,

as deduced from two

_independent

artificial selection

_experiments,

in which we have been able to increase

_pupation

height

in D simulans and to increase or decrease it in D

_melanogaster

_(Casares

and

Carracedo,

1986b;

Garcia-Florez et

_al,

_1989).

The lines obtained after artificial selection are useful material with which to

or behaviours involved in the observed

_high

and low

_pupation

_{height phenotypes ?}

In the first

place,

one could

_suspect

that artificial selection could have

_changed

the larva’s

_perception

of

_humidity

in the

_high

_and/or

low

_pupation

_height

_lines,

since

_humidity

is the main factor

_accounting

for

pupation height

in the

_laboratory

(Sameoto

and

Miller, 1966, 1968; Mensua,

1967;

Wallace,

1974).

Also,

perception

of

_light

_(usually

_placed

above the

_animal)

and

_gravity

could have been modified

by

selection,

in this way

affecting

the

displacement

of the larvae to the

top

or

the bottom of the vials.

_Finally,

larval locomotion could be

_important

in

_pupation

behaviour because larvae from the

_high

_pupation

lines must travel a

_greater

distance

than those from the low lines to reach the

_{highest places}

in the vials.

In the

_present

work we have examined the influence of

_{humidity, light}

and

_gravity

on the larval behaviour of lines selected for

high

and low

_pupation

sites. We have also

examined whether selection has

_brought

about correlated

_changes

in two other well-known larval behaviours:

_digging

_{(Godoy-Herrera, 1978)}

and

_foraging

_(Sokolowski,

1980).

MATERIALS AND METHODS

The material consisted of lines of D simulans and D

_melanogaster

from two

exper-iments of selection for

_high

_(H)

and low

_(L)

_pupation

sites.

In D

_simulans,

selection for

_increasing

_pupation

_height

was

_{successful, giving}

rise to the

_high

lines SH1 and SH2

_(Casares

and

_Carracedo,

_1986b)

with means

of

_{approximately}

110 mm of

pupation height. Response

to selection for

_decreasing

pupation

height

was not

statistically

successful. The two low

lines,

SLI and

SL2,

had mean

_pupation

_heights

of about 25 mm. Selection was

_{accomplished using}

36 or more

_{mating pairs}

_per line

_{along eight}

_generations.

The

_inbreeding

coefficient of the

_resulting

selection lines was calculated to be less than F = 0.084 in the

_high

lines

_{(Falconer, 1989)}

and even smaller in the low lines.

In D

_{melanogaster,}

selection succeeded in both directions

_{(Garcia-Florez}

et

_al,

1989),

the

_pupation

_heights

of the

_high,

low and control lines

_being

around

_120,

4 and 15 mm,

respectively.

The two

_high

and two low selection lines are called

MH1,

MH2 and

_{ML1, ML2, respectively.}

The base

_population

MC is also used here as a

control. In this

_species,

selection was carried out

_using

at least 48

_{mating pairs}

_per line

_during

11

_generations,

and

_inbreeding

of the selection lines was calculated as

F = 0.076 for the

_high

lines and smaller in the low lines.

In all

_experiments

detailed

below,

the culture medium was

_{prepared by boiling}

bakers’

yeast

(20%),

sugar

(5%)

and agar

(1.4%)

in water, and

_{adding propionic}

acid

_(0.5%)

as a mold inhibitor.

Experiment

I: influence of

_humidity

Virgin

flies from each line were collected

immediately

after

eclosion,

then sexed and

aged separately

for 3

_days.

On

_day

_4,

mature males and females were allowed to

mate,

then groups of 15-20 females were

placed

for 6 h in

_petri

dishes

_containing

a thin

_layer

of food to

_lay

_eggs. Around 9 000

_pairs

of flies were used. From the

It has been found that the

_higher

the larval

_density,

the

_greater

the

_tendency

of food to

_liquefy

and release water as larvae eat it

_(Sokal

et

_{al, 1960;}

Sameoto

and

_{Miller, 1968; Wallace, 1974;}

Casares and

_Carracedo,

_1984a).

Because the vials

exchange

water with the

_{outside, humidity}

on the inside can be controlled in an

approximate

manner

_by

_{using plugs}

of different size and texture

_(Mensua,

_1967;

Casares and

_Carracedo,

_1984b).

In the

_present

_experiment,

we established three different

_humidity

levels -

_low,

intermediate and

_{high -}

as follows: we used vials 25 mm in diameter and 200 mm

in

_length,

with standard bakers’

_yeast

food

₍₂₅

mm in

_height).

Low

_humidity

was

achieved

using

unplugged

vials that facilitated the

_interchange

of water with the

climatic chamber in which all the vials were

_kept

_(40%

_RH);

_humidity

in these vials was

_low,

as deduced from the

_dryness

of the vial walls

_during

_pupation.

To achieve

_{high humidity,}

some vials were

_plugged

under _pressure with foam dishes 35 mm in

_{diameter, through}

which water

_interchange

with the outside was difficult.

During

pupation

in these

vials,

a thin

_layer

of water

_covering

the walls to a

_height

of around 90 mm above the food was

_{observed, suggesting}

that relative

_humidity

inside the vials was close to

_100%,

a value which was similar to that observed in the selection

_experiments

from which the lines

_originated.

Intermediate

_humidity

was

_{accomplished by}

_plugging

vials with foam dishes 25 mm in

_diameter,

the water

layer

attaining

here around 15 mm in

_height

and relative

_{humidity supposedly being}

between 40 and

100%.

Sixteen vials were used for each line and

humidity.

Nine

_days

after

_seeding,

all larvae had

_pupated

and the

_height

of the pupae in the

vials was measured with a

_transparent

_{plastic cylinder}

that was

_laterally

_printed

with

_marking

ink into 10 mm divisions. The

_cylinder

was

_externally

attached to the

vials,

allowing

a

_quick

classification of _{pupae into} different

_height

classes

_(Casares

and

_Carracedo,

_1986a).

The mean value of

_pupation

_height

_per vial was used as the raw measure in all statistical

_analyses.

Experiment

II: influence of

_light

Cages

(40

x 40 x 30

_cm)

made of wood

_{(two sides)}

and

_glass

_{(four sides)}

were used.

A hole 8 cm in diameter was made in the two wooden sides. In half of the _cages, the

_glass

walls were

_perfectly

sealed with matt black _paper, so that total darkness

was achieved inside. In the other _cages,

_light

_penetrated

_through

the

_glass.

Forty-two vials

₍₁₀

x 2

_cm)

_per line were seeded with five first instar larvae

_following

the

procedure

detailed in

_experiment

I. Half of the vials were introduced

_through

the hole into the

_lighted

_{cage and the other half into the dark cage. The holes} were

plugged

with dense foam

_cylinders.

The cages were incubated side

_by

side in a

chamber at 21 °C with constant illumination.

Pupation

height

was measured as in

experiment

I.

Experiment

III: larval

_mobility

Adult flies from each line were sexed and

_aged

as detailed earlier.

_Eight

_pairs

of

ma-ture,

3-day-old

virgin

flies were

_put

into a vial

(12

x 2.5

_cm)

with food and allowed

to mate and

_oviposit

for 24 h and then discarded. Five

_days

_later,

one larva of the third instar was

carefully

_picked

_up, washed in distilled water for a few

_seconds,

Previously,

a thin

_layer

of _agar medium

(1.2%

_{agar in} distilled

water)

slightly

stained

with

_methylene

blue had been

_poured

into the dishes. Larvae were left for 15 s

to recover from

_{manipulation.}

_Mobility

was measured in individual larvae

by

the

number of forward and backward movements in 60 s

following

the

methodology

of

Sokolowski

_(1980).

Observations were carried out under a

_{stereomicroscope}

at 21 °C.

_Fifty

D

simu-lans larvae and 70 D

_{nzelanogaster}

were scored from each line.

Experiment

IV:

_digging

behaviour and effect of

_gravity

Third instar larvae were obtained and

petri

dishes

prepared

as described in

experiment III,

with the difference that a softer medium with

1%

_agar was used here

to facilitate larval

_digging.

Two tests were

performed simultaneously.

In one

_test,

five larvae were

placed

in the centre of the

dish,

observed for 60 s and classified as

diggers

or

_non-diggers.

Two hundred larvae _per line were

_assayed.

In the other

_test,

all

things

were

similar,

_except

that once the five larvae were

placed

in the

dish,

this

was inverted and

_put

_upside

down under the

_microscope;

in this _case, observations

were made

_through

the bottom wall of the dish. One hundred larvae _per line were

measured in this test. Thus in one test

_digging

was favoured

_{by gravity,}

while in

the other test the

_opposite

was true.

RESULTS

Experiment

I: influence of

_humidity

No differences in larval to adult

_viability

and

_development

time were detected

between the three humidities. The two

_replications

of each selection line in each

humidity

were

_compared

_by

a Student’s t-test. Since all

_comparisons

were

non-significant

(data

not

_shown),

the two

_replications

were

_pooled.

The

_pupation

_height

values so obtained are

_given

in

_figure

1. In both

_species

and in all

_lines,

_height

was

much

greater

at

_increasing

humidities. Within each

_{line, comparisons}

of

_pupation

height

between humidities were carried out

_by

_one-way

_analyses

of variance

_(Sokal

and

Rohlf,

1981).

All tests were

_significant

_(results

are not

_shown),

as could be

expected

from the

_large

differences in mean values and the small standard errors

that appear in

figure

1. Not all lines

_responded

to increased

_humidity

with the same

intensity.

To _prove

_this,

we carried out a

_two-way

analysis

of variance with lines and

humidities as the sources of variation. As

_expected,

the line x

_humidity

interaction

was

_significant

(MS

interaction = 1 996.7 with 16

df;

MS _error 61.2 with 405

df;

F =

32.6,

P _<

0.001).

This

experiment

shows that larvae of both

_{species perceive}

and

_respond

to

changes

in

_{humidity regardless}

of their

_having

a

_phenotype

for

_high

or low

_pupation

sites.

_Although

between-line differences were

_generally

_greater

at the

_highest

humidity,

differences

persisted

at a relative

_humidity

as low as

40%

_indicating

that

Experiment

II: influence of

_light

Figure

2 shows the

_height

attained

_by

the larvae in

_light

and darkness.

_Pupation

height

was almost unaffected

_{by light.}

_Only

one out of the nine lines

_(the

ML1

line)

showed a

_{statistically}

different

_height

in the two environments

_(t-test

=

3.74;

df =

_40;

P _<

0.001).

This result

_strongly

_contrasts with that of Markow

(1979),

who stated that most

_populations

of D

_melanogaster

and D simulans exhibit

_higher

pupation

sites in dark conditions. On the other

hand,

Schnebel and Grossfield

₍₁₉₈₆₎

found that D

_melanogaster

_pupated

_higher

in the

_light

and that D simulans was

unaffected

_by

this factor. The

_{discrepancies}

between these authors and between their results and ours are

probably

due to the use of different

_{methodologies}

or

strains. Markow used different chambers for

_{measuring pupation}

_height

under dark and

_light

conditions. Whether

_humidity,

which is

_undoubtedly

the main factor

affecting

pupation

height

(Sameoto

and Miller

_{1966; 1968; Mensua, 1967; Wallace,}

1974;

Casares and

_Carracedo,

_1984b),

was the same in the two chambers is unknown. The same

_{problem might apply}

in the Schnebel and Grossfield

methodology,

for these authors used the same chamber for

_measuring

the two conditions but darkness was

_accomplished

in a box inside the chamber with the

_consequent

risk of a difference in

humidity.

In

_contrast,

as described

above,

our

light

and dark

environments were

_produced

_{simultaneously,}

in the same

_type

of

_chamber,

at the

The conclusion of this

_experiment

is

_that,

with the

_exception

of the MLI

line,

the larvae of the selection lines have a similar

_phototactic

_response

_during

_pupation.

Therefore,

the observed between-line differences in

pupation

height

are

_independent

of the presence or absence of

_light.

_Concerning

the ML1

_line,

the _response to

selection for low

_pupation

_height

was

_due,

at least

partly,

to indirect selection for a diminished

_phototactic

behaviour.

Experiment

III: larval

_mobility

The means and standard errors of larval

_mobility

of the different lines and

_species

are

depicted

in

figure

3.

One-way

_analyses

of variance demonstrated

significant

between-line differences in both

_species

_(MS

between = 2 739 with 3

df;

within = 237

with 196

_df;

F =

11.56;

P _< 0.01 for D

simulans;

between = 5178 with 4

df;

within = 245 with 345

df;

F =

_21;

P _< 0.01 for D

melanogaster).

The between-line differences

found,

although significant,

are not so

_great

as

those found in the ’rover-sitter’ larval

_polymorphism

described

_by

Sokolowski

(1980),

who found that the

_crawling

scores for sitters and rovers were around 35

and

140,

respectively,

in 6 min of observation. Since our scores are between 40

and 70 in

_only

1

_min,

it is clear that all our lines

_display

the rover

_behaviour,

although

this result must be taken with caution as our substrate differs from that

of Sokolowski in

_composition

and

_{density. However,}

Bauer and Sokolowski

₍₁₉₈₄₎

state that classification of larvae into

_only

two

_mobility

classes

_{(rover/sitter)}

is

an

oversimplification

that does not

_correspond

with the

_mobility

scores found in

A contrast between the

_high

and low lines of D simulans

_by

Scheffe’s method

(Sokal

and

Rohlf,

1981)

gave a

_significant

result

(contrast ±

95%

confidence limits:

11.15 !

_6.14)

and the same result for D

_melanogaster

when

_comparing

the control and the two

_high

lines

_{(13.18 ! 7.10).}

_Therefore,

the

_{high pupating}

lines of both

species

show

_greater

_mobility

than the

_{corresponding}

controls. This is an

_important

result because larvae

_pupating

in the

_highest

sites of the vial must travel a

_longer

distance than those

_pupating

in the lowest _ones, which

_suggests

that locomotion could

_{play a significant}

role in this larval

displacement.

The same

_applies

to the

comparatively

small

_mobility

showed

_by

the low

_pupation

line ML2.

Experiment

IV:

_digging

behaviour and effect of

_gravity

The

percentages

of

_diggers

of the different lines are shown in

_figure

4. It is clear

_that,

in D

_simulans,

_digging

was more

_frequent

in the low than in the

_high

lines: a STP

procedure

for

_homogeneity

_(Sokal

and

_Rohlf,

₁₉₈₁₎

_applied

to the four

_percentages

of

_digging

in normal food gave the

following:

where two lines not

_{joined by}

a

_straight

line are different at a

5%

or lower

_probability

level.

Thus,

selection for

_{increasing pupation}

_height

in D simulans was

_accompanied

by

a clear decrease in the number of

_digger

larvae.

In

_contrast,

the

_digging

behaviour of the

_high

lines of D

_melanogaster

was more

The

_analysis

now shows how selection for

high

_pupation

in D

melanogaster

did

not

_{modify digging,}

whereas it was increased when selection was for low

_pupation

sites. This

_species

difference in the selection _response

_suggests

that both

_species

have been

_subjected

to different selection pressures on

digging

behaviour in the

wild.

In _summary, artificial selection for

high pupation

sites decreased the

_digger

trait

in D simulans but not in D

_{melanogaster.}

In this

_{species, however,}

selection for low

pupation

height

was

_{accompanied by}

an increase in

_digging.

A common fact is that

in both

_species

the

_high

pupating

lines show less

_digging

than the low

_pupating

ones, a result that _agrees with data of

Godoy-Herrera

in D

melanogaster

(1978)

and in D

_pavani

and D

_gaucha

_(1986).

Another

_species

difference _appears when

_comparing,

_by

_{contingency chi-squares,}

the

_percentages

of

_digging

achieved in food

_placed

under or above the animal. In

D

_simulans,

the number of

_diggers

decreased in the two low lines when food was

above the animal

_(x

₂

=

6.99;

df =

1;

P = 0.008 for

SL1;

x

2

=

4.07;

P = 0.04 for

_SL2;

see

_fig

4).

This means that larvae are

_geopositive,

as

_digging

is favoured

if food is

_placed

under them. In D

_{melanogaster,}

the differences affect the two

high

pupating

lines

₍

_X

₂

=

4.22;

df =

_1;

P = 0.04 for

MH1;

X

Z

=

7.28;

df =

1;

P = 0.007 for

MH2),

in which food inversion increases the number of

diggers.

Here

this is

_interpreted

as larvae

being geonegative.

The

remaining

lines have the same

behaviour in normal and inverted

food,

which argues that their larvae are neutral

with

_respect

to

_gravity.

Interestingly,

a characteristic trait of D simulans

populations

is the

tendency

of

clear

contrast,

populations

of D

_melanogaster

_pupate

much

_higher,

which

_suggests

they

are

_geonegative

or neutral. It seems that in the selection

experiments

that

gave rise to the lines

examined,

selection for

high

pupation

sites in D simulans

eliminates the

_geopositive

_tendency

present

in the base

population

and in the low

lines,

whereas selection for low

_pupation

sites in D

_melanogaster

was

accompanied

by

selection of

_geopositive

larvae.

DISCUSSION

The

_study

of behavioural

traits,

because of their

_{singularity, requires}

researchers

to

_apply

careful control of environmental variables and to

_{perform well-designed}

experiments

that allow

_objective

measurements

_(Ehrman

and

_Parsons,

_1976).

Another

_question

to consider could be the

_complexity

of a trait. Most animal

behaviours,

before

_and/or

_during

their

_{manifestation,}

are influenced

by

a

battery

of stimuli that are

_{simultaneously perceived}

and

_{processed by}

the

_organism.

When

examining

a

_{given behaviour,}

the

question

is whether the

experimenter

is

_measuring

only

one well-defined behaviour or whether the _response of the animal is the result

of a

_complex

mixture of several other

_simpler

behaviours. An

_example

is offered

_by

Drosophila geotaxis.

After 181

_generations

of selection for

_{negative geotaxis}

in the

laboratory, Murphey

and Hall

₍₁₉₆₉₎

found that the flies had also been selected for resistance to starvation

_and/or

desiccation in the maze, reduced locomotor

activity

and low

_{claustrophobia}

levels. Thus an

_{apparently simple}

behaviour influences other

undetected behaviours that affect the final measure.

The above

_question

is of

_great

_importance

if one is concerned with the

_genetic

basis of a

_complex

behavioural trait. If the trait is based on other

_simpler

behaviours or

_taxes,

its

_genetic

basis could be difficult to determine since _any

_analysis

would

in fact be the

_analysis

of several traits

possibly

inherited to different extents

_(Hay,

1985).

On the other

_hand,

if different laboratories use different

_apparatus

or achieve

different control of the environmental variables

affecting

the

_{underlying simpler}

behaviours,

different

_genetic

architecture could be found

_by

different

_{investigators.}

In

_studying

_phototaxis

in

_Drosophila,

Rockwell and

_Seiger

₍₁₉₇₃₎

conclude that

there is no a

_priori

reason to suppose that one

particular design

is better to measure

’true’ wild

_phototaxia.

That is to _say, different environments could reveal different

genetic

systems.

In the

present

work we have found several correlated _responses to selection for

pupation

height.

Henderson

₍₁₉₈₉₎

_suggested

that differences between

_high

and low selection lines for traits other than the

originally

selected could be attributed

to

_genetic

drift instead of to a

_genetical

_relationship

with the selected trait. As

previously indicated,

our selection lines were not

_exposed

to severe

_drift,

and their

inbreeding

coefficient values have been estimated to be under

10%.

On the other

hand,

our

_hypotheses

for a causal

_relationship

between

_pupation

_height

and the

traits examined in the

present

work are

_reasonable,

and for some of them we have found

_strong

experimental

support

in the sense indicated

by

Henderson

(1989),

that

_is,

_predictions

are confirmed

_by

the differences exhibited

_by

the

_high

and low

lines in both D simulans and D

_melanogaster

_species.

For

_instance,

an increase in

locomotion was detected in four of the

_eight

selected

_lines,

and these were

_precisely

Therefore,

we can conclude that larval

_pupation

_height

is a

_complex

behaviour

influenced

_by,

at

_least,

_humidity,

_{locomotion, digging behaviour,}

and

_gravity.

Thus the larvae with

_{higher humidity}

_sensitivity

pupate

higher,

as well as those

_having

greater

locomotion. It is also evident that selection has modified the

_digging

behaviour of some lines

_through

changes

in

_{gravity perception}

_by

the

_larvae;

thus,

D

_melanogaster

larvae

_{showing high pupation}

are

_geonegative,

whereas low

pupating

larvae of D simulans are

_geopositive.

_Owing

to the mixed influence of these

factors,

it is not too

_surprising

that selection lines

_showing

the same

_pupation

height

phenotype

had been selected for different levels of

_{locomotion, gravity,}

etc. In this

way, different

genotypes

attained

through

modifications of very different

genetic

systems

produce

similar

_pupation

_{height phenotypes.}

This conclusion stresses the need for caution when

_interpreting

the results of

_genetic

_{analyses performed}

on

complex

behavioural traits whose

_underlying

basic mechanisms are unknown. It is

clear from our results that

_depending

on the larval behaviour more

_directly

involved

in the observed

pupation

height

in the

_laboratory,

the

genetic

analyses

would reveal

different genes.

Finally,

it is common to

_infer,

from the results of a

_genetic

analysis,

the

_type

of natural selection that has acted in the

_past

on the

_population

_(Broadhurst

and

Jinks,

1974).

In this _sense,

_general

conclusions drawn from a

_{single population}

about the nature of differences in a

_{complex trait,}

such as

_pupation

_height,

can be

_misleading

if natural

_populations

have variation for different behavioural

components,

and each author is

_measuring

different

_things.

ACKNOWLEDGMENT

This work was

_{supported by}

the

_Ministry

of Education and Science of

Spain

(DGICYT

Grant No

_PB94-1347).

REFERENCES

Barker JSF

₍₁₉₇₁₎

Ecological

differences and

_competitive

interactions between

Drosophila

melanogaster

and D simulans in small

_{laboratory populations.}

_lEcologia

8, 139-156 Bauer

_SJ,

Sokolowski MB

₍₁₉₈₄₎

Larval

foraging

behaviour in isofemale lines of

Drosophila melarcogaster

and D

_P

_{seudoobscura.}

J

_{Heredity 75,}

131-134

Broadhurst

_PL,

Jinks

_JL,

₍₁₉₇₄₎

What

_genetical

architecture can tell us about the natural

selection of behavioural traits. In: The Genetics

_{of Behaviour}

_(Van

Abeleen

JHF,

ed),

North-Holland, Amsterdam,

43-63

Casares

_P,

Carracedo MC

_(1984a)

_{Comportamiento}

de

_pupacion

en

_Drosophila

melano-gaster y D simulans. I.

Respuestas

ante el cambio de densidad y el

tipo

de alimento.

Rev Biol Univ Oviedo 2, 11-20

Casares P, Carracedo MC

_(1984b)

Comportamiento

de _pupacion en

_Drosophila

melano-gaster and D simulans. II. Influencia de la humedad en la altura de

pupacion.

Rev Biol

Univ Oviedo

_2,

21-28

Casares

_P,

Carracedo MC

_(1986a)

Genetic variation in pupation

height

in a

_population

of

_Drosophila

simulans. Genetica

_70,

17-22

Casares

P,

Carracedo MC

_(1986b)

On

selecting

for

pupation height

in

_Drosophila

Casares P, Rubio J

₍₁₉₈₄₎

_{Competencia interespecifica}

entre los

_preadultos

de

_Drosophila

melanogaster

y D simulans. Medio Ambiente 7, 3-8

Ehrman

_L,

Parsons PA

₍₁₉₇₆₎

The Genetics

_of

Behavior. Sinauer

_{Ass, Sunderland,}

Massachusetts

Falconer DS

₍₁₉₈₉₎

Introduction to

Quantitative

Genetics.

Longman

Sci &

Tech,

Third ed.

Harlow, England

Garcia-Florez

_L,

Casares

P,

Carracedo MC

₍₁₉₈₉₎

Selection for

_pupation

_height

in

Drosophila melanogaster. Genetica, 79,

155-160

Godoy-Herrera

R

₍₁₉₇₈₎

Selection for

_digging

behaviour in

_{Drosophila melanogaster}

larvae. Behav Genet

_8,

475-479

Godoy-Herrera

R

₍₁₉₈₆₎

The

_development

and

_genetics

of

_digging

behaviour in

_Drosophila

larvae.

_Heredity

56, 33-41

Grossfield J

₍₁₉₇₈₎

Non-sexual behavior in

_Drosophila.

In: The Genetics and

_{Biology of}

Drosophila,

vol 2a

_(M

Ashburner,

TRF

_Wright,

_eds),

Academic

_Press,

London

Hay

DA

₍₁₉₈₅₎

Essentials

_{of Behaviour}

Genetics. Blackwell Sci

Publ,

Melbourne Henderson ND

₍₁₉₈₉₎

_Interpreting

studies that compare

high-

and low-selected lines on

new characters. Behav Genet

_19,

473-502

Manning A,

Markow TA

₍₁₉₈₁₎

_{Light-dependent pupation}

site

_preference

in

_Drosophila.

II.

_{Drosophila melanogaster}

and D simulans. Behav Genet 11, 557-563

Markow TA

₍₁₉₇₉₎

A survey of intra- and

interspecific

variation for

pupation height

in

Drosophila.

Behav Genet

9,

209-217

Mensua JL

₍₁₉₆₇₎

Some factors

affecting pupation height

in

_Drosophila.

Dros

_{Inf Serv 42,}

76

Murphey

RM, Hall CF

₍₁₉₆₉₎

Some correlates of

_{negative geotaxis}

in

_Drosophila

melanogaster.

Anim Behav

_17,

181-188

Ringo JM,

Wood D

₍₁₉₈₃₎

_Pupation

site selection in

_{Drosophila melanogaster.}

Behav Genet 13,17-27

Rizki T, Davis C

₍₁₉₅₃₎

_Light

as an

_ecological

determinant of

_{interspecific}

_competition

between

Drosophila

willistoni and D

_{melanogaster.}

Am Nat

_87,

273-278

Rockwell

RF, Seiger

MB

₍₁₉₇₃₎

Phototaxis in

Drosophila:

A critical evaluation. Am Sci

61,

339-345

Sameoto

DD,

Miller RS

₍₁₉₆₆₎

Factors

_controlling

the

_productivity

of

_Drosophila

melano-gaster and D simulans.

_Ecology

47, 695-704

Sameoto

DD,

Miller RS

₍₁₉₆₈₎

Selection of

pupation

site

_{by Drosophila melanogaster}

and

D simulans.

Ecology 49,

177-180

Schnebel

EM,

Grossfield J

₍₁₉₈₆₎

The influence of

light

on

_{pupation height}

in

_Drosophila.

Behav Genet 16, 407-413

Sokal

RR,

Erhlich

PR,

Hunter

PE, Schlager

G

₍₁₉₆₀₎

Some factors

affecting pupation

site in

_Drosophila.

Ann Ent Soc Am 53, 174-182

Sokal RR, Rohlf FJ

₍₁₉₈₁₎

Biometry.

WH Freeman and

Co,

San

Francisco,

2nd ed

Sokolowski MB

₍₁₉₈₀₎

_{Foraging strategies}

of

Drosophila melanogaster:

A chromosomal

analysis.

Behav Genet 10, 291-302

Sokolowski MB

₍₁₉₈₅₎

Genetics and

_ecology

of

_{Drosophila melanogaster}

larval

_foraging

and

_pupation

behaviour. J Insect

_{Physiol 31,}

857-864

Sokolowski

_MB,

Bauer

_{SJ, Wai-Ping V, Rodriguez L, Wong}

JL, Kent C

₍₁₉₈₅₎

Ecolog-ical

_genetics

and behaviour of

_{Drosophila melanogaster}

larvae in nature. Anim Behav

34, 403-408

Spieth HT, Ringo

JM

₍₁₉₈₃₎

Mating

behavior and sexual isolation in

_Drosophila.

In: The Genetics and

_{Biology of Drosophila}

_(M

Ashburner,

HL

Carson,

JN

Thompson Jr,

eds),

vol 3c, Academic

_Press,

New York

Wallace B

₍₁₉₇₄₎

Studies on intra- and

_{interspecific competition}

in

_{Drosophila. Ecology}