Breeding structure of Drosophila buzzatii in relation to competition in prickly pears (Opuntia ficus-indica)

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Breeding structure of Drosophila buzzatii in relation to

competition in prickly pears (Opuntia ficus-indica)

Je Quezada-Díaz, H Laayouni, A Leibowitz, M Santos, A Fontdevila

To cite this version:

Original

article

Breeding

structure

of

_Drosophila

buzzatii

in

relation

to

_competition

in

_prickly

_pears

_(Opuntia

_{ficus-indica)}

JE

_{Quezada-Díaz}

H

_Laayouni

A

_Leibowitz,

M

Santos

A Fontdevila

Departament

de Genetica i de

_{Microbiologia,}

Universitat Autonoma de Barcelona, 08193

_{Bellaterra, Barcelona, Spain}

(Received

15 October 1996;

accepted

22

_April

₁₉₉₇₎

Summary -

Rotting Opuntia ficus-indica

fruits

_{(prickly pears)}

are used as

_breeding

sites

for _{up to} four

Drosophila

species

(D

melanogaster,

D

_simulans,

D buzzatii and D

_hydei)

in southern

_Spain.

A field

_experiment

showed that the larvae of D buzzatii are resource

limited in

_Opuntia

fruits available for

_oviposition

for 108 h.

_Experimental

fruits infested

with D larvae were divided into two

_halves;

the larvae in one half were allowed to

develop

normally,

while those in the other half were

_provided

with extra food.

_{Approximately}

five times as _{many D} buzzatii

emerged

from the

_supplemented

as from the control

_halves,

and the flies

_emerging

from the

_supplemented

halves were, on _average,

_larger

than those

emerging

from the control halves. F-statistics were estimated from

_allozyme

data for the

D buzzatii flies. The values obtained from the

_{supplemented halves, coupled}

with _computer

simulations _{to compare} these estimates with the expected values

_{generated by}

a limited number of

_{mating pairs contributing}

_{progeny to} a

_fruit,

_suggest an effective size of about 30

individuals. Even

_though

95%

_bootstrap

confidence intervals for

_F

_IS

estimates

_comparing

the

supplemented

and control halves do not

_overlap,

_computer simulations _suggest that we cannot support the

_hypothesis

that selection is _acting on

_allozyme

variation.

body size

_/

_{cactophilic Drosophila}

_/

competition

/

density-dependent mortality

/

population structure

Résumé - Structure _génétique des _populations de _Drosophila buzzatü en situation de _compétition dans les figues de barbarie

_{(Opuntia ficus-indica).}

Les

_fruits

pourris

d’Opuntia

ficus-indica

_(,figues

de

_Barbarie)

sont utilisés comme sites de

_reproduction

_par quatre

espèces

de

drosophiles

(D

melanogaster,

D

_simulans,

D buzzatii et D

_hydei!

dans le sud de

l ’Espa

9

ne.

Une

_{expérimentation}

sur le terrain a montré que les larves de D buzzatii ont des ressources limitées dans les

_{fruits d’Opuntia disponibles pour la}

_ponte

_pendant

108 h. Des

_{fruits expérimentaux infestés}

de larves de

_drosophiles

ont été divisés en deux

moitiés : dans la

première,

les larves ont pu se

_développer

normalement et, dans la

_seconde,

*

on a

_ajouté

de la nourriture.

À

_peu

près

_cinq

_{fois plus}

de D buzzatii sont sorties des moitiés

complémentées

en _comparaison aux moitiés de

référence,

et les mouches sortant

des moitiés

complémentées

ont été en _moyenne

_{plus grandes}

_que celles sortant des moitiés de

référence.

Des _statistiques F ont été estimées à partir de données sur

allozymes

_pour les mouches D buzzatii. Les valeurs obtenues à _partir des moitiés

_{supplémentées, couplées}

avec des simulations sur ordinateur pour comparer ces estimées avec les valeurs

espérées

générées

par un nombre limité

_{d’accouplements}

contribuant au

_peuplement

d’un

_fruit

suggèrent

un

_{effectif ef,!’ccace}

d’environ 30 individus. Même si les intervalles de

_confiance

de

_F

_I

_g

donnés _par la méthode de

_bootstrap

pour les moitiés

supplémentées

et de

_référence

ne se recouvrent pas, les simulations ne _{permettent pas}

_{d’appuyer l’hypothèse}

selon

laquelle

la sélection s’exerce sur la variation

_allozymique.

taille

_/

drosophile cactophile

/

compétition

/

mortalité

_/

structure de population

INTRODUCTION

Populations

of many

organisms,

particularly

insects,

are subdivided in the sense

that females

_lay

_{eggs in} discrete and

_ephemeral

resources, each used as a

_breeding

site

_by

a small number of individuals

(Heed,

1968;

Jaenike and

Selander, 1979;

Shorrocks,

1982;

Brncic, 1983; Lacy, 1983;

Hoffmann et

_al,

_1984;

Santos et

_{al, 1989;}

Thomas and

_Barker,

_1990;

_Santos,

_1997).

A

_strong

motivation to

_study

the effects of such a

_population

structure relates to the

_pervasive

idea that environmental

heterogeneity - arising

because selection

_proceeds

in different directions in different

places,

because there are

complementary

interactions among

_genotypes,

or because

there is an

_aggregated

distribution of _eggs over

_{patches -}

can maintain

_genetic

heterogeneity

(Levene,

1953;

Hoffmann and

Nielsen, 1985; Hedrick, 1986;

Gillespie

and

_{Turelli, 1989; Gillespie, 1991;}

_Dytham

and

_{Shorrocks, 1992,}

_1995).

A basic

ingredient

in most

_genetic

models is the existence of crowded conditions within

patches

(ie,

selection is

_{’soft’, meaning}

that

_{density regulation}

occurs within each

patch

separately).

If

_competition

is

_absent,

environmental

_{heterogeneity}

_might

be

irrelevant to

_{explain genetic}

variation.

The presence of

competition

in natural

populations

of

Drosophila

has been

inferred in several cases

(eg,

Fellows and

_{Heed, 1972;}

Atkinson, 1979;

Prout

and

_Barker,

_1989),

but a clear

_experimental

demonstration was first

_provided

by

Grimaldi and Jaenike

_(1984).

These authors collected mushrooms infested with larvae and divided each mushroom into

_two;

the larvae in one half were

allowed to

_develop

_normally,

while those in the other half were

_provided

with

extra food

_(see

also Jaenike and

_James,

_1991).

_They

showed that there is

density-dependent mortality

in natural

_populations,

and that flies

_emerging

from halves

of

supplemented

mushrooms are

_larger

than flies

_emerging

from control halves. An

_important

conclusion to be obtained from this

_experiment

is that in natural

populations

of

_Drosophila

there is the

_opportunity

for selection

_(Crow,

_1958;

Arnold

and

Wade,

1984).

From an

_evolutionary

_{perspective, however,}

the

_{important point}

is not to show that there is

_opportunity

for

_selection,

but that selection does indeed

differentially

affect the various

_genotypes.

We

_report

here an

_{experiment designed}

to

_investigate

the incidence of compe-tition in

_Opuntia

_ficus-indica

fruits

_(prickly

_pears)

in the

_{field, together}

with an

rotting Opuntia cladodes,

which are not

_{significantly}

utilized as

_breeding

sites

_by

other

_drosophilids

in the Old World

_(at

least

_during

the summer

_months,

Santos

et

_{al, 1988,}

_1992).

In contrast to

_this,

the

_Opuntia

fruits can be

_{exploited by}

other

Drosophila

species

in southern

_Spain.

These fruits are _sweet,

_{fleshy, frequently}

vis-ited

_by

_Drosophila

adults after

_falling

from the

_plant,

and _{very easy} to

_manipulate

experimentally. Although

they

are

individually

small

(from

approximately

30 to

90 _g of wet

_weight),

’en masse’

_they

can form habitats of considerable size. We do

not have an estimate of the relative contributions of

_Opuntia

fruits and cladodes to

the total

population

density

of D

_buzzatii,

but

_during

the fruit season

(from

August

to November in southern

_Spain)

it is

_quite

_likely

that a

_significant

_proportion

of

D buzzatii flies come from

Opuntia

fruits. We

investigated

the

allozyme

_genotypes

of

D buzzatii

_emerging

from the

_fruits,

and obtained estimates of F-statistics. Because under field conditions we are never sure of what fraction of the

genetic

differenti-ation is attributable to drift

_(founder

events of individual

_patches)

or to

_selection,

the flies raised from halves of

_supplemented

fruits

₍₌

’non-limited

_resource’,

see

below)

were used to obtain

_empirical

distributions of F-statistics

_likely

to be due

to drift. Measures of

_inbreeding

were then used to estimate the effective number of

_parents

_contributing

_gametes

to each

_fruit,

and to see whether or not there is

genetic

differentiation between

_breeding

sites as a result of selection.

Materials and methods

Description

of collections

Samples

were collected in

September

1993 from a disused

Opuntia fccus-indica

plantation

(Carboneras,

SE

_Spain),

described in detail elsewhere

_(Ruiz

et

_al,

_1986).

At that time of the year there are abundant

_Opuntia

fruits which are

exploited by

D

_{melanogaster,}

D

_simulaus,

D

_buzzatii,

and D

_hydei.

On 3

_September,

300

_undamaged

mature fruits were harvested from the

_Opuntia

stems,

labelled with coloured

bands,

and

_placed

at random in the

experimental

area on 4

_September

after

_cutting

a small slice at the

_top

to allow for

_oviposition

by

Drosophila

females. After various

_periods

of time in the field these fruits were

recollected and

_placed

_separately

in

_jars

on a bed of

_sand,

covered with _gauze.

After 24

_h,

30 labelled fruits were collected. After 48

h,

another 30 fruits were

collected and divided in half

_{longitudinally.}

One half of each fruit

_{(’control’ half)}

was left untreated while half of a

_fresh,

uncolonized

_fruit,

was added to the other

’supplemented’

(’=

non-limited

_resource’)

half. This

_{approximately}

doubled the wet mass of food available to larvae. The same

_procedure

was followed for an

additional

_sample

of 30 fruits collected after 72 h.

Finally,

on 9

September

we

randomly

collected 124 fruits out of the

_remaining

210 fruits that had been left in

the field for 108 h. Control and

_supplemented

halves were

_produced

for 63 of those

fruits,

whereas the

_remaining

61 were

placed

into

_separate

_jars

without

_cutting.

These fruits served as the control for the

_cutting

treatment.

The

_experimental

fruits were

_kept

at room

_temperature

(22-27 °C)

in the

makeshift

_laboratory

near the field site and checked

_regularly

for

_emergent

adults.

From the time of first adult _emergence

₍₁₃

_September),

all

_jars

were examined

D buzzatii

flies,

which were

_kept

alive in vials

_containing

5 mL of standard

cornmeal-agar-yeast

food.

D

_melanogaster

and D simulans males were

distinguished

by

the differences in

their external

genitalia

(Sturtevant,

1919).

No

_attempt

was made to

_distinguish

between females of these two

_species

and their numbers were

_grouped

_together

into

a

_single

class. For each

_species

that

_emerged

from the two halves of the 63 fruits that remained in the field for 108

_h,

the

_wing

_length

of _up to five males _per collection was

measured

_(see

Leibowitz et

_al,

_1995).

_{Wing length}

was used as an index of adult

body

size because it is a more convenient measure when flies can be killed. The

average

wing

length

from each control and

_supplemented

half fruit was calculated

by weighting

the mean of each collection

_by

the number of males of each

_species

in

that collection.

Allozyme

electrophoresis

From most of the 108 h

_Opuntia

fruits that

_yielded

D buzzatii

_flies,

a random

sample

of individuals was

_assayed

for four

_polymorphic

enzyme loci

(Est-2,

Aldox,

Pept-2

and

_Adh-1).

Details of the

_{electrophoretic techniques,}

allele nomenclature

[standardized

following

Barker and

_Mulley

_(1976),

and Barker et al

_(1986)],

chromosome

mapping,

and

_gametic

associations between these loci and between them and the

_polymorphic

inversions in the

_population

of

Carboneras,

are

_given

elsewhere

_{(Quezada-Diaz}

et

_{al, 1992; Quezada-Diaz,}

_1993;

Betran et

_al,

_1994).

Briefly,

Est-2

_segregates

for five

_alleles,

and the other three loci

segregate

for two

alleles each. Est-2 and Aldo! are linked to the inversions on the second

_chromosome,

while

Pept-2

is outside the inverted

_fragments.

There are

_strong

_{linkage disequilibria}

(sensu

Lewontin and

_Kojima,

₁₉₆₀₎

between alleles of Est-2 and Aldo! with the second chromosome

_{arrangements.}

_Thus,

alleles Est-2a and

Est-!b

are

_segregating

within the gene

arrangements

2st and

_2j,

with the former allele at

_higher

_frequency

in !st and the latter in

_higher

_frequency

in

_2j.

Allele

Est-2°

+

is fixed in the _gene

arrangement

2jq

7

,

and alleles Est-2C and

Est-2

d

are

_only

_present

in the inversion

!jz3.

Allele

!o!o:E!

is associated with 2st. Adh-1 is located on the third chromosome which lacks

_polymorphic

inversions

_(Labrador

et

_al,

_1990).

Statistical

_analyses

for the

_allozyme

data

Analyses

of allelic

_frequencies

and the calculation of F-statistics

_using

the methods of Weir

₍₁₉₉₀₎

were

_accomplished

with the GENEPOP

_{(v. 1.2)}

_{population genetics}

software

_(Raymond

and

_Rousset,

_1995).

Associations _among alleles at two loci were measured

_by

the

_composite

_digenic

disequilibrium

coefficient

_A

_AB

_(Weir

and

_Cockerham,

_1989;

_Weir,

_1990).

Alleles other than Est-2a were

_{grouped together}

into a

_single

class. This coefficient is

RESULTS

Evidence of

_competition

in natural substrates

A total of 34 745 individuals of the four

Drosophila species emerged

from the exper-imental fruits of

Opuntia

,ficus-indica

(39.6%

D

_{melanogaster,}

54.4%

D

_simulans,

4.3%

D buzzatii and

1.6%

D

_hydei).

Table I shows their _average numbers per

fruit,

together

with Wilcoxon

_matched-pair

_signed-rank

tests

_(Siegel

and

Castellan,

1988)

comparing

the number of flies in each half. After 108 h in the

_field,

_{approximately}

five times as _many D buzzatii

_emerged

from the

supplemented

as from the control

halves

_(only

187 D buzzatii

_emerged

from the control

_halves,

whereas 845

_emerged

from the

_supplemented

_halves).

A

_{potential problem}

with ’the

_cutting

treatment’

might

be

_that,

for _any reason

_(eg,

control halves dried-out

quicker),

a lower number

of adult flies

_emerged

from the control fruits. A

comparison

of the average number of

males and females

_{(D melanogaster/D}

simulans females were

pooled)

of

Drosophila

species

that

_emerged

from the 108 h whole fruits with twice as _many

_emerging

from

the 108 h control halves

suggests

that this is

_probably

not the case here

(Wilcoxon-Mann-Whitney

tests

_ranged

from z =

_0.03,

P =

_0.972;

for D

melanogaster males,

to z =

1.18,

P =

0.236;

for D simulans

males).

Except

for D

hydei,

the

Spearman

rank correlations between the emergence numbers were

_positive

and

_{statistically}

significant

for all

_pairs

of

_species

in the 108 h

_supplemented

halves.

_However,

in

both the 108 h control halves and 108 h whole fruits the D buzzatii-D

_melanogaster

and D buzzatii-D simulans rank correlations were _very low and

statistically

non-significant, probably

owing

to the increase in

_mortality

suffered

_by

D buzzatii.

The

_only

differences in size distributions between flies

_emerging

from the 108 h control and

supplemented

halves were found for D buzzatii

(table

II).

Because _up to

five males _per fruit per collection were measured for each

species

(see

Material and

methods),

in the 108 h control halves we had an index of

_body

size for most of the D buzzatii males that

_emerged.

Therefore,

we could _{carry out} a

_{multiple regression}

analysis

of the effect of each

_{species’ density}

_(estimated

as the number of males that

emerged

in a

_given

fruit)

on the individual

wing

length

of each D buzzatii male.

Forward

_{stepwise regression}

coefficients were

_{statistically}

_significant

for D buzzatii

(,8i

n

t

ra

= -0.018 P =

0.008), D

simulans

(0i,,

ter

=

_-0.004,

P _<

0.001)

and

D

_melanogaster

_(Winter

=

0.002,

P =

0.006),

the

negative regression

for D buzzatii

suggesting

that

_{intraspecific competition}

is

_occurring

within the

_breeding

sites.

The

negative

correlation between the

_wing

_length

of D buzzatii and the number of D simulans

_might

also indicate the occurrence of

interspecific

competition,

but some care must be taken with such an

_{interpretation}

because it is

possible

that conditions that enhance the numbers of D simulans may

adversely

affect the

body

size of D buzzatii.

_Conversely,

some conditions _may enhance the numbers of

D

_melanogaster

and the

body

size of D

_buzzatii,

which could

_explain

the

_highly

significant

positive

correlation found between both variables.

Figure

1 shows the number of male flies that

_emerged

from the 108 h control and

supplemented

fruits

_through

time. It is obvious that D

_melanogaster

and D simulans have shorter

_development

times than D buzzatii and D

_hydei,

which

clearly

suggests

that

_they

would

always

be at a

_{competitive advantage}

at

_high

larval densities

_(see

Analysis of population

structure

Table III

_presents

_summary F-statistics for the D buzzatii flies that

_emerged

from the 108 h

_Opuntia

fruits

_(the

raw data are available _upon

_request

to

the

_{corresponding}

_author).

For

_selectively

neutral

_loci,

the extent of

genetic

differentiation of the

_{subpopulations}

in the

_present

_situation,

where there is

_only

one round of drift and random

_mating

in the

_population

at

_large

_{(Quezada-Diaz}

et

_al,

_1992;

Barbadilla et

_al,

_1994),

is characterized

_by

_N,,

the effective number of

_{locally breeding}

adults

_(Wade

and

_McCauley,

_1988).

All confidence intervals for F values in the 108 h

_supplemented

fruits included zero, which

suggests

that

N

e

is

_{relatively large}

_{(see below).}

On the other

_hand,

_F

_IS

and

_F

_ST

values were

different from zero in the 108 h control

_{fruits, indicating}

that there is an excess

of

_{heterozygotes}

_within,

and a substantial differentiation _among, limited resource

fruits. The confidence intervals for

_F

_IS

do not

_overlap,

and this could be taken

as a real difference between the

_supplemented

and the control fruits.

_However,

some caution is needed with this

_{interpretation}

because it could be

_argued

that with four loci

_{(as here),}

and under the null

_hypothesis

_H

_o

_{: F}

_IS

=

0,

estimates

of this

_parameter

are

_negative

for all loci with a

_probability

of

_(1/2)

₄

=

0.0625,

and zero will be included in the confidence interval.

_Raymond

and Rousset

_(1995b)

have

_recently

stressed that

_{bootstrap resampling}

to build a confidence interval is

incorrect when the number of loci is small.

Exact tests for

_population

differentiation

_(Raymond

and

_{Rousset, 1995a,}

_b)

com-paring

control and

supplemented

fruits

_provided

no evidence for allelic

heterogene-ity

at any locus.

_Analysis

of two-locus

_linkage

disequilibrium

coefficients showed

only eight significant disequilibria

out of 179

_{possible comparisons.}

Four of these

pairs

have both members located on the second

_chromosome,

and the other four involved the Adh-1 locus.

_Thus,

no clear

_patterns

were

found,

and this result could

be taken as another reflection of

_large

effective

_population

size within

_Opuntia

fruits

Estimate of the number of

_parents

_breeding

on a

_single

fruit

Given that flies from the

_supplemented

halves have

_developed

in ’non-limited

resources’,

it is

_important

to recall that the

_{corresponding}

F-values in table III do

not mix drift and selection but

_probably

reflect the

_sampling

effect.

_Therefore,

we

used

_computer

simulations to _compare F-statistics from

supplemented

fruits with the

_expected

values

_{generated by}

various numbers of

_{mating pairs}

_contributing

to a

_breeding

site. The main

_steps

of our

_reasoning

for

disentangling

the effects

of drift and selection are summarized in

_figure

2. Different numbers of

_mating

pairs

were selected

randomly

to contribute to 19

_breeding

_sites,

and a number of

offspring

equal

to that obtained for the

_supplemented

fruits

_(between

5 and 80 individuals per

fruit)

was

_sampled

at random from each

breeding

site. Even

_though

theoretical

_predictions

about the

_expected

F-values could be

_easily

made from the above

_assumptions

_{(see below),}

we think _{it may} be useful to have an idea of the

standard deviations of their distributions in this

_particular

case

(ie,

equal

number of

fruits and

_emerging

adults than in the actual

_sample).

F-statistics were calculated

according

to the methods of Weir

_(1990),

and 100 simulations were undertaken for

each set of conditions. The interactive matrix

_algebra

program MATLAB

(V

4.0 for

_Windows)

was used for

_computations

on a 486

(66 Mhz)

_{PC-compatible.}

For

simplicity,

we considered four biallelic loci

_(alleles

other than Est-2’ were

_grouped

Fs

T

and

_F

_IS

values

_approach

zero

_(fig

₃₎

as more

_{mating pairs}

contribute to a

_site,

and

_comparisons

with the actual values in table III

_(ie,

those from the

_supplemented

halves)

suggest

that at least 15 females

_oviposit

on a site. The

_expected

values

_{of Fis}

in

_figure

3 could also be

approximated

by -1/(2

N

e

-

1)

(Kimura

and

_Crow,

_1963).

Thus,

for

five,

ten and 15

_{mating pairs}

_F

_IS

_approximates

to

_-0.053,

-0.026 and

- 0.017, respectively,

which are _very close to the numerical values. The

_simulations,

however,

assume that females contribute

equally

to the total number of eggs of each

fruit,

but the actual estimate is for an effective number of

_mating

_pairs.

Let us

_accept

the

figure

of 15

_{mating pairs}

contributing

_{progeny to} a site. It

would be

possible,

then,

to overcome the difficulties with the

bootstrap

method

and build a confidence interval for the

F

IS

value in the control fruits under the

hypothesis

that selection has a

_negligible

effect on the

_allozyme

variation. To do

this,

we

generated

500

_{independent samples}

that matched the situation for the control

fruits, ie,

the

_{mating pairs}

were selected

_randomly

to contribute to 12

_breeding

_sites,

and a number of individuals

equal

to the actual

_sample

size was taken at random

per site.

Figure

4 shows the distribution of the

F

IS

values obtained. We find that

the

_probability

of

F

IS

being

less than or

equal

to -0.0687

_(ie,

the actual value in

the control

_fruits)

is

_0.162,

and it is clear that we cannot

_support

the

_hypothesis

of a

_heterozygote

excess in the control fruits caused

_by

selection with the evidence

DISCUSSION

We have shown that the larvae of D buzzatii are resource limited in

_Op!ntia

_fruits,

and this

species

suffers

density-dependent mortality

in the field

_{(table I).}

In addition

to larval

viability,

resource limitation also has a

significant

effect on

body

size,

a trait

phenotypically positively

correlated with adult fitness

_components

_(Santos

et

_al,

1988, 1992;

Leibowitz et

_al,

_1995).

The relevance of resource structure for

expected

mating

success

and/or

fertility

of the

fly

is

clearly

indicated

by

the substantial

differences found in the individual

_body

size due to

_properties

of the

_breeding

sites in which it _grew as a larva

_(Prout

and

_Barker,

_1989),

and both of these effects can be

_interpreted

as

_{demonstrating}

that there is a

_large

_opportunity

for selection

under natural conditions. The same conclusion had been

_previously

reached

_by

Grimaldi and Jaenike

₍₁₉₈₄₎

for

_{mushroom-feeding}

_Drosophila,

which

_suggests

that such

_phenomena

have to be considered as

_general

in natural conditions. It should

be noted that the fruits in our

_experiment

were

_only

available for

oviposition

for

at most 108 h. If

_oviposition

continues

_beyond

this

_time,

resource limitation in

undisturbed

_Opuntia

fruits _may

_actually

be

_greater

than our data

_suggest.

When resources are _scarce, those

_species

(or

individuals within a

species)

with

a short

_{developmental period}

are

_presumably

at a

_competitive

_advantage

because

larvae are more

likely

to

_{complete development}

before their

_patch

is exhausted

Santos et

_al,

_1994).

It

_is,

_therefore,

_interesting

that one of the two slow

_developers

in our

sample, namely

D

buzzatii,

shows a

significant

effect of additional food. The

numbers of D

_hydei

flies

_emerging

from the

_OPu!atia

fruits were too low to obtain

meaningful

conclusions.

_Breeding

_{opportunities}

for

Drosophila species

in

_Opuntia

sites are

frequent

during

the fruit season

(from

August

to November at

_Carboneras),

and we could

_hypothesize

that

_{interspecific}

_competition

would exclude D buzzatii

from

_Opuntia

fruits because the most common

_{coexisting species,}

D

simulans,

is

at a

_competitive

_advantage.

_However,

even

_though

the

_wing

_length

of D buzzatii

was

_{significantly negatively}

correlated with the

density

of D simulans in 108 h control

fruits,

the relative intensities of intra- and

_{interspecific}

_competition

cannot

be assessed from our data. Some recent

_experiments

on larval

_competitive

effects

measured on semi-natural 0

_ficus-indica

fruit food at 25 °C

suggest

that both D

_melanogaster

and D simulans

_{significantly}

reduce larval

_performance

of D buzzatii

(A

Galiana,

pers comm

1995)

We can think of

_{Drosophila population}

structure as

_consisting

of an _array of local

breeding populations

with

_high

extinction and recolonization rates. The fraction of

genetic

variance due to the

_sampling

effect of colonization among the

newly

founded

1

populations

is

_F

_ST

=

2

Ne

,

2

_N!

N

e

being

the effective number of adults

breeding

on

a

_{single patch}

(Wade

and

_McCauley,

1988).

This would be an

_equilibrium

value

because the successional

_changes

that take

_place

in the discrete

_breeding

sites are

so

rapid

that there is often time for

_only

a

_single

_generation

before the

_patch

becomes unusable. Under this

_{simple model,}

the effect of

_breeding

structure on

genetic

variation is

_expected

to be uniform over all alleles and

loci,

whereas natural

selection _may be

_expected

to act

_differently

on each allele and locus

(Lewontin

and

_Krakauer,

_1973).

Estimates of F-statistics

_{(table III)}

_suggest

that there is an excess of

_{heterozygotes within,}

and a substantial differentiation _{among, limited} resource fruits when

_compared

with the values obtained from the

_supplemented

halves. This could be taken as evidence for selection

_acting

in some _way on the

allozyme

variation.

_However,

even

_though

the

95%

confidence intervals for the

_F

_IS

estimates do not

_overlap,

the conclusion that both values are

_{statistically}

different is

not warranted

_{(fig 3).}

We would like to

_point

out that

_comparisons

of F-statistics

is a somewhat indirect _way of

_{detecting selection,}

and other methods

_might

be

more

_appropriate.

The

_present

_{experimental design}

_could,

in

_theory,

allow us to

estimate both

_input

_(from

_supplemented

_halves)

and

_output

_(from

control

_halves)

zygotic

ratios in natural

_breeding

substrates. Measures of larval

_viability

could thus be obtained in each site from the

_{cross-product}

estimator

_(Manly,

_1985),

_together

with confidence intervals.

_However,

the number of D bv,zzatii flies that

_emerged

from the control halves was not

_large

_enough

to allow

_meaningful

fitness estimates. In

_conclusion,

our data show that there is

_{density-dependent mortality}

in the

field for D

buzzatii,

and

_suggest

that about 30 individuals contribute progeny to

each fruit. The data do

_not,

_{however, provide}

an

_unambiguous

answer with

_respect

ACKNOWLEDGEMENTS

We thank B Shorrocks and two _{anonymous reviewers} for constructive criticisms on

previous

drafts. This work was funded

_by

_grant PB89-0325 from the Direcci6n General de

Investigaci6n

Cientffica y T6cnica

(DGICYT,

Spain)

to

AF,

Contract No

CHRX-CT92-0041 from the Commission of the

_European

_Communities,

and _grant CE93-0019 from the DGICYT.

REFERENCES

Arnold

_SJ,

Wade MJ

₍₁₉₈₄₎

On the measurement of natural and sexual selection:

_theory.

Evolution 38, 709-719

Atkinson WD

₍₁₉₇₉₎

A field

_{investigation}

of larval competition in domestic

Drosophila.

J Anim

Ecol 48,

91-102

Bakker K

₍₁₉₆₁₎

An

_analysis

of factors which determine success in

_competition

for food

among larvae of

Drosophila melanogaster.

Arch Neerl

_{Zool 14,}

200-281

Bakker K

₍₁₉₆₉₎

Selection for rate of

growth

and its influence on _competitive

_ability

of larvae of

_{Drosophila melanogaster.}

Netherl J

Zool 19,

541-595

Barbadilla

_A,

Ruiz

A,

Santos

_M,

Fontdevila A

₍₁₉₉₄₎

_Mating

_pattern and fitness

com-ponent

analysis

associated with inversion

_polymorphism

in a natural

_population

of

Drosophila

buzzatii. Evolution

_48,

767-780

Barker

_{JSF, Mulley}

JC

₍₁₉₇₆₎

Isozyme

variation in natural

_populations

of

Drosophila

buzzatii. Evolution 30, 213-233

Barker

JSF,

East

PD,

Weir BS

₍₁₉₈₆₎

Temporal

and

microgeographic

variation in

_allozyme

frequencies

in a natural

population

of

Drosophila

buzzatii. Genetics

_112,

577-611 Betran

E, Quezada-Diaz JE,

Ruiz

A,

Santos M, Fontdevila A

₍₁₉₉₄₎

The

evolutionary

history

of

_Drosophila

buzzatii. XXXII.

_{Linkage disequilibrium}

between

_allozymes

and

chromosome inversions in two

_{colonizing populations. Heredity}

_74, 188-199

Brncic D

₍₁₉₈₃₎

_Ecology

of

_{flower-breeding Drosophila.}

In: The Genetics and

_{Biology of}

Drosophila,

vol 3d

_(M

Ashburner,

HL

_Carson,

JN

_{Thompson Jr,}

_eds),

Academic

_Press,

London,

333-382

Crow JF

₍₁₉₅₈₎

Some

_{possibilities}

for

_measuring

selection intensities in man. Hum Biol

30, 1-13

Dytham C,

Shorrocks B

₍₁₉₉₂₎

_{Selection, patches}

and

_genetic

variation: a cellular automaton

modelling Drosophila populations.

Evol

_{Ecol 6,}

342-351

Dytham C,

Shorrocks B

₍₁₉₉₅₎

_Aggregation

and the maintenance of

_{genetic diversity:}

an individual-based cellular model. Evol

_{Ecol 9,}

508-519

Fellows

DP,

Heed WB

₍₁₉₇₂₎

Factors

affecting

host

plant

selection in

_{desert-adapted}

cactiphilic Drosophila. Ecology

53, 850-858

Gillespie

JH

₍₁₉₉₁₎

The Causes

of

Molecular Evolution. Oxford Univ Press, New York

Gillespie JH,

Turelli M

₍₁₉₈₉₎

_{Genotype-environment}

interaction and maintenance of

polygenic

variation. Genetics

121,

129-138

Grimaldi

_D,

Jaenike J

₍₁₉₈₄₎

_Competition

in natural

_populations

of

_mycophagous

Drosophila. Ecology 65,

1113-1120

Hedrick PW

₍₁₉₈₆₎

Genetic

_polymorphism

in

_{heterogeneous}

environments: a decade later. Ann Rev Ecol

_Syst

17, 535-566

Heed WB

₍₁₉₆₈₎

_Ecology

of the Hawaiian

_{Drosophilidae.}

Univ Texas

_{Publ 6818,}

387-419 Hoffmann

AA,

Nielsen KM

₍₁₉₈₅₎

The effect of resource subdivision on

_genetic

variation

Hoffmann

AA,

Nielsen

_KM,

Parsons PA

₍₁₉₈₄₎

_Spatial

variation of biochemical and

ecological phenotypes

in

_{Drosophila: electrophoretic}

and quantitative variation. Devel

Genet

4, 439-450

Jaenike

J,

James AC

₍₁₉₉₁₎

_Aggregation

and the coexistence of

_{mycophagous Drosophila.}

J Anim Ecol _60, 913-928

Jaenike J, Selander RK

₍₁₉₇₉₎

_{Ecological generalism}

in

_{Drosophila falleni: genetic}

evi-dence. Evolution

33,

741-748

Kimura

M,

Crow JF

₍₁₉₆₃₎

The neasurement of effective

population

number. Evolution

17, 279-288

Labrador

M,

Naveira

H,

Fontdevila A

₍₁₉₉₀₎

Genetic

mapping

of the Adh locus in the

repleta

group of

Drosophila by

in situ

hybridization.

J Hered 81, 83-86

Lacy

R

₍₁₉₈₃₎

Structure of

genetic

variation within and between

populations

of

my-cophagous Drosophila.

Genetics

_104,

81-94

Leibowitz

A,

Santos

M,

Fontdevila A

₍₁₉₉₅₎

_Heritability

and selection on

_body

size in a natural

_population

of

_Drosophila

buzzatii. Genetics 141, 181-189

Levene H

₍₁₉₅₃₎

Genetic

_equilibrium

when more than one

_ecological

niche is available. Am Nat 87, 331-333

Lewontin

_{RC, Kojima}

K

₍₁₉₆₀₎

The

_{evolutionary dynamics}

of

_{complex polymorphisms.}

Evolution _14, 458-472

Lewontin

_RC,

Krakauer J

₍₁₉₇₃₎

Distribution of gene

frequency

as a test of the

_theory

of the selective

_neutrality

of

_{polymorphisms.}

Genetics

_{74, 175-195}

Manly

BFJ

₍₁₉₈₅₎

The Statistics

_of

Natural Selection on Animal

_{Populations. Chapman}

and

_Hall,

London

MATLAB

@

Reference Guide

₍₁₉₉₂₎

The MathWorks

_Inc,

Massachusetts

Mueller LD

₍₁₉₈₈₎

_{Density-dependent population growth}

and natural selection in food-limited environments: the

_Drosophila

model. Am Nat

_132,

786-809

Nunney

L

₍₁₉₈₃₎

Sex differences in larval

_competition

in

_{Drosophila melanogaster:}

the

testing

of a

_competition

model and its relevance to

_{frequency-dependent}

selection. Am

Nat

_121,

67-93

Partridge L,

Fowler K

₍₁₉₉₃₎

_Responses

and correlated responses to artificial selection on thorax

_length

in

_{Drosophila melanogaster.}

Evolution

_47,

213-226

Prout T, Barker JSF

₍₁₉₈₉₎

_Ecological

aspects of the

_heritability

of

_body

size in

_Drosophila

buzzatii. Genetics 123, 803-813

Quezada-Diaz

JE

₍₁₉₉₃₎

Estructura

_poblacional

_y

_patr6n

de

_{apareamientos}

de la

_especie

cact6fila

Drosophila

buzzatii. PhD

diss,

Universitat Aut6noma de

Barcelona, Spain

Quezada-Diaz JE,

Santos

_M,

Ruiz

A,

Fontdevila A

₍₁₉₉₂₎

The

_{evolutionary history}

of

Drosophila

buzzatii. XXV. Random

mating

in nature.

_{Heredity 68,}

373-379

Raymond

M, Rousset F

_(1995a)

GENEPOP Version 1.2, a

_{population genetics}

software for exact tests and ecumenicism. J Hered 86, 248-249

Raymond M,

Rousset F

_(1995b)

An exact test for

population

differentiation. Evolution

49, 1280-1283

Ruiz

A,

Fontdevila

A,

Santos

M,

Seoane

M, Torroja

E

₍₁₉₈₆₎

The

evolutionary history

of

_Drosophila

buzzatii. VIII. Evidence for

endocyclic

selection

acting

on the inversion

polymorphism

in a natural

_population.

Evolution _40, 740-755

Santos M

₍₁₉₉₇₎

Resource subdivision and the

_advantage

of

_{genotypic diversity}

in

Drosophila. Heredity

78, 302-310

Santos

_M,

Ruiz

_A,

Barbadilla

_{A, Quezada-Diaz}

JE, Hasson E, Fontdevila A

₍₁₉₈₈₎

The

Santos

M,

Ruiz

A,

Fontdevila A

₍₁₉₈₉₎

The

_{evolutionary history}

of

Drosophila

buzzatii.

XIII. Random differentiation as a

partial explanation

of chromosomal variation in a structured natural

_population.

Am Nat 133, 183-197

Santos

M,

Fowler

K, Partridge

L

₍₁₉₉₄₎

Gene-environment interaction for

body

size and larval

_density

in

_{Drosophila melanogaster:}

an

_{investigation}

of effects on

_development

time, thorax

length

and adult sex ratio.

_Heredity

_72, 515-521

Santos

M,

Ruiz

A, Quezada-Diaz JE,

Barbadilla

A,

Fontdevila A

₍₁₉₉₂₎

The

_evolutionary

history

of

Drosophila

buzzatii. XX. Positive

_phenotypic

covariance between field adult fitness components and

body

size. J Evol Biol 5, 403-422

Shorrocks B

₍₁₉₈₂₎

The

_breeding

sites of _temperate woodland

_Drosophila.

In: The Genetics and

_{Biology of Drosophila,}

vol 3b

_(M

_Ashburner,

HL

_Carson,

JN

_Thompson

Jr,

eds),

Academic

_{Press, London,}

385-428

Siegel S,

Castellan Jr NJ

₍₁₉₈₈₎

Nonparametric

Statistics

for

the BehavioraL Sciences.

McGraw-Hill,

New

_York,

2nd ed

Sturtevant AH

₍₁₉₁₉₎

A new

species closely resembling Drosophila melanogaster. Psyche

16, 153-155

Thomas

_RH,

Barker JSF

₍₁₉₉₀₎

_Breeding

structure of natural

_populations

of

_Drosophila

buzzatii: effects of the distribution of larval substrates.

_{Heredity 64,}

355-365

Wade

MJ, McCauley

DE

₍₁₉₈₈₎

Extinction and recolonization: their effects on the

_genetic

differentiation of local

_populations.

Evolution 42, 995-1005

Weir BS

₍₁₉₉₀₎

Genetic Data

_{Analysis. Sinauer,}

Massachusetts

Weir

_BS,

Cockerham CC

₍₁₉₈₉₎

_Complete

characterization of

_{disequilibrium}

at two

loci. In: Mathematical

_{Evolutionary Theory}

_(ME

_Feldman,

_ed),

Princeton Univ Press,