Effects of selection for early and late reproduction in low and high larval density populations of the bean weevil (Acanthoscelides obtectus)

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Effects of selection for early and late reproduction in low

and high larval density populations of the bean weevil

(Acanthoscelides obtectus)

I Gliksman, N Tucić

To cite this version:

Original

article

Effects of selection for

_early

and late

_reproduction

in low

and

_high

larval

_{density populations}

of the bean weevil

(Acanthoscelides

obtectus)

I

Gliksman

N Tuci&jadnr;

University of Belgrade,

Institute

_{for Biological}

Research and

_{Faculty of Science,}

_Yugoslavia

(Received

25

_May

1990;

accepted

30 November

₁₉₉₀₎

Summary - This

_study

concerns an

_analysis

of

_{one-generation}

selection for

early

and

late

reproduction

in

_populations

maintained for 10

_generations

at low and

_high

levels of larval

_density.

We have obtained direct responses to selection in both the

early

and late

reproducing

parents. There was,

however,

no correlation between

_fecundity

and

_longevity.

Phenotypic

responses suggest that

_parental

_age could

_change

the

_fecundity

schedule in

the _way

_{predicted by antagonistic pleiotropy theory.}

However, the absence of

_{statistically}

detectable

_negative

_genetic

correlation between

_early

and late fecundities made our

_study

inconclusive

with _respect to

_{possible genetically}

based trade-off among

fecundity

indices.

Acanthoscelides obtectus

_/

larval density

/

parental age effect

/

fitness components

Résumé - Effets de la sélection pour une _{reproduction précoce} et tardive dans

des populations de bruche du haricot

_{(Acanthoscelides obtectus)}

à haute et faible densité larvaire. Cette étude concerne

_l’analyse

d’une

_génération

de sélection _pour une

reproduction précoce

ou tardive de

populations

maintenues

pendant

10

_{générations}

à une

densité larvaire

_faible

ou élevée. Nous avons obtenu des

réponses

directes à la

sélection,

chez les parents à

_{reproduction précoce}

et à

_reproduction

tardive. Il

_n’y

avait

_cependant

_pas de corrélation entre la

_fécondité

et la

_longévité.

Les

_{réponses phénotypiques suggèrent}

_que

l’âge

des _parents

_{pourrait modifier}

l’évolution de la

_fécondité

au cours de la vie comme le

prédit

la théorie de la

_{pléiotropie antagoniste. Cependant,}

l’absence de corrélation

_négative

statistiquement

détectable entre les

_{fécondités précoces}

et tardives ne nous _{permet pas} de

conclure _quant à des

_oppositions

de nature

_génétique

entre ces

_{2 fécondités.}

Acanthoscelides obtectus

_/

densité larvaire

_/

_âge des parents

/

valeur adaptative

*

INTRODUCTION

Since natural selection

_operates

on several traits

_{simultaneously}

the

_optimal

life

his-tory

has been seen as &dquo;the best

_{compromise given}

a set of

_{options&dquo;}

_(Sibly

and

_Calow,

1986).

Starting

from this

_point,

a

_great

deal of the

theory

of life

history

evolution

(see

eg

Charlesworth, 1980; Reznick, 1985;

Scheiner et

_al,

₁₉₈₉₎

is based on the

assumption

of &dquo;trade-offs&dquo; or

_{negative genetic}

_pleiotropy

_among life

_history

fitness

components

(fitness

components

are

_largely

_synonymous with life

_history

parame-ters;

see _eg

_Istock,

_1983).

For

_{example, according}

to the best-known

_evolutionary

theory

of senescence

_{(Williams, 1957),}

senescence is

_envisaged

most

_accurately

as a

_by-product

of genes with

pleiotropic

effects on the fitness

_components,

such that increases

_early

in the life _span in one set of fitness

_components

are

accompanied

by

decreases in other fitness

_components

later in life.

Despite

the central

position

of

_antagonistic

_pleiotropy

in life

_{history theory,}

experimental

demonstrations of

_genetic

trade-offs were until _very

_recently

almost

absent. Several recent studies

_(eg

Rose and

Charlesworth,

1981a, b;

Rose,

1984;

Luckinbill et

_{al, 1984;}

Tucié et

_al,

₁₉₈₈₎

have

_provided

evidence of

_{negative genetic}

correlations among traits at

_early

and late

stages

of life

_history.

However,

Giesel

et al

_(1982),

Stearns

_(1983),

Mitchell-Olds

₍₁₉₈₆₎

and

_Engstrom

et al

_(1989),

for

_example,

found no evidence of

_{negative genetic}

correlations among fitness

components.

Independently

of any mechanism of

genetic

control

_proposed,

the fundamental

question

of the

_evolutionary

_theory

of senescence is &dquo;how do

long

and short

life spans evolve?&dquo;

(Luckinbill

and

Clare,

1985).

Having

in mind the

_suggestion

provided by

MacArthur and Wilson

₍₁₉₆₇₎

that

_{density dependent}

selection could be

_responsible

for much of the observed

_diversity

of life

_history

_strategies

_among

taxa,

Luckinbill and Clare

₍₁₉₈₅₎

used 2

_experimental

treatments of larval

_density

in a

_long-term

selection

_study

for increased

_longevity

of

_Drosophila

_{melanogaster.}

By

selecting

for late

_reproduction

when larval

_density

was held

_low,

_they

were

unable to detect a

_significant

increase in

_longevity.

_However,

_populations

with

_high

and uncontrolled numbers of the

_developing

larvae

responded

strongly

to selection

for late

_{reproduction,}

with the

_{longevity increasing}

_by

_?

50%.

Thus Luckinbill and

Clare

_{demonstrated,}

_showing

that the larval environment could alter the

_expression

of genes for adult

_longevity,

existence of

_genetic

variation for

_sensitivity

of fitness to

density.

Service et al

₍₁₉₈₈₎

have corroborated

these

findings.

Indeed,

low

_rearing

density

seems to reduce response to selection for increased

longevity

in

_Drosophila

populations.

Motivated

_by

these studies on

_Drosophila,

as

_surprisingly

little

_experimental

work on

_genetic

variation for _response to

_conspecific

_{density exists,}

we conducted an

_experiment

to examine the

_phenotypic

and

_genetic

responses in bean weevil

(Acanthoscelides obtectus)

life

_history

traits that occur when

_{density dependent}

selection is

_{acting. Specifically,}

in this paper we

_present

the results of a

_study

of

one-generation

selection for

early

and late

_reproduction

in

_populations

maintained

MATERIALS AND METHODS

Life

_history

of Acanthoscelides obtectus

The bean

_weevil,

Acanthoscelides obtectus

_(Say),

is a well known

_{leguminous-feeding}

beetle of the

_family

Bruchidae

_{(Coleoptera).}

The females

_lay

_eggs on the surface of

host seeds. The _eggs

_hatch,

and the first instar larvae bore into the seed where

_they

feed. The final instar larvae excavate a chamber

_just

below the seed _{testa, and} the presence of a larva _may be detected

_by

a small &dquo;window&dquo;.

_Pupation

occurs in this

chamber and adults must chew a hole

_through

the testa in order to _{emerge. Larval}

stages

feed

_entirely

within a

single

seed.

Competition

_among larvae in overcrowded

circumstances can be _severe; from a

_single

bean seed

(about

20 mm

sized)

as _many as 45 adults can _emerge

_{(personal observation).}

The adults do not

_require

to feed in

order to mate and

_oviposit.

This feature

_{(non-feeding adults)}

of the bean weevil life

history

is

_particularly

attractive for the

_analysis

of

_genetic

trade-offs since adults have a finite amount of resource that _may be allocated between

_fecundity

and

maintenance.

The

_population

used here was

_{&dquo;synthetic&dquo;, originating}

from different local

populations

of

_Yugoslavia.

It was maintained in culture : 3 _yr

_prior

to these

experiments.

In all

_experiments

Phaseolv,s

_vulgaris

cv

_{&dquo;gradistanac&dquo;}

beans were

used. The size of all seeds used in the

present

experiment

was ! 20 mm. All the

beans were

_brought

in bulk at one time from one source.

Density dependent

selection

The

_following

summarizes the method of selection used to obtain the low

_density,

high

density

and control

_populations.

The low

_{density regime}

was

_designed

to be uncrowded for the

_developing

larvae.

At the start of this

_treatment,

320 beetles were chosen

_randomly

from the base

population

and reared in 10

_separate

bottles with 100 bean seeds

_{(ie each}

bottle

contained 32 weevils whose sex ratio has been determined

_by

_chance).

After

! 3 wk these bottles were monitored

daily

until the first eclosion of adults

began

(the

eclosion is

_recognized

_{by getting}

&dquo;windows&dquo;

_black;

otherwise windows at the seed testa are

_grey).

At that time beans with 1 to 3 windows

_(which

indicate low larval

_density)

were

separated.

Since the

probability

of larvae from same bean

being

sibs is

_higher

when the number of larvae per bean is small

(which

could cause

inbreeding

depression

over

generations)

we

_employed

the

_following

_procedure.

Beans

with low larval

_density

from all 10 bottles were

_kept

_together

in a

_single

bottle. The

seeds with

_higher

larval

_density

were discarded. From the

_{newly emerged}

adults

(usually !

1000

_{individuals),}

in the batch of low larval

_density

seeds we

_chose,

again randomly,

10 _groups with 32

_beetles,

in order to establish a new

_generation.

This

_procedure

was

_repeated

for 10

_generations.

A

_high

_{density population}

was maintained under

_high

larval

_density.

The

procedure

and

_propagule

size

_(ie

32 beetles per

bottle)

were as described

above,

except

that new

_generations

were founded from beans

_containing

10-20

_(rarely

more).windows.

Thus,

the

_only

difference between the 2 selection

_regimes

was the

In the control treatment we did not control larval

_density.

In all other

_aspects

the

_{experimental procedures}

were identical to those in the

_previous

2 treatments.

Analysis

of the

parental

age effects

After 10

_generations

200

_pairs

of beetles were chosen

randomly

from each treatment.

Individual females were

_put

into _separate Petri dishes

_containing

3 beans and

an unrelated

_male,

for

_oviposition.

These Petri dishes were checked

_{daily. Upon}

death of the

female,

her life span and

daily

fecundity

were recorded. In order

to demonstrate

_parental

_age effects in the

_population

selected for different larval

density,

the

_following

_1-generation

artificial selection for

_age-specific

modification of

_fecundity

was

_imposed.

This selection

proceeded

by choosing

the females with

_{highest 3-day fecundity}

record,

from 1-3 as the

_{&dquo;young&dquo;}

_parents,

and from 7-10 as the &dquo;old&dquo;

_parents.

According

to our

_previous

results

_(Tuci6

et

_al,

₁₉₉₀₎

these 2

_periods

are the most

convenient for

_{analysing age-specific}

_pattern

of

_reproduction

in the

_non-feeding

bean weevils.

_Thus,

within the

_population

selected for different larval

_densities,

the

_experiments

were

_designed

to enforce an

_early

versus late

_age-specific

_pattern

of

reproduction.

The best

25%

of females

_laying

were chosen.

_However,

for late

reproduction

more intense selection was

_imposed

_(!

15%

of females

_laying

were

chosen as the

parents)

because of the low

_fecundity

of the old females or some

other reason.

Daily fecundity

was measured on 4

_{newly emerged}

adult

_females,

chosen at

random from each selected

_female,

in _separate Petri dishes with 3 bean seeds and

a male sib. The number of beetles which died was counted _every

_{day. Upon}

death of the

_female,

her

_longevity

and different

_fecundity

indices have been recorded. We

chose the

_following

indices of

_{fecundity: egg-laying}

d

_1-3,

_egg-laying

d

7-9,

egg-laying

d 10 or _more; total

_fecundity

_(over

life

_span),

first

_day

of egg

laying,

last

day

of _egg

_laying,

_age of

_peak

_fecundity

and

_laying

rate

_{(total fecundity/last}

_day

of _egg

laying).

All cultures were maintained in an incubator at 30°C and !

70%

_humidity.

Experimental

adults were

_subjected

to starvation.

Narrow sense

_heritability

for each trait was estimated

_{by regression analysis}

of mean

offspring

values and maternal values

_{(Falconer, 1981).}

In

_general,

it is desirable to _regress

_offspring

values of the male

_parents,

because the estimated covariance between mother and

_offspring

can be inflated

_by

maternal effects.

Unfortunately,

regression

on male

_parent

is

_impossible

here because we

_analysed

age-specific fecundity

pattern

in females.

_Thus,

the

_confounding

of maternal and

genetic

covariance in the

_{parent-offspring}

_regression

is unavoidable. In

_addition,

broad sense

_heritability

was also calculated

_using

a least _squares

_analysis

of variance

model

_(Sokal

and

_Rohlf,

_1981).

RESULTS

In table I the means and standard errors of different life

_history

traits for each

density

selected

_population

in the first

_generation

of

_reproduction

at _young or

using Tukey’s

test. The

_high

larval

_density

population

for both

parental

ages shows lower average

longevity

relative to the

_{corresponding}

parental

age in the low

density

groups, as well as in control

_populations.

These differences are

significant

at the 0.01 level in all but 1 case

_(low

_density,

_young

_{parents vs}

_high

_density,

old

_parents).

In all

other cases the differences between average

longevities

are

insignificant.

In the

high

larval

density population

all

fecundity indices,

except

the

fecundity

7-9

_days,

were

significantly

different between the

_offspring

_{of the young and old}

_parents.

_However,

when the same

_comparisons

are made within low

_density

or control

_populations,

the

picture

which _emerges is

_quite

different. Here

_Tukey’s

test has shown that

_fecundity

indices of the

_{differently aged}

parents, except

for the first

_day

of

_laying

within the

low

_density

population,

are not

_detectably

different from each other

_{(table I).}

All

the other

_comparisons

show that

_fecundity

indices of the young

parent

offspring

in the

_high

_{density population}

are most

_{frequently statistically}

different from the other treatment.

It is worth

_considering

the effect of larval

_density

on the

_longevity

and

fecun-dity

irrespective

of the

_parental

age. Table II compares the means of these life

populations

with low and

_high

_{larval densities. The average}

_{longevity, fecundity}

>

10

d,

last d of egg

laying,

age of

peak

fecundity

and

_laying

rate were

_higher

in

the low

_{density population}

than in the

_high

_{density population.}

In all above cases

the differences between traits at low and

_{high density}

were

_significant,

as shown

_by

the t-test. The observed pattern among life

history

traits,

for weevils selected for

different larval

_density,

is

_{quite opposite}

to that

_{predicted by}

r- and K-selection

regimes

(see

Pianka,

1970).

An outcome such as we obtained is

_expected

in models

of life

_history

in which the effects of variable environment are

_relatively

_greater

on

mortality

of

_juveniles

than adults. Stearns

₍₁₉₇₆₎

discusses these

_predictions

under the

_heading

of

_{&dquo;bet-hedging&dquo;.}

It seems from table I that

parental

age at the time of

reproduction

can also

affect the

_analysed

traits. To test for

_significance

between

_early

and

_{late-reproduced}

parents,

taking simultaneously

into account the effects of larval

_density,

a

_2-way

analysis

of variance was made

(since

we had an unbalanced data

set,

we used the

method described in Steel and

_Torrie,

_1960).

The results of such an

_analysis

of

variance for

_longevity

and different

_fecundity

indices are

_presented

in table III. In all these cases there was a

_{small, statistically}

_{insignificant}

_{&dquo;parental}

_age x

_{density&dquo;}

interaction, suggesting

that these 2 variables are

largely

_independent

in

_moulding

parental

age and

population density

on

_longevity

and

_fecundity

seem to be different.

For all

_fecundity

indices we found

_significant

influence of

_parental

_age

(table III).

Significant

effects of larval

_density

have been found for

_longevity,

as well as for

several

_fecundity

indices

_(see

table

_III).

Figure

1 shows the overall

pattern

of

_{survivorships}

in different treatments. In

order to compare different

_survivorship

curves we have

_employed

the test

_suggested

by

Rose

_(1984).

This test is based on the

_comparison

of

_mortality

rates.

_Concerning

mortality

rate differences there are 2

_hypotheses

to test.

_First,

statistical

_significance

of the _average

_mortality

rate differences between

_paired

_groups.

_{Second, analysis}

of trend with

_respect

to _{age in}

_mortality

rate differences. The raw data for

_testing

both

_hypotheses

have been obtained as follows:

_(1),

age

specific mortality

rates were

calculated for each of 5 4-d

_intervals,

from d 1 to d 20 of the _assay

_(after

d 20 there

were too few

_{individuals); (2),}

_mortality

rate differences between the

_paired

groups

have been divided

_by

the total

_mortality

rate in that

_{age-interval;}

_(3),

the obtained

data were then used to calculate the mean

mortality

rate differences between

_paired

groups.

Through

each

_pair’s

_mortality

rate data the least square linear

regression

None of the estimated average

mortality

rate differences between any

pair

of

compared

populations

is

_{significantly}

different from zero. In

addition,

except

for the

comparison

within control

population

(between

offspring

of _young and old

_parents)

there is no

_apparent

_age

specificity

in

mortality

rate differences between

compared

groups.

(Space

considerations

preclude

the detailed

presentation

of these

results,

but

_they

are available from the senior

author.)

The overall

_patterns

of the mean

_daily

_fecundity

in all treatments are shown in

figure

2.

_Although

there is no

appropriate

statistical test for the entire

fecundity

pattern,

it seems that

_postponement

in

reproductive

_output

is

_present

in the

offspring

of old relative to young

parents

in all treatments.

The

_magnitudes

of the actual responses to

_{one-generation}

selection for

_early

and late

_fecundity

in terms of the

_intensity

of selection

_(i),

selection differentials

_(S),

selection response

(R)

and realized

heritability

(h

2

)

are

_given

in table IV. The

change

in the

_{expected phenotypic}

value in

_offspring

as

_{a result}

of

_{one-generation}

selection that

_chiefly

interests us here is the realized

_{heritability.}

The

patterns

for

both traits

_(early

and late

_fecundities)

suggest

that realized heritabilities are

_greater

in the

_{high density}

population.

A

_comparison

of realized heritabilities between

early

and late

_reproduced

groups indicates that realized heritabilities are

higher

for

early

fecundity

in all

_{experimental populations.}

Narrow and broad sense

_heritability

estimates for

longevity

and

fecundity

data

for

_{early reproduced}

weevils within each

_population

are

_given

in table V. The

heritabilities for late

_reproduced

parents

have been

omitted,

since a valid estimation

of

_genetic

variation with a small number of families

(here

10-16 per

population)

is not

_{possible. Only}

2

_traits,

both within the low

density population,

showed

significant

narrow sense heritabilities

(table

V). Among

all full-sib estimates of

heritability

7 were

significantly

different from zero.

Genetical variation is a _necessary

prerequisite

for the existence of

genetic

correlation between different traits. Since we found no

_significant

heritabilities in most

_analysed

traits,

there was a small

_possibility

of

finding

a

_{significant genetic}

correlation among them.

Nevertheless,

we calculated the additive

_genetic

correlation

among traits for

offspring

of young

parents

within each of 3

_populations.

As

predicted,

none of the estimated correlations are

_{significantly}

different from zero

(for

this reason we omitted their

_{presentation,}

but

_they

are available from the

_authors).

DISCUSSION

According

to Luckinbill and Clare

₍₁₉₈₅₎

_{specific change}

in the external

environ-ment

_{during preadult}

_stages

_(ie

_high

larval

_density)

and the internal environment

during

adult

_stages

_(ie

_reproduction

at the later

_stages)

_give

rise to increased

longevity

in

_{Drosophila melanogaster.}

The results of

_long-term

selection for

de-layed

senescence obtained

by

Rose and Charlesworth

_(1981b),

Rose

_(1984),

Mueller

(1987)

and Service et al

_(1988),

also with D

melanogaster,

are in

_qualitative

accord with the above-cited

_expectation

_regarding

the effects of

_parental

_{age and larval}

density.

It is to be

_{expected, therefore,}

that

_1-generation

selection for late

repro-duction in a

_population

selected 10

_generations

for

_high

larval

_density

will show

obtectus,

we,

however,

have not been successful in

_detecting

_significant

response to

combined effects of late

_parental

age and

high

larval

density

with

respect

to adult

longevity

(table I).

In

_fact,

we have obtained

only

direct _responses to

_1-generation

selection in both

early

and late

_reproducing

_parents

_{(table IV).}

However,

there was no correlation

between the

_fecundity

responses and

longevity. Though

the average

longevities

in late

_reproduced

_{parents suggest}

some enhancement in

longevity,

the overall effects

of this treatment do not reveal

_significant

differences

_{(table III).}

_Finally,

mortality

rate differences between treatments were not observed either.

The outcome of our

previous study

with the bean weevil

(Tuci6

et

al,

1990),

in

which

_long-term

selection for

_early

and late

_reproduction

was

imposed,

has been

consistent with the

_present

results.

Moreover,

these

_findings

are in

_agreement

with

the observation of

Moller

et al

_(1989),

who found a similar

_relationship

between

longevity

and

_fecundity

in another weevil

_species,

Calosobruchus maculatu,s. The

absence of the direct link between

_fecundity

and

_longevity

in our

_experiments

implies,

therefore,

that differences in

_fecundity

of the bean weevils need not involve genes with

pleiotropic

effects on

longevity.

Though

quantitative genetic analyses

seem futile when selection

_experiments

fail to detect a cost of

reproduction trade-off,

we have

_found,

at

_least,

2 reasons for such

analyses. First,

besides the

_{predictive value, analyses}

of the

_genetic

variation and covariation among fitness traits have

_{important descriptive}

value.

_Second,

and of

more interest in the

present context,

there is a

possibility

that observed

trade-offs between

_early

and late

_fecundity

have a

_genetic

basis.

Indeed,

the results

in tables I and III

_suggest

that

_parental

_age could

_{really change}

the

_fecundity

schedule in the way

predicted by antagonistic pleiotropy theory

(ie

that

offspring

of _{the young}

_parents

have

_{higher early fecundity}

than

_offspring

of the old

_parents,

and vice

_versa).

But the absence of

_{statistically}

detectable

_negative

correlations

between

_early

and late fecundities made our

_study

inconclusive with

_respect

to

the

_{possible genetically}

based trade-off among

fecundity

indices. There is another

argument against

antagonistic

pleiotropy

between

_early

and late

fecundity.

Having

in mind our selection

regimes,

it is _very

likely

that

offspring

of the young

parents

lack some

_genetic

variants,

increasing

the late

reproduction,

which are

_present

in

offspring

of the old

_parents.

_Similarly,

the

_high

late

_reproduction

in

_offspring

of the old

_parents

would not be

causally

related to their low

_{early reproduction}

effort

(Partridge,

1987,

has used the same

_arguments

_{against negative genetic pleiotropy}

in the

_{interpretation}

of the above-cited

_long

term selection studies with

_Drosophila).

Hence,

the

_simplest

_{interpretation}

of our results is in terms of

_phenotypic

plasticity

for

_longevity

and

_fecundity.

It means that _every bean weevil female _may

face a

phenotypic

(physiological)

trade-off in the allocation of limited resources

between

_fecundity

and

_longevity,

but in the absence of

_genetic

variation in this

allocation,

the

_genetical

trade-off is undetectable

_{by long-term}

selection

_experiments

or

_{by genetical analyses.}

We could say then that differences between individuals are

based on

_phenotypic

_plasticity

(which

is a

_{short-term, contingent}

_response of an

individual to immediate _{circumstances which may increase}

_fitness),

and a

_genetical

ACKNOWLEDGMENTS

We thank D Marinkovic and the anonymous reviewers for their

helpful suggestions

and

comments on the

manuscript.

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