Laboratory evolution of host plant utilization in the bean weevil (Acanthoscelides obtectus)

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Laboratory evolution of host plant utilization in the

bean weevil (Acanthoscelides obtectus)

N Tucić, D Milanović, S Mikuljanac

To cite this version:

Original

article

Laboratory

evolution of host

_plant

utilization in the bean weevil

(Acanthoscelides

obtectus)

N Tuci&jadnr;

D

Milanovi&jadnr;, S

_Mikuljanac

University of Belgrade,

Institute

_{for Biological}

Research and

Faculty of Science,

Belgrade,

Serbia

(Received

4

_August

_1994;

_accepted

29

_August

₁₉₉₅₎

Summary -

Populations

of the bean weevil

_{(Acanthoscelides obtectus)}

were

_subjected

to 35

generations

of artificial selection for characteristics

_affecting

host utilization when females

were

_exposed

to a choice between 2 hosts

(Phascolus

_vulgaris

and Cicer

arietinum),

or

exposed

to natural selection for the same

period,

when

_only

one host was available. We

obtained a

_positive

_response for the _percentage of eclosed adults on the

chickpea

seeds

in the ‘Cicer choice’ lines but not in the Phaseolus lines. In the

_{post-selection}

test we

demonstrated that oviposition

preference

and

_{egg-to-adult viability}

in the Cicer lines

were

_higher

on the

_chickpea

than on the bean seeds. Lines that had been selected for female

_{oviposition preference}

on

_{chickpea displayed}

the same

_preference

for this host after

selection was terminated as lines that had been maintained on

_chickpea

seeds without

choice.

Acanthoscelides obtectus _/ larval _{density /} selection

_/

_{oviposition preference}

*

Corresponding

address: Institute of

Zoology, Faculty

of

Science,

PO Box _550, Studentski

trg 16, 11000

_Belgrade,

Serbia

Résumé - Évolution en laboratoire de l’utilisation de la plante hôte chez le charançon

du haricot

_{(Acanthoscelides obtectus).}

Des

populations

de

charançon

du haricot

!Acan-thoscelides

_obtectus)

ont été soumises à 35

_{générations}

de sélection pour des caractères

affectant

l’utilisation de la

_plante

hôte, avec un choix

_possible

_pour les

_femelles

entre 2

hôtes

_(Phaseolus

_vulgaris

ou Cicer

ariesinum),

ou soumises à la sélection naturelle durant le même nombre de

_{générations}

sans choix de l’hôte. Une

_réponse

_positive en _pourcentage

d’adultes éclos est obtenue dans les

_lignées

choisissant

_Cicer,

mais non dans celles

choisis-sant Phaseolus. Dans les _comparaisons

_effectuées

à l’issue de la

_sélection,

on _{montre que,}

dans les

lignées Cicer,

la

préférence d’oviposition

et la viabilité du stade

ceuf

au stade adulte

sont

_{plus grandes}

sur les

_grains

de Cicer _que sur ceux de Phaseolus. De

plus,

les

lignées

sélectionnées pour une _oviposition

_{préférentielle}

sur Cicer

manifestent

après sélection la même

préférence

pour cet hôte que des

lignées

maintenues sur Cicer sans

_possibilité

de choix de l’hôte.

INTRODUCTION

There have been many observations of

significant variability

and

_potential

for host

change

in

_phytophagous

insects

_(see

recent reviews in

_{Via, 1990;}

Jaenike and

_Holt,

1991).

These studies have shown that both the behavioural traits which influence the choice of

_{plant species}

for

_feeding

or

_oviposition

(’preference’)

and

_{physiological}

or

_{morphological}

traits that affect

_growth

_and/or

_reproduction

on a

_particular

host

plant

(’performance’)

may have a

_genetic

basis.

Due to its

_special

_evolutionary

_importance,

several studies have

_{separated genetic}

and environmental variance in host

_preference

_(Tabashnik

et

_{al, 1981; Rausher,}

1983;

Jaenike, 1985, 1986, 1989; Lofdahl, 1987;

Singer

et

_{al, 1988; Fox,}

₁₉₉₃₎

or

performance

(Rausher,

_{1984; Via, 1984;}

Hare and

_Kennedy,

_{1986; Futuyma}

and

_Philippi,

_1987;

James et

_al,

_1988).

Some of these short-term

experiments

have demonstrated a

_genetic

correlation between

_preference

and

_performance

(Tavormina,

1982;

Via,

1986;

Singer

et

_{al, 1988;}

_Jaenike,

_1989).

_However,

long-term

_{experiments examining}

_{evolutionary changes}

in host

_preference

_and/or

host utilization

_ability

in insect

_populations

are

largely lacking

(but

see

Gould, 1979;

Wasserman and

_Futuyma,

_1981;

_Fry,

_1990).

We used this

_approach

in a

_{laboratory study}

with bean weevils

_{(Acanthoscelides}

obtectus).

We

_investigated

the

_changes

in the traits

_affecting

host utilization that occurred when bean weevils were

_exposed,

over 35

_generations,

to a choice of 2 hosts

_(bean

and

chickpea

seeds)

and when

only

1 host was available. Here we are in

a

_position

to assess whether the evolution of 2 host

_species

utilization is more

_likely

to

_proceed

_by

the

_propensity

of A obtectus females to

_accept

hosts for

_oviposition

or

by changes

in the

physiological

traits that affect the

ability

of weevil larvae to

use different hosts.

MATERIALS AND METHODS

Life

_history

of the bean weevil and

_experimental

conditions

A obtectus

_(Say)

is a bruchid

_species

that attacks seeds of various

leguminous

_crops.

The

_primary

host of this weevil is the common bean

(Phaseolus vulgaris).

The weevil also attacks

_chickpeas

_{( Cicer arietinum)}

and other stored

_legumes

_(see

Milanovi6

et

_al,

_1991).

The females

_deposit

eggs in clusters under or

_nearby

_single

seeds. The first instar

larva bores into a seed where the beetle

spends

its larval and

pupal

stages.

The final instar larvae excavate a chamber

_just

below the seed testa and the presence of the

larva _may be detected

_by

a small ’window’. After eclosion the adult chews a hole

in the seed coat and

_pulls

itself out of the

_seed,

_ready

to mate. Adults do not feed on the seeds.

_{Moreover, they}

need neither food nor water to

_produce

viable _eggs.

(For

more details of A obtectus life

_history,

see Tuci6 et

_al,

_1991.)

For the

experiments reported here,

we made use of different A obtectus lines established from a base

_population

that had been maintained in the

_laboratory

since 1986.

_(This

is the

_{’synthetic’ population}

established from 3 local

_populations

Serbia.)

The base

_population

was reared at

_{large population}

size

_(about

5 000 individuals in each

_generation)

on P

_vulgaris,

cv

_{’gradištanac’}

seeds.

The

_experiments

were conducted in a dark incubator at 30 °C and a relative

humidity

of about

70%.

All seeds were

_bought

in bulk from one source. Seeds were

frozen before their use in

_experimental

treatments. No food or water was offered to

the adults in the

_experiment.

Experiment

l: selection

_procedures

Four selection

_regimes,

with 2

_replicates

_per

_regime,

were used: 2 ’no-choice’ and 2

’choice’ treatments.

In ‘no-choice’ treatments

_only

one

_species

of host was

offered;

2

replicates

(the

’Phaseolus

_lines’)

were reared on common bean

_seeds,

the other 2

(the

‘Cicer

lines’)

were maintained on

_chickpea

seeds. Since no selection for host

_preference

was

imposed,

the weevils should have

experienced

natural selection for larval

_adaptation

to host

_species.

Each

_replicate

line was initiated with 100

_randomly

chosen adults

from the base

_population.

These

_replicates

were

_kept

in

_separate

bottles

_containing

200 seeds of the

_appropriate

host. This

_procedure

was

_{repeated during}

35

non-overlapping

generations.

’Choice’ treatments also

_produced

_replicated

Phaseolus and Cicer lines which

_began

with 10 groups, each

comprising

10

pairs

of

one-day-old

weevils. The weevils were

placed

in Petri dishes

(50

mm

diameter)

which contained

equal

numbers of bean seeds and

_chickpea

seeds of about the same size

(7 mm).

Seeds were

_place

in each dish so that bean covered one half and

_chickpeas

the other half of the dish. The Petri dishes were

_kept

in a dark incubator and from about 3 weeks onward were checked

_daily

until the eclosion of adults started.

(The

eclosion

is

_{recognized by}

the ’windows’ on the seed testa

_becoming

_black;

otherwise the windows are

grey.)

At that

_time,

beans and

_chickpeas

were

_separated.

In 2

_replicate

lines

_(the

’choice Phaseolus

_lines’),

bean seeds from all 10 dishes were

_{kept together}

in a

_single

bottle. The number of eclosed adults from beans and

_chickpeas

was then

counted. From the

_newly

_emerged

adults from bean seeds we

_{chose, again randomly,}

10 groups with 10

_pairs

of

_beetles,

in order to establish a new

_generation,

which was

again

offered a choice between beans and

_chickpeas.

This

_procedure

was

_repeated

for 35

_generations.

In the ’choice Cicer line’ the same

procedure

was

_applied,

_except

that new

_generations

were founded

_by

adults

_emerging

from the

_chickpeas.

In ’choice’

_treatments,

the selection criterion was the

_percentage

of eclosed adults which

_originated

from the

_appropriate

host

_(hereafter

denoted as ’the

_percentage

of

eclosed

_adults’).

We have chosen this

_composite

trait

_(which

includes the number of _eggs

_laid,

larval

_preference

and larval

_performance)

because the _eggs are

_usually

deposited

underneath the host

seeds,

and therefore

_they

are difficult to count

without

_harming

them.

_Thus,

the information on the

_{oviposition preference}

and larval

_performance

is crucial for the

_{understanding}

of the

_changes

in the utilisation

by

A obtectus under

applied

selection

_regimes.

_(On

the basis of our

_exploratory

experiment

(unpublished data),

we believe that larval

_preference

does not determine host choice because first instar larvae are not _very

_vagile

_(only

this instar has

legs

and can walk to find a

_place

to enter the

_seeds;

_usually

_they

remain underneath the seed where the _eggs were

laid).)

In order to determine how much selected lines

_diverged

from each other in

was

_designed

to measure the

_oviposition

preference,

whereas in the second larval

performance

of different lines was estimated.

Experiment

2:

_{oviposition preference}

To rule out the effect of

_plant

seeds where weevils fed

_during

the larval

stages

as a

_possible

reason for the

_divergence

_among

_lines,

we reared all lines in both host

seeds after the end of the

_selection,

and then tested their

_offspring

with

_regard

to

oviposition preference

in a mixed-host environment. Two _groups of about 50

_pairs

of

_newly

_emerged

weevils from each

_replicate

line were collected. The first _group

was reared for one

generation

on beans and the second on

_chickpeas.

After

_that,

15 5

pairs

of weevils within each

_line/host

seed treatment were tested

individually

for

oviposition preference

in Petri dishes

_containing

_equal

numbers of bean and

_chickpea

seeds.

_Although

the seeds were not distributed

_randomly

in the Petri dishes

_(we

applied

the same conditions as in the ’choice’ selection

regimes),

it is very

unlikely

that this could

_produce

_any bias in the

_{oviposition preference}

because weevil females exhibited a

_{pre-oviposition period}

_(see,

for

_{example, Pouzat,}

_1978).

The number of

eggs

deposited

on each host seed were counted.

Oviposition preference

was measured

only during

the first 4 d of female life _span

_(this

_period

covers about

3/4

of the

female’s

fecundity,

see Tuci6 et

_al,

_1990).

Experiment

3: larval

_performance

To determine whether larval survival differed among the

lines,

the

_egg-to-adult

viability

and

_{pre-adult developmental}

time were also tested after the termination of the selection

experiment.

A

_sample

of about 100

one-day-old

adults was collected

randomly

from one

_replicate

within each

_treatment,

and weevils were mated in

groups. These 4 groups were

_kept

in

_separate

Petri dishes

_containing

bean seeds

only.

Females were allowed to

_lay

_eggs for 24 h. After removal of the

weevils,

the

eggs were counted and collected from the dish bottom and the surface of the beans

using

a

_paint

brush.

_Eggs

collected from each line were divided into 2

_{equal batches,}

one

being

set _up on 5 Petri dishes

_containing

bean seeds and the other on Petri dishes

_containing

_chickpeas.

To

_prevent

differential larval

_densities,

each Petri dish contained 20 seeds and 50 _{eggs. In}

_addition,

a

_density

of about 2-3 larvae _per

_grain

is too low to _express

_pronounced

effects on either survival or

pre-adult

development

(Aleksi6

et

_al,

_1993).

_Thus,

the total number of eggs used for the estimation of the

egg-to-adult

viability

and

_{pre-adult developmental}

time for each line and host was

50 x 5 = 250. The

egg-to-adult viability

is defined _as the

_percentage

of adults _on

each host seed. The duration

_{(in days)}

from

_deposition

of _eggs to _emergence of adults was used to estimate

_{preadult developmental}

time.

Statistical

procedures

Multiway analyses

of variance for the

_percentage

of _eggs laid on

_chickpeas

and for the 4 d

_fecundity

_{(Experiment 2)}

were

_{performed by using}

the PC-EMS

program

(Dalla, 1985).

We have dealt here with 4 factors: selection treatment

(factor

A-fixed),

host

_(B-fixed),

_replicate

lines

_(C-random)

and

_rearing

host

interaction. The model

_description

was: A + B + AB +

_C(AB)

+ D + AD + BD +

ABD +

_C(AB)D,

with mean and error terms not stated

_explicitly.

In this model we have 3 ’error terms’

(numbers

correspond

to levels in the model under ’source of variation’ in table II

_below):

₍₁₀₎

error

(within groups), (9) C(AB)D

and

(8)

C(AB).

In

_F-tests,

terms

_{D, AD,}

BD and ABD and

_C(AB)

were tested

_against

(9)

and terms

_A,

B and AB were tested

_against

(8).

The

_{egg-to-adult viability}

and

_{pre-adult developmental}

time

_(Experiment

₃₎

were

analysed

by 3-way

ANOVA. The factors were: selection treatment

_{(factor A-fixed),}

host

_(B-fixed)

and

_rearing

host

_(C-fixed).

Since here we are

_dealing

with a Model I

(fixed

effects

_model)

_ANOVA,

each F value refers to the error MS.

RESULTS

Responses

to selection

The

_percentages

of eclosed adults on

_chickpeas

or beans over the choice selection

regimes

are

_presented

in

figure

1. It is evident that increases in the

_percentages

of eclosed adults on

_chickpeas

did occur in both

_replicates

of the Cicer line

_{(fig 1B),}

but not in the choice Phaseolus line

_{(fig 1A).}

For each

_replicate

we estimated

regression

coefficients for the

_percentage

of eclosed adults on the

_appropriate

host

(after

arcsin

_{transformation)}

on

_generation

of selection. The coefficients were

_highly

significant

for both choice Cicer line

_(b

_l

= 0.67

f 0.17;

P _<

0.001;

b

2

= 0.86

t 0.19;

P <

_0.001).

A test of

_equality

of

_regression

coefficients did not reveal

_significant

differences between these 2

_replicates

_(F

_S

=

_0.56;

P _>

0.05).

In

contrast,

both

regression

coefficients for the Phaseolus line were

_{non-significant}

_(b,

= 0.07 +

0.14;

b

2

=

-0.04

_{+ 0.16).}

Post-selection tests

The mean

_percentages

of _eggs laid on

_chickpeas

of all the

_lines,

reared on both hosts and tested in a mixed-host

_environment,

are listed in table I. A

_striking

feature of these data is the

_large

difference between hosts where selection was

imposed.

The mean

_percentages

of _eggs laid on

_chickpeas

in the Cicer

_lines,

no matter what the

treatment or the

_rearing

host _were, were

higher

than those in the Phaseolus lines.

This

_finding

was

_{statistically}

confirmed

_by

the results of the factorial

analysis

of

variance

_(table

_II).

In

_addition,

the host seeds where females

_developed

_during

larval

stages

(’rearing

host’ in table

_II),

and interaction ’selection treatment x

_rearing

host’

_(A

x D in table

_II),

contributed

_{significantly}

to host

_preference

variation. The mean

_percentages

of _eggs laid on

_chickpeas

in the ’no-choice’ and ’choice’ treatments

were,

however,

not different from each other

_(F

=

0.95;

P _>

0.05,

table

II).

Although

the results in table I

_suggested

that the mean number of eggs laid per female is

higher

in Phaseolus than in Cicer

lines,

this was not confirmed

_by

the

_analysis

of variance

_{(table II).}

Since the average of the 4 d

fecundity

varied

considerably

across

_rearing

host x

_replicate

cells

(see

line 9 of table

II),

none of the main effects

_{(’selection}

_{treatments’,}

’host seeds’ and

_’rearing

_host’;

table

_II)

was

The number of tested females is

_equal

₍₁₅₎

in all groups.

The average viabilities were estimated from 5 Petri

dishes,

each

_containing

50 eggs. The

numbers of tested individuals for

developmental

time are

_given

in

_parentheses.

viability

but not for the

_{pre-adult developmental}

time. On the beans the average

viabilities

_(pooled

over ’no-choice’ and ’choice’

treatments)

are 62.4 and

50.6%

in the Phaseolus and Cicer

_{lines, respectively.}

On the

_chickpeas

these _averages are

54.8%

(the

Phaseolus

_line)

and

57.4%

_{(the Cicer line).}

This trend is

_{statistically}

confirmed

by

the

_significant

’host x

_rearing

host’ interaction term in the

_analysis

of variance

(table V).

All other effects did not contribute

_{significantly}

to the observed variation of the

_{egg-to-adult viability.}

A

_3-way

ANOVA of the

_{pre-adult developmental}

_time,

on the other

_hand,

showed

_significant

effects of selection

_treatment,

_rearing

host and all interactions

_except

those between host and

_rearing

host

_(B

x C in table

_V).

DISCUSSION

We obtained

_positive

responses to selection for the

_percentage

of eclosed adults on

the

_chickpea

seeds in both ’choice’ Cicer lines

_(fig

_1).

The absence of _{any responses}

in the Phaseolus lines most

_likely

reflects the fact that local

_populations,

from which the base

population

has been established

_(see

Materials and

_methods),

used bean seeds.

_Hence,

we have observed substantial

_genetic

variance for the use of

_chickpea

seeds,

which are not available in the area where the weevils were collected. Our results resemble the data of Lofdahl

₍₁₉₈₇₎

who worked with

_{Drosophila rnojavensis}

offered a novel cactus

_species.

There is one more

_interesting

_aspect

of the data

depicted

in

figure

1. In the

chickpea

selected lines

_{(fig 1B),}

the first

_generations

showed a

_preference

for beans as

_only

about

20%

of eclosed adults

_originated

from the

_{chickpea. By}

the end of the

_experiment

these weevils

_expressed

a

_preference

for

_chickpea,

with more than

50%

of the adults

_emerging

from this host. It seems,

therefore,

that the actual rank

order

_preference

has been

_changed

as a result of selection. These observations do

not

_support

_prediction

of a

_’general

model for individual host selection’

postulated

by

Courtney

et al

_(1989).

These authors _argue that

_changes

in host use are due to

changes

in overall threshold for

_acceptage

of _any

_host,

and that

_changes

in rank order

_preference

are not

_expected.

_Contrary

to our

_results,

2 studies

_(Harrison,

1987; Prokopy

et

_al,

_1988),

_{however, support}

the

_Courtney

et al

₍₁₉₈₉₎

model of evolution of host utilization. Both of these studies have considered host

acceptance

in

_populations

where

_ancestry

is

_known,

and where derived

_populations

have evolved novel host utilization.

The observed _responses to selection indicate the presence of additive

genetic

variation in one or both of the 2 constituent traits of the selection criterion: the

oviposition behaviour,

which determines whether or not the females

_accept

the

host,

and

_egg-to-adult

survival on different hosts. In the

_{post-selection}

test, we

demonstrated that in Cicer lines both the

_{oviposition preference}

_{(table I)}

and egg-to-adult

_viability

_{(table IV)}

were

_higher

on the host where selection was

_imposed.

Accordingly,

it could be concluded

_that,

as a result of the

_{long-term rearing}

of

weevils on the

chickpeas,

females tend to choose

_oviposition

sites in which their

offspring

have a

_{higher probability}

of

_surviving.

Although

some theoretical

_analyses

_predict

that

_genetic

correlation between

preference

and

_performance

could be

_responsible

for the maintenance of

_genetic

variation in habitat selection

_(Bush,

_1974;

but see review in Jaenike and

_Holt,

correlated variation in

_performance

and vice versa

_(eg,

_{Gould, 1979;}

Wasserman

and

_{Futuyma, 1981; Tabashnik,}

_1983;

_Futuyma

and

_Moreno,

_1988).

In this

_study,

however,

we have

_presented

evidence for the

_genetic

correlation between

_preference

and

_performance.

Such correlations _may be attributable either to a fortuitous

pleiotropic relationship

due to biochemical

_and/or

developmental

processes common

to

_oviposition

_preference

and

_egg-to-adult

_viability

or to natural selection

_building

up

linkage disequilibrium

between genes that influence these traits.

Singer

et al

(1988)

envisaged

a _{way in} which a correlation between

_preference

and

_performance

could be

produced.

We believe that their

_{interpretation}

could be

_applied

to our

data as well. Their scenario favours the

_linkage

_{disequilibrium hypothesis}

and the

primacy

of

_{physiological}

adaptation

over the host selection behaviour.

Although

this does not influence our conclusion

_{given above,}

a

post-selection

test of another fitness-related

_trait,

pre-adult developmental

time

_yielded

a more

complicated

picture

(table

IV).

Pre-adult

_{developmental}

_time,

which could be influenced

_by

some

_{physiological}

characters

_(eg,

the

_ability

to overcome certain

toxic

_compounds,

assimilation

_efficiency,

_etc),

differed

_{significantly}

between the ’no-choice’ and ’choice’ selection

_treatments,

but not between host seeds on which

selection was

_imposed

(bean

vs

chickpea).

_Bearing

in mind our methods of

selection,

this was

_quite

an

_expected

result. In order to collect a sufficient number of

one-day-old weevils

₍₁₀₀

females and 100

_males)

for the ’choice’

_treatments,

new

_generations

were

established, usually,

from the first 200

newly

emerged

weevils.

Hence,

the faster

pre-adult development

in the ’choice’ lines was the result of inadvertent selection

for fast

_development

in these lines.

A third fitness-related trait

_(fecundity)

was also measured at the end of the selection

_experiments

_(table

_I).

We counted the number of eggs laid

by

individual

females

_during

the first 4

d,

so this is a kind of ’realized

_fecundity’

(Wasserman

and

_Futuyma,

_1981).

Since the realized

_{fecundity mainly depended}

on nutritional

history during

the larval

_stage,

our data

_{suggest, contrary}

to

_expectation,

that the

primary

host

_(bean

_seeds)

is not

_{nutritionally}

_superior

to the

chickpea.

However,

we have observed that

_only

in the Phaseolus lines more fecund females

laid

_{significantly}

fewer _eggs on the

_chickpeas

than on the beans

_{(table III).}

A

negative

correlation between realized

_fecundity

and

_{oviposition preference}

was more

_pronounced

in the Phaseolus choice line. These

_negative

correlations between

oviposition

and realized

_fecundity

can be

_{tentatively explained}

if increased choice

requires

females to

_spend

more time

_searching

for the

_{preferred host,}

thus

_reducing

laying

time.

_Also,

the lower

_magnitude

of these correlations in ’no-choice’ lines

may be

explained

through

selection

favouring

females that do not

spend

time

searching

for the

_{lacking host,}

as well as

_through

a correlated _response to selection for

_adaptation

to the host. It _seems,

_therefore,

that &dquo;selection on host choice

may be one factor

_{maintaining genetic}

variance in

_fecundity,

an

_important

fitness

component&dquo;

(Courtney

et

_al,

_1989).

_Hence,

an

_important

implication

of the results

presented

in table III would be in the

_explanation

for the

_high

level of

_genetic

variance for

_fecundity

observed

_previously

in A obtectus

_(Tuci6

et

_al,

₁₉₉₀₎

and

Drosophila populations

(Roff

and

_{Mousseau, 1987;}

Tuci6 et al

₁₉₈₈₎

and which

appears to contradict Fisher’s

(1930)

Fundamental Theorem

(ie

the

expectation

that

_populations

at selective

equilibria

have little or no heritable variation for traits

The selection

_experiments

with Callosobruchus maculatus

_(Wasserman

and

Futuyma,

1981)

showed that a

population

that had been selected for female

oviposition

preference

on a

_given

host

species displayed

the same

_preference

for this

host after the selection as a

population

that had been maintained without choice on the same host. These

_findings

are in

_agreement

with our observation for the

no-choice and choice Cicer lines

_{(table I).}

_According

to Wasserman and

_Futuyma

(1981),

the main reason

why

a

population

will evolve a

preference

for the host to

which it is

_exposed

lies either in &dquo;a

_lowering

of a

_specific

threshold of _response

to the

_particular

host with which

_they

are

_confined,

or a lower

_general

_response

threshold,

so that less

_{discriminatory}

_genotypes

_(which

_may come to

_accept

_any of several host

_species)

are favoured&dquo;

(p 615).

ACKNOWLEDGMENTS

We thank the _{anonymous reviewers} for their

_helpful

_suggestions and comments on the

manuscript.

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