Latitudinal clines of allelic frequencies in Mediterranean populations of Ceratitis capitata (Wiedemann)

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Latitudinal clines of allelic frequencies in Mediterranean

populations of Ceratitis capitata (Wiedemann)

A Kourti, P Hatzopoulos

To cite this version:

Original

article

Latitudinal clines of

allelic

_frequencies

in

Mediterranean

_populations

of

Ceratitis

_capitata

_(Wiedemann)

A

Kourti

P

_Hatzopoulos

₂

1

Agricultural University of Athens, Laboratory of Genetics;

2

Agricultural University of Athens, Laboratory of Molecular Biology, Department

of

Agricultural Biology

and

_{Biotechnology,}

Iera

_{Odos, 75,}

11855

Athens,

Greece

(Received

28

_{February 1994; accepted}

3 October

₁₉₉₄₎

Summary - Collections of Ceratitis capitata from 6 different areas from 4 Mediterranean

countries were

analysed

for

_genetic

variation. Six out of the 25 loci tested were found to

be

_polymorphic.

Allelic

_frequencies

were estimated and the

populations

were found to be

panmictic

for 4 out of those 6 loci. Two of the

_polymorphic

loci showed a

significant

clinal

pattern in gene

frequency changes by Spearman

rank correlation with latitude. At 4

_loci,

a _steep

_gradient

in allele

_frequency

_(in

Idh from 1.00 to

_0.63)

is observed in the axis of the north-south transect.

Ceratitis

_capitata

_{/ allelic frequency}

_{/ cline}

Résumé - Clines latitudinaux de _{fréquences alléliques} dans des _populations

médi-terranéennes de Ceratitis capitata

(Wiedemann).

Des échantillons de la mouche des

fruits,

Ceratitis

_capitata,

provenant de 6

_{régions différentes de}

_{4 pays méditerranéens,}

ont été examinés pour leur

polymorphisme génétique

à 25 locus

enzymatiques.

Six des locus se sont avérés

polymorphes

et les

_{fréquences génotypiques}

observées

_à

4 de

ces locus

sont

conformes

à

_{l’hypothèse}

de panmixie. Un cline latitudinal

_significatif

des

fréquences

alléliques

a été mis en évidence

_pour

2 de ces

_{gènes polymorphes}

_par un test de corrélation de rang de

Spearman. Pour

4 locus,

les variations de

fréquences alléliques

dans l’axe

nord-sud sont importantes, par

exemple

la

fréquence

de l’allèle 1.00 du

_gène

Idh varie de

_1,00

à 0,63 le

long

de cet axe.

INTRODUCTION

The Mediterranean fruit

_fly

Ceratitis

_capitata

is a

_polyphagous

and multivoltine

tropical

and

subtropical species,

and one of the most serious

_pests

of fruits and

vegetables.

During

the last 150 years,

medfly

has been established in several countries

_including

those of the Mediterranean basin from its

_proposed

_equatorial

African

_origin

_(Fletcher,

_1989).

Even

_though

the first

report

of this

pest

within the

_European

Mediterranean area dates from the middle of the last

century,

the

_medfly

has

_{expanded throughout}

this basin because this

_region

offers an

increasing

number of host fruits of different cultivars

_and/or

_species

_(De

_Breme,

1842; Martelli, 1910; Fimiani,

1989).

Medfly

is a serious threat to

_important

fruit

production

centers

_throughout

the world.

_However,

the

_biology

of this

_species

is

poorly

known,

especially

its

_population

_genetics.

Genetic

_{variability might}

also

provide

information on the

spread

of C

_capitata

(Kourti

et

al, 1990;

Gasperi

et

_al,

_1991).

Recent results showed

_large

_genetic

differences between introduced

populations

and those

_postulated

to be their ancestral African ones. The average

heterozygosity

of

_medfly

populations

within the Mediterranean basin is about

5%

versus

22%

in the African

_populations

_(Huettel

et

_{al, 1980;}

Kourti et

_al,

_1990;

Gasperi

et

al,

1991).

This

_{significant heterozygosity}

of African

_populations

_(22%)

is

comparable

to that of other insect natural

_populations,

_eg,

_Drosophila

_(Ayala

et

_al,

1972; Lewontin, 1974; Prakash, 1977; Hyytia

et

_al,

_1985).

The

_similarity

_displayed

in allele

_frequencies

among

medfly

or

Drosophila populations

could be

expected

under the neutral

_theory,

_provided

that some

_exchange

of

_genetic

material takes

place

between

_populations

_(Kimura

and

_{Marayama, 1971;}

Kimura and

_Ohta,

_1971).

In _{many cases,} it was concluded that the

_{subcosmopolitan}

status of

_medfly

was

recently achieved,

due to human

transport,

and that any

genetic

divergence

between

geographically

distant

_populations

has occurred over a _very short time.

_However,

C

capitata

appeared

to be

_adapted

to different environmental

conditions,

being

able to constitute

_large

_populations

in both the

_tropics

and the

_temperate

_regions.

Since the

_polymorphism

of the introduced

_populations

is low

_compared

with that of their African

counterparts,

we focus our effort on

elucidating

this

discrepancy

among

populations originating

from different Mediterranean areas. This

_study

was

designed

to detect latitudinal clines in gene

frequencies.

For this _purpose, 6 natural

populations

from

_distantly

located areas were examined.

MATERIALS AND METHODS

Origin

of

_populations

Six

_populations

of C

_capitata

from the Mediterranean basin were used in this

_study.

The

_{Spanish population}

was

represented

by

a

sample

of _pupae on

_peaches

collected

in October 1986 in the area of Castellon

_(S).

The Greek

_populations

were:

a)

from

Attiki,

where _pupae were collected on

peaches

in

July

of 1986

(G1);

and

b)

from the

island of

_Crete,

where _pupae were also collected on

_peaches

in

_July

1986 in the area

of Chania

_(G2).

The Israelian

_population

was from the area of Best

_Dagan,

where

pupae were collected on

_apricots

in June 1984

(I).

The

_{Egyptian populations}

were

Kalubia

_(E1);

and

_b)

pupae on

_apricots

collected the same

_period

in the area of El

Fayum

(E2).

Collections of wild flies from these Mediterranean

_populations

were

made

_by

_harvesting

infested fruits from the

_ground.

An effort was made to collect

samples

from 1 food source: the stone fruits

_(peaches

or

apricots).

Electrophoretic

studies

The

_preparation

of

_samples,

and

_{electrophoretic}

and

_staining

_procedures

have been described

_by

Kourti et al

_(1990).

For each

individual,

25

_enzymes’

loci were tested

in the 6

_populations.

These were:

Mpi, To,

Diaph-1

and

_Dia!h-2,

Adh, Ak, Odh,

G-6-pd, 6-pgd,

Idh, Hk-1,

Hk-2, Got-1,

Got-2, Fum, Est, Lap, Me,

Mdh,

Phi,

Pgm,

a-Gpdh,

Pep-1, Pep-2

and

Pep-3.

From these the

_following

_enzymes were

poly-morphic: MPI,

G-6-PD,

IDH, PEP-1,

PEP-2 and PEP-3. In a limited number of collections EST was also

_polymorphic.

Statistics

Chi-square

tests were

_performed

to _compare observed numbers with those

_expected

under

_{Hardy-Weinberg}

_equilibrium.

The

_Chi-square

test of the correlation between genes

(Barker

et

_al,

₁₉₈₆₎

was

_applied

for loci with more than 2 alleles. The

_degree

of

relationship

between clinal

_patterns

was determined

_by

_Spearman

rank correlation

(Statgraph

Statistical

_Program).

The

_strategy

is to look at conditional

frequencies

and their correlations with latitude as clinal measures.

RESULTS

The allelic

_frequencies

at the 6

_polymorphic

_loci,

the collection sites and the number of individuals

_analysed

are listed in table I. Most loci have 2 alleles with the

exception

of

_G-6-pd.

All the

_peptidases

are

_unmapped

while the

position

of the other 3 loci is known

_(Malacrida

et

_al,

_1987).

The

_Mpi

and

_{Pep-1 loci}

are

_polymorphic

in all

_places,

while the others were found

_monomorphic

at least in 1 of the collection

sites. In order to have a constant

_scoring

and

_recording

of

_data,

the relative

_mobility

of each allele was defined as

previously

described

_by

Kourti et al

_(1990).

The

genotypes

were

_pooled

into 2

_classes,

_homozygotes

and

_{heterozygotes}

in order to

perform

a uniform statistical

_comparison

of observed versus

_expected

_genotypes.

In table

_II,

the numbers of

_homozygotes

and

_{heterozygotes}

observed are

_compared

with the

expected

ones.

_Using

the allelic

_frequencies

determined at each

_locality,

_expected

genotypic proportions

(Hardy-Weinberg equilibrium)

were

_compared

with those

observed

_{(table II).}

The

_x

₂

value

is never

_significant

for the biallelic

_loci,

whereas in the case of

_G-6-pd

2

_samples

showed

_significant

deviation from

_{Hardy-Weinberg}

expectation.

The correlation coefficient rs between the common allele

_frequency

and latitude is

_given

in table III. The

_Spearman

rank correlation coefficient

_(r

_s

₎

is

given

in table III. This

_analysis

showed that 2 loci

_G-6-pd

and Idh exhibit

_significant

correlation between the common allele of these loci with latitude. Recent results

obtained from

_only

a few

_{populations regarding}

Est

_{polymorphisms}

also showed

Alleles in order of

increasing

anodal

_mobilities;

n, number of individuals

analysed.

Populations according

to country of

_origin.

The numbers in the row below the countries

represent latitudinal

positions.

At the 2

_{loci, G-6-pd}

and Idh

_{showing significant}

correlations with

latitude,

the common allele

_frequencies

exhibit a

gradual change

from different localities of

collections, indicating

the _presence of a latitudinal cline

(fig 1).

In all _cases, there is a

large

frequency

difference between the third site of

collection

_(Crete)

and fourth

_{(Israel) (fig 1).}

The same difference in

G-6-pd

is

smaller. In

_contrast,

the

_{frequency changes}

of the most common allele at the locus

Mpi

are rather smooth with the

_exception

of the

_population

collected from

Spain

(fig la).

For loci

_showing

the smallest

_frequency

difference, Pep-1

and

_Pep-2,

these

G-6-pd,

Mpi,

Pep-3

and

_Idh,

are

_{19.9, 21.3,}

27.3 and

36.7%

_{respectively.}

The

_Mpi

frequency

difference between natural

_populations

collected from

_Egypt

and from Attiki is

33%

and is one of the

_highest

observed.

DISCUSSION

An extensive

_analysis

has shown that the

_polymorphism

_(at

_{least 16 genes} out of

₂₅₎

found in the African

_populations

of C

_capitata

was

high.

However,

only

6 loci out of

medfly

(Kourti

et

_al,

_1990).

It is

_interesting

to note that these 6

_{loci, Mpi,}

_G-6-pd,

Idh,

Pep-1,

Pep-2

and

_Pep-3

maintain variants at substantial

_frequencies

and from

these,

the

frequencies

of

_Mpi,

_G-6-pd,

Idh and

Pep-3

change steadily

with latitude. When

_neighboring

_populations

of a

_species

are

_compared,

one finds that

_{they usually}

differ from one

_{another, slightly}

or

_appreciably,

in a number of

characteristics,

eg,

size,

color or _any other

morphological

or

physiological

character

(Halkka

et

al,

1975;

Rhomberg

and

_Singh,

_1989).

_Huxley

₍₁₉₄₂₎

introduced the term ’cline’ for such a character

_gradient.

The

_gradual

_change

of the common allele

_frequencies

for at least these 4 loci

_corresponds

to the latitudinal cline definition.

_However,

only

2

_loci,

_G-6-pd

and

_Idh,

have shown a

_significant

_rs. The

cyclic

behavior of

Mpi,

found

_by

Malacrida et al

₍₁₉₉₂₎

in another Mediterranean

_region

_(Italy),

corroborates the results of the latitudinal cline observed

_through

the Mediterranean

basin.

_{Alternatively,}

observed variations

_(ie

_Mpi)

in allelic

_frequencies

could be the combined effect of latitude and season. Several loci have been

_reported

as

latitudinally

clinal in other insect natural

_populations.

The list includes

Est-6,

Adh, a-Gpd,

G-6-pd, Odh,

6-Pgd,

Aph,

Est-C,

and Mdh

_(Voelker

et

_{al, 1977, 1978;}

Anderson,

1981;

Oakeshott et

_{al, 1981, 1982; David, 1982;}

Anderson and

_Oakeshott,

1984).

The seasonal

_cycle

in

_Mpi

and the latitudinal clines of other loci

_suggest

that alternative alleles have different

_optimal

_temperatures

or other environmental

variables

_(see

also Tomiuk and

_Wohrmann,

_1984).

The

_working

_hypothesis

is that the cline results from a

_{geographically}

variable selective factor of the environment

(Halkka

et

_al,

_1975).

A concordance of clines for different characters is

_{normally only}

found where ranges are

essentially

latitudinal and where the various environmental

gradients

(temperature

and

_humidity

for

_example)

run more or less in a

_parallel

way. The

study

of

geographic

variations has revealed that many of them are clinal.

Clines are,

ultimately,

the

_product

of 2

_conflicting

_forces,

_selection,

which would make every

population uniquely adapted

to its local

environment,

and gene

flow,

which would tend to

_homogenize

all

_populations.

While some alleles

sampled

at

_{high frequency}

in African

_populations

are also

found in Mediterranean

_{populations, others, especially}

the uncommon _ones, are

locally

or

_regionally

lost

(Kourti

et

_al,

_1990).

This new

_pattern

_{of allozyme frequency}

is maintained at a

_relatively

stable level.

selection for which the

equilibrium

shifts with climate. Therefore the

_adaptively

neutral or

_nearly

neutral variation is

_expected

to be

_purged,

most

_likely

_by

the

repeated

passage of local

populations through

bottlenecks

(Kourti

et

al,

1990).

Moreover,

the lower

_polymorphism

of Mediterranean

_populations

compared

with the African

_populations

resembles the

_general

_pattern

of

_{marginal populations}

of a

species

submitted to

_genetic

drift. Loci that are under rather

_strong

_balancing

selec-tion _{manage to} maintain their total

variability

while others become

_monomorphic.

Identification of such

loci,

however small in

_proportion,

as in the case of

medfly,

may be more

_important

than

_performing

_generalized

tests of the neutral

_theory

(Tomiuk, 1987).

This

explanation

of

widespread monomorphism,

the

_difficulty

of

establishing

and

_maintaining

alleles

_expect

_by

_balancing

selection,

is an alternative

to the

_hypothesis

of broad

_adaptability

of

_’general

_purpose

_genotypes’

_(Parker

et

al, 1977;

Angus

and

Schutz, 1979;

Jaenike et

al, 1980;

Lynch,

1983).

Environment

_(latitude

_and/or

_season)

could exert its force on

allozyme

frequen-cies. Since it is reasonable to

_expect

that some loci

_might

be

_important

selective

targets,

and that there is a

_relationship

between

genotype

and environment

_(often

confirmed),

as

_previously

_discussed,

_neutrality

could not

_{satisfactorily explain}

the data. Similar results have been

_reported

for other natural

populations

(Johnson

et

al, 1969;

Johnson and

Schaffer, 1973;

Voelker et

al,

1978).

In most cases there are

correlations between allelic

_frequencies

and environmental variables

_{clearly favoring}

the selection

_hypothesis

_(Schaffer

and

Johnson,

1974).

In

_conclusion,

the

_genetic

latitudinal variations of C

_capitata

could be

considered,

as a

_whole,

as a _consequence of natural selection. Further

_{studies, comparing}

tropical

and

temperate

populations

from other

_parts

of the

_world,

will determine whether such clines exist worldwide.

ACKNOWLEDGMENTS

We would like to thank K Krimbas for critical

_reading

and attentive comments of this manuscript. AK is thankful to M Loukas for

_introducing

her to the C

_capitata

_system.

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