Molecular Ecology (2005) 14, 869–878
doi: 10.1111/j.1365-294X.2005.02457.x
Is genome size influenced by colonization of new
environments in dipteran species?
Blackwell Publishing, Ltd.
C . N A R D O N ,* G . D E C E L I E R E ,* C . L Œ V E N B R U C K ,* M . W E I S S ,† C . V I E I R A * and C . B I É M O N T *
*Laboratoire de Biométrie et Biologie Evolutive, UMR C.N.R.S. 5558. Université Lyon 1, 69622 Villeurbanne cedex, France,
†Centre de Génétique Molèculaire et Cellulaire, UMR C.N.R.S. 5534. Université Lyon 1, 69622 Villeurbanne cedex, France
Abstract
Genome size differences are usually attributed to the amplification and deletion of various
repeated DNA sequences, including transposable elements (TEs). Because environmental
changes may promote modifications in the amount of these repeated sequences, it has been
postulated that when a species colonizes new environments this could be followed by an
increase in its genome size. We tested this hypothesis by estimating the genome size of
geographically distinct populations of Drosophila ananassae, Drosophila malerkotliana,
Drosophila melanogaster, Drosophila simulans, Drosophila subobscura, and Zaprionus
indianus, all of which have known colonization capacities. There was no strong statistical
differences between continents for most species. However, we found that populations of
D. melanogaster from east Africa have smaller genomes than more recent populations. For
species in which colonization is a recent event, the differences between genome sizes do
not thus seem to be related to colonization history. These findings suggest either that
genome size is seldom modified in a significant way during colonization or that it takes
time for genome size of invading species to change significantly.
Keywords: colonization, Drosophila, flow cytometry, genome size, population
Received 11 October 2004; revision received 19 November 2004; accepted 5 December 2004
Introduction
Genome size, which varies considerably among organisms,
and by more than 200 000-fold between different eukaryotes,
is not linked to biological complexity (Gregory 2001). This
variation results mainly from the amplification, deletion,
and divergence of various kinds of repetitive sequences,
including the transposable elements (TEs), which constitute
a large fraction of the genome (Keyl 1965; Flavell 1980; Black
& Rai 1988; Warren & Crampton 1991; Pagel & Johnstone
1992; Johnston et al. 1996; SanMiguel et al. 1996; Uozu et al.
1997; Gregory & Hebert 1999; Petrov 2001). Intraspecific
variation has also been reported and is related to environmental conditions, suggesting either that genome size is a
selective trait associated with cell and body size (Gregory
2001) or that TEs may be mobilized under stressful environmental conditions (McDonald 1989, 1995; Brezinsky et al.
1992; Arnault & Dufournel 1994; Vieira et al. 1999; Hagan
et al. 2003), as has been clearly detected in bacteria and
Correspondence: Christian Biémont, Fax: (33) 4 72 43 13 88; E-mail:
[email protected]
© 2005 Blackwell Publishing Ltd
yeast (Arnault & Dufournel 1994; Kidwell & Lisch 1997), or
by specific genetic conditions (Wisotzkey et al. 1997; Biémont
et al. 1999; Labrador et al. 1999), in some cases following
horizontal transfer (Kidwell & Lisch 1997).
Several sets of evidence suggest an association between
TE mobilization and the colonization processes, as shown
by the high copy number of the Uhu and LOA elements in
Hawaiian Drosophila, which suggests that the colonization
of new islands from older islands has resulted in a significant
increase in copy number of these TEs (Wisotzkey et al.
1997). The unusual distribution of Osvaldo in populations
from the Iberian Peninsula compared to that in populations from Argentina is also thought to be a consequence of
the colonization process (Labrador et al. 1998). It has thus
been suggested that inbreeding (Brookfield & Badge 1997)
and changes in regulatory networks (Bhadra et al. 1997)
could result in an increase in transposition rate each time a
new area is colonized (Labrador et al. 1998, 1999). TE transposition and amplification have also been seen in hybrid
Drosophila (Evgen’ev et al. 1982; Labrador et al. 1999) and
Australian wallaby (O’Neill et al. 1998) as a result of hybridization between populations that had not previously met,
870 C . N A R D O N E T A L .
and it has been shown that the mutation rate is higher in
crosses between previously geographically separated strains
of Drosophila (Thompson & Woodruff 1980). However,
no difference in the abundance of mariner elements was
observed within native and newly derived populations
of the medfly, Ceratitis capitata (Torti et al. 2000), suggesting
that the rapid colonization process of this pest (Davies et al.
1999) has not yet affected the copy number of these TEs.
This could mean that the time since invasion is important,
but it could also simply reflect the specific behaviour of
one element. There is indeed no reason why all the TEs of
a given species should follow the same pattern. This is
especially evident in Drosophila melanogaster, natural populations of which show an increase in their global amount of
TEs that parallels their geographical distance from Africa,
the birthplace of this species (Biémont et al. 1999; Vieira &
Biémont 2004), whereas no such tendency is found for each
TE taken individually. A recent increase in the activity of
some TEs has also been reported in association with the
colonization patterns of Drosophila simulans (Biémont et al.
2003; Vieira & Biémont 2004), and not all the TEs have a
high copy number in the population in Canberra, which is
characterized by a large genome (Vieira et al. 2000, 2002).
Although change in TE amount really affects genome size,
it has been shown, however, that other repeated DNA
sequences must also be involved (Vieira et al. 2002).
If an increase in genome size can result from an increase
in repeated DNA amount, which in turn could be promoted
by environmental stress, populations submitted to novel
environmental conditions, as may happen when populations invade new habitat, can be expected to experience the
mobilization or amplification of these DNA sequences and
a consequent increase in genome size (Arnault & Dufournel
1994; Wisotzkey et al. 1997; Labrador et al. 1999; Kalendar
et al. 2000; Vieira et al. 2002; Vieira & Biémont 2004). We tested
this hypothesis by determining the genome size, estimated
by flow cytometry, of various populations of Drosophilidae strains known to have different colonization histories:
Drosophila ananassae, Drosophila malerkotliana, Drosophila
melanogaster, Drosophila simulans, Drosophila subobscura, and
Zaprionus indianus (Lemeunier et al. 1986). We thus checked
for change in genome size in the range of what was already
reported between populations in D. melanogaster and D.
simulans (Vieira et al. 2002; Vieira & Biémont 2004).
Materials and methods
Species used
The flies came from natural populations of various species
collected by M. L. Cariou (Drosophila malerkotliana and
Drosphila ananassae), J. David (Drosphila subobscura, Zaprionus
indianus), A. Brehm, J.A. Castro, G. Gilchrist, R. Huey and
W. Miller (D. subobscura), and by various other researchers
(Drosphila melanogaster and Drosphila simulans). The flies were
either maintained as isofemale lines or small mass-mated
populations. See Figs 1 and 2 for the original locations and
map positions of the various population samples analysed.
For the detailed origin and geographical distribution of the
various species see Lemeunier et al. (1986), Lachaise et al.
(1988, 2003) and the succeeding summary.
D. melanogaster and D. simulans. D. melanogaster is a strict
human commensal that is thought to have colonized the
entire globe a long time ago, following human colonization
from their ancestral African home area. The worldwide
dispersal of D. simulans from tropical Africa, its cradle, is
thought to have occurred more recently, perhaps as little as
10 000 bp (David & Capy 1988; Lachaise et al. 1988), and this
species is still assumed to be in the process of geographical
spread (Lachaise et al. 1988; Hamblin & Veuille 1999). Because
D. simulans overall has a lower level of morphological,
chromosomal, enzymatic, and mitochondrial polymorphism
than D. melanogaster but has higher nucleotide polymorphism, its effective size is considered greater than that of D.
melanogaster (see Lachaise et al. 2003; Lachaise & Silvain 2004).
D. melanogaster is considered to be highly differentiated
into geographical populations, but DNA data suggest that
some differentiation and structuring may also exist among
populations of D. simulans, mostly between Africa and the rest
of the world (see Lachaise & Silvain 2004; Veuille et al. 2004).
D. malerkotliana and D. ananassae. The two species D.
ananassae and D malerkotliana are very widespread and
subcosmopolitan in their distribution. D. malerkotliana
is Oriental, Afrotropical, and Neotropical. D. ananassae is
circumtropical (Lemeunier et al. 1986; Vogl et al. 2003), and
was so far the only representative species of the ananassae
subgroup in North America. The ananassae complex is
thought to have arisen in South-East Asia, where nine species
are endemic, and then radiated into the Pacific islands.
The arrival of D. malerkotliana in Afrotropical regions is a
recent event. For example, this species was introduced into
the Seychelles during the 20th century (Louis & David 1986),
and is now very abundant in South America as a result
of a primary invasion in the 1950s and 1960s (Val & Sene
1980). It has only recently been collected in North America
(Birdsley 2003; Medeiros et al. 2003).
D. subobscura. D. subobscura is a Palaeartic species of the
obscura subgroup of Drosophila, which is widely distributed
throughout Europe, with the exception of central and
northern Scandinavia. It is also present in the Middle East,
northern Africa and the Atlantic islands of the Azores,
Madeira and the Canaries (Castro et al. 1999). This species
has recently colonized the North American continent, where
a population was first identified in the early 1980s (Ayala
et al. 1989; Balanya et al. 1994; Huey et al. 2000).
© 2005 Blackwell Publishing Ltd, Molecular Ecology, 14, 869–878
G E N O M E S I Z E O F D I P T E R A N S P E C I E S 871
Fig. 1 Map localization of the population
samples of Drosophila melanogaster, Drosophila
simulans, and Zaprionus indianus.
Zaprionus indianus. We have no good evidence for the exact
origin of Z. indianus, but some authors consider it to be of
Afrotropical origin (Tsacas 1985; J.R. David, personal
communication). This species was introduced into Brazil
in 1998 (Vilela 1999; Santos et al. 2003), spread very rapidly
(Castro & Valente 2001; Tidon et al. 2003), has adopted a pest
behaviour, displacing other drosophilid species (Santos et al.
2003), and is now abundant in most places up to the equator
(Tsacas 1985; J.R. David and M. Martins, unpublished),
© 2005 Blackwell Publishing Ltd, Molecular Ecology, 14, 869–878
Genome size estimation
Nuclei were extracted from five heads of freshly hatched
male flies. The heads were crushed in a small, siliconized
Eppendorf vial containing 200 µL labelling solution (0.1 g
trisodium citrate, 0.01 mL Triton X100, 0.05 mg RNase-A,
water UHQ 100 mL) with 1 µg/mL propidium iodide
(Aldrich) (see Vieira et al. 2002; Nardon et al. 2003). Tetraodon
(Tetraodon nigroviridis) blood was used as the internal
872 C . N A R D O N E T A L .
Fig. 2 Map localization of the population
samples of Drosophila ananassae, Drosophila
malerkotliana, and Drosophila subobscura.
standard for diploid genome size, and was labelled by
exposure to 2 µg/mL of the fluorescent dye 5 – 6-carboxyfluorescein diacetate succinimidyl ester (CFSE, Molecular
Probes) in PBS for 15 min at 37 °C. The reaction was stopped
by adding an equal volume of cold fetal calf serum, and the
vial was kept in ice. Twenty microliters of stained tetraodon
blood was then added to the solution of fly head nuclei.
The mixture was incubated for 10 min in ice, and filtered
across 140-micron then 30-micron nylon meshes. Six hundred
microliters of the initial labelling solution was then added
to achieve the appropriate dilution. The resulting solution
was analysed on a FACSCalibur flow cytometer (Becton
Dickinson Instruments) fitted with an argon laser at 488 nm
wavelength. About 10 000 nuclei were analysed, with an
average rate of 500–800 events/s for each determination of
the diploid genome size of the flies. Although the diploid
genome size of the tetraodon is usually reported to be 0.8 pg,
we have found significant variation around this value for
different fish species. Hence, the blood used throughout the
experiment all originated from a single fish. The genome
© 2005 Blackwell Publishing Ltd, Molecular Ecology, 14, 869–878
G E N O M E S I Z E O F D I P T E R A N S P E C I E S 873
size of the flies was thus estimated in terms of the fish
blood used as the internal control, and we determined the
relative fluorescence intensity (the ratio of fly fluorescence
intensity over tetraodon fluorescence intensity) rather than
the absolute genome size of the fly nuclei. For examples
illustrating the flow distributions of fluorescence intensity
of fly and tetraodon nuclei, see Nardon et al. (2003).
In all experiments in which different populations of a
given species were to be compared, we added the chemicals
to the Eppendorf vials following a random order of the
population samples. This eliminated any bias because of a
possible cytometer drift over time and differing time of
exposure to the chemicals, which could have happened if
the various populations had been tested successively in the
same order (Nardon et al. 2003).
Genome size estimation is very sensitive to various
environmental factors (temperature, humidity, age of the
flies, see Nardon et al. 2003), and so the flies of the different
species were maintained under our laboratory conditions
at 25 °C for two to three generations before their genome
size was estimated. Because the variance in genome size was
low between sets of five heads coming from a given vial but
was high between vials (Nardon et al. 2003), we estimated
genome size on two independent experiments for each
species, and on two lines per population when available.
Statistical analysis
We investigated whether the estimated genome size within
a given species differed in different geographical regions
by anova or by the Wilcoxon Rank Sum test applied to
data from groups of populations: west and east Africa,
South and North America, Asia, Europe, and Oceania. The
data were tested for the normality of their distribution by
a Kolmogorov–Smirnov test, and for homoscedasticity by
the Bartlett’s test.
Results
Table 1 and Fig. 1 summarize the mean values and distribution of genome size in all the population samples investigated. There was a significant tendency for populations
to have smaller genomes in India in the case of Zaprionus
indianus (F2,41 = 43.2, P < 0.001). Drosophila malerkotliana had
a smaller genome size in Asia but the statistical test was not
significant (F3,17 = 1.24, P = 0.31) because the sample size
was too small. There was no statistical difference between
continents for Drosophila ananassae (F4,61 = 0.12, P = 0.97),
Drosphila subobscura (F2,21 = 1.13, P = 0.34), and Drosophila
melanogaster (F4,39 = 2.21, P = 0.09). For the last species,
however, a Wilcoxon Rank Sum test showed that values in
east Africa were statistically different from both those for
the rest of the world (U6,38 = 15, P = 0.0001) and those for west
Africa (U6,8 = 3, P = 0.005). There was a small difference in
© 2005 Blackwell Publishing Ltd, Molecular Ecology, 14, 869–878
genome size between continents in Drosophila simulans
(F5,44 = 2.4, P = 0.05) attributable to the fact that the mean
size of the populations were smallest in South America, but
this overlooked the small genome size in some African
populations. And while the genome size of D. ananassae
showed no overall significant difference between continents,
the smallest genome sizes were observed in some populations from Asia.
Discussion
Because genome size has been found to be associated with
environmental conditions (altitude, latitude, temperature,
etc.) (Turpeinen et al. 1999; Temsch & Greilhuber 2001) and
with cell and body size (Gregory 2001), it has been suggested
that selection could be involved in regulating genome size.
However, a change in size could be primarily because of the
ability of the genome to tolerate repeated sequences (Pagel
& Johnstone 1992) or of its ability to mobilize them (Vieira
et al. 2002), possibly in response to environmental conditions,
as has been suggested in plants (Kalendar et al. 2000),
Drosophila (Vieira et al. 1999, 2002) and pocket gophers
(Thomomys) (Sherwood & Patton 1982). Any stressful conditions known to mobilize transposable elements, such as
UV light, temperature, breeding conditions, etc. (Arnault &
Dufournel 1994) could therefore constitute factors that are
able to influence genome size. The mobilization of repeated
DNA sequences may therefore be a strategy for adapting to
a changing environment, as proposed by Walbot & Cullis
(1983, 1985) or could simply be a by-product of the effects
of the stresses encountered during the colonization process.
Drosphila melanogaster, which has a smaller genome size in
its probable region of origin, east Africa (Lemeunier et al. 1986;
Lachaise et al. 1988, 2003), could thus be a good illustration
of the hypothesis that colonization is followed by an increase
in genome size.
Drosophila simulans, Drosophila subobscura, and Drosophila
ananassae do not display obviously smaller average genomes
in their probable region of origin, although smaller sizes have
been observed in some populations within these regions.
Because D. melanogaster is thought to have colonized the
entire globe a long time ago, whereas D. simulans is thought
to have dispersed more recently, this suggests that the time
since colonization occurred may matter, and that change in
genome size, when it happens, is a very slow process. This
led us to wonder why the smallest genome size in D. simulans
was observed in populations from South America. This
situation may reflect the facts that firstly this species is
thought to have been introduced into the Americas after
the European conquest around 500 bp (Wall et al. 2002) and,
that secondly South America was colonized by flies from
the African populations, which have a small genome size.
If the time since colonization is important in determining
genome size change, then it would appear that Zaprionus
874 C . N A R D O N E T A L .
Table 1 Estimated relative mean genome size (pg) of two samples per population of the various Drosophila species investigated
Species
D. melanogaster
Origin
Europe
St Cyprien
(France)
Czechoslovakia
Chicharo
(Portugal)
Madeira
Greece
West Africa
Senegal
D. simulans
D. malerkotliana
Genome
size
Origin
Genome
size
Origin
0.4765
(0.018)
0.4554
(0.006)
0.4585
(0.004)
0.4693
(0.001)
0.4611
(0.005)
0.3869
(0.003)
0.4078
(0.005)
0.3957
(0.004)
0.3888
(0.002)
Moscow
(Russia)
St. Cyprien
(France)
Czechoslovakia
Madeira
Bissao
(Guinea)
0.4589
(0.005)
0.4498
(0.001)
Nasrallah
(Tunisia)
Marrakech
(Morocco)
0.3899
(0.001)
0.4085
(0.006)
Brazzaville
(Congo)
0.4693
(0.005)
Brazzaville
(Congo)
0.3841
(0.007)
East Africa
Djamena
(Chad)
Kenya
0.4582
(0.012)
0.4449
(0.005)
Mamoudzou 0.4408
(Mayotte)
(0.005)
Tanta
(Egypt)
Kwale
(Kenya)
Meru
(Kenya)
0.3981
(0.005)
0.4080
(0.017)
0.3967
(0.003)
Madagascar
Makindu
(Kenya)
Arusha
(Tanzania)
Johannesburg
(South
Africa)
Mayotte
0.3902
(0.001)
0.3932
(0.003)
0.3931
(0.001)
0.4490
(0.007)
Antananarivo
(Mada
gascar)
D. ananassae
Genome
size
Origin
D. subobscura
Genome
size
Origin
Genome
size
Origin
Genome
size
0.4384
(0.001)
0.4356
(0.023)
0.4180
(0.004)
0.4724
(0.025)
0.3937
(0.005)
Madeira
0.6477
(0.014)
Principe
(Cap Verde)
Abidjan
(Côte
d’Ivoire)
Cotonou
(Dahomey)
0.6322
(0.004)
0.6228
(0.008)
0.5499
(0.032)
0.6069
(0.025)
0.6087
(0.034)
Sao Tomè
0.6204
(0.011)
0.6317
(0.001)
0.6286
(0.009)
0.5675
(0.029)
0.5486
(0.027)
0.5814
(0.019)
Nairobi
(Kenya)
Kenya
Helsinki
(Finland)
Moscow
(Russia)
Vienna
(Austria)
Prunay
(France)
Calvia
(Balearic
Islands)
Madeira
Taô
0.5681
(Côte d’Ivoire) (0.019)
Sao Tomè
0.5907
(0.020)
Comoros
0.5837
(0.021)
Cameroon
Taô
(Côte
d’Ivoire)
Bouakè
(Côte
d’Ivoire)
Brazzaville
(Congo)
Jeffa
(Benin)
Nago
(Benin)
Mayotte
Maurice
Island
St Louis
(Maurice
Island)
Z. indianus
0.5490
(0.021)
0.5837
(0.018)
0.5409
(0.027)
0.4244
(0.019)
Marrakech 0.4020
(Morocco) (0.003)
Pointe Noire
(Congo)
Congo
Madagascar
0.6398
(0.004)
0.6288
(0.000)
0.6362
(0.001)
0.6184
(0.014)
Antananarivo 0.6291
(Madagascar) (0.002)
Reunion
0.6371
Island
(0.004)
Maurice
0.6352
Island
(0.005)
0.3975
(0.002)
0.3977
(0.009)
© 2005 Blackwell Publishing Ltd, Molecular Ecology, 14, 869–878
G E N O M E S I Z E O F D I P T E R A N S P E C I E S 875
Table 1 Continued
Species
D. melanogaster
Origin
D. simulans
Genome
size
Origin
North America
Seattle
0.4549
(USA)
(0.009)
D. malerkotliana
Genome
size
Origin
Porto
Rico
Lima
(Peru)
0.3839
(0.003)
0.3788
(0.003)
Costa Rica
Bolivia
0.3967
(0.007)
0.3842
(0.002)
French
Guiana
Kourou
(French
Guiana)
Bolivia
Virasoro
(Argentina)
Asia
Guangzhou
(China)
Vietnam
D. subobscura
Z. indianus
Genome
size
Origin
Genome
size
Origin
Genome
size
Origin
Genome
size
0.5857
(0.024)
0.5840
(0.031)
0.5529
(0.024)
0.6010
(0.031)
0.4321
(0.001)
0.4118
(0.000)
Bahia
(Brazil)
Rio de
Janeiro
(Brazil)
Tijuca
(Brazil)
Campinas
(Brazil)
0.6330
(0.000)
0.6365
(0.005)
Standard
(Brazil)
(0.6410
(0.009)
0.5952
(0.027)
0.5157
(0.021)
Delhi
(India)
Bangalore
(India)
0.6061
(0.002)
0.5991
(0.007)
0.5425
(0.028)
Chandigarh
(India)
Raidah
(India)
0.6016
(0.011)
Seattle
0.3861
(USA)
(0.001)
Washington 0.3944
(USA)
(0.001)
South America
Cuba
0.4547
(0.001)
Guadeloupe 0.4532
(0.003)
0.4709
(0.002)
0.4689
(0.006)
D. ananassae
Porto
Alegre
(Brazil)
Martiniquè
French
Guiana
Martinique
French
Guiana
Porto Rico
0.5658
(Caribbean) (0.045)
Porto
0.5620
Viego
(0.007)
(Argentina)
0.4611
(0.003)
Santiago
(Chile)
Puyehue
(Chile)
0.6399
(0.001)
0.6252
(0.005)
0.4561
(0.005)
0.4436
(0.001)
0.4575
(0.001)
0.4721
(0.004)
Higa
(Japan)
0.3875
(0.000)
Saiging
(Myanmar)
0.5661
(0.023)
Delhi
(India)
Bubaneshivar
(India)
Rohtak
(India)
0.5823
Saiging
(Myanmar) (0.060)
Beruwala
0.5490
(Sri Lanka) (0.028)
Colombo
0.6166
(Sri Lanka) (0.030)
Nepal
0.6080
(0.050)
Borneo
0.5765
Island
(0.054)
Oceania
Canberra
(Australia)
0.4056
(0.002)
Papeete
(French
Polynesia)
Hawaii
0.3927
(0.001)
0.3965
(0.011)
The standard deviation is shown in parentheses.
© 2005 Blackwell Publishing Ltd, Molecular Ecology, 14, 869–878
Tahiti
(French
Polynesia)
Moorea
(French
Polynesia)
Hawaii
0.6098
(0.016)
0.5410
(0.028)
0.5529
(0.004)
0.5983
(0.001)
876 C . N A R D O N E T A L .
indianus, which has the smallest size in some populations
of India, is also an old invader, or at least an older invader
than D. ananassae, D. malerkotliana, and D. subobscura. Note
that we have no good evidence for the exact origin of
Zaprionus indianus, but its smaller genome in India could
indicate that India is the region from which this species
originally came.
The origin of the two widespread species D. ananassae
and D. malerkotliana is considered to be Southeast Asia. The
variability of genome size in India, especially in the case of
D. ananassae, is however, as high as it is between populations from different continents. This suggests that the
colonization routes of this species did not follow a consistent
direction. We cannot, however, rule out the possibility that
the environmental conditions encountered by the various
populations may have determined their genome size, and
differed as much between populations within the region of
origin as between populations on other continents, leading
to a highly variable genome size.
Genome size estimates of D. subobscura vary considerably
between populations, with no significant differences between
continents. This species is mainly concentrated in a narrower area than the other species investigated, and so we
cannot be sure that there are no intercontinental differences
in genome size. However, the genome size of the populations from North America, which are not globally smaller
than that of populations from the other continents, suggests
that genome size does not change quickly after a colonization event. Similar data were obtained with D. pseudoobscura,
which shows differences in genome size between populations without any geographical tendency (Dawley in Powell
1997).
Our findings show that genome size estimates differ
considerably between populations of the same species. There
is a tendency in some species towards smaller genome size
in populations from the region where the flies is believed
to originate, only in some species. However, the time since
colonization could be a major factor in these changes, suggesting that genome size may change very slowly in some
species, maybe resulting from DNA loss and gain (Petrov
2001, 2002), although it can greatly differ between populations of a same species. It may be that the few centuries that
separated the native and the newly established populations were not sufficient to modify genome size to any great
extent, or that back colonization and admixture between
not completely isolated populations have blurred the data.
The increase in the genome size of maize has been attributed to the amplification of transposable elements, but this
has occurred within the last 2– 6 Myr (SanMiguel & Bennetzen
1998). We do not yet understand the dynamics of this amplification, its frequency or the phenomena responsible for
it, which are not so easily addressable experimentally. The
question of how differences in genome size between species
arise is therefore still open, and will not be easy to explore
experimentally if thousands of years are necessary for any
drastic change in the size and composition of the genome to
be observable. Extensive sampling in the regions of species
origin, molecular data to better know the colonization
route of the species (Gasperi et al. 2002) could be the first
steps towards a better understanding of genome size
evolution.
Acknowledgements
We would like to thank A. Brehm, M. L. Cariou, J. A. Castro, J.
David, G. W. Gilchrist, R. Huet, F. Lemeunier, W. Miller, and
various researchers for their collection of flies and their help in
collecting new ones. We thank Monika Ghosh for reviewing the
English text. This work was supported by the Centre National de
la Recherche Scientifique (UMR 5558 and 5534, GDR 2157).
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This research is part of a larger project being conducted under the
supervision of Cristina Vieira and Christian Biémont, which is
intended to understand the evolution of genome size in natural
populations. Christiane Nardon, Catherine Lœvenbruck and
Michèle Weiss are CNRS and University scientists specializing in
flow cytometry and image analyses. Grégory Deceliere is a PhD
student specializing in statistics and software.
© 2005 Blackwell Publishing Ltd, Molecular Ecology, 14, 869–878