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Downloaded from http://rspb.royalsocietypublishing.org/ on May 4, 2017
Proc. R. Soc. B
doi:10.1098/rspb.2009.1167
Published online
Spatial swarm segregation and reproductive
isolation between the molecular forms of
Anopheles gambiae
Abdoulaye Diabaté1, *, Adama Dao2, Alpha S. Yaro2,
Abdoulaye Adamou2, Rodrigo Gonzalez1, Nicholas C. Manoukis1,
Sékou F. Traoré2, Robert W. Gwadz1 and Tovi Lehmann1
1
Laboratory of Malaria and Vector Research, National Institute of Allergy and Infectious Diseases,
National Institutes of Health, 20852 Rockville, MD, USA
2
Malaria Research and Training Center, Bamako, Mali
Anopheles gambiae, the major malaria vector in Africa, can be divided into two subgroups based on genetic
and ecological criteria. These two subgroups, termed the M and S molecular forms, are believed to be
incipient species. Although they display differences in the ecological niches they occupy in the field,
they are often sympatric and readily hybridize in the laboratory to produce viable and fertile offspring.
Evidence for assortative mating in the field was recently reported, but the underlying mechanisms awaited
discovery. We studied swarming behaviour of the molecular forms and investigated the role of swarm segregation in mediating assortative mating. Molecular identification of 1145 males collected from 68
swarms in Donéguébougou, Mali, over 2 years revealed a strict pattern of spatial segregation, resulting
in almost exclusively monotypic swarms with respect to molecular form. We found evidence of clustering
of swarms composed of individuals of a single molecular form within the village. Tethered M and S
females were introduced into natural swarms of the M form to verify the existence of possible mate recognition operating within-swarm. Both M and S females were inseminated regardless of their form under
these conditions, suggesting no within-mate recognition. We argue that our results provide evidence that
swarm spatial segregation strongly contributes to reproductive isolation between the molecular forms in
Mali. However this does not exclude the possibility of additional mate recognition operating across the
range distribution of the forms. We discuss the importance of spatial segregation in the context of possible
geographic variation in mechanisms of reproductive isolation.
Keywords: molecular forms; Anopheles gambiae; reproductive isolation; swarms
1. INTRODUCTION
Ecologically based divergent selection is a process in
which different phenotypes are favoured by different
environments. If the variation between phenotypes has a
genetic basis, different environments will favour different
alleles, resulting in ecologically based divergent evolution.
Ultimately, reproductive isolation evolves as a consequence of this selection. The process is known as
ecological speciation and it might occur in allopatry or
in sympatry (Schluter 2001). Rundle and Nosil (2005)
separated ecological speciation into three components:
an ecological base of divergent selection, a mechanism
of reproductive isolation, and a linkage between them.
Recent results have revealed that divergent selection
between the molecular forms of Anopheles gambiae is
mediated by predation pressure (Diabaté et al. 2008), in
accordance with the first component defined by Rundle
and Nosil. Here, we investigate the second component,
i.e. the mechanisms of reproductive isolation that restrict
gene flow between the forms.
An. gambiae, the major malaria vector in Africa, is
undergoing speciation (Coluzzi et al. 2002; della Torre
et al. 2002). Early studies based on chromosomal inversions of An. gambiae in West Africa found five partially
isolated populations based on combinations of paracentric
inversions on the right arm of chromosome 2. These were
named Forest, Savanna, Bamako, Mopti and Bissau chromosomal forms (Bryan et al. 1982; Coluzzi et al. 1979,
1985; Touré et al. 1998). The chromosomal forms exhibit
different degrees of gene flow between them, and their
spatial and seasonal distribution indicates that they are
adapted to different niches.
The distribution range of the chromosomal forms
overlaps extensively, except in the semi-desert belt of
West Africa, where the Mopti chromosomal form occurs
exclusively (Touré et al. 1998; Lehmann & Diabaté
2008). The Forest chromosomal form is found in the
humid forest belt of West and Central Africa. The
Bamako chromosomal form is restricted to the upper
Niger river basin and is associated with laterite rock
pools as its main larval habitat (Touré et al. 1998;
Manoukis et al. 2008; Sogoba et al. 2008).
Subsequent studies revealed two ‘molecular’ forms
(M and S) characterized by fixed nucleotide differences
in the intergenic spacer of the ribosomal DNA (della
Torre et al. 2001; Favia et al. 2001). The relationship
between the molecular and chromosomal forms is
* Author for correspondence ([email protected]).
Received 6 July 2009
Accepted 12 August 2009
1
This journal is q 2009 The Royal Society
Downloaded from http://rspb.royalsocietypublishing.org/ on May 4, 2017
2 A. Diabaté et al. Swarm segregation in Anopheles gambiae
complex and depends on geography. The M-form genotype is associated with the chromosomal forms Mopti,
Savanna, Forest and Bissau, whereas the S genotype is
associated with the chromosomal forms Savanna,
Bamako and Forest. In Mali and Burkina Faso, the M
form strictly corresponds to Mopti and the S form strictly
corresponds to Savanna and Bamako chromosomal forms
(della Torre et al. 2001). The reproductive isolation
between the molecular forms is independent of their chromosomal constitution (Wondji et al. 2002). Therefore,
chromosome inversions are not linked to the mate
recognition system, whereas they are believed to contain
genes conferring ecotypic adaptations (Coluzzi et al.
2002; della Torre et al. 2005).
Typically the S form peaks in the rainy season, exploiting rain-dependent puddles as larval sites, whereas the
M form predominates in more arid conditions and in
association with irrigated sites such as rice fields (Diabaté
et al. 2002, 2003, 2004; della Torre et al. 2005). Genetic
differentiation between the molecular forms is high only
in two or three tiny genomic areas named the ‘speciation
islands’ (representing less than 1% of the total genome)
with low or no differentiation found across most of the
genome (Gentile et al. 2001; Mukabayire et al. 2001;
Wondji et al. 2002; Lehmann et al. 2003; Stump et al.
2005; Turner et al. 2005; Turner & Hahn 2007). The
absence of differentiation across most of the genome is
probably due to ongoing gene flow between the molecular
forms that continues to homogenize regions of the
genome not directly involved in the speciation process
(della Torre et al. 2002). The rate of natural hybridization
between the molecular forms is below 1 per cent (della
Torre et al. 2001; Wondji et al. 2005), although 7 to 20
per cent hybridization was found in restricted locations
in Gambia and Guinea-Bissau (Caputo et al. 2008;
Oliveira et al. 2008). Whether this deficit of hybrids
reflects hybrid inferiority in the field is not known, but
laboratory studies have found no evidence for reduced fitness in hybrids (Diabaté et al. 2007). Strong assortative
mating between the molecular forms in the field has
been described (Tripet et al. 2001), but its underlying
mechanisms are not known.
An. gambiae mates in flight at specific mating stations,
and very often over specific landmarks known as swarm
markers (Downes 1969; Charlwood et al. 2002; Yuval
2006). The swarms are composed of males; females typically approach a swarm, acquire a mate and leave in
copula. Insects use a variety of stimuli to bring males and
females together for mating, including pheromones, visual
signals and sound signals, which can operate over long
and short ranges (Clements 1999). The way the sexes are
attracted to each other may contribute to the specific mate
recognition systems, which facilitate species identification
and prevent hybridization (Clements 1999). The hypothesis
that flight-tone is used for differential mate recognition was
not supported by experiments in the laboratory (Tripet et al.
2004). Additionally, a recent study using a mark–release
experiment of M and S forms in natural houses (absence
of swarm markers) found no evidence for assortative
mating when mating occurs indoors (Dao et al. 2008),
suggesting that chemical and sound cues are not involved,
at least under these conditions.
Studies on mate recognition between the molecular
forms and especially the absence of hybrids and the
Proc. R. Soc. B
evidence for assortative mating lead us to hypothesize
that reproductive isolation between the molecular forms
is associated with mating swarms. In a previous study of
swarm composition in Burkina Faso, we found that
swarm composition was not random and that the frequency of mixed swarms was far smaller than expected
by chance (Diabaté et al. 2006), suggesting that swarm
segregation contributes to reproductive isolation. However, inference based on that study was limited because
we only found swarms of S forms exclusively or mixed
swarms, but no swarms of the M form, possibly because
of a low abundance of M males (3.2%) at that location
and time. Here, we address this hypothesis by further
evaluating the contribution of spatial swarm segregation
to reproductive isolation between the molecular forms.
We show that, in Mali, segregation of swarms is an important mechanism that restricts gene flow between the
molecular forms.
2. MATERIAL AND METHODS
(a) Study area
A study on swarming behaviour of the molecular forms of
An. gambiae was conducted in August and September
2006 and 2007 in Donéguébougou, Mali (128 480 3800 N;
78 590 500 W), located 29 km northeast of Bamako on the
edge of a temporary stream surrounded by hills with a
small rice cultivation area. During the wet season (1998),
An. gambiae ss. population in this village comprised 11 to
30 per cent of the Bamako chromosomal form, 4 to 44
per cent of the Savanna form and 33 to 63 per cent of
the Mopti form (Touré et al. 1998).
(b) Swarm composition
A survey of swarms was undertaken by trained observers in
Donéguébougou, starting at sunset and looking towards the
lightest part of the sky from 0.5 to 4 m above the ground.
Once located, swarms were collected using an insect net.
Mosquitoes were aspirated into cups, killed with chloroform,
identified and kept in 80 per cent ethanol in 1.5 ml tubes.
The location of the swarm, time of collection, landmark
and height above ground were recorded. Observations were
made on 19 swarm sites spread throughout the entire village,
where swarms were observed forming every evening. Samples
were taken from swarms that formed in the same locations
over several evenings. Swarm locations were mapped using
a global positioning system (GPS) with measurements of latitude and longitude accurate to within 2 m. Collected
specimens were identified by polymerase chain reaction
(PCR) to the level of species and molecular forms (Fanello
et al. 2002), and swarms of the S form were subsequently
identified with respect to whether they were of the Bamako
or Savanna chromosomal forms (Coulibaly et al. 2007).
Mating pairs were also collected as they fell or flew out of
swarms in the 2007 survey. Males and females from these
pairs were subsequently identified to species and molecular
forms (Fanello et al. 2002).
(c) Indoor resting composition
Pyrethrum spray collection was performed indoors throughout the village to estimate the relative frequency of the
different molecular forms. The collection was done in
September the day after the last swarm collection to avoid
affecting swarm compositions with the pyrethrum spray. To
ascertain that the pattern of swarm distribution across the
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Swarm segregation in Anopheles gambiae
village was not a by-product of spatial distribution of the
forms within the village, 2– 4 houses, located within 10 m
of each swarming site, were selected for indoor collections.
All specimens were identified, preserved and subsequently
identified to species/molecular form as described above.
(d) Form recognition within swarm: tethered
females experiment
The experiment was conducted in the village of Sokourani,
located in a large ricefield area in the district of Niono in
northeast Mali (see details in Sogoba et al. 2007). The rice
irrigation area is occupied exclusively by the M form of
An. gambiae. Virgin females were produced in the laboratory
from egg batches of wild-caught blood-fed and gravid
females collected in Donéguébougou. Three- to five-day-old
F1 virgin females of one or the other form were individually
tethered by gluing a fine line (50 cm long) to the scutum
(dorsal face of the thorax), which was tied onto a 2-m pole.
After confirming the flying ability of the tethered female,
she was introduced into a natural swarm for 5 min. Pairing
between the tethered female and a male from the swarm
was noted and subsequently the female was dissected to
determine if mating was successful (presence of sperm in
her spermatheca). The same experiment was also performed
in Donéguébougou, but, in contrast to Sokourani, pairing
occurred rarely and the number of females inseminated
(one) did not allow further interpretation. Hence, only the
data collected in Sokourani is presented. The low pairing in
Donéguébougou, as opposed to Sokourani, is probably due
to the small size of the swarms. During the experiment we
noted that the rate of pairing was higher in large swarms
than in small swarms. Swarm size in Sokourani ranged from
100–1000 males, whereas the size in Donéguébougou rarely
reached or exceeded 100 males.
3. RESULTS
(a) Swarm observations and collections
Swarms began to form 2 –5 min after sunset with one or
two males observed in zigzag flight, which were then
joined by other males, and lasted for 20 –40 min.
Swarms remained stationary, flying within a 1.5 m
radius of an imaginary centre throughout their duration.
Swarm height ranged from 0.5 to 3 m above ground,
although sometimes they reached up to 4 m for short
intervals. Swarms were observed at the same sites repeatedly. Swarms that were observed in the same site on
different days were treated as distinct swarms.
A total of 1145 males were collected from 68 swarms
(19 sites) from Donéguébougou between August and
September in 2006 and the same period in 2007, when
both forms coexisted in that village. During swarm collection, the S form comprised 68.30 per cent, the M form
31.61 per cent and Anopheles arabiensis 0.09 per cent of
the total. Sample size per swarm varied from 5 to 74
males (median ¼ 13). In 2006, 99.02 per cent (203/205
from 13 swarms) of the S specimens were of the Savanna
chromosomal form, and the remainder were the Bamako
chromosomal form. In 2007, 100 per cent (154/154 from
11 swarms) were of the Savanna chromosomal form.
(b) Within-swarm form composition
Swarms were sampled when swarm size was near its peak,
between 10 and 20 min after sunset. Swarms usually
Proc. R. Soc. B
A. Diabaté et al. 3
appeared in the same location every evening. In 2006,
identification of 901 males from 47 swarms revealed
complete swarm segregation, with every swarm being
composed exclusively of either M or S males
(figure 1a). Three swarms (swarms 1, 2 and 17;
figure 1a) were sampled three times in the same evening
(2 min apart) to assess temporal change in male composition. Overall, 29, 23 and 74 specimens, respectively,
were sampled from these swarms (sample size range per
time point 2– 30), and composition remained 100 per
cent of the S form. The composition of all swarm sites
sampled at different dates remained unchanged except
for one swarm (swarm 17), which consisted exclusively
of S males on four evenings (sample size range: 23 – 74
specimens), but consisted of M males on one evening
(sample size, 13 specimens; figure 1a).
To further test this pattern, swarm sampling was also
performed in the same period in 2007. A total of 244
males were sampled from 21 swarms. Eight of the 13
swarm sites located in 2007 were from the same sites
identified in 2006 (swarms 2, 3, 4, 6, 7, 8, 14 and 17).
A similar pattern of swarm segregation was observed in
2007, with 20 pure-form swarms and one mixed swarm
(figure 1b). The sample from swarm 14 had two of the
M form and 22 of the S form. The sample from swarm
33 had 14 of the M form and one An. arabiensis.
A total of 27 mating couples were collected. Five
couples were collected from two M swarms and 22
couples were collected from six S swarms. All couples
were homogeneously paired (male and female being of
the same molecular form) and were of the same form
as the males from the swarms from which they were
collected.
(c) Distribution of the molecular forms
resting inside houses
In 2006 and 2007 indoor collections resulted in 394 and
169 An. gambiae specimens, respectively (45% males and
55% females). An. arabiensis represented 10 per cent of
the total collection in 2006 and 2 per cent in 2007.
Considering only the molecular forms, the rarer form in
2006 was the S form (47%), whereas in 2007 it was the
M form (30%). Importantly, An. arabiensis and the molecular forms of An. gambiae were spread over the village
and found co-inhabiting houses in the vicinity of the
different swarms (figure 1a). The S molecular form was
predominantly of the Savannah chromosomal form
(97.6% in 2006, n ¼ 167; 100% in 2007, n ¼ 116).
Based on the indoor composition of the molecular
forms and the number of observed swarms in 2006
and 2007, the expected frequency of mixed swarms
was calculated for both years under the assumption of
random mixing (no spatial segregation). This expected
number was found to be substantially higher than the
number of mixed swarms observed (p , 0.0001;
table 1) both in 2006 and 2007, suggesting a strong segregation in the swarming behaviour of the two forms
(table 1).
(d) Swarm markers
To understand the role of ground markers in swarm site
selection by the molecular forms, all swarm sites were
characterized (figure 2). All swarms of the S form were
Downloaded from http://rspb.royalsocietypublishing.org/ on May 4, 2017
4 A. Diabaté et al. Swarm segregation in Anopheles gambiae
(a)
(b)
Figure 1. (a) Spatial segregation of swarms of the molecular forms (shaded ovals) and indoor composition of the molecular
forms collected in the vicinity of the swarms (vertical bars) in 2006. With the exceptions of swarms 0, 11, 16 and 18, all
swarms were sampled more than once (2 –8 evenings) at the same site. Swarm sizes ranged from 5 to 74. (b) Spatial segregation
of swarms of the molecular forms (shaded ovals) in 2007. Locations of swarms 1, 11, 15, 16 and 18 are seen on the map, but
these swarms were not sampled in 2007. Swarm 33 and swarm 14 are mixed swarms respectively of the M form and
An. arabiensis, and of the S and M forms.
collected over bare ground, whereas the M form was
strongly associated with markers consisting of contrasting dark/light pattern, such as the intersection of
vegetation (dark) and footpath (light), a water well
(dark) surrounded by bare ground (light), and a physical
object such as a donkey cart, a chicken house, or a wall
on a lighter background (figure 3). Although one M
Proc. R. Soc. B
form swarm was found over bare ground, the association
between swarm markers and swarm molecular form was
highly significant (x 2 ¼ 56.92, d.f. ¼ 3, p , 0.0001).
The mixed S/M swarm (14) was found over bare
ground whereas the mixed M/An. arabiensis swarm
(33) occurred over an intersection of grassland and
footpath.
Downloaded from http://rspb.royalsocietypublishing.org/ on May 4, 2017
Swarm segregation in Anopheles gambiae
A. Diabaté et al. 5
Table 1. Observed and expected number of mixed swarms.
indoor composition
swarm composition mix / total
year
nb
M//S (%)
observedc
n
expectedd
pd
Aug–Sep 2006
Aug–Sep 2007a
394
169
54//46
30//70
0 / 46
1 / 21
901
243
45.7
19.7
,0.0001
,0.0001
a
A single collection was obtained in 2006, 2 weeks after collection of the first swarm, coinciding with the collection of the last swarms. The
two collections in 2007 (1–2 September and 13 –14 September) were pooled because there was no significant difference in form
composition between them (x2 ¼ 1.8208; d.f. ¼ 1; p . 0.17).
b
Total number of mosquitoes collected. Indoor samples include males and females pooled because there was no significant difference
between them (p . 0.1). Swarm samples consisted of males only.
c
The number of mixed swarms of the total number of swarms sampled.
d
Expected number of mixed swarms based on binomial samples drawn from a population with corresponding indoor form composition.
Each sample represents a swarm and is of the same sample size as that swarm. Ten thousand simulated sets of swarm samples, each
representing the same number of swarms (and the same number of mosquitoes from each swarm) as the actual collection of swarms, were
used to enumerate the mixed swarms expected. A mixed sample has at least one member of each swarm (without regard to degree of
mixing).
(e) Within-swarm form recognition
Overall, 455 tethered virgin females were introduced in
94 swarms of the M molecular form in the village of
Sokourani during August and September of 2006 and
during the same period in 2007. Of these, 47 females
(10.33%) were inseminated and no significant difference
in the rate of insemination was found between the
forms (x 2 ¼ 2.38, d.f. ¼ 1, p ¼ 0.122; table 2).
4. DISCUSSION
In this study, we found differences in the swarming behaviour of the molecular forms of An. gambiae that help to
explain their reproductive isolation (Tripet et al. 2001;
Diabaté et al. 2006). A robust pattern of spatial segregation between swarms was found, revealing distinct
form-specific mating units in sharp contrast to the
mixed composition of the molecular forms indoors. Our
results suggest that spatial swarm segregation in Mali is
virtually complete, so it probably contributes strongly to
the assortative mating between the forms. This mechanism of reproductive isolation could most easily be
effective if females discriminate between swarms similarly
to males. Some evidence in support of this hypothesis was
obtained from analysis of 27 mating couples collected
from swarms in Donéguébougou, all of which were of
the same form. These results suggest that females also
discriminate between swarms of their own versus the
other form, although further study is needed to confirm
this hypothesis. If intra-swarm recognition indeed plays a
decisive role, it would be difficult to explain the sharp
male segregation and the absence of ‘wrong’ females
among couples collected from different swarms. Moreover,
if males discriminate between swarms, and humans can
use ground markers to correctly predict the form of the
swarm, it is reasonable that females too can discriminate
among swarms, especially because they are expected to
incur a higher cost than males for cross mating. Assuming
that the fitness of hybrid is reduced in nature, females are
supposed to pay the highest cost in the case of cross
mating, because they mate only once in their lifetime,
whereas males can mate several times.
It is possible that a low rate of cross mating occurs
during indoor mating, as suggested by the absence of
form recognition in experiments conducted in natural
huts (Dao et al. 2008). Indirectly, it suggests that mate
Proc. R. Soc. B
recognition does not operate well outside swarms. Dao
et al. (2008) found direct evidence for indoor mating
only in an allopatric M population and proposed that in
areas of sympatry, males and females of the S form
depart houses before indoor mating starts. The absence
of form recognition in tethered female experiments
and in indoor mating provides additional evidence
against the existence of within-swarm form recognition
mechanisms in Mali.
In Burkina Faso, however, the absence of hybrids
(Diabaté et al. 2006), despite the relatively high rate of
mixed swarms (approx. 15%), indicates that within-swarm
form recognition must operate. Although the expected
frequency of mixed swarms (by chance) in Burkina Faso is
substantially greater than that observed (Diabaté et al.
2006), we suggest that at least one additional withinswarm recognition mechanism is involved. Direct studies
on the role of chemical and auditory signals will be rewarding (e.g. Gibson & Russell 2006). The repeated failure of the
tethered female experiment in an area of sympatry
(Donéguébougou) as opposed to the allopatric M
population in Niono (only 300 km away) probably reflects
yet another difference in mating behaviour between populations and suggests that the importance of mechanisms of
reproductive isolation may vary geographically.
The coexistence of the Bamako and Savanna chromosomal forms within the S molecular form in Mali and not
in Burkina Faso (della Torre et al. 2001) could contribute
to this contrast in the mating behaviour between the two
populations. However, because 99 per cent of the S form
specimens collected from swarms in this study were of the
Savanna chromosomal form, which is the only form
found in Burkina Faso, this consideration cannot explain
the differences. In both populations, the observed
barriers operate primarily between Savanna and Mopti
chromosomal forms.
Our results stress the role of ground markers as a determinant of swarm segregation in the molecular forms of
An. gambiae. Several studies on swarming insects have
found that males aggregate at certain stations (Downes
1969; Savolainen 1978; Titmus 1980; Charlwood et al.
2002; Yuval 2006). Consistent with our results, an allopatric S form population in Tanzania swarmed exclusively
on bare ground (Marchand 1984), whereas an allopatric
M form population in São Tomé used patterns of contrast
as marker (Charlwood et al. 2002).
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6 A. Diabaté et al. Swarm segregation in Anopheles gambiae
Figure 2. Pictures of representative swarm markers. The arrow indicates the exact placement of the swarm in each site.
40
markers (%)
present
absent
M form
90.3 (28)
9.7 (3)
S form
0 (0)
100 (35)
35
number of swarms
30
25
20
15
M form/An. arabiensis 100 (1)
0 (0)
M/S forms
100 (1)
0 (0)
P < 0.0001
M form
mixed swam M form/An. arabiensis
Mixed swarm S/M form
S form
10
5
0
physical object
patch of grass
well
bare ground
swarming station
Figure 3. Association between landmarks and swarm of the molecular forms. The M forms swarm above areas of contrast on
the landscape, whereas the S form uses no such contrast (table incorporated in figure). The figure gives a brief description
of the swarming sites on the x-axis.
That only one An. arabiensis male was collected from
swarms, despite the fact that this species comprises
10 per cent of the indoor population, suggests that
An. arabiensis mates at specific sites not covered in our
survey. Similarly, in Tanzania, no single pure swarm of
An. arabiensis was found in an area where An. arabiensis
and An. gambiae coexisted (Marchand 1984); however,
swarms of An. arabiensis could be seen in a village
where An. arabiensis was the only species present. The
author concluded that in sympatry, An. arabiensis changes
its swarming behaviour or mates without swarming.
The extent of reproductive isolation within
An. gambiae has been the focus of much debate, although
Proc. R. Soc. B
recent theoretical (Lehmann & Diabaté 2008 and references therein) and empirical (Turner & Hahn 2007)
studies have resolved many of the issues. Our data provide
evidence that swarm segregation strongly contributes to
the reproductive isolation of the two forms. The question
remains as to how this isolation mechanism has evolved.
Recent studies suggest that divergent selection
between the forms has acted on larval traits (Diabaté
et al. 2008). Larvae of the M form predominate in permanent larval habitats such as rice fields, whereas S larvae
predominate in temporary puddles (Diabaté et al. 2002,
2003, 2004; della Torre et al. 2005). Larvae of the M
form outperform S larvae in predator-rich habitats
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Swarm segregation in Anopheles gambiae
Table 2. Within-swarm discrimination using tethered
females introduced into natural M swarms. p ¼ 0.122.
insemination
M form
S form
a
No
Yes
Total
87.6%
(212)a
92%
(196)
12.4%
(30)
8%
(17)
100%
(242)
100%
(213)
number of mosquito females.
(i.e. permanent habitats), whereas S larvae outperform M
larvae in the absence of predators (i.e. in temporary habitats;
Diabaté et al. 2008). We propose that M larvae are better
adapted to avoid predators than S larvae, whereas the S
larvae are better adapted for competition under low predator pressure (Diabaté et al. 2008). Rundle and Nosil
(2005), in their review on ecological speciation, stated that
speciation is facilitated when genes under divergent
selection cause reproductive isolation pleiotropically. The
most convincing example is when reproductive isolation
evolves as a direct consequence of habitat selection,
assuming that individuals mate in their preferred habitat.
The molecular forms of An. gambiae do not mate near
their preferred larval habitats, and it is therefore unlikely
that the genes under divergent selection in the molecular
forms also cause reproductive isolation. We presume that
linkage exists between genes conferring adaptive differences
at the larval stage and those that influence swarming site
selection. The role of divergent natural selection in speciation has been demonstrated in many species, including
Bombina toads. Specifically, Bombina bombina prefers
semi-permanent ponds with a higher density of aquatic predators, rather than the temporary puddles typically used by
B. variegata. Similarly, behavioural differences in predator
avoidance were reported between them in accordance with
their habitat distribution (Kruuk & Gilchrist 1997). The
authors presumed that the differential adaptation to cope
with predation pressure led to differential choice of habitat,
and indirectly to preference for alternative breeding habitats.
Although no post-mating reproductive isolation has
been found in the laboratory (Diabaté et al. 2007), the fitness of hybrids in nature has not been tested. It is possible
that hybrid inferiority contributes to reproductive barriers
between the forms. In ecological speciation, post-zygotic
isolation can arise when hybrids are not well adapted to
either parental environment and, in effect, fall between
the niches (Schluter 2001; Rundle & Nosil 2005).
Uncovering the ecological and genetic mechanisms
involved in speciation is key to understanding how
biological diversity is generated. Genetic differentiation
between the molecular forms of An. gambiae and its
distribution across the genome has been extensively
studied, but phenotypic differences between them, the
evolutionary forces that generated divergence and
the mechanisms that maintain their genetic isolation have
only recently been addressed (Lehmann & Diabaté
2008). Our study provides evidence that swarm spatial segregation strongly contributes to the reproductive isolation
between the molecular forms of An. gambiae in Mali,
although this does not exclude the possibility that more
than one mechanism of form recognition operates across
Proc. R. Soc. B
A. Diabaté et al. 7
the range of the molecular forms. This study extends our
understanding of the behavioural components of the speciation process and may eventually facilitate the
development of new strategies for vector control.
This work was supported by the Intramural Research Program
of the Division of Intramural Research, National Institute of
Allergy and Infectious Diseases, National Institutes of Health.
We are grateful to F. Tripet and D. Huestis for useful
comments and suggestions to improve the manuscript and to
NIAID intramural editor B. R. Marshall for assistance.
Special thanks go to M. Diallo, K. Yaya and the villagers from
Donéguébougou for their help in collecting swarms.
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