Behavioral Ecology Vol. 13 No. 1: 28-31
© 2002 International Society for Behavioral Ecology
Lack of inbreeding avoidance in the Argentine ant Linepithema humile
Institute of Ecology, University of Lausanne, Bâtiment de Biologie, 1015 Lausanne, Switzerland
Address correspondence to L. Keller. E-mail: laurent.keller{at}ie-zea.unil.ch . D. Fournier is now at the Département de Biologie Animale, Université Libre de Bruxelles, Av. F.D. Roosevelt, 50, B-1050 Bruxelles, Belgium.
Received 15 August 2000; revised 1 October 2000; accepted 8 February 2001.
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ABSTRACT |
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Although workers might increase their inclusive fitness by favoring closer over more distant kin, evidence suggest that nepotism generally does not occur within colonies of social insects. It has been suggested that this may be due to the cost of recognition errors. We tested whether recognition occurs in a system where a better than random ability to recognize kin should be selected for. Using DNA microsatellites, we show that sexuals of the Argentine ant Linepithema humile fail to use genetic cues to avoid sib-mating. When offspring of two queens were allowed to mate, the percentage of matings among siblings was not significantly lower than expected under the hypothesis of random mating. The finding that sexuals fail to use genetic cues to avoid sib-matings cannot be attributed to the cost of recognition errors because any recognition system that would lead to a better than random ability to avoid sib-mating should be selected for when there are costs to inbreeding. These data are thus consistent with the view that kin recognition mediated solely by genetic cues might be intrinsically error prone within colonies of social insects.
Key words: Argentine ants, inbreeding, kin recognition, Linepithema humile, recognition errors, sib-mating.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Individuals of many species can distinguish kin from non-kin, usually on the basis of proximate cues based on familiarity or association within nests (Fletcher and Michener, 1987
).
Considerable attention has recently focused on whether individuals can
distinguish kin from non-kin within colonies of social insects because such an
ability would set the stage for numerous kin conflicts among colony members
(Bourke, 1997;
Bourke and Franks, 1995;
Heinze et al., 1994;
Keller and Chapuisat, 1999;
Keller and Reeve, 1999;
Ratnieks and Reeve, 1992). For
example, colonies of social insects frequently contain multiple reproductive
queens, or queens that have mated with more than one male
(Bourke and Franks, 1995;
Chapuisat et al., 1997;
Crozier and Pamilo, 1996;
Keller, 1993). In such
colonies, workers belong to several genetically distinct lineages, which are
groups of more related individuals, such as full sisters. Hence, workers might
benefit from behaving nepotistically—that is, favoring the individuals
most related to them. Earlier studies on honeybees suggested that workers
favored full sisters over half sisters (e.g.,
Getz and Smith, 1983;
Page et al., 1989). However,
these studies have been justifiably criticized on a number of grounds
(Breed et al., 1994;
Frumhoff, 1991;
Hogendoorn and Velthuis, 1988;
Oldroyd et al., 1990), and new
empirical studies using molecular markers failed to demonstrate differential
family composition during swarming (Kryger
and Moritz, 1997). Moreover, studies in a number of ant and wasp
species also failed to detect nepotism within colonies
(DeHeer and Ross, 1997;
Keller, 1997;
Strassmann et al., 1997).
Two recent studies showed that subfamilies (offspring from different
fathers) tend to have different cuticular hydrocarbon profiles in the honeybee
(Arnold et al., 1996,
2000). As cuticular
hydrocarbons are probably the chemical labels used by workers to discriminate
nest mates from non-nest mates (Arnold et
al., 1996; Bonavita-Cougourdan
et al., 1987; Lahav et al.,
1999; Lenoir et al.,
1999), this raises the possibility that such labels can be used
for within-colony discrimination. However, the study by Arnold et al.
(1996) also showed considerable
overlap in the chemical profile of workers from different patrilines,
suggesting that these chemical labels would at best provide a moderately
efficient system of recognition for with-in-colony discrimination.
Two general explanations may account for the lack of nepotism within insect
societies (Reeve, 1998). One
is that selection has favored uniform treatment of colony members because
differential treatment of kin classes incurs costs in colonies where all
individuals are related (albeit to a variable degree). For example, the
preferential treatment of full sisters over half sisters in a colony where the
queen is mated with several males likely will affect colony performance and
lead to decreased fitness of half sisters. Hence, nepotism will be selected
against if the cost incurred by less related individuals outweighs the
benefits provided to more related individuals.
Alternatively, kin-biased behaviors may be disfavored because of the cost
of recognition errors. Because no recognition system is perfect, the decision
of an individual to behave nepotistically depends on the probability of
correctly identifying desirable and undesirable recipients. Recognition
mediated by genetic cues might be unstable and error prone. Theoretical
studies indicate that allelic diversity of recognition systems may vary over
time, and in some ecological circumstances more frequent alleles may be
continually favored until fixation, resulting in a loss of diversity at the
recognition system (Crozier,
1988, Ratnieks,
1991). Finally, it has been suggested that colony members may also
benefit from reducing or eliminating information about kinship within the
group when nepotism entails a cost for colony productivity
(Reeve, 1998). This phenomenon
illustrates the fact that mechanisms may evolve at the colony level to prevent
the outbreak of conflicts (Keller and
Chapuisat, 1999; Keller and
Reeve, 1999; Ratnieks and
Visscher, 1989; Reeve,
1998).
The aim of this study was to determine whether genetic cues can be used to
discriminate kin from non-kin within colonies as a means to avoid inbreeding.
As in most other animals, inbreeding is probably detrimental and rare in ants
and other eusocial Hymenoptera (Crozier,
1980; Crozier et al.,
1984; Pamilo,
1983; Ross and Carpenter,
1991; Ross and Fletcher,
1986). Dispersal is probably the primary mechanism preventing
inbreeding. In most ants, winged males and females emerge from many nests
simultaneously and undergo large mating flights before mating, thus greatly
decreasing the probability of close relatives contacting one another
(Baudry et al., 1998;
Crozier, 1980). However,
inbreeding seems to occur in a few monogynous (single queen colonies) ants
(Cole and Wiernasz, 1997; but
see Cole and Wiernasz, 1999;
Pamilo, 1991). In polygynous
(multiple queen colonies) ants, females (and sometimes males) have reduced
dispersal abilities, and mating occurs in the nest (e.g.,
Hölldobler
and Wilson, 1990; Passera,
1994). In species where mating occurs exclusively between nest
mates, preferential mating between related or non-related individuals would
have to be based on genetically determined cues.
A previous study suggested that male and female sexuals of the Argentine
ant Linepithema humile (previously Iridomyrmex humilis) may
use genetic cues to avoid inbreeding. Colonies of this species typically
contain multiple queens, and most reproductive individuals mate within their
parental nest without dispersing (Keller
and Passera, 1993). When females were presented simultaneously
with a brother that was reared in the same colony until the pupal stage and an
unrelated male produced in another colony, they mated preferentially with the
unrelated male (Keller and Passera,
1993). These experiments raised the intriguing possibility that
sexuals use genetically derived cues to avoid sib-mating, although it could
not be ruled out that colony odor may have been learned during the larval
stage. Males of the social bee Lasioglossum zephyrum are able to
determine the relatedness between females to which they have been exposed and
preferentially mate with females that are unrelated to each other
(Smith, 1983;
Smith and Ayasse, 1987).
However, in that case, discrimination is based on learning kin identity of
females. To test whether genetic cues are used to avoid sib-mating in the
Argentine ant, we used micro-satellites
(Krieger and Keller, 1999) to
determine whether the proportion of sib-matings was lower than expected by
chance in unmanipulated colonies containing reproductive individuals produced
by two unrelated queens.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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We collected L. humile in Port-Leucate, near Perpignan, southern France and set up 120 nests, each with 2 queens and 300 workers but no eggs or brood. Queens in the nest were most likely unrelated, as they were collected from several nests and randomly paired (moreover, the relatedness between nest-mate queens is very low in this population, r = 0.004; M. Reuter, F. Balloux, L. Lehman, and L. Keller, unpublished data). These colonies were maintained in the laboratory under standard laboratory conditions (Keller and Passera, 1992
,
1993). To ensure that colonies
contained only brood from the two queens, colonies were regularly checked for
the first 2 weeks and any brood removed. After another 2 weeks we removed the
two queens and froze them at -20°C for genetic analyses. The workers then
reared the brood that had been produced in the previous 2 weeks. Queenless
colonies generally rear male and female sexuals which typically mate in the
nest (Keller and Passera,
1992,
1993). We collected all female
reproductives that shed their wings, a behavior generally associated with
mating, and stored them at -20°C for genetic analyses.
The two mother queens of each colony and the sperm stored in their
spermathecae were genotyped at eight micro-satellite loci (Lhum-11,
Lhum-13, Lhum-19, Lhum-28, Lhum-35, Lhum-39, Lhum-52, Lhum-62)
(Krieger and Keller, 1999). To
isolate sperm DNA, the queen's abdomen was dissected in distilled water and
her intact spermatheca removed and placed in extraction buffer. The
spermatheca was then ruptured with forceps and the sperm collected with a
micropipette for genetic analysis. Queens of this species mate only once
(Krieger and Keller, 2000),
and in 30 of the 120 colonies the 2 mother queens and their mate (i.e., sperm
in their spermatheca) had genotypes that allowed unambiguous maternity
assignment of all their offspring. These colonies produced 324 female sexuals
that mated and dealated. These females and the sperm in their spermathecae
were genotyped at the informative loci (inferred from the genotype of their
mothers and fathers) to determine which females mated with a brother versus an
unrelated male. Genetic analyses showed that more than five males and five
females from each queen were present in 26 out of the 30 colonies. We used
only these 26 colonies in further analyses (which represent a total of 273
matings).
Deviation from the null hypothesis of random mating between siblings and
unrelated individuals was tested in two ways. First, we estimated for each of
the i colonies the probability of sib-mating
(Reli) which depends on the proportion of male
and female offspring (that successfully mated) produced by each of the two
queens. The probability of sib-mating (Reli) is
 |
) to compare these values with the observed proportion of
sib-mating in each of the 26 colonies where both queens contributed to the
production of reproductive individuals (all values were arcsine transformed
before analysis). We also conducted an analysis on the whole data set by
comparing with a binomial test the observed and expected number of matings
between siblings under the assumption of random mating, the latter value
being:
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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There was no evidence that sexuals avoided mating with siblings in the 26 colonies in which we could unambiguously assign maternity of both the male and female sexuals (total number of matings = 273; see Table 1). A binomial test showed that the percentage (45.1%) of matings among siblings was not significantly lower (p >.05) than expected (48.6%) under the hypothesis of random mating. A power analysis demonstrated that the probability of detecting even a relatively weak avoidance of sib-mating (g = 0.1; that is, 40% or fewer of the matings occurring between siblings) with an
value = 0.05 and a sample size of 273 was high (.93
< p <.97; the power analysis was done by using a 50%
theoretical value of sib-mating, the closest value to the expected value in
our experiments; Cohen, 1988).
An analysis conducted at the colony level
(Table 1) also showed no
significant difference between the observed (46.6 ± 14.0, n =
26) and expected percentages (49.4 ± 12.6, n = 26) of matings
between siblings (paired t test, df = 25, p =.344).
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The finding that sexuals fail to use genetic cues to avoid sib-matings
cannot be attributed to the cost of recognition errors, in contrast to other
studies that failed to demonstrate nepotistic behavior
(Keller, 1997;
Reeve, 1998). This is because
any recognition system that would lead to a better than random ability to
avoid sib-mating should be selected for when there are costs to inbreeding
(e.g., the costs of diploid male production;
Cook and Crozier, 1995;
Ross et al., 1993;
Ross and Fletcher, 1985). In
the Argentine ant, the operational sex ratio is highly male biased because
there are many more males than females per colony and because both sexes mate
only once (Keller and Passera,
1992). Hence, females can easily have access to mates, and they
should avoid mating with sibs. Thus, a recognition system that would allow
females to mate less frequently with sibs than expected by mating randomly
with the males in their colony should be selected for. It is important to
note, however, that the selective pressure to avoid sib-matings might not be
very strong in our study population because colonies contain a high number of
queens (Keller et al., 1989),
leading to a relatively low probability of sib-mating even if matings would
occur randomly between sexuals. Selection for a mechanism preventing
sib-mating is presumably higher in native populations (Argentina and Brazil)
where queen number per colony is frequently much lower than in introduced
French populations (Pedersen J, Giraud T, and Keller L, unpublished data).
The lack of kin recognition also cannot be attributed to a reduction of
recognition alleles following a bottleneck because microsatellite analyses
revealed a relatively high genetic diversity in the population under study
(Krieger and Keller, 1999).
That introduced populations have retained genetic variability is also
supported by the finding that some populations from the Mediterranean coast
are highly aggressive with other populations and that the level of aggression
is strongly associated with the genetic differentiation between populations
(Giraud T, Pedersen J, and Keller L, unpublished data). Hence, our results are
consistent with the view that within-colony recognition mediated by genetic
cues is error prone (Crozier,
1988; Ratnieks,
1991), possibly because of meiotic shuffling and/or loss of
diversity at the recognition system over evolutionary time
(Crozier, 1988;
Ratnieks, 1991).
Alternatively, low efficiency of kin recognition systems within insect
colonies may also stem from colony members benefiting from and actively
reducing or eliminating information about kinship within the group (by
scrambling recognition labels; Reeve,
1998), if, for example, nepotism would decrease colony
productivity. Whatever the mechanism underlying the lack of inbreeding
avoidance in the Argentine ant, the demonstration that sexuals fail to avoid
sib-matings suggests that genetically mediated recognition cues may not
readily be used for within-colony kin recognition.
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ACKNOWLEDGEMENTS |
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We thank A. Bourke, B. Brown, M. Chapuisat, P. Christe, J. Goudet, J. Parker, J. Perdersen, F. Ratnieks, J. Strassmann, and an anonymous reviewer for comments on the manuscript. This work was supported by grants from the Swiss National Science Foundation and the Fondation du 450e of the University of Lausanne.
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