ArticlePDF Available

Do Cuticular Hydrocarbons Provide Sufficient Information for Optimal Sex Allocation in the Ant Formica exsecta?

Authors:
  • Statistics Netherlands (CBS)
  • The Ella and Georg Ehrnrooth Foundation, Helsinki, Finland

Abstract and Figures

Split sex ratio theory predicts that when kin structure varies among colonies of social insects, in order to maximize the inclusive fitness, colonies with relatively high sister-sister relatedness should specialize in producing reproductive females, whereas in those with relatively low sister-sister relatedness workers should bias their sex ratio towards males. However, in order to achieve this, workers need to be able to reliably assess the type of colony in which they live. The information on colony kin structure may be encoded in cuticular hydrocarbons (CHCs), assuming that genetic variability translates accurately into chemical variability. However, in genetically heterogeneous colonies, too accurate information may encourage the pursuit of individual interests through nepotistic behavior and reduce colony efficiency or cause social disruption. In this study, we estimated how well variability of CHC recognition cues reflects colony kin structure in the ant Formica exsecta. Our results show that CHC variability does not covary with kin structure or the overall genetic diversity of the colony, and that patrilines and matrilines can have distinct CHC profiles in some but not all colonies. However, within-colony relatedness remains the key determinant of colony sex ratios. Based on our results, CHC variability cannot serve as accurate information on within-colony relatedness, kin structure, or full-sib affiliation, nor do workers seem to use colony CHC variability as a proxy for sex-ratio adjustment. The use of this type of information thus could lead workers to make mistakes, and it remains unclear how colonies of Formica exsecta adjust offspring sex ratio to their optimal value.
Content may be subject to copyright.
Do Cuticular Hydrocarbons Provide Sufficient Information
for Optimal Sex Allocation in the Ant Formica exsecta?
Jelle S. van Zweden &Emma Vitikainen &
Patrizia dEttorre &Liselotte Sundström
Received: 31 July 2011 /Revised: 9 October 2011 /Accepted: 7 November 2011 /Published online: 23 November 2011
#Springer Science+Business Media, LLC 2011
Abstract Split sex ratio theory predicts that when kin
structure varies among colonies of social insects, in order to
maximize the inclusive fitness, colonies with relatively high
sister-sister relatedness should specialize in producing
reproductive females, whereas in those with relatively low
sister-sister relatedness workers should bias their sex ratio
towards males. However, in order to achieve this, workers
need to be able to reliably assess the type of colony in
which they live. The information on colony kin structure
may be encoded in cuticular hydrocarbons (CHCs),
assuming that genetic variability translates accurately into
chemical variability. However, in genetically heterogeneous
colonies, too accurate information may encourage the
pursuit of individual interests through nepotistic behavior
and reduce colony efficiency or cause social disruption. In
this study, we estimated how well variability of CHC
recognition cues reflects colony kin structure in the ant
Formica exsecta. Our results show that CHC variability
does not covary with kin structure or the overall genetic
diversity of the colony, and that patrilines and matrilines
can have distinct CHC profiles in some but not all colonies.
However, within-colony relatedness remains the key deter-
minant of colony sex ratios. Based on our results, CHC
variability cannot serve as accurate information on within-
colony relatedness, kin structure, or full-sib affiliation, nor
do workers seem to use colony CHC variability as a proxy
for sex-ratio adjustment. The use of this type of information
thus could lead workers to make mistakes, and it remains
unclear how colonies of Formica exsecta adjust offspring
sex ratio to their optimal value.
Key Words Insects .Social evolution .Nepotism .Kin
selection .Levels of selection .Genetic variance .Alkanes .
Alkenes
Introduction
Reproductive division of labor is the key contributor to the
ecological success of social insects (Wilson, 1971). How-
ever, despite an impressive degree of cooperation, social
insect colonies are rife with potential conflict, which arises
because colony members, not being clonal, each have an
incentiveto pursue their individual fitness interests
(reviewed in Ratnieks et al., 2006). The worker-queen
conflict over sex ratio is contingent on the fact that the
haplodiploid system of sex determination of Hymenoptera
Electronic supplementary material The online version of this article
(doi:10.1007/s10886-011-0038-x) contains supplementary material,
which is available to authorized users.
J. S. van Zweden (*):P. dEttorre
Centre for Social Evolution, Department of Biology,
University of Copenhagen,
Universitetsparken 15,
2100 Copenhagen, Denmark
e-mail: jvzweden@bio.ku.dk
E. Vitikainen :L. Sundström
Department of Biosciences, University of Helsinki,
PO Box 65, Helsinki 00014, Finland
Present Address:
E. Vitikainen
Centre for Ecology and Conservation, University of Exeter,
Tremough Campus,
TR10 9EZ(Penryn, UK
Present Address:
P. dEttorre
Laboratoire dEthologie Expérimentale et Comparée (LEEC),
University of Paris 13,
99 av. J.B. Clément,
93430 Villetaneuse, France
J Chem Ecol (2011) 37:13651373
DOI 10.1007/s10886-011-0038-x
creates asymmetric genealogies, such that full sisters are
more closely related to each other, than they are to their
brothers (Trivers and Hare, 1976). This relatedness asym-
metry, however, decreases when the queen has mated more
than once (polyandry) or if colonies contain several related
queens (polygyny) (Boomsma, 1993). Consequently, when
kin structure varies among colonies in the same population,
theory predicts that workers can enhance their inclusive
fitness by specializing on the sex to which they are most
related relative to the population average, i.e., females in
monogynous/monandrous colonies (with high relatedness
asymmetry), and males in polyandrous or polygynous
colonies (with low relatedness asymmetry) (split sex ratio
theory; Boomsma and Grafen, 1990,1991). Indeed,
empirical studies, mainly of ant species, have found sex
allocation patterns consistent with worker control in
response to variation in relatedness asymmetry (Chan and
Bourke, 1994; Sundström, 1994; Sundström et al., 1996),
although other studies have found sex allocation patterns
inconsistent with worker control in response to variation in
relatedness asymmetry (Brown and Keller, 2000; Liautard
et al., 2003; Bonckaert et al., 2011).
Precise adjustment of sex ratios by colony workers in
response to colony kin structure requires a mechanism by
which they can assess the genetic diversity within colonies,
and manipulate brood composition accordingly, e.g., by
removing male larvae (Chapuisat et al., 1997). A parsimo-
nious hypothesis is that recognition cues are at least partly
genetically determined, so that within-colony recognition
cue variability to a certain extent reflects genetic diversity
and, thus, relatedness asymmetry. Ants and other social
insects use cuticular hydrocarbons (CHCs) for discriminat-
ing colony members from strangers to maintain colony
integrity. Typically, each colony is characterized by specific
proportions of a set of CHCs (reviewed in van Zweden and
dEttorre, 2010), the exact relative compositions of which
can have strong genetic underpinnings (e.g., Stuart, 1988;
van Zweden et al., 2009,2010). On the other hand, if the
individualsblends of CHCs accurately reflect their genetic
lineage within a colony, this could allow workers in
colonies that contain different patrilines or matrilines to
favor close kin to the detriment of more distant kin, and so
promote their own evolutionary interests. Such nepotistic
discrimination is expected to carry costs with reduced
colony efficiency and social disruption in its wake (Keller,
1997), and may lead to selection against highly diverse
recognition cues (Ratnieks et al., 2007). Therefore, the level
of information encoded in social insect recognition cues is
likely to be the result of these two opposing evolutionary
forces, being good enough to allow efficient discrimination
against non-nestmates but noisy enough to prevent precise
discrimination within colonies (van Zweden et al., 2010).
Evidence to date indicates that this is indeed the case in the
ant Formica truncorum (Boomsma et al., 2003) and the
wasp Vespa crabro (Danietal.,2004), where CHC
variation explained by patriline affiliation is variable among
colonies, although a recent study on Acromyrmex octospi-
nosus shows higher kin specificity (Nehring et al., 2010).
Not all components of the CHC profile are of equal
importance in inter- and intra-colony recognition. Methyl
branched alkanes and alkenes have been especially impli-
cated in nestmate recognition, whereas the role of linear
alkanes in nestmate recognition is less clear (Dani et al.,
2001,2005; Akino et al., 2004; Greene and Gordon, 2007;
Martin et al., 2008; Guerrieri et al., 2009). Conversely, the
relative abundance of linear alkanes appears, for example,
to be involved in within-colony recognition of (worker)
castes in harvester ants (Wagner et al., 2001; Greene and
Gordon, 2003; Martin and Drijfhout, 2009). Therefore, we
hypothesize that CHCs associated with nestmate recogni-
tion may be uniform within colonies, whereas those
involved in the assessment of colony kin structure may be
more variable (c.f., van Zweden et al., 2010).
In the ant Formica exsecta, both the number of queens
that head the colony (monogyny vs. polygyny) and the
number of males that mate with a queen (monandry vs.
polyandry) vary. A previous study showed sex ratio
specialization consistent with the predictions from split
sex ratio theory (Sundström et al., 1996). This implies that
workers can assess colony kin structure also when the
relatedness asymmetry varies solely due to queen mating
frequency. The CHC profile of F. exsecta typically consists
of five to six linear alkanes (n-C
21
:n-C
31
) and five to six
alkenes ((Z)-9-C
21:1
:(Z)-9-C
31:1
), of which the latter have
been implicated as nestmate recognition cues (Martin et al.,
2008). In this study, we compared the CHC variability
among workers in monogynous/monandrous, monogynous/
polyandrous, and polygynous colonies of F. exsecta,to
assess whether workers have enough information to act in
their best inclusive fitness interests. We predicted that CHC
variability would be greater in polyandrous and polygynous
than in monogynous/monandrous colonies, thus reflecting
lower relatedness asymmetry. The second prediction was
that greater CHC variability corresponds to colonies with
male-biased sex ratios. We then combined the information
on CHC and genetic diversity with sex ratio data for the
same colonies to evaluate the degree to which sex
allocation corresponds to colony relatedness on the one
hand and CHC variability on the other.
Methods and Materials
Study Organisms and General Procedures Workers of F.
exsecta were collected from 9 colonies in June 2007 and
from 6 colonies in June 2009 (Table 1), on islands off the
1366 J Chem Ecol (2011) 37:13651373
Tvärminne Zoological Station, southwestern Finland. The
population comprises largely monogynous colonies headed
by queens mated with 1 to 3 males, with the exception of a
few polygynous colonies (Haag-Liautard et al., 2009). This
is the same population where Sundström et al. (1996) found
that colonies headed by a singly mated queen specialize in
female brood, and those headed by a multiply mated queen
specialize in male production. Altogether we chose 5
colonies with a single queen that had mated once
(monogynous-monandrous, MG/MA), 5 colonies where
the queen had mated multiply (monogynous-polyandrous,
MG/PA) and 5 colonies with multiple queens (polygynous,
PG) (Table 1). From each colony, 1824 workers were
collected and killed by freezing at 20°C for later genetic
and chemical analysis.
Sex ratios of the study colonies were assessed by
sampling 5060 sexual pupae from each colony, in June-
July 2007 and 2009 after all the larvae had pupated and
before any sexuals had emerged. The sex of the pupae was
determined based on morphology, and the sex ratio was
calculated as the proportion of queens of all sexual brood
(c.f., Vitikainen et al., 2011).
Genetic Analysis To confirm the kin structure of the
sampled colonies and to determine patriline or matriline
affiliation, the 1824 workers of each colony were
genotyped at ten loci: Fe11, Fe13, Fe17, Fe37, Fe38,
Fe42, Fe49 (Gyllenstrand et al., 2002), Fl21 (Chapuisat,
1996), P22 (Trontti et al., 2003), and Fy3 (Hasegawa and
Imai, 2004). Further details of PCR conditions are equal to
Haag-Liautard et al. (2009). The PCR-products were
separated by using automated capillary sequencer (Mega-
BACE 1000) and sized against ET400-R standard (GE
Healthcare). The genotypes were scored with the program
Fragment Profiler v1.2 (GE Healthcare), and allele calling
was confirmed manually.
We used maximum likelihood methods to confirm the
genetic composition of the colonies, and to assign workers
into groups of full- and half-sisters, as implemented in the
program COLONY 1.3. (Wang, 2004). Average within-
colony relatedness was calculated as the pedigree related-
ness, based on the equations in Bourke and Franks (1995),
assuming r=0.75 for MG/MA colonies, and using the
observed paternity shares in MG/PA colonies. For the PG
colonies, we inferred the number of matrilines from the
genotype data assuming the queens were unrelated (r =0).
The true value may be higher, so we ran each test also with
higher values without finding any differences in the
outcome. Pairwise relatedness estimates between individu-
als and colonies were calculated based on individual
genotypes at each of the ten loci using Relatedness 5.0.8
(Goodnight and Queller, 1999), and colony inbreeding
(HL) following the procedure by Aparicio et al. (2006). We
used the background population genetic data from 102
colonies (c.f. Haag-Liautard et al., 2009) to enhance the
accuracy of allele frequency and relatedness estimates.
Table 1 Basic data for the sampled colonies
Colony Year NSex ratio Inbreeding
coefficient
Gene
diversity
Allelic
richness
Nr. of matri/
patrilines
Pedigree
relatedness
All compounds (Z)-9-Alkenes n-Alkanes
MG/MA 1 2007 20 0.85 0.29 0.513 2.275 1 m/1p 0.750 1.664 1.021 1.525
MG/MA 2 2007 18 0.17 0.04 0.548 2.279 1 m/1p 0.750 1.676 0.838 1.469
MG/MA 3 2007 19 1.00 0.04 0.513 2.214 1 m/1p 0.750 1.860 0.843 1.214
MG/MA 4 2009 24 0.96 0.26 0.479 2.130 1 m/1p 0.750 1.871 0.423 1.154
MG/MA 5 2009 23 0.00 0.00 0.585 2.462 1 m/1p 0.750 1.365 0.569 1.017
MG/PA 1 2007 20 0.13 0.26 0.516 2.194 1 m/2p 0.503 2.817 1.337 1.562
MG/PA 2 2007 19 0.55 0.39 0.408 1.983 1 m/2p 0.540 1.946 1.191 1.259
MG/PA 3 2007 19 1.00 0.16 0.647 2.805 1 m/2p 0.590 2.701 1.308 1.970
MG/PA 4 2009 24 0.00 0.12 0.577 2.488 1 m/2p 0.674 1.499 0.534 1.074
MG/PA 5 2009 23 0.00 0.11 0.583 2.415 1 m/2p 0.516 1.531 0.277 1.262
PG 1 2007 18 0.15 0.25 0.718 3.373 8 m 0.094 1.940 0.759 1.063
PG 2 2007 19 0.00 0.28 0.622 2.930 7 m 0.107 1.914 0.856 1.368
PG 3 2007 20 0.00 0.28 0.714 3.305 9 m 0.083 2.774 0.848 1.890
PG 4 2009 23 0.29 0.33 0.446 2.180 4 m 0.083 1.475 0.579 1.343
PG 5 2009 23 0.04 0.40 0.604 2.933 7 m 0.048 1.321 0.683 1.129
Colony name also reflects colony type (MG/MA monogynous-monandrous, MG/PA monogynous-polyandrous, PG polygynous). Nequals the
number of individuals screened per colony. Sex ratio is expressed as the number of females as a proportion of the total number of sexual offspring.
Pedigree relatedness is calculated based on the assumption that queen relatedness is zero. All compounds, (Z)-9-Alkenes, and n-Alkanes refer to
cuticular hydrocarbon (CHC) variability measures based on the respective sets of compounds.
J Chem Ecol (2011) 37:13651373 1367
Colony-specific gene diversity and allelic richness were
calculated with colony defined as population using Fstat
2.9.3 (Goudet, 2001). As the two measures correlated
strongly (r=0.94, df=15, P< 0.001), we used the factor
scores from the first axis in a PCA encompassing the two
measures. Comparisons between the different categories of
colonies were done with a one-way ANOVA.
Chemical Analysis Cuticular lipids were obtained by
individually immersing the 1824 freshly killed workers
(before genotyping them) in 200 μl HPLC-grade pentane
for 10 min, with gentle vortexing for 15 sec at the start and
end of the 10 min. The pentane was left to evaporate at
room temperature under a laminar air-flow hood. Extracts
were resuspended in 50 μl pentane, 2 μl of which were
injected into an Agilent Technologies 6890 N gas chro-
matograph (GC), connected to an Agilent Technologies
5975 mass selective detector (MS), using 70 eV electron
impact ionization. The GC was equipped with an HP-
5MS capillary column (30 m×0.25 mm ID, 0.25 μm
film thickness) and a split-splitless injector. The carrier
gas was helium at 1 ml/min. After an initial hold of
1 min at 70°C, the temperature rose to 180°C at a rate
of 30°C/min, and then to 320°C at 4°C/min with a final
hold of 5 min.
The areas of five (Z)-9-alkenes and five linear alkanes
found on the cuticle of all workers (c.f. Martin et al., 2008)
were integrated for further analysis with Agilent ChemSta-
tion v. D.02.00.237. Peak areas were normalized for each
individual by using a Z-transformation (Aitchison, 1986).
Statistics We estimated within-colony CHC variability both
for single compounds and for sets of multiple compounds
[all compounds (10 variables), (Z)-9-alkenes only (5
variables), and linear alkanes only (5 variables)]. For single
compounds, we calculated the within-colony standard
deviation from the Z-transformed values. For sets of
multiple compounds, we first scaled the Z-transformed
variables to zero mean and unit variance, and then
calculated the absolute Euclidean distance to the mean of
the colony. CHC variability then was expressed per colony
as the mean distance of its individuals.
To assess which factors may influence within-colony
CHC variability, we used ANCOVAs with within-colony
CHC variability as the dependent variable, and colony kin
structure (MG/MA, MG/PA, PG) or within-colony related-
ness, year of sampling (2007 or 2009), within-colony
genetic diversity (as captured in the factor scores for gene
diversity and allelic richness combined), and colony
inbreeding (HL) as the predictor variables. We included
HL in the analysis, given that two earlier studies have
indicated a significant effect of inbreeding on sex ratios
(Haag-Liautard et al., 2009; Vitikainen et al., 2011). To test
the effects of within-colony CHC variability, relatedness,
genetic diversity, and inbreeding on colony sex ratio we
used a multiple regression with stepwise backward elimi-
nation. These analyses were done in Statistix 9 (Analytical
Software, USA).
To test for an association between genetic and CHC
similarity of colonies and individuals within colonies, we
created distance matrices, one with the pairwise genetic
relatedness based on the ten genotyped loci, and one with
the pairwise Euclidean CHC distance. Pairwise Euclidean
CHC distance was calculated for sets comprising either all
compounds (10 variables), (Z)-9-alkenes only (5 variables),
or linear alkanes only (5 variables). The correlation
between the genetic and CHC matrices then was tested
using Mantel tests in the program Genodive (v. 2.0b20)
(Meiermans, 2010). Correlations between relatedness and
CHC distances were performed both between individuals
within colonies (i.e., for each colony separately) and
between colonies.
Finally, to estimate variation among hydrocarbon pro-
files explained by patriline or matriline affiliation, we
performed a PCA using the MGPA and PG colonies, based
on the three sets of multiple compounds [all compounds,
linear alkanes only, and (Z)-9-alkenes only]. This was
followed by a MANOVA in the program R (v. 2.12.0) with
those PCs explaining >95% of the variation as dependent
variables, and colony and patri- or matriline nested within
colony as the explanatory variables. We repeated this
analysis for each colony separately, except that colony
was not used as an explanatory variable in the MANOVA.
We corrected the above analyses for multiple testing by
using the false discovery rate as appropriate (Benjamini and
Hochberg, 1995).
Results
All putative MG/MA colonies were confirmed to be
composed of full sisters only. All five MG/PA colonies
had two patrilines, and the samples from PG colonies
contained between 4 and 9 matrilines (Table 1). The
diversity of (Z)-9-alkenes was in both sampling years
significantly lower than that of n-alkanes, and the diversity
of both was lower in 2009 than in 2007 (full factorial
ANOVA, compound type: F
1,26
=46.3, P<0.001, year: F
1,26
=
23.5, P<0.001, compound type x year: F
1,26
=1.08, P=0.31).
We found no significant differences in CHC variability
between PG, MG/PA, and MG/MA colonies, neither when
all compounds were considered together, nor when the
subsets of (Z)-9-alkenes or n-alkanes were considered
separately [Fig. 1; one-way ANOVA with colony type as
explanatory variable: (Z)-9-alkenes: F
2,12
=0.57, P=0.58,
1368 J Chem Ecol (2011) 37:13651373
n-alkanes: F
2,12
=0.31, P=0.74, all compounds: F
2,12
=0.84,
P=0.46]. Similarly, in the more comprehensive model,
neither within-colony relatedness, genetic diversity (factor
scores for the combined effects of allelic diversity and gene
diversity), nor colony inbreeding were significantly associ-
ated with CHC variability in any of the three multi-
compound sets [all compounds, (Z)-9-alkenes, n-alkanes;
Table 2], or any of the individual compounds (Appendix 1).
The outcome was qualitatively the same when colony type
(PG, MG/PA, and MG/MA) was used instead of within-
colony relatedness (Appendix 1). Indeed, within-colony
CHC variability was not significantly correlated to any
of the other measures, except for sampling year
(Table 2). However, genetic diversity did not significantly
differ among the three colony types either (PG, MG/PA,
MG/MA) (F
2,12
=3.25, P=0.07), suggesting that the
increased number of genetic lineages in MG/PA and PG
colonies did not necessarily increase genetic diversity.
We found a portion of CHC variation was explained by
patri- and matrilines when all compounds or (Z)-9-alkenes
only were considered (all compounds: Wilksλ=0.333,
approx. F
148,632.1
=1.36, P<0.01; (Z)-9-alkenes: Wilksλ=
0.420, approx. F
111,477.1
=1.44, P<0.01; n-alkanes: Wilks
λ=0.521, approx. F
111,477.1
=1.05, P=0.367), although
these differences were only pronounced in some of the
colonies (Table 3). Matrilines differed significantly in their
CHCs in one of the five PG colonies, regardless whether all
compounds or only (Z)-9-alkenes were used. Similarly, in
one of the five MG/PA colonies patrilines differed
significantly with respect to the (Z)-9-alkenes (all corrected
for false discovery rate; Table 3). We also found a negative
correlation between pairs of colonies for pairwise related-
ness and (Z)-9-alkene distance after correction for false
discovery rate (r=0.342, P=0.001), but not for n-alkanes
or when all compounds were considered (r=0.021, P=
0.401, and r=0.213, P=0.016, respectively) (Fig. 2). This
shows that the relative abundance of (Z)-9-alkenes has a
strong genetic component with respect to between-colony
variation. However, within colonies, we found no signifi-
cant associations between genetic and CHC distances for
any of the considered compound sets (Appendix 2).
The sex ratios of all PG colonies were highly male
biased, as expected based on their low relatedness (Fig. 3).
The monogynous colonies were more variable. Two of the
five MG/PA colonies produced a female biased sex ratio,
and two of the five MG/MA colonies produced mainly
males (Fig. 3). When the effects of colony-specific CHC
variability on sex ratio were considered in conjunction with
within-colony relatedness and colony inbreeding (HL), the
only significant determinant of colony sex ratios was
within-colony relatedness (full model including relatedness
and HL as explanatory factors: R
2
=0.40, F
2,12
=4.04, P=
0.045; HL: F
2,12
=3.13, P=0.102; relatedness F
2,12
=8.07,
P=0.015; all other variables, year, all compounds, (Z)-9-
alkenes, n-alkanes: F
2,12
<0.35, P>0.50).
Discussion
In this study, we showed that in Formica exsecta the
within-colony CHC variability is not contingent on colony
kin structure, yet genetically related colonies have more
similar chemistry, showing a genetic component to the
cuticular chemistry without a direct link with the genetic
composition of the colony. Nonetheless, genetic lineages
were chemically distinct in some but not all colonies. This
suggests that chemical cues by which workers can assess
colony kin structure are occasionally available, but that
these cues generally are unreliable, so that workers would
make too many mistakes when using them as a proxy for
the sex ratio to be produced by the colony.
Polygynous and monogynous/polyandrous colonies
showed no higher amounts of CHC variability than
monogynous/monandrous ones. This is in agreement with
the result obtained by S.J. Martin et al. (unpublished data)
for the same species, but stands in stark contrast with the
general expectation of greater CHC variability in geneti-
cally more heterogeneous, i.e., polygynous and/or mono-
Fig. 1 CHC variability, colony type and CHC group (mean ±95%CI;
MG/MA monogynous-monandrous, MG/PA monogynous-polyandrous,
PG polygynous)
Table 2 Ancova results for effects on CHC variability
Effect All compounds (Z)-9-Alkenes n-Alkanes
F P FPFP
Year 5.63 0.04 20.09 0.001 3.71 0.08
Relatedness 0.12 0.73 0.33 0.58 0.01 0.92
Genetic diversity 0.39 0.55 0.12 0.73 0.07 0.80
Inbreeding 0.25 0.63 1.64 0.23 0.06 0.81
Within-colony relatedness was calculated assuming zero relatedness
among queens, and genetic diversity as the combined effects of
allelic richness and gene diversity captured in their factor scores.
df1=1, df2=10 in all cases.
J Chem Ecol (2011) 37:13651373 1369
gynous/polyandrous societies, which has formed the foun-
dation for assuming lower discrimination abilities in
polygynous societies (e.g., Fletcher and Michener, 1987;
Starks et al., 1998). However, we also found that the
within-colony genetic diversity was not higher in the
polygynous and the monogynous/polyandrous colonies
than in the monogynous/monandrous ones. Indeed, al-
though within-colony relatedness varied 15-fold, the
corresponding ranges for both gene diversity and allelic
richness were only 1.7 fold in both cases (Table 1). Thus,
the genetic basis for a greater diversity is not present, which
may explain the lack of correlation between colony kin
structure and CHC variability.
The (Z)-9-alkenes overall were less variable within
colonies than n-alkanes. This supports the existing evidence
that mainly alkenes mediate nestmate recognition in F.
exsecta, whereas alkanes are less informative in this respect
(Martin et al., 2008). Moreover, the between-colony CHC
distance for (Z)-9-alkenes, but not the n-alkanes, decreased
significantly with increasing between-colony pairwise
relatedness. This, in conjunction with the result that
within-colony CHC variability showed no clear association
with genetic diversity, suggests a strong genetic foundation
of the (Z)-9-alkene profile that is to a large extent blurred
within colonies by odor cue transfer between nestmates (c.f.
van Zweden et al., 2010). It also suggests unequal mixing
Fig. 3 Sex ratios (numerical proportion of females in the brood) as a
function of within-colony relatedness, grouped according to colony
type (MG/MA monogynous-monandrous, MG/PA monogynous-
polyandrous, PG polygynous)
Z
Fig. 2 Correlations between colony pairwise relatedness and colony
CHC distances. Association is significant, after correction for false
discovery rate, for (Z)-9-alkenes (Mantels test; (Z)-9-alkenes, r=0.342,
P<0.001), but not when n-alkanes or all compounds were considered
(n-alkanes, r=0.021, P=0.401; all compounds, r=0.213, P=0.016)
Table 3 Discriminant analyses between genetic lineages within MGPA and PG colonies
Colony All compounds (3 PCs) (Z)-9-Alkenes (2 PCs) n-Alkanes (2 PCs)
WilksλFPWilksλFPWilksλFP
All colonies 0.333 F
148,632
=1.36 0.007 0.420 F
111,477
=1.44 0.005 0.521 F
111,477
=1.05 0.367
MG/PA 1 0.795 F
3,16
=1.38 0.285 0.891 F
2,17
=1.05 0.373 0.858 F
2,17
=1.41 0.272
MG/PA 2 0.502 F
3,14
=4.63 0.018 0.810 F
2,15
=1.76 0.206 0.577 F
2,15
=5.49 0.016
MG/PA 3 0.730 F
3,15
=1.85 0.182 0.711 F
2,16
=3.25 0.065 0.851 F
2,16
=1.40 0.276
MG/PA 4 0.732 F
3,20
=2.44 0.094 0.539 F
2,21
=8.99 0.002 0.829 F
2,21
=2.16 0.140
MG/PA 5 0.718 F
3,19
=2.49 0.092 0.821 F
2,20
=2.18 0.140 0.902 F
2,20
=1.09 0.355
PG 1 0.098 F
20,20.7
=1.23 0.323 0.482 F
14,16
=0.50 0.899 0.221 F
14,16
=1.29 0.310
PG 2 0.290 F
18,28.8
=0.88 0.606 0.255 F
12,22
=1.80 0.113 0.325 F
12,22
=1.38 0.246
PG 3 0.220 F
24,26.7
=0.76 0.746 0.413 F
16,20
=0.70 0.767 0.379 F
16,20
=0.78 0.690
PG 4 0.088 F
9,41.5
=7.92 <0.001 0.023 F
6,36
=33.52 <0.001 0.468 F
6,36
=2.77 0.026
PG 5 0.323 F
18,40.1
=1.09 0.394 0.351 F
12,30
=1.72 0.111 0.436 F
12,30
=1.29 0.277
A PCA was performed on the Z-transformed CHC data of either all colonies together or single colonies; PCs explaining >95% of the variation
were then included as the dependent variable in a MANOVA with genetic lineage (nested within colony, when all colonies were considered
together) as the explanatory variable.
1370 J Chem Ecol (2011) 37:13651373
of CHCs, which could be achieved by accumulating
alkenes to a greater extent than alkanes in the postpha-
ryngeal gland (PPG; Soroker et al., 1994). The PPG does
not synthesize hydrocarbons, but acts as a reservoir for
these compounds to be homogenized and exchanged via
grooming and trophallaxis (Soroker et al., 1994). Thus,
inequalities in diversity could arise if alkenes are released
more readily, or workers metabolize alkanes more rapidly,
with alkenes becoming better mixed than alkanes among
colony members. Altogether, these results suggest a pattern
similar to that observed in F. rufibarbis, where cues used in
nestmate recognition are genetically determined but prefer-
entially mixed among nestmates (van Zweden et al., 2010).
Although the within-colony CHC variability showed no
correlation with colony kin structure, relatedness, or any of
the measures of genetic diversity, we found significant
CHC divergence between genetic lineages within colonies.
This was, however, only the case in two of the ten colonies
that comprised more than one lineage. Earlier analyses in
the wasps Polistes dominulus,Vespa crabro (Dani et al.,
2004), and Dolichovespula saxonica (Bonckaert et al.,
2011), and the ant Acromyrmex octospinosus (Nehring et al.,
2010), found more consistent CHC differences between
matrilines and patrilines. However, the CHC profile of F.
exsecta exhibits far fewer hydrocarbons with fewer structural
groups than these species (P. dominulus:44compounds;V.
crabro:25compounds;D. saxonica:56compounds;A.
octospinosus: 38 compounds), and so offers less scope for
both divergence and statistical discrimination between
genetic lineages. However, in F. truncorum,Boomsmaet
al. (2003) nonetheless were able to discriminate a greater
fraction of patrilines although the analysis entailed only nine
compounds.
At least two mutually non-exclusive explanations for the
relative lack of CHC discrimination among genetic lineages
are possible. First, active cue scrambling may render the
colony odor more uniform or blur the recognition code. This
may be an evolved response that either mitigates the
opportunities for workers to selfishly favor close kin and
disfavor more distant kin with social disruption in its wake, or
increases discrimination between members of different colo-
nies (Keller, 1997; van Zweden et al., 2010). Theory predicts
depauperate CHC profiles and cue mixing when potential
conflicts are rife in the colony (e.g., Sundström and
Boomsma, 2001). This is indeed the case in F. exsecta,but
many more compounds have been found in F. truncorum
(Akino, 2006), which goes against this interpretation.
Second, given that inbreeding is rife in the population of F.
exsecta studied here (Sundström et al., 2003;Haag-Liautard
et al., 2009; Vitikainen et al., 2011), genetic variation for cue
diversity may have been purged. We found no significant
effects of inbreeding on CHC variability, but the measure we
used as a covariate in the analyses does not reflect the
absolute level of inbreeding, only the relative level. Thus, a
significant effect of inbreeding may not be detectable.
Within-colony CHC variability had no significant pre-
dictive value on sex ratio, but colonies with higher
relatedness produced more females, when also accounting
for colony inbreeding. This suggests that workers do not
use the CHC variability of adult individuals as a proxy for
sex ratio decisions, in contrast to the results obtained by
Boomsma et al. (2003) for F. truncorum. It begs the
question by what mechanism this is achieved. All poly-
gynous colonies produced a male-biased brood, but against
expectations based on the kin value of brood, one
polyandrous colony produced only female brood and one
monandrous colony only male brood. In the case of
polyandrous colonies, errors in the assessment of colony
kin structure may occur if the two fathers are closely
related, and so mediate similar CHC profiles to their
offspring. However, the polyandrous colony that produced
an all-female brood had the lowest relatedness between
fathers, and not an exceptionally unequal contribution by
the two fathers (Table 1). It remains to be shown what
information the workers use to assess colony kin structure,
and why such apparent errors occur. A possibility is that
CHC information on eggs or larvae more accurately reflects
the genetic make-up of the colony, owing to no or less
transfer of cues between genetic lineages, and that nurse
workers bias the sex ratio accordingly. Another possibility
is that nurse workers can assess the number of queens
directly or by colony egg production, and use this as a
proxy to adjust sex ratio in polygynous colonies. This is
consistent with split-sex ratio theory (Boomsma, 1993),
because polygynous colonies will tend to have higher egg-
production and a lower than population-level-average
relatedness asymmetry, and also with the idea that
specialization in the cheaper sex (males weigh less in F.
exsecta; Vitikainen et al., 2011) requires greater initial egg-
production for equal total biomass production. On the other
hand, this does not explain the tendency of monogynous/
polyandrous colonies to specialize in male production.
Environmental factors also are likely to influence the
optimal sex ratio for a colony, and hence may explain the
discrepancies between the observed pattern and the out-
come expected based on purely inclusive fitness arguments
(Nielsen et al., 1999; Liang and Silverman, 2000).
In summary, we found no support for the suggested
association between colony kin structure and variability of
recognition cues (c.f. Fletcher and Michener, 1987), but this
lack of relationship may be explained by the uniform levels
of genetic variation we found among the different colony
types. The breeding patterns and the genetic structure of the
population, which indicate non-trivial levels of inbreeding
(Sundström et al., 2003; Haag-Liautard et al., 2009;
Vitikainen et al., 2011), may have contributed to the
J Chem Ecol (2011) 37:13651373 1371
depauperate chemical profile through genetic purging, a
question which will have to be addressed in future studies.
We also found that the within-colony CHC variability does
not allow accurate assessment of colony kin structure, and
that workers do not seem to use any single compound or set
of compound as a proxy for sex ratio adjustments. Rather,
genetically informative cues appear to be mixed among
workers to create a colony odor that is used in between-
colony discrimination. The cuticular hydrocarbon variabil-
ity of Formica exsecta may thus be subject to balancing
selection on the accuracy of genetic variability, allowing
colony members to serve their collective but not selfish
fitness interests, possibly blended with loss of genetic
diversity owing to inbreeding in this population.
Acknowledgements We thank V. Nehring for his help in analyzing
the chemical data, J.J. Boomsma for critical discussions, and all
members of the Copenhagen Centre for Social Evolution for providing
a stimulating working environment. This work was supported by the
EU Marie Curie Excellence Grant CODICES-EXT-CT-2004-014202
(to PdE), Academy of Finland grants 206404,121216, and 135975 (to
LS), LUOVA graduate school in Helsinki (to EV), and the NordForsk
research network Social evolution in insects. JSvZ was also
supported by a postdoctoral fellowship from the Danish Council for
Independent Research (09066595).
References
AITCHISON, J. 1986. The statistical analysis of compositional data. The
Blackburn Press, Caldwell.
AKINO, T. 2006. Cuticular hydrocarbons of Formica truncorum
(Hymenoptera: Formicidae): Description of new very long
chained hydrocarbon components. Appl. Entomol. Zool.
41:667677.
AKINO, T., YAMAMURA, K., WAKAMURA, S., and YAMAOKA, R. 2004.
Direct behavioral evidence for hydrocarbons as nestmate recog-
nition cues in Formica japonica (Hymenoptera: Formicidae).
Appl. Entomol. Zool. 39:381387.
APARICIO, J. M., ORTEGO, J., and CORDERO, P. J. 2006. What should
we weigh to estimate heterozygosity, alleles or loci? Mol. Ecol.
15:46594665.
BENJAMINI,YandHOCHBERG, Y. 1995. Controlling the false
discovery rate: A practical and powerful approach to multiple
testing. J. Roy. Stat. Soc. B 57:289300.
BONCKAERT,W.,VA N ZWEDEN, J. S., DETTORRE, P., BILLEN, J., and
WENSELEERS, T. 2011. Colony stage and not facultative policing
explains pattern of worker reproduction in the Saxon wasp. Mol.
Ecol. 20:34553468.
BOOMSMA, J. J. 1993. Sex ratio variation in polygynous ants, pp. 86
109, in L. Keller (ed.), Queen number and sociality in insects.
Oxford University Press, New York.
BOOMSMA, J. J. and GRAFEN, A. 1990. Intraspecific variation in ant
sex ratios and the Trivers-Hare hypothesis. Evolution 44:1026
1034.
BOOMSMA,J.J.andGRAFEN, A. 1991. Colony-level sex ratio
selection in the eusocial Hymenoptera. J. Evol. Biol. 4:383407.
BOOMSMA, J. J., NIELSEN, J., SUNDSTRÖM, L., OLDHAM, N. J.,
TENTSCHERT, J., PETERSEN, H. C., and MORGAN, E. D. 2003.
Informational constraints on optimal sex allocation in ants. Proc.
Natl. Acad. Sci. USA 100:87998804.
BOURKE, A. F. G. and FRANKS, N. R. 1995. Social evolution in ants.
Princeton University Press, Princeton.
BROWN, W. D. and KELLER, L. 2000. Colony sex ratios vary with
queen number but not relatedness asymmetry in the ant Formica
exsecta. Proc. R. Soc. B 267:17511757.
CHAN,G.L.andBOURKE, A. F. G. 1994. Split sex ratios in a
multiple-queen ant population. Proc. R. Soc. B 258:261266.
CHAPUISAT, M. 1996. Characterization of microsatellite loci in
Formica lugubris B and their variability in other ant species.
Mol. Ecol. 5:599601.
CHAPUISAT, M., LISELOTTE, S., and KELLER, L. 1997. Sex-ratio
regulation: The economics of fratricide in ants. Proc. R. Soc. B
264:12551260.
DANI,F.R.,JONES,G.R.,DESTRI,S.,SPENCER, S. H., and TURILLAZZI,S.
2001. Deciphering the recognition signature within the cuticular
chemical profile of paper wasps. Anim. Behav. 62:165171.
DANI, F. R., FOSTER, K. R., ZACCHI, F., SEPPÄ, P., MASSOLO, A.,
CARELLI, A., ARÉVALO, E., QUELLER, D. C., STRASSMANN, J.,
and TURILLAZZI, S. 2004. Can cuticular lipids provide
sufficient information for within-colony nepotism in wasps?
Proc. R. Soc. B 271:745753.
DANI, F. R., JONES, G. R., CORSI, S., BEARD, R., PRADELLA, D., and
TURILLAZZI, S. 2005. Nestmate recognition cues in the honey
bee: differential importance of cuticular alkanes and alkenes.
Chem. Senses 30:477489.
FLETCHER, D. J. C. and MICHENER, C. D. 1987. Kin recognition in
animals. Wiley, New York.
GOODNIGHT, K. F. and QUELLER, D. C. 1999. Computer software for
performing likelihood tests of pedigree relationship using genetic
markers. Mol. Ecol. 8:12311234.
GOUDET, J. 2001. FSTAT, a program to estimate and test gene
diversities and fixation indices. Version 2.9.3.
GREENE, M. J. and GORDON, D. M. 2003. Cuticular hydrocarbons
inform task decisions. Nature 423:3232.
GREENE, M. J. and GORDON, D. M. 2007. Structural complexity of
chemical recognition cues affects the perception of group
membership in the ants Linepithema humile and Aphaenogaster
cockerelli.J. Exp. Biol. 210:897905.
GUERRIERI, F. J., NEHRING, V., JØRGENSEN, C. G., NIELSEN, J.,
GALIZIA, C. G., and DETTORRE, P. 2009. Ants recognize foes
and not friends. Proc. R. Soc. B 276:24612468.
GYLLENSTRand, N., GERTSCH,P.J.,AND PAMILO, P. 2002. Poly-
morphic microsatellite DNA markers in the ant Formica exsecta.
Mol. Ecol. Notes 2:6769.
HAAG-LIAUTARD, C., VITIKAINEN, E., KELLER, L., and SUNDSTRÖM,
L. 2009. Fitness and the level of homozygosity in a social insect.
J. Evol. Biol. 22:134142.
HASEGAWA, E. and IMAI, S. 2004. Characterization of microsatellite
loci in red wood ants Formica (s. str.) spp. and the related genus
Polyergus.Mol. Ecol. Notes 4:200203.
KELLER, L. 1997. Indiscriminate altruism: unduly nice parents and
siblings. Trends Ecol. Evol. 12:99103.
LIANG, D. and SILVERMAN, J. 2000. Youarewhatyoueat:Diet
modifies cuticular hydrocarbons and nestmate recognition in
the Argentine ant, Linepithema humile.Naturwissenschaften
87:412416.
LIAUTARD, C., BROWN, W. D., HELMS, K. R., and KELLER, L. 2003.
Temporal and spatial variations of gyne production in the ant
Formica exsecta.Oecologia 136:558564.
MARTIN, S. J. and DRIJFHOUT, F. 2009. Nestmate and task cues are
influenced and encoded differently within ant cuticular hydro-
carbon profiles. J. Chem. Ecol. 35:368374.
MARTIN, S. J., VITIKAINEN, E., HELANTERÄ, H., and DRIJFHOUT,F.P.
2008. Chemical basis of nest-mate discrimination in the ant
Formica exsecta.Proc. R. Soc. B 275:12711278.
MEIERMANS P. G. 2010. GenoDive. Version 2.0b20.
1372 J Chem Ecol (2011) 37:13651373
NEHRING, V., EVISON, S. E. F., SANTORELLI, L. A., DETTORRE,P.,
and HUGHES, W. O. H. 2010. Kin-informative recognition cues in
ants. Proc. R. Soc. B 278:19421948.
NIELSEN,J.,BOOMSMA,J.J.,OLDHAM,N.J.,PETERSEN,H.C.,
and MORGAN, E. D. 1999. Colony-level and season-specific
variation in cuticular hydrocarbon profiles of individual workers
in the ant Formica truncorum.Insectes Soc. 46:5865.
RATNI EK S ,F.L.W.,FOSTER, K. R., and WENSELEERS, T. 2006. Conflict
resolution in insect societies. Ann. Rev. Entomol. 51:581608.
RATN I EK S ,F.L.W.,HELANTERÄ, H., and FOSTER, K. R. 2007. Are
mistakes inevitable? Sex allocation specialization by workers
can reduce the genetic information needed to assess queen
mating frequency. J. Theor. Biol. 244:470477.
SOROKER,V.,VIENNE,C.,HEFETZ, A., and NOWBAHARI, E. 1994. The
postpharyngeal gland as a gestaltorgan for nestmate recogni-
tion in the ant Cataglyphis niger.Naturwissenschaften 81:510
513.
STAR KS ,P.T.,WATS O N,R.E.,DIPAOLA,M.J.,andDIPAOLA,C.P.
1998. The effect of queen number on nestmate discrimination
in the Facultatively Polygynous Ant Pseudomyrmex pallidus
(Hymenoptera: Formicidae). Ethology 104:573584.
STUART, R. J. 1988. Collective cues as a basis for nestmate
recognition in polygynous leptothoracine ants. Proc. Natl. Acad.
Sci. USA 85:45724575.
SUNDSTRÖM, L. 1994. Sex ratio bias, relatedness asymmetry and
queen mating frequency in ants. Nature 367:266268.
SUNDSTRÖM, L. and BOOMSMA, J. J. 2001. Conflicts and alliances in
insect families. Heredity 86:515521.
SUNDSTRÖM, L., CHAPUISAT, M., and KELLER, L. 1996. Conditional
manipulation of sex ratios by ant workers: A test of kin selection
theory. Science 274:993995.
SUNDSTRÖM, L., KELLER, L., and CHAPUISAT, M. 2003. Inbreeding and
sex-biased gene flow in the ant Formica exsecta.Evolution
57:15521561.
TRIVERS, R. L. and HARE, H. 1976. Haplodiploidy and the evolution
of the social insects. Science 191:249263.
TRONTTI, K., TAY, W. T., and SUNDSTRÖM, L. 2003. Polymorphic
microsatellite markers for the ant Plagiolepis pygmaea.Mol.
Ecol. Notes 3:575577.
VAN ZWEDEN, J. S., BRASK, J. B., CHRISTENSEN, J. H., BOOMSMA,J.
J., LINKSVAYER, T. A., and DETTORRE, P. 2010. Blending of
heritable recognition cues among ant nestmates creates distinct
colony gestalt odours but prevents within-colony nepotism. J.
Evol. Biol. 23:14981508.
VAN ZWEDEN, J. S. and DETTORRE, P. 2010. Nestmate recognition in
social insects and the role of hydrocarbons, pp. 222243, in G. J.
Blomquist and A.-G. Bagnères (eds.), Insect hydrocarbons:
biology, biochemistry, and chemical ecology. Cambridge Uni-
versity Press, Cambridge.
VA N ZWEDEN, J. S., DREIER, S., and DETTORRE, P. 2009. Disentangling
environmental and heritable nestmate recognition cues in a
carpenter ant. J. Insect Physiol. 55:158163.
VITIKAINEN, E., HAAG-LIAUTARD, C., and SUNDSTRÖM, L. 2011.
Inbreeding and reproductive investment in the ant Formica
exsecta.Evolution 65:20262037.
WAGNER,D.,TISSOT,M.,andGORDON,D.2001.Task-related
environment alters the cuticular hydrocarbon composition of
harvester ants. J. Chem. Ecol. 27:18051819.
WANG, J. 2004. Sibship reconstruction from genetic data with typing
errors. Genetics 166:19631979.
WILSON, E. O. 1971. The insect societies. Harvard University Press,
Cambridge.
J Chem Ecol (2011) 37:13651373 1373
... To overcome this apparent contradiction, van Zweden et al. (2010) proposed that patriline signals might be encoded in n-alkanes, which they hypothesized would be less affected by mixing than other CHCs, such as alkenes, thereby allowing them to persist as a weak patriline signal. However, van Zweden et al. (2011) more recently showed that there was little evidence of heritability in the n-alkanes in the ant Formica exsecta, whereas there was greater evidence of heritability of a patriline signal in the alkenes. This signal was lost in colonies, presumably as a consequence of a gestalt mechanism. ...
... These colony recognition profiles remain stable over years (Martin et al. 2012). In this species, pariline-specific Z9-alkene profiles were detected in only two of ten colonies studied by van Zweden et al. (2011) using PCA analysis performed on the z-transformed CHC data. Formica exsecta colonies contain a single queen (monogyny) or multiple queens (polygyny) that can each be mated with one (monandry), two, or three males (polyandry). ...
... Isolation allowed us to detect significant patriline variation in both the n-alkanes and Z9-alkenes of individuals from doubly mated colonies. Van Zweden et al. (2011), studied the same F. exsecta population and found distinct CHC patriline profiles in nonisolated ants from only two of ten study colonies, and again only in the Z9-alkenes. By contrast, our study detected differences in all isolated ants, which is of crucial importance because it may facilitate the evaluation of genetic diversity at an appropriate developmental stage. ...
Article
Full-text available
Chemical recognition cues are used to discriminate among species, con-specifics, and potentially between patrilines in social insect colonies. There is an ongoing debate about the possible persistence of patriline cues despite evidence for the mixing of colony odors via a "gestalt" mechanism in social insects, because patriline recognition could lead to nepotism. We analyzed the variation in recognition cues (cuticular hydrocarbons) with different mating frequencies or queen numbers in 688 Formica exsecta ants from 76 colonies. We found no increase in the profile variance as genetic diversity increased, indicating that patriline effects were absent or possibly obscured by a gestalt mechanism. We then demonstrated that an isolated individual's profile changed considerably relative to their colony profile, before stabilizing after 5 days. We used these isolated individuals to eliminate the masking effects of the gestalt mechanism, and we detected a weak but statistically significant patriline effect in isolated adult workers and also in newly emerged callow workers. Thus, our evidence suggests that genetic variation in the cuticular hydrocarbon profile of F. exsecta ants (n-alkanes and alkenes) resulted in differences among patrilines, but they were obscured in the colony environment, thereby avoiding costly nepotistic behaviors.
... However, given that the chemical component in recognition cues often appears to have a genetic component , cue diversity may co-vary with kin structure. Thus, assuming that low relatedness is associated with high genetic diversity (Giraud et al., 2001;Helanterä et al., 2011, but see Trontti et al., 2007;van Zweden et al., 2011), low-relatedness societies may have a greater diversity of recognition cues (Hölldobler and Wilson, 1977;. To date, six studies have demonstrated colonyspecific profiles in resident queens (Formica fusca, Hannonen et al., 2002;Polistes metricus, Layton et al., 1994;Vespa crabro, Butts et al., 1995), or young sexuals (F. ...
... An earlier study on F. fusca workers found no association between kin structure and the diversity of odor cues . Similarly, studies on workers of other Formica species also failed to find such a correlation van Zweden et al., , 2011, with one exception (Boomsma et al., 2003). The discrepancy between the present study and these others implies that the chemical signatures of worker and reproductive castes may depend to a different degree on the genetic composition of individuals. ...
Article
Full-text available
Communication between organisms involves visual, auditory, and olfactory pathways. In solitary insects, chemical recognition cues are influenced mainly by selection regimes related to species recognition and sexual selection. In social insects, chemical recognition cues have an additional role in mediating recognition of society members and, thereby, allowing kin selection to operate. Here, we examined whether cuticular hydrocarbon profiles are sex-specific and whether males and young queens of the ant Formica fusca have colony-specific profiles. We also investigated whether there is a relationship between genetic relatedness and chemical diversity within colonies. We demonstrated that female and male sexuals do not have unique sex-specific compounds, but that there are quantitative chemical differences between the sexes. Out of the 51 cuticular hydrocarbon compounds identified, 10 showed a significant quantitative difference between males and females. We also showed that both males and females have a significant colony-specific component in their profiles. Finally, we found a negative correlation between within-colony relatedness and within-colony chemical diversity of branched, but not linear compounds. This suggests that colonies with multiple matri- or patrilines also have a significantly greater chemical diversity.
... This species occurs in two distinct social forms: colonies with a single egg-laying queen (monogynous) that are founded through temporary social parasitism of other Serviformica ant species, and colonies with multiple queens (polygynous) that presumably arise from monogynous colonies through adoption of daughter queens. The chemical profile of this species has been extensively studied across Europe (Martin et al. 2008aMartin et al. , b, 2009Martin et al. , 2011Martin et al. , 2012aMartin et al. , b, c, 2013 van Zweden et al. 2011), demonstrating a simple and consistent CHC profile, composed of homologous series dominated by three n-alkanes (C 23 , C 25 , and C 27 ) and three (Z)-9-alkenes (C 23:1 , C 25:1 , and C 27:1 ), with a fourth (C 29 and C 29:1, respectively) of each series always present in lower quantities (Fig. 1a). Only the (Z)-9-alkenes have been shown to act as nest-mate recognition cues (Martin et al. 2008aMartin et al. , 2012b), with changes in n-alkanes corresponding with task differences in F. exsecta (), other ants (Wagner et al. 1998), and honeybees (Kather et al. 2011). ...
... On a larger spatial scale, across islands, we found no segregation of (Z )-9-alkenes and significant segregation for n -alkanes, yet we found correlations between genetic and chemical distance for both groups of compounds, such that chemical distance increased with genetic distance. This suggests a genetic influence on the CHC profile, as demonstrated earlier in a smaller data set (van Zweden et al. 2011). The positive correlation between spatial and chemical distance for n-alkanes was somewhat unexpected, as generally only nest-mate cues are expected to be closely associated with genotype. ...
Article
Full-text available
Phenotypic variation arises from interactions between genotype and environment, although how variation is produced and then maintained remains unclear. The discovery of the nest-mate recognition system in Formica exsecta ants has allowed phenotypic variation in chemical profiles to be quantified across a natural population of 83 colonies. We investigated if this variation was correlated or not with intrinsic (genetic relatedness), extrinsic (location, light, temperature), or social (queen number) factors. (Z)-9-Alkenes and n-alkanes showed different patterns of variance: island (location) explained only 0.2 % of the variation in (Z)-9-alkenes, but 21–29 % in n-alkanes, whereas colony of origin explained 96 % and 45–49 % of the variation in (Z)-9-alkenes and n-alkanes, respectively. By contrast, within-colony variance of (Z)-9-alkenes was 4 %, and 23–34 % in n-alkanes, supporting the function of the former as recognition cues. (Z)-9-Alkene and n-alkane profiles were correlated with the genetic distance between colonies. Only n-alkane profiles diverged with increasing spatial distance. Sampling year explained a small (5 %), but significant, amount of the variation in the (Z)-9-alkenes, but there was no consistent directional trend. Polygynous colonies and populous monogynous colonies were dominated by a rich C23:1 profile. We found no associations between worker size, mound exposure, or humidity, although effect sizes for the latter two factors were considerable. The results support the conjecture that genetic factors are the most likely source of between-colony variation in cuticular hydrocarbons. Electronic supplementary material The online version of this article (doi:10.1007/s10886-013-0366-0) contains supplementary material, which is available to authorized users.
... In contrast, no kinship information was detectable in facultatively polygynous ant species (Martin & al. 2009 although the profiles of their queens differ between monogynous and polygynous colonies (Eliyahu & al. 2011, Johnson & Sundström 2012. Often, differences among patrilines or matrilines are apparently too weak to allow within-colony kin recognition, and may be lost in the colony environment (Boomsma & al. 2003, Nehring & al. 2011, van Zweden & al. 2011). Overall, nepotism seems to be rarer than expected (Keller 1997, Leonhardt & al. 2016, possibly because nepotism would lead to intracolonial conflicts, such that affected colonies would be less fit and hence selected against. ...
Article
Full-text available
The body surface of nearly all insects, including ants, is covered with a lipid layer that largely consists of cuticular hydrocarbons (CHC). They fulfil several functions, the two best-studied ones being communication and protection against water loss. CHC profiles are astonishingly diverse as even a single individual can possess more than 100 different hydrocarbon molecules. Species vastly differ in their CHC composition, but also within species, CHC profiles vary among individuals of different sex, caste, fertility, age, health state, etc. This variation has been intensely studied especially in eusocial insects like ants, where differences are likely to have a signalling function. However, with so many sources of variation in CHC profiles, it is easy to lose track of which factors are more important than others, which patterns can be generalised, and which are idiosyncratic. Thus, we need a deeper understanding of how precisely different factors influence CHC variation. In this review, we aim to provide an overview of what is known to date about fixed and plastic CHC variation and discuss sources of variation on the level of individuals, social insect colonies, populations, and species. We focus on abiotic and biotic environmental factors, social structure and the genetic background as sources of CHC variation. Finally, we discuss how variation can be adaptive and how it can be constrained by biophysical and biosynthetic mechanisms. Focusing on clearly defined CHC traits will help us to build a predictive framework to understand how CHC profiles are shaped by multiple selection pressures, to identify how different sources affect fixed and plastic CHC variation, and to determine the adaptive value of CHC traits.
... L'étude de cette communication a commencé chez les insectes, et particulièrement chez les fourmis il y a plus de 100 ans . La première source de phéromones découverte chez les insectes sont les hydrocarbures cuticulaires (CHC) (van Zweden et al., 2011;Johansson and Jones, 2007;Howard and Blomquist, 2005). Les CHC sont des hydrocarbures à longue chaînes produits par des cellules spécialisées appelées oenocytes via la chaîne de synthèse des acides gras et transportés ensuite jusqu'à la cuticule des insectes Howard and Blomquist, 2005) (Figure 1.11). ...
Thesis
Full-text available
Inbreeding is well known by biologists to lower the fitness of individuals by for example decreasing survival or fertility. Therefore, natural selection should favour behaviours preventing the reproduction of genetically-related individuals or mitigating the harmful consequences, called inbreeding depression. Inbreeding depression is particularly visible in Hymenoptera with a sex-determination system called single-locus Complementary Sex Determination (sl-CSD), where it leads to the production of diploid males that are either unviable or sterile. My PhD work has thus been devoted to the study of sib-mating avoidance in natural populations of a parasitoid with sl-CSD, Venturia canescens, and to understand the cues used by females to recognize their kin. We first studied the link between habitat type (continental, island or captive), genetic diversity and diploid male production in 11 V. canescens populations. Indeed, a theoretical framework called "Diploid male extinction vortex" predict a negative correlation between populations’ isolation, genetic diversity and diploid male production that could lead to the extinction of hymenopteran populations.We actually showed a negative correlation between genetic diversity and diploid male production in isolated populations. Previous studies have furthermore demonstrated kin discrimination and sib-mating avoidance by V. canescens females in the laboratory. We therefore studied the sib-mating avoidance behaviour in natural populations of this species by genotyping more than 450 wild individuals and their offsprings. We demonstrated that females tolerated inbreeding in the wild as well as in the laboratory when several males were present. We highlighted the importance of environmental conditions on mate choice. At last, we were interested in the kin recognition system and researched the chemical cues used by females in two ecological contexts, mate choice and superparasitism avoidance. This allowed us to identify similarities in the composition of the two chemical signals and find that they were not interchangeable between the two studied ecological contexts. In the end, the results we obtained shed new light on the necessary conditions for the apparition of sib-mating avoidance in natural populations, as well as on the cues used for kin recognition in a parasitoid.
... It has morphologically well-separated queen and worker castes, and forms perennial nests of some thousands of workers ( Brown, Keller & Sundström, 2002;Vitikainen, Haag-Liautard & Sundström, 2011;Vitikainen, Haag-Liautard & Sundström, 2015). Genetic and social nest structure varies greatly, and each nest can have one or several queens that have mated with one or many males, which makes it a model species for kin conflicts ( Sundström, Chapuisat & Keller, 1996;Brown, Liautard & Keller, 2003;Kummerli & Keller, 2007), social recognition ( Martin et al., 2008;Martin et al., 2009;Van Zweden et al., 2011), and causes and consequences of inbreeding ( Sundström, Keller & Chapuisat, 2003;Haag-Liautard et al., 2009;Vitikainen, Haag-Liautard & Sundström, 2011;Vitikainen, Haag-Liautard & Sundström, 2015). We characterise the transcriptome of Formica exsecta, including different sexes, life stages, and castes to obtain a good representation of expressed genes with their possible isoforms, potentially useful as a genomic resource for future studies on ants, as well as other insects. ...
Article
Full-text available
Transcriptome resources for social insects have the potential to provide new insight into polyphenism, i.e., how divergent phenotypes arise from the same genome. Here we present a transcriptome based on paired-end RNA sequencing data for the ant Formica exsecta (Formicidae, Hymenoptera). The RNA sequencing libraries were constructed from samples of several life stages of both sexes and female castes of queens and workers, in order to maximize representation of expressed genes. We first compare the performance of common assembly and scaffolding software (Trinity, Velvet-Oases, and SOAPdenovo-trans), in producing de novo assemblies. Second, we annotate the resulting expressed contigs to the currently published genomes of ants, and other insects, including the honeybee, to filter genes that have annotation evidence of being true genes. Our pipeline resulted in a final assembly of altogether 39,262 mRNA transcripts, with an average coverage of >300X, belonging to 17,496 unique genes with annotation in the related ant species. From these genes, 536 genes were unique to one caste or sex only, highlighting the importance of comprehensive sampling. Our final assembly also showed expression of several splice variants in 6,975 genes, and we show that accounting for splice variants affects the outcome of downstream analyses such as gene ontologies. Our transcriptome provides an outstanding resource for future genetic studies on F. exsecta and other ant species, and the presented transcriptome assembly can be adapted to any non-model species that has genomic resources available from a related taxon.
... However, different matrilines' cuticular compound profiles provide sufficient information for matriline discrimination in the wood ant Formica truncorum and the wasps Polistes dominula [dominulus] and Vespa crabro (Boomsma et al., 2003;Dani et al., 2004b). In monogyne colonies composed of different patrilines, full sisters and half sisters differ in their cuticular hydrocarbon profiles in Acromyrmex octospinosus (Nehring et al., 2011), Formica truncorum (Boomsma et al., 2003), Formica exsecta (van Zweden et al., 2011), the hornet Vespa crabro (Dani et al., 2004b), and several species of Apis honeybees (Getz and Smith, 1986;Arnold et al., 1996). Although the information conveyed by cuticular compounds may be accurate enough to permit potential withinpatriline cooperation in A. octospinosus, the heritable component of recognition cues appears to be too weak and error-prone in F. truncorum, F. exsecta, and V. crabro. ...
Article
Animal communication is very complex as it usually requests several sensory modalities. Chemical communication is universal and plays a fundamental role for species continuity (individual survival, reproduction...). This chapter is mainly focused on pheromones and their role in social insect species. Social insect organization can be very complex with different castes involved in various tasks. The division of labor - polyethism - is essential for colony cohesion, efficiency and production. Moreover pheromones contribute to maintain colony division of labor. Chemical cues are fundamental in social insects and I will illustrate the different levels of recognition as they can be specific to the species, the colony, the caste or the individual. Nestmate discrimination capacity can lead to social affinity which conducts individuals to gather and cooperate, but do we have such evidence in social insects species? Moreover, chemical cues can have a genetic and/or environmental basis. Case studies will be provided showing that in species with environmentally-based chemical cues, social interactions are essential to maintain colony cohesion. The cohesion is based on interaction networks and exchanges, thereby favoring pathogen spread among colony members. Nevertheless, social insects present behavioral adaptations limiting the spread of pathogens which will be discussed as well. Finally the directions to follow in the near future will be discussed.
... Social insect colonies where sufficient CHC profile variation exists for sister groups to be reliably identified are rare in the literature (Keller 1997; van Zweden et al. 2010 van Zweden et al. , 2011 but see Page et al. 1991; Arnold et al. 1996; Arnold et al. 2000; Bonckaert et al. 2011; Nehring et al. 2011). Uniform colony odors may be selectively advantageous because they facilitate nest-mate recognition, particularly in large groups (Boomsma and d'Ettore 2013 ). ...
Article
Full-text available
Relatedness is predicted to be a key determinant of cooperative behavior, but kin discrimination within social insect colonies is surprisingly rare. A lack of reliable cuticular hydrocarbon (CHC) cues is thought to be responsible, but here we show that in a high-profile paper wasp model, kin recognition cues are available for some individuals that found nests with nonrelatives. Thus, unrelated Polistes dominulus helpers could potentially recognize themselves as such. On this basis, we reanalyzed a behavioral data set to investigate whether foraging effort, defense contributions and aggression toward nest mates might thus reflect CHC profiles. Both foraging behavior and aggression varied with genetic relatedness, but genetic relatedness itself was a better predictor of this variation than differences in CHC profiles. We propose that wasps use specific components of the CHC profile, the identity of which is as yet unknown, to identify relatives among nest mates. Our data provide the first evidence of within-nest kin discrimination in primitively eusocial wasps but leave open the question of which cues are responsible.
... However, different matrilines' cuticular compound profiles provide sufficient information for matriline discrimination in the wood ant Formica truncorum and the wasps Polistes dominula [dominulus] and Vespa crabro (Boomsma et al., 2003;Dani et al., 2004b). In monogyne colonies composed of different patrilines, full sisters and half sisters differ in their cuticular hydrocarbon profiles in Acromyrmex octospinosus (Nehring et al., 2011), Formica truncorum (Boomsma et al., 2003), Formica exsecta (van Zweden et al., 2011), the hornet Vespa crabro (Dani et al., 2004b), and several species of Apis honeybees (Getz and Smith, 1986;Arnold et al., 1996). Although the information conveyed by cuticular compounds may be accurate enough to permit potential withinpatriline cooperation in A. octospinosus, the heritable component of recognition cues appears to be too weak and error-prone in F. truncorum, F. exsecta, and V. crabro. ...
Article
Full-text available
Chemical messengers are the primary mode of intracolony communication in the majority of social insect species. Chemically transmitted information plays a major role in nestmate recognition and kin recognition. Physical and behavioral castes often differ in chemical signature, and queen effects can be significant regulators of behavior and reproduction. Chemical messengers themselves differ in molecular structure, and the effects on behavior and other variables can differ as a consequence of not only molecular structure of the chemical messenger itself but also of its temporal expression, quantity, chemical blends with other compounds, and effects of the environment. The most studied, and probably the most widespread, intracolony chemical messengers are cuticular hydrocarbons (CHCs). CHCs are diverse and have been well studied in social insects with regard to both chemical structure and their role as pheromones. CHCs and other chemical messengers can be distributed among colony members via physical contact, grooming, trophallaxis, and contact with the nesting substrate. Widespread intracolony distribution of chemical messengers gives each colony a specific odor whereby colony members are integrated into the social life of the colony and non-members of the colony are excluded. Colony odor can vary as a function of genetic diversity within the colony, and the odor of a colony can change as a function of colony age and environmental effects. Chemical messengers can disseminate information on the presence of reproductives and fertility of the queen(s) and workers, and queen pheromone can play a significant role in suppressing reproduction by other colony members. New analytical tools and new avenues of investigation can continue to expand knowledge of how individual insects function as members of a society and how the society functions as a collective.
Article
Sex allocation theory predicts parents should adjust their investment in male and female offspring in a way that increases parental fitness. This has been shown in several species and selective contexts. Yet, seasonal sex ratio variation within species and its underlying causes are poorly understood. Here we study sex allocation variation in the wood ant Formica pratensis. This species displays conflict over colony sex ratio as workers and queens prefer different investment in male and female offspring, owing to haplodiploidy and relatedness asymmetries. It is unique among Formica ants because it produces two separate sexual offspring cohorts per season. We predict sex ratios to be closer to queen optimum in the early cohort but more female-biased and closer to worker optimum in the later one. This is because the power of workers to manipulate colony sex ratio varies seasonally with the availability of diploid eggs. Consistently more female-biased sex ratios in the later offspring cohort over a three-year sampling period from 93 colonies clearly support our prediction. The resulting seasonal alternation of sex ratios between queen and worker optima is a novel demonstration how understanding constraints of sex ratio adjustment increases our ability to predict sex ratio variation. This article is protected by copyright. All rights reserved
Article
Full-text available
Cuticular hydrocarbons of R)rmicaj'aponica mainly consisted of pairs of n-alkanes and alkenes from C23 to C33, but those of E tnincon{m consisted of alkadicnes, alkenes, n-alkanes, and methyl alkanes from C25 to C48, with relative ratios ofO.3%, 8.2%, 28.4%, and 63.2%, respectively. V℃ry long-chained hydrocarbons, larger than C34, accounted fbr 55.6Y6 of the whole hydrocarbons, Such hydrocarbons were not previously fbund presumably because of differences in analytical methods. Even after these very long-chained hydrocarbons were addcd to the analysis, the hydrocarbon profiles remained similar among colony members, but different enough between colonies to allow identification .
Article
The common approach to the multiplicity problem calls for controlling the familywise error rate (FWER). This approach, though, has faults, and we point out a few. A different approach to problems of multiple significance testing is presented. It calls for controlling the expected proportion of falsely rejected hypotheses — the false discovery rate. This error rate is equivalent to the FWER when all hypotheses are true but is smaller otherwise. Therefore, in problems where the control of the false discovery rate rather than that of the FWER is desired, there is potential for a gain in power. A simple sequential Bonferronitype procedure is proved to control the false discovery rate for independent test statistics, and a simulation study shows that the gain in power is substantial. The use of the new procedure and the appropriateness of the criterion are illustrated with examples.
Article
We present evidence indicating that the level of nestmate discrimination in the facultatively polygynous ant Pseudomyrmex pallidus varies predictably with the number of queens in the colony. P. pallidus workers were introduced into observation arenas in either nestmate or non-nestmate pairs. During the 5-min period immediately following the first contact between test ants, all interactions and relative distances were recorded. Aggression between non-nestmates was negatively correlated with the number of queens in the colony and distance between nestmates was positively correlated. These results are consistent with predictions of Reeve's (1989) optimal acceptance threshold model.
Article
We consider worker-controlled sex investments in eusocial Hymenoptera (ants in particular) and assume that relatedness asymmetry is variable among colonies and that workers are able to assess the relatedness asymmetry in their own colony. We predict that such "assessing" workers should maximize their inclusive fitness by specializing in the production of the sex to which they are relatively most related, i.e., colonies whose workers have a relatedness asymmetry below the population average should specialize in males, whereas colonies whose workers have a higher than average relatedness asymmetry should specialize in making females. Our argument yields the expectation that colony sex ratios will be bimodally distributed in ant populations where relatedness asymmetry is variable owing to multiple mating, worker reproduction, and/or polygyny. No such bimodality is expected, however, in ant species where relatedness asymmetry is known to be constant, or in cases where relatedness asymmetry is supposed to be irrelevant due to allospecific brood rearing under queen control, as in the slave-making ants. Comparative data on colony sex ratios in ants are reviewed to test the predictions. The data partly support our contentions, but are as yet insufficient to be considered as decisive evidence.
Article
HAMILTON'S rule and the principle of inclusive fitness1 provide a theoretical basis for understanding the evolution of social behaviour, and a framework for predicting reproductive characteristics of social insect colonies2-1. Sex allocation in social insects (especially ants) has become a central factor in tests of inclusive fitness theory5-9. The most powerful such test is the analysis of individual colonies where the predicted sex allocation varies depending on variation in worker fitness functions10,11. Recently developed models5,12 predict that workers may enhance their inclusive fitness by biasing sex ratios in response to the degree of related-ness asymmetry in each colony. Here I provide the first empirical evidence of facultative sex ratio biasing in response to relatedness asymmetries caused by inter-colony variations in queen mating frequencies. In a Finnish population of the ant Formica truncovum, colonies have a single queen mated to one or several males. Colonies show a bimodal distribution of sex ratios, with a significantly greater proportion of males in colonies headed by a multiply mated queen.
Article
Cuticular hydrocarbon profiles of individual workers of the ant Formica truncorum were measured and found to contain relatively few hydrocarbons. Pentacosane, heptacosane, nonacosane and hentriacontane dominated the mixture, but small amounts of the corresponding alkenes were also present. Principal component analysis and nested analysis of variance showed that workers from different colonies varied significantly in quantitative aspects of their cuticular hydrocarbon profiles. Furthermore, differences between habitat-patches within populations and (to some extent) between populations were also detected. Finally, workers from the same colony, sampled only a few months apart, were found to be different in the quantitative composition of their cuticular hydrocarbon profiles, emphasising the importance of collecting samples from a colony at a single point in time.