Evolutionary and functional insights into reproductive strategies of aphids

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Evolution / E
´volution
Evolutionary and functional insights into reproductive strategies
of aphids
Variation du mode de reproduction chez les pucerons : aspects e
´volutifs et fonctionnels
Jean-Christophe Simon *, Solenn Stoeckel, Denis Tagu
INRA, UMR 1099, INRA/Agrocampus Ouest/Universite
´Rennes 1, BiO3P (Biologie des Organismes et des Populations applique
´ea
`la Protection des Plantes), BP 35327,
Domaine de la Motte-au-Vicomte, 35653 Le Rheu cedex, France
C. R. Biologies 333 (2010) 488–496
ARTICLE INFO
Article history:
Available online 13 May 2010
Keywords:
Acyrthosiphon pisum
Clonal reproduction
Sexual reproduction
Photoperiodic changes
Phenotypic plasticity
Evolution of sex
Genomics
Mots cle
´s:
Acyrthosiphon pisum
Reproduction clonale
Reproduction sexue
´e
Photope
´riodisme
Plasticite
´phe
´notypique
E
´volution du sexe
Ge
´nomique
ABSTRACT
Aphids are among the few organisms capable of reproducing either sexually or asexually.
This plasticity in reproductive mode is viewed as an adaptive response to cope with
seasonal changes. Clonal reproduction occurs during the growing season allowing rapid
population increase, while sexual reproduction occurs during late summer and leads to
frost-resistant eggs that can survive winter conditions. This shift between these two
extreme reproductive modes is achieved by using the same genotype, i.e. within the same
genetic clone, and is triggered by photoperiodic changes perceived by the aphid brain or
visual system. Advances have been made recent ly to depict genetic programs that relate to
the regulation of reproductive modes in aphids. These studies have benefited from the
rapid development of genomic and post-genomic resources obtained through the
International Aphid Genomics Consortium. Here, we underline the importance of several
candidate genes in the switch from clonal to sexual reproduction in aphids and whose
roles await full validation. Besides reproductive mode variation expressed at the genotypic
level, aphid species also frequently encompass lineages which have lost the sexual phase
and hence the alternating clonal and sexual reproductive phases of the life cycle. This
coexistence of sex and asexual reproduction within the same species raises questions on
its evolutionary and ecological significance. We summarize the knowledge accumulated to
date on the maintenance of sex as well as on the origin and evolution of asexuality in
aphids. By combining functional genomics, genetic and ecological approaches on
reproductive plasticity and polymorphism, we hope to obtain an integrative view of
the evolutionary forces shaping aphid reproductive strategies, from gene to population
and species levels.
ß2010 Acade
´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
RE
´SUME
´
Les pucerons font partie des rares organismes a
`pouvoir se reproduire de manie
`re sexue
´e
ou asexue
´e. Cette plasticite
´phe
´notypique est conside
´re
´e comme une re
´ponse adaptative a
`
la saisonnalite
´: les populations de pucerons sont clonales pendant la belle saison et
forment des œufs re
´sistants au froid par voie sexue
´ea
`la fin de l’e
´te
´. Ce changement de
mode reproducteur est re
´alise
´par le me
ˆme ge
´notype et de
´clenche
´par la longueur de la
nuit perc¸ue par le cerveau ou le syste
`me visuel des pucerons. Re
´cemment, des progre
`s ont
e
´te
´enregistre
´s sur les programmes ge
´ne
´tiques implique
´s dans la re
´gulation photope
´r-
iodique du mode de reproduction. Ces nouveaux acquis ont be
´ne
´ficie
´des ressources
* Corresponding author.
E-mail address: jean-christophe.simon@rennes.inra.fr (J.-C. Simon).
Contents lists available at ScienceDirect
Comptes Rendus Biologies
www.sciencedirect.com
1631-0691/$ – see front matter ß2010 Acade
´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.crvi.2010.03.003

Author's personal copy
1. Introduction
While the vast majority of eukaryotes reproduce
sexually, involving meiosis at each generation, a small
category of diverse and intriguing organisms exists that
deviate from the general rule by using alternative modes of
reproduction. Briefly, these reproductive strategies range
from a combination of sexual and asexual reproduction in
the organism’s life cycle to strict clonal reproduction with
no recombination events, as far as is known [1]. This
diversity in reproductive mode can be expressed at two
levels. Firstly, facultative asexual organisms use both
sexual and asexual reproduction depending on environ-
mental conditions. This means that specific environmental
cues trigger a change in reproductive mode within the
same individual genotypes, involving phenotypic plasticity
and relying on changes in expression of genes that regulate
the observed reproductive orientation. Canonical exam-
ples of facultative asexual animals are found in cladocer-
ans, insects, mites and trematodes [2]. Secondly, asexual
and sexual reproduction can coexist at the population
level. In this case, reproductive mode variation relies on a
polymorphism whereby genetically determined sexual
and asexual lineages coexist within the same species [3].
Reproductive mode variation at these two levels raises
questions not only in relation to its evolutionary and
ecological significance but also on the regulatory mechan-
isms of reproductive plasticity involved at the molecular
level. Furthermore, how exactly polymorphism and
phenotypic plasticity are related evolutionarily or func-
tionally remains unclear [4,5]. Here, we propose to review
these various aspects on aphids which are among the few
organisms to use both sexual and asexual reproduction
within the same genotype and involving reproductively
specialized lineages (Fig. 1).
2. Alternation of clonality and sexuality is the typical
aphid reproductive mode
Aphids form a monophyletic group that evolved around
280 million years ago (late Carboniferous or early Permian)
[6] and are presently represented by around 4500 species
worldwide. The vast majority of aphid species propagate
by apomictic parthenogenesis (clonal or asexual repro-
duction) as the main or exclusive mode of reproduction.
Asexual reproduction was therefore seemingly acquired in
the very early steps of aphid evolution, presumably in an
all-sexual ancestor, and has been kept in association with
sexual reproduction until now [7]. The typical aphid
reproductive mode indeed involves a succession of usually
numerous parthenogenetic generations, followed by a
single sexual one within the annual life cycle [8,9]. This
reproductive system is also known as cyclical partheno-
genesis in animals and is found in other invertebrates such
as cynipid wasps, midges, water fleas (Daphnia) and a wide
range of trematodes [2]. The asexual phase of aphids
occurs during the growing season (spring and summer)
and up to 20 asexual generations can be produced if
climatic conditions are favorable. In addition, asexual
reproduction in aphids is associated with viviparity and
telescoping of generations [7]. The former trait means that
a newly born aphid has already started to develop within
mother’s abdomen, in contrast with oviparous organisms
ge
´nomiques et post-ge
´nomiques de
´veloppe
´es dans le cadre du Consortium International
pour la Ge
´nomique des Pucerons. Nous pre
´sentons ici les dernie
`res avance
´es sur les
me
´canismes de re
´gulation ge
´ne
´tique associe
´s au changement du mode reproducteur. En
plus d’une variation du syste
`me reproducteur a
`l’e
´chelle ge
´notypique, les pucerons
posse
`dent e
´galement des ligne
´es qui ont perdu la phase sexue
´e et qui co-existent avec
celles alternant clonalite
´et sexualite
´dans leur cycle annuel. La co-existence de ligne
´es
sexue
´es et asexue
´es au sein d’une me
ˆme espe
`ce pose questions sur sa signification
e
´cologique et e
´volutive. Nous faisons la synthe
`se des connaissances accumule
´es sur les
raisons du maintien de la reproduction sexue
´e chez les pucerons et sur l’origine et le
devenir des ligne
´es asexue
´es. En combinant des approches de ge
´nomique fonctionnelle,
de ge
´ne
´tique et d’e
´cologie sur la plasticite
´et le polymorphisme du mode de reproduction
des pucerons, nous espe
´rons disposer dans un futur proche d’une vision inte
´gre
´e des
forces e
´volutives agissant sur leurs strate
´gies reproductives des ge
`nes aux populations.
ß2010 Acade
´mie des sciences. Publie
´par Elsevier Masson SAS. Tous droits re
´serve
´s.
Fig. 1. Simplified representation of the two types of reproductive mode
variation expressed in aphids. ‘‘Sexual’’ lineages are cyclical
parthenogenetic populations that regularly alternate a clonal and a
sexual phase in their annual life cycle. The shift in reproductive mode is
determined by environmental factors. Asexual lineages may coexist with
‘‘sexual’’ lineages within the same aphid species and refer to obligate
parthenogenetic populations that have abandoned the sexual phase in
their life cycle.
J.-C. Simon et al. / C. R. Biologies 333 (2010) 488–496
489

Author's personal copy
which lay eggs that generally hatch after a certain delay.
The latter trait is more specific to aphids and refers to the
fact that an adult viviparous female not only contain the
next generation as fully developed embryos, but also the
subsequent one as early embryos within these developed
embryos. Asexual reproduction associated with viviparity
and telescoping of generation confers aphid populations
with an extremely high rate of increase, leading sometimes
to outbreaks of billions of individuals within a single field.
Therefore, aphid reproductive mode is a key component of
the damage caused by these economically important crop
pests [10].
The sexual generation is produced in late summer in
response to shortened day length. Parthenogenetic females
give birth to sexuparae (pre-sexual females) which in turn
produce wingless oviparous sexual females and males
(winged or wingless depending on species). After mating,
sexual females lay frost-resistant eggs that hatch the
following spring after several months of diapause [11].
New parthenogenetic females arise from these sexually
produced eggs and generate clonal offspring that give birth
in turn to other parthenogenetic generations, until the next
sexual episode. Asexual and sexual phases generally occur
on plants of the same genus or family. In 10% of aphid
species however, clonal generations develop on herba-
ceous hosts while sexual forms mate on woody plants [12].
Cyclical parthenogenesis in such species is thus associated
with host alternation, resulting in large population
reshuffling induced by obligate dispersal between unre-
lated hosts (see [13] in this issue for more details on host
alternation in aphids). Thus, in theory, aphids combine the
best of both worlds in their life cycle. Asexual viviparous
reproduction ensures high population increase during the
growing season, natural selection giving the fittest
genotypes a considerable demographic advantage due to
sustained clonality. Oviparous sexual reproduction leads
to the production of genetically recombined individuals via
cold resistant forms, the eggs, which are absent in the
asexual phase of Aphididae (but still present in the sister
families Adelgidae and Phylloxeridae), allowing aphid
populations to resist harsh winters and to produce an
array of new genotypic combinations each year ([7,9] for
further details on aphid life cycles).
3. From clonal to sexual reproduction in response to
seasonal photoperiodism
It is known from the work of Marcovitch [14] that the
shift from clonal to sexual reproduction in aphids is
triggered by photoperiodic changes, although temperature
mitigates the photoperiodic response in some cases. Clonal
aphids perceive the increase in night length (scotophase)
that occurs by late summer and throughout autumn. They
respond to this photoperiodic change via a transgenera-
tional process involving production of sexual forms
(oviparous females and males). Aphids are thereby able
to measure accurately the length of the scotophase, and a
critical number of consecutive dark and light periods are
needed for inducing the production of sexual forms.
Scotophase measurement is presumably achieved by an
internal clock and counter system, which has to be
identified [15]. The photoperiodic signal is detected
directly by the aphid brain or via the visual system
through the head cuticle, although embryos might also
sense environmental cues through their mother’s abdo-
men [16]. It has been shown experimentally that a group of
neurosecretory cells localized in the anterior dorsal region
of the protocerebrum (pars intercerebralis) is required for
perception and early transduction of the photoperiodic
signal [17]. How the signal is then transferred to the
ovaries for orientation of the reproductive pathways
(clonal vs. sexual) is still unresolved but probably involves
at some stages hormones of the endocrinal system such as
melatonin or juvenile hormones [18–20].
The final consequences of this photoperiodic response
take place in the aphid reproductive tract, more specifically
in the germ cells or oocytes located in the germarium of
presumptive embryos. Under short night conditions,
asexual reproduction is promoted: embryos of partheno-
genetic females develop within the ovariole from a diploid
oocyte after a single modified mitosis division whereupon
bivalent chromosomes are paired and aligned through
metaphase but maintained as separate strands along their
length (i.e. there is no chromosome recombination as such)
[21]. By contrast, long night conditions induce the
production of sexual females which contain oocytes
blocked in metaphase I of the meiotic division. Males
are also produced under long nights by a modified mitosis,
but at the end of the division, one X-chromosome is
eliminated [22]. Males thus differ from females by their
chromosome set which contains one X-chromosome but
the same set of autosomes. All these morphs proceeding
from the same mother are predominantly genetically
identical (although it is now known that mutations
frequently arise in such lineages, even at short generation
times [23,24]). The progeny resulting from mating of these
sexual morphs forms an array of diverse parthenogenetic
lineages that will expand the next spring.
4. Early progress on regulatory mechanisms of
reproductive mode variation in aphids
Much of what is known on the physiological aspects of
reproductive orientation in aphids dates back to the 1970s
and early 80s, especially due to the pioneered works of
A.D. Lees and colleagues at Silwood Park, Imperial College,
London. Recently, through the initiative of the International
Aphid Genomics Consortium (IAGC) the shared efforts of the
aphid scientific community have allowed the acquisition of
considerable genomic and post-genomic resources for the
pea aphid, Acyrthosiphon pisum (Harris) as a model system,
including the recent annotation of its complete genome,
allowing searching for candidate genes and determination
of putative gene functions by genome-wide comparisons
[25]. These resources have been used to tackle different
aspects of aphid biology ([26], this issue), including their
peculiar reproductive mode. So far, genomic and functional
analyses on reproductive plasticity in aphids have mainly
concentrated on the early steps of photoperiod signal
transduction.Hereafter, we present an overview of the main
findings obtained on the regulation of reproductive mode
variation in the pea aphid (Fig. 2).
J.-C. Simon et al. / C. R. Biologies 333 (2010) 488–496
490

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Since the aphid brain is considered as the center of
photoperiod signal reception and early transduction,
comparative gene expression analyses and protein profil-
ing have been made on aphid heads exposed to either long
(sex induction) or short (clonal induction) night regimes.
Changes in gene expression and protein profiles were
studied during the course of the transgenerational process,
tracking variation at different developmental stages of the
two generations involved [16,27]. In addition, more
specific analyses have been conducted on candidate
regulatory mechanisms that could control reproductive
plasticity in the pea aphid. These include the micro RNA
machinery and the epigenetic DNA methylation system
[28,29] which are both known to be involved in caste
determination in social insects [30,31].
4.1. Visual and photoreception systems
Aphid photoreceptors occupy an extra-optic situation
and locate in a light-sensitive region in the protocerebrum,
beneath the cuticle [17,32]. However, none of them has yet
been characterized and a location of such receptors in the
eyes is still not completely ruled out [16]. Transcriptomic
analyses of aphid heads exposed to contrasted photoperi-
odic regimes allowed detection of several regulated genes
implicated in the photoreception or visual systems. These
studies suggest a particular involvement of rhodopsins
(visual pigments) in response to day length shortening, in
both the compound eyes and the protocerebrum [33]. The
photoactivation of rhodopsins by the light source might
therefore well play a role in the transduction of the
photoperiodic signal responsible for reproductive shift
from asexuality to sexuality.
4.2. Unexpected changes in cuticular transcripts in response
to photoperiod
Three independent transcriptomic studies on aphid
heads or whole bodies exposed to either short or long
nights unexpectedly identified cuticular proteins as being
regulated by photoperiodic changes [16,27,34]. Cuticular
protein genes were among the most highly regulated,
particularly those coding for cuticular proteins that
contain chitin-binding domains and whose expression
level was up or down to 500 fold. Transcriptomic data thus
suggest a softening of the cuticular network between
cuticular proteins and chitin that occurs in the heads of
aphids reared under short-days [16]. This change in cuticle
structure with photoperiodism could be interpreted in two
ways (reviewed in [35]). Firstly, aphids perceived photo-
periodic variation in a light-sensitive region containing
putative photoreceptors and located very close to the
surface of the brain. Cuticle modifications mediated by the
photoperiod signal could therefore affect the quality and
quantity of light through change in cuticle filtering
characteristics, resulting in different reproductive orienta-
tions. Secondly, the cuticle stores several metabolites
among which N-
b
-alanin dopamine (NBAD) is responsible
for cuticle hardening or sclerotization. Transcriptomic
analyses show that NBAD biosynthetic pathway is down-
regulated in aphids exposed to long nights, suggesting a
modification of dopamine concentration in the aphid brain
in response to photoperiod. Dopamine plays a critical role
in melanization and sclerotization of the cuticle, as well as
in neurotransmission. Complementary experiments in-
volving semi-quantitative PCR showed that genes required
for dopamine biosynthesis are down-regulated in long
Fig. 2. Schematic representation of the early steps of sensing and transduction of the photoperiod in aphids. Night length is responsible for the fate between
the production of oviparous sexual females and viviparous parthenogenetic females. The photoperiod signal is sensed in the head and the transduction
occurs through a group of neurosecretory cells. Neuroregulation probably triggers hormonal modifications that will be sensed by targeted cells in the
ovarioles.
J.-C. Simon et al. / C. R. Biologies 333 (2010) 488–496
491

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night reared aphids, arguing for a down-regulation of the
dopamine concentration in aphids giving birth to sexual
individuals [35].
4.3. Transduction of the photoperiod signal through the
neuro-endocrine system
Global transcriptomic approaches also detected
changes in expression in genes involved in the nervous
system in response to photoperiodic cues. Several regulat-
ed genes showed homology to proteins associated with
axon guidance or nervous system development [16].In
particular, the expression of two genes of the insulin
pathway is affected by shortening of photoperiod: a
putative insulin receptor transcript is down-regulated
and a degrading insulin enzyme transcript is up-regulated.
These results suggest a down-regulation of the insulin
pathway in long night reared pea aphids. In Drosophila
melanogaster, a decrease in the insulin pathway is
correlated to a decrease in the juvenile hormone pathway
[36]. In addition, the insulin pathway is regulated during
diapause of Culex pipiens also induced by photoperiod
shortening, in relation with the juvenile hormones
pathway [37]. As juvenile hormones regulate photoperiod
signal transduction in aphids [20], it is possible that insulin
acts as an upstream regulator of juvenile hormones in long
night conditions. Bioinformatic analyses of the pea aphid
genome identified 10 different genes coding insulin-
related peptides [38]. Molecular analyses are currently
underway to check whether some of these insulin genes
are regulated during the switch of reproductive mode.
Rapid progress has thus been made on hypothetical
genetic programs and biosynthetic pathways involved in
the photoperiod response and in the switch of reproduc-
tive mode. Cauterization of secretory cells impairs the
production of parthenogenetic morphs under short night
conditions. Thus, the default mechanism, when the
neurosecretory cells do not function, seems to be the
production of sexual morphs, even in short night. One
hypothesis is that parthenogenesis under short night has
been acquired by the capacity of cells to secrete molecules
that change the fate of the oocytes (Table 1). More
functional analyses on these different pathways are
required to test these hypotheses.
5. Loss of sex in aphids: reproductive mode variation at
population and species levels
Although cyclical parthenogenesis (i.e. regular alterna-
tion of clonal and sexual generations) was acquired in the
very early steps of aphid evolution, a few species,
representing about 3% of the aphid fauna, appear to
reproduce by permanent all-female parthenogenesis [9].
These asexual species have therefore secondarily lost the
sexual phase from their life cycle and are insensitive to
photoperiodic changes that normally trigger the produc-
tion of sexual forms in most other aphids which have a
facultative life cycle. In addition, many species (30–40% of
the total) show a variation in reproductive mode within or
between populations [8]. This variation ranges from
lineages with a regular alternation of clonal and sexual
reproduction each year (referred to as sexual lineages for
simplicity) to lineages that have completely abandoned
sex and persist as pure clonal populations (referred to as
asexual lineages). In between, lineages with different
investments in sexual reproduction and sex-ratios can be
found along the continuum [39].
The possible coexistence of sexual and asexual lineages
within the same species and even the same population
raises questions about the selective advantage gained by
sexual lineages to compensate for their lower population
growth rate. Indeed, sexual lineages pay the general two-
fold cost of sex because of male production [40], and more
specific to aphids, they suffer another demographic
drawback over asexual lineages because of their develop-
ment arrest as overwintering eggs. By contrast, asexual
lineages potentially multiply all year round as all-female
populations, and because they have no need to seek sexual
partners, have, in theory at least, a significant fitness
advantage over sexual lineages.
This seeming paradox of sexual maintenance despite
lack of competitiveness is not specific to aphids and has
been the subject of much debate in recent years and a vast
literature now exists on the topic (see, for example, [41] for
a review). So far, there is no general agreement as to the
resolution of this paradox, although a plethora of
hypotheses have been proposed involving deleterious
mutation elimination, adaptation to environmental
changes, as well as changes in chromosome telomere
length and re-setting [42]. Because of the frequent
coexistence of sexual and asexual lineages in many distinct
species, aphids represent an excellent system to test some
of these explanations and to address the evolution and
adaptive potential of sexuality and asexuality.
6. Sexual reproduction provides aphids with ecological
advantages
Resolving the paradox of sex in aphids requires, as for
a more general understanding, consideration of the
Table 1
Hypotheses on the molecules regulating the switch of reproductive mode in aphids in response to photoperiodic changes.
Sexual reproduction Parthenogenesis Reference
Short nights Long nights Marcovitch, 1924 [14]
Oviparous females Viviparous females –
Meiosis No meiosis Blackman, 1987 [21]
Low level of juvenile hormones High level of juvenile hormones Corbitt and Hardie, 1985 [18]
Low level of melatonin High level of melatonin Gao and Hardie, 1997 [19]
Low level of insulin High level of insulin Le Trionnaire et al., 2009 [16]
Low level of dopamine High level of dopamine Le Trionnaire et al., 2009 [16]
J.-C. Simon et al. / C. R. Biologies 333 (2010) 488–496
492

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evolutionary process, both in the short- and the long-term
[9]. In cyclically parthenogenetic aphid populations or
‘‘sexual’’ lineages, the sexual phase that takes place by late
summer involves the production of sexual forms from
clonal females, and egg-laying by mated oviparous sexual
females, as earlier mentioned. Since these overwintering
eggs are frost-resistant and can survive temperatures as
low as 35 8C[11], sexual reproduction in aphids is in
some way linked with cold resistance [22]. By contrast,
aphids spending winter as live parthenogenetic individuals
such as those found in asexual lineages, are susceptible to
frost because they do not resist temperatures below 88C
or 10 8C[11]. With this difference in mind, we expect
sexual lineages to be favoured in climatic zones with
regular harsh winters, whereas asexual lineages should
dominate in zones with mild winter temperature. In areas
with fluctuating winter severity, mixed (bet-hedging)
reproductive strategies would be promoted, as predicted
by modeling works [43,44]. These predictions have now
been validated in several aphid species for which the
distribution of reproductive variants have been assessed
along climatic gradients or in zones with contrasted
climatic conditions [45–47]. In addition, we expect that the
distribution of sexual and asexual lineages would vary
from year to year depending on the harshness of preceding
winter, a prediction that has been recently confirmed in
the cereal aphids Sitobion avenae (F.) and Rhopalosiphum
padi (L.) [48,49].
From the above theoretical and empirical studies, it is
clear that aphid reproductive strategies are predominantly
selected for by winter climate. However, winter climate
alone hardly accounts for the frequent local coexistence of
sexual and asexual aphid lineages exposed to the same
climatic regime. Ecological factors other than climate may
further act on reproductive mode variation, such as
environmental heterogeneity, including variation in biotic
interactions e.g. parasitic pressure [50,51]. This seems to
be the case for sympatric host adapted races of the pea
aphid, A. pisum which differ in terms of sexual investment,
those living in spatially and temporally heterogeneous
habitats being the most sexual [50]. Similarly, it has been
found that aphid crop pests show a higher incidence of
asexuality, a result that could be linked with environmen-
tal uniformity and reduced complexity that especially
characterize current agricultural landscapes [49].
7. The evolutionary fate of asexually reproducing
lineages of aphids
While asexual lineages of aphids are counter-selected in
zones with regular harsh winter, they can show high
ecological success in vast areas with a mild climate (e.g.
oceanic or tropical climates). Several papers reported in
various aphid species the existence of asexual lineages
represented by few highly frequent clones distributed over
large geographic and ecological ranges [39,52,53].These
dominant clones also termed ‘‘super-clones’’ may also
persist for more than a decade [49,54]. However, the
replacement of asexual genotypes i.e. clonal turn-over by
fitter ones, is also commonly found in these studies,
demonstrating that recombination and genetic diversity
provide a greater adaptive potential than plasticity alone
[39]. Although we mentioned earlier that no general
consensus has emerged for explaining the short-term
maintenance ofsex, there is a large agreement on the forces
favoring sexual reproduction on longer evolutionary time-
scales, as reflected by the supremacy of sex on the tree of life
[55]. Sex (involving chromosomal recombination via
crossing-over) provides populations with new genotypic
combinations on which natural selection operates, and
facilitates removal of deleterious mutations that otherwise
accumulate inexorably in asexual genomes [3].The
theoretical prediction that asexual lineages constitute
evolutionary dead-ends is confirmed on phylogenetic trees
relating sexual and asexual taxa: asexual taxa typically
show a sexual ancestor and occupy terminal nodes of the
trees. Only rare asexual taxa, referred to as ancient asexuals
and in effect ‘‘evolutionary scandals’’ by Maynard-Smith
[56], succeeded in persisting as asexual lineages for millions
of years (My). The most famous and documented case are
the Bdelloid rotifers which have been reproducing without
sex for 35 My, although doubt persists as to the actual
antiquity of such asexual lineages [57].
This expectation of low ‘‘evolvability’’ in asexuals also
applies to aphids, as now briefly summarized. To begin with,
comparing the genetic diversity of ‘‘sexual’’ and asexual
lineages in distinct aphid species has allowed quantification
of a two third reduction in genotypic variation in clonal
relatives in comparison with recombining populations
(Table 2). Furthermore, experimental evolution studies
and sequence analysis have indicated a rapid acceleration of
deleterious mutation accumulation in asexual aphids
[58,59]. Lastly, phylogenetic methods involving DNA
sequencing, applied to test for long-term asexuality in
two putatively ancient asexual taxa (the Tramini tribe and
asexual lineages of the aphid R. padi)haverevealedsignsof
recombination in both taxa, suggesting a recent lossof sex in
these asexual lineages [60,61].
8. Asexual aphid lineages arise repeatedly from diverse
mechanisms
The presence of coexisting asexual and sexual lineages
within species belonging to various aphid genera and
Table 2
Comparison of genotypic diversity in recombining and clonal lineages of three aphid species.
Aphid species Recombining lineages Clonal lineages References
Myzus persicae 0.97 0.27 Vorburger et al., 2003 [53]
Rhopalosiphum padi 1.00 0.35 Delmotte et al., 2003 [60,62]
Sitobion avenae 0.97 0.28 Papura et al., 2003 [46]
Recombining lineages refer to aphid populations that undergo regular sexual reproduction once a year. Genotypic diversity was calculated as the ratio of
number of multilocus genotypes detected with molecular markers (microsatellites) to number of genotyped individuals.
J.-C. Simon et al. / C. R. Biologies 333 (2010) 488–496
493

Author's personal copy
tribes is evidence for multiple origins of obligate asexuality
in these groups [8,9]. Phylogenetic analyses of sexual and
asexual lineages using rapidly evolving markers (e.g.
microsatellites, mitochondrial DNA sequences) confirms
independent sex loss events below the species level
[62,63]. This shows that sexual reproduction is a rather
labile genetic system and raises questions on the
mechanisms by which asexual lineages originate.
Intensive studies combining ecological, experimental
and phylogenetic approaches have demonstrated the
existence of at least three not mutually exclusive routes
to asexuality in aphids (reviewed in [3]). Firstly, spontane-
ous loss of sex could occur when the conditions necessary to
induce sexual reproduction are not met, as for example in
invasive populations colonizing zoneswhere environmental
cues are insufficient to elicit the reproductive switch [64].In
this case, any mutation on genes involved in the cascade of
events from photoperiod perception to reproductive orien-
tation (see above for candidate genes) would partially or
completely alter the sexual function [62]. Alternatively,
chromosomal changes, which are very common in aphids,
can potentially prevent sexual reproduction by generating a
meiotic barrierdue mainly to disjunctions on the metaphase
plate [64]. Secondly, hybridization between closely related
species is a major route to asexuality in animals. This
mechanism of asexual lineage generation is also found in
aphids, although its importance is unknown [60]. Thirdly,
asexual lineages may arise secondarily from pre-existing
asexual lineages, formed upon one or both of the above
mechanisms, as a result of incomplete reproductive
isolationbetween sexual lineages and their asexualcounter-
parts. New asexual lineages can thus be generated through
‘‘contagious’’ gene flow into the sexual pool. Contagious
parthenogenesis has not only been detected in laboratory
crosses involving sexual and essentially asexual lineages
[65,66] but also in natural populations surveyed with
molecular markers and biological assays [67,68].Micro-
organisms such as Wolbachia induce obligate parthenogen-
esis in many arthropods. Although aphids harbor a large
community of bacteria, including some known to manipu-
late their host reproduction [67], there is no evidence so far
for a role of micro-organisms in sex loss in this insect group
[69].
The variety of processes involved in transitions to
asexuality translates into a wide range of asexual
genotypes with different genetic and ecological properties,
and deeply affects the outcome of competition between
sexual and asexual lineages, both in the short- and the
long-term [9].
9. Conclusions
Because of their uniquely evolved reproductive strate-
gies, which explain to a large extent their economical
impacts [10], aphids are excellent systems to study
reproductive biology in an integrative manner, from
molecules involved in alternative pathways of sexual
and clonal reproduction to ecological factors that shape
their evolution.
The discovery of parthenogenesis in aphids was made
by Charles Bonnet (1720–1793), a Swiss naturalist and
philosopher, as long ago as 1745 [70]. Yet, despite the great
strides made in aphid biology since that time, including in
physiological and molecular studies, the exact molecular,
cellular and physiological processes implicated in the
cascade of mechanisms from the perception of the
photoperiodic signal to the production of clonal or sexual
forms remain largely unknown. The recent development of
genomics and post-genomics has tremendously revitalized
the field of aphid reproductive biology that originally
prospered some 20–30 years ago. Hypothetical scenarios
of reproductive mode regulation by seasonal photoperi-
odism are available but they now require full validation
through functional analysis of candidate genes and
effectors. Genome-wide changes in gene expression can
also be tracked during the course of reproductive shifts by
means of exhaustive micro-arrays and high-throughput
sequencing technologies. Coupled with quantitative trait
loci (QTL) approaches, these methods are powerful tools in
current efforts to identify regulatory networks of poly-
morphic genes. Their application to aphids will greatly
help to test whether reproductive mode variation is
determined by a few genes or genome-wide chromosomal
interactions and will surely allow exploration of the
interplay between phenotypic plasticity and polymor-
phism at the gene level.
Acknowledgements
We thank J.L. Bonnemain for inviting us to contribute to
this special issue on aphids. We are also indebted to many
colleagues for stimulating exchanges on evolution of sex
theories, in particular B. Bengtsson, F. Delmotte, F. Halkett,
H. Loxdale, M. Plantegenest, C. Rispe, P. Sunnucks
C. Vorburger and A. Wilson. G. Le Trionnaire is acknowl-
edged for Fig. 2.
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