Biological Journal of the Linnean Society, 2015, 116, 519–529. With 3 figures.
REVIEW ARTICLE
Fast chromosomal evolution and karyotype instability:
recurrent chromosomal rearrangements in the
peach potato aphid Myzus persicae (Hemiptera:
Aphididae)
GIAN CARLO MANICARDI, ANDREA NARDELLI and MAURO MANDRIOLI*
Dipartimento di Scienze della Vita, Universita di Modena e Reggio Emilia, Via Campi 213/d, 41125,
Modena, Italy
Received 28 April 2015; revised 3 June 2015; accepted for publication 3 June 2015
The occurrence of karyotype variations with respect to both chromosome number and structure has been
frequently reported in aphids. Here, we review recent data attesting to the presence of recurrent chromosomal
changes in the karyotype of the peach potato aphid Myzus persicae, where clones presenting metaphases with
different chromosome number (from 12 to 17) have been observed, also comparing plates obtained within the
same embryo. According to the available data, M. persicae autosomes 3 and 1 are the chromosomes mostly
involved in changes compared to other autosomes, suggesting that they could have sites more susceptible to
fragmentation. Chromosomal fissions involving the X chromosomes have also been observed, suggesting that they
may have fragile sites located at the termini opposite to the nucleolar organizer regions-bearing telomere. The
presence of holocentric chromosomes and reproduction by apomictic parthenogenesis, together with a constitutive
expression of telomerase, could explain the inheritance of the observed chromosomal instability in aphids.
Considering that chromosomal changes may affect the host choice and could also favour speciation, it would be
intriguing to confirm whether the observed karyotype variants have effects over short temporal and spatial
scales. © 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 116, 519–529.
ADDITIONAL KEYWORDS: aphid holocentric chromosomes – fissions – nucleus architecture – stability –
translocations.
INTRODUCTION
Aphids represent an interesting experimental model
in cytogenetics because they possess holocentric
chromosomes showing a centromeric activity spread
along the whole chromosomal axis (Hughes-Schrader & Schrader, 1961; Blackman, 1987; Mandrioli
& Manicardi, 2012; Manicardi, Mandrioli & Blackman, 2015). These chromosomes are also termed
holokinetic because, during mitotic anaphase, they
behave as if the spindle attachment is not localized
such that chromatids move apart in parallel and do
not form the classical V-shaped figures usually
observed during the movement of monocentric ones
(Blackman, 1987). Aphids could be particularly
*Corresponding author. E-mail: [email protected]
useful for of the study of the architecture of holocentric chromosomes because mitotic chromosomes
can easily be obtained from their embryonic tissues
(Blackman, 1987; Mandrioli & Manicardi, 2012,
2014) (Fig. 1).
For several decades, holocentrism has been considered as a great drawback with respect to karyotype analyses because holocentric chromosomes
lack primary and/or secondary constrictions such
that, in conventionally stained preparations, homologues can only be recognized on the basis of their
size (Blackman, 1987; Hales et al., 1997; Manicardi
et al., 2002). Indeed, G-banding staining of aphid
chromosomes was not clear or sufficiently consistent
for use in karyotype comparisons (Blackman, 1985)
such that only changes affecting the number of
chromosomes or their size (such as nonreciprocal
© 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 116, 519–529
519
520
G. C. MANICARDI ET AL.
A
C
E
B
D
F
Figure 1. Myzus persicae chromosomes stained with propidium iodide (A, D, E) and silver nitrate (C) demonstrating
the holocentric nature of the aphid chromosomes (A), as shown in (B). Different chromosomal markers have been identified on M. persicae chromosomes, such as the subtelomeric DNA repeat (D) and the C0T fraction (E) that mapped on
mostly on X chromosomes (indicated by arrows): nucleolar organizer regions (NORs) (N) (mapped by silver staining in
C) are localized on a telomere; highly repeated subtelomeric satellite DNAs (ST) map at the telomere opposite to the
NOR one; the Hind200 highly repeated satellite DNA (H) labels three intercalary bands, as summarized in (F). Scale
bar = 10 lm. Arrows indicate X chromosomes.
translocations of large portions of chromosomes)
could be detected.
Despite this limitation, several chromosomal rearrangements have been recently reported in aphids
mainly involving autosomal translocations and fissions (Blackman, 1980; Brown & Blackman, 1988;
Monti et al., 2012a; Rivi et al., 2012; Mandrioli,
Bandinelli & Manicardi, 2014a; Mandrioli, Zanasi &
Manicardi, 2014b) and it has been suggested that they
played a large part in aphid evolution because they
could affect the host choice, as reported in the corn
leaf aphid Rhopalosiphoum maidis (Fitch) feeding in
barley and sorghum (Brown & Blackman, 1988).
The interest in cytogenetic analyses of aphid chromosomes is also emphasized by the observed relationship between some chromosomal rearrangements and
the carboxylesterase-based resistance to some insecticides (i.e. a partial reciprocal translocation between
the first and third autosome pairs, observed as
heterozygous in karyotypes with a standard chromosome number 2n = 12), which also makes aphid chromosomal rearrangements relevant from an economic
point of view in agriculture, as reported for the peach
potato aphid Myzus persicae (Field et al., 1999).
Indeed, aphids are lymph-sucking insects and they
have serious implications for agriculture not only in
view of their parasitic action against crops, but also
because they represent active vectors of crop viruses
such that extensive chemical control programmes are
applied against them. As Loxdale et al. (2011) noted
in their review, the presence of M. persicae on different plants of agricultural interest could make this species an ideal experimental model for analyzing rapid
evolution (i.e. measured over a perceptible time scale)
because the agricultural practices could act as a
strong selection pressure favouring evolutionary
changes over short periods.
Up until a few years ago, chromosome number and
the structure of aphid karyotypes were regarded generally as stable within genera, although karyotype variations were relatively common within some species and
may perhaps be associated with host plant specialization, as reported, for example, in the corn leaf aphid
R. maidis (Fitch) feeding in barley and sorghum
(Brown & Blackman, 1988). In this respect, the considerable increase in chromosome number found to occur
as a consequence of autosome fissions in Amphorophora spp. has been related to the rise of host-associated forms that evolved to the point of speciation
(i.e. feeding on raspberry and blackberry, respectively)
(Blackman, 1980, 1987). High levels of chromosomal
rearrangements have also been found in aphids of the
genera Trama that show a large variation in chromosome number, with few corresponding morphological
and genetic changes (Blackman, 1980).
Among pest crop insects, M. persicae is, without
any doubt, one of the most important species, and a
number of classical and molecular cytogenetic studies have been carried out to analyze its genome and
karyotype (Blackman, 1980; Manicardi et al., 2002,
© 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 116, 519–529
KARYOTYPE INSTABILITY IN THE APHID M. PERSICAE
2015; Monti et al., 2012a, b; Rivi et al., 2012). The
standard karyotype of the M. persicae consists of 12
chromosomes, including five couples of autosomes
and two X chromosomes, although some variations
in chromosome number and structure have been
observed (Blackman, 1980). Recently, several
M. persicae clones presenting different chromosome
numbers have been described in Italy and Greece
and it is timely to review the main information collected over recent years in relation to that previously obtained by means of both classical and
molecular genetics approaches. We hope to provide
an updated analysis of the available data together
with the suggestion of recently identified chromosomal markers as new tools for the study of chromosomal rearrangements in M. persicae, aiming to
better understand the evolution of the aphid in the
light of current selection as a result of management
practices in the field.
FROM CYTOGENETICS TO INSECTICIDE
RESISTANCE: EFFECTS OF THE A1–3
CHROMOSOMAL TRANSLOCATIONS ON
ESTERASE GENE EXPRESSION
The most common chromosomal variant described in
M. persicae consists of a reciprocal translocation
521
between autosomes 1 and 3, resulting in female
karyotypes with 2n = 12 with a marked structural
differences in heterozygosis (Blackman, 1980)
(Fig. 2). The worldwide presence of this chromosomal
rearrangement is a result of its linkage with organophospate and carbamate resistance (Blackman,
Takada & Kawakami, 1978). Indeed, biochemical
and molecular approaches have demonstrated that
this resistance is related to the increased production
of one of two esterase forms, named E4 and FE4,
because of gene amplification (Devonshire, 1989;
Field, Crick & Devonshire, 1996). Field et al. (1996)
showed that the translocated clones have an array of
around 12 copies of the esterase E4 form, which code
for an enzyme able to confer resistance by sequestering insecticide molecule. Cytogenetic studies demonstrated that this array is located in a terminal
portion of the autosome 3 in proximity to the translocation breakpoint (Blackman et al., 1996). Fluorescence in situ hybridization (FISH) experiments
carried out with a subtelomeric DNA repeat indicated that the location of the E4 gene array coincides
with or is very close to that of subtelomeric repeats
translocated to 3 from autosome 1 (Blackman et al.,
1996). In M. persicae, amplification of E4 genes is
widespread in North Europe populations and always
occurs in conjunction with the A1–3 translocation
(Blackman et al., 1978, 1995), whereas strains that
A
B
C
D
Figure 2. Karyograms (A, C) and in situ hybridization with the subtelomeric repeat (B, D) in Myzus persicae clones
with standard (A, B) and translocated karyotypes (C, D). Scale bar = 10 lm. Arrows indicate X chromosomes. Arrowheads indicate autosomes 1 and 3 involved in the reciprocal translocation and the breakage sites at the basis of the
translocation, whereas asterisks indicate autosomes 1 and 3 not involved in rearrangements. Modified from Blackman
et al. (1999).
© 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 116, 519–529
522
G. C. MANICARDI ET AL.
have the amplified FE4 genes (mainly diffused in
Mediterranean regions) apparently possess a normal
karyotype (Blackman et al., 1995). Recently, this
chromosomal rearrangement has also been found in
Italy, where it is not related to increased insecticide
resistance because Italian M. persicae populations
have the unamplified FE4 genes only (Rivi et al.,
2013). This observation lead to the hypothesis that
the origin of this chromosomal rearrangement could
be unrelated to the esterase-based resistance but,
instead, is the result of a fragmentation occurring at
a fragile site located on autosome 3 that has been followed by esterase gene amplification in some strains
only (Rivi et al., 2013).
Interestingly, Kati et al. (2014) reported M. persicae clones with a translocation of a complete autosome A3 onto A1, such that the chromosome number
was 11 instead of the normal 2n = 12. As a whole, it
appears that an A3 fragment or the complete A3
autosome frequently translocated on one telomere of
autosome 1, which evidently meant that it was more
prone to accept this fragment compared to the other
chromosomes. According to data referring to the
structure of the interphase nuclei, Mandrioli et al.
(2014a) suggested that autosomes 1 and 3 cluster in
tight proximity in the M. persicae interphase nuclei
such that chromosomal breakages could favour the
recombination of their terminal portions resulting in
fissions and/or translocations.
FISH experiments also suggested that chromosomal rearrangements could alter the expression level
of the E4 esterase genes. Indeed, the location of
unamplified (wild-type) esterase genes is near a subtelomeric block of heterochromatic repetitive DNA on
autosome 1, whereas, after A1–3 translocation, esterase genes map within a euchromatic region (Blackman et al., 1999). Consequently, the A1–3
translocation affects the expression level of the E4
esterase genes, enhancing their expression such that
aphids combining this rearrangement and the esterase gene amplification produce a greater amount of
the enzyme, thereby becoming resistant to
organophosphates and carbamates (Blackman et al.,
1978; Foster, Devine & Devonshire, 2007; Loxdale,
2009). This hypothesis has been recently supported
by data reporting that the heterochromatin enriched
in the subtelomeric repetitive DNA is silent because
it is associated with the nuclear membrane through
lamina-interacting proteins (Mandrioli et al., 2014a).
By contrast, M. persicae strains with a FE4-based
resistance possess multiple FE4 clusters located on
different chromosomes (Field & Blackman, 2003)
such that specimens with a normal karyotype could
escape insecticide treatment as a consequence of
esterase gene amplification without any positioneffect variegation.
The worldwide diffusion of this translocation is not
only a result of the common use of organophosphates
and carbamates for the chemical control of aphids in
the field, but also is favoured by the apomictic
parthenogenesis that is typical of aphids and the
anholocycly that enables aphids to escape sexual
reproduction allowing the inheritance of rearranged
karyotypes.
M. PERSICAE POSSESSES A HIGHLY
DYNAMIC KARYOTYPE
Until approximately 10 years ago, few studies
reported the presence of M. persicae clones with a
rearranged karyotype in place of the standard
2n = 12 complement. The observed variations
involved both chromosome number and structure
and were mainly the result of chromosomal translocations and, occasionally, fragmentations leading to
an increased chromosome number (Blackman, 1980;
Lauritzen, 1982; Spence & Blackman, 1998). For
example, M. persicae populations with 13 chromosomes have been identified in different countries as
the result of autosome 3 fission (Blackman, 1980).
Interestingly, at least two independent and diverse
fragmentations of the autosome 3 were reported
(Blackman, 1980; Lauritzen, 1982), suggesting that
different naturally occurring rearrangements of the
same chromosome may be observed in the M. persicae karyotype (Blackman, 1980; Lauritzen, 1982). In
some M. persicae populations, a further fission of
autosome 2 give raise to karyotype consisting of
2n = 14 chromosomes (Blackman, 1980; Lauritzen,
1982), making this species a good experimental
model for the study of chromosome rearrangements
in aphids (Blackman, 1980, 1987; Lauritzen, 1982;
Fenton, Woodford & Malloch, 1998; Spence & Blackman, 1998; Loxdale, 2007; Monti et al., 2012a, b;
Mandrioli et al., 2014a, b).
Recent studies based on chromosome length measurements, combined with FISH experiments with a
subtelomeric probe, revealed that several variant
karyotypes can be observed in M. persicae as a result
of recurrent fragmentations or translocations involving not only autosomes 1 and 3 (including a nonreciprocal translocation of an autosome A3 onto an A1
not described in the previous studies), but also the X
chromosomes (Monti et al., 2012a; Mandrioli et al.,
2014a, b) (Fig. 3).
Interestingly, karyotype variations (with chromosome numbers ranging from 12 to 17) have been
observed also in embryos from different individuals
within the same asexual lineage, between embryos
from the same individual, and even within the same
embryo (Monti et al., 2012b). Chromosome instability
© 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 116, 519–529
KARYOTYPE INSTABILITY IN THE APHID M. PERSICAE
523
A
B
Figure 3. Myzus persicae karyotype after in situ hybridization with the subtelomeric repeat (in green) showing fissions
involving a single X chromosome and an autosome 3 (A) and after double in situ hybridization with a 28S rDNA probe
(in green) and the histone genes (in blue) showing an A1–3 translocation (B). Asterisks indicate the chromosomal breakage sites on chromosomes X and 3. Arrowheads indicate autosomes 1 and 3 involved in the translocation. C, at present,
two chromosomal markers can be used on autosome 1: the repeated subtelomeric satellite DNA (ST) that labels the two
subtelomeric regions and the histone genes (HS) that cluster in a single intercalary band.
within individuals is an almost unique finding in the
animal kingdom at the level of population and species, with the exception of malignant cells in humans
and, among insects, in ants where some intraspecific
chromosomal rearrangements have been observed
(Imai, Crozier & Taylor, 1977; Imai, Taylor & Crozier, 1994; Karnik et al., 2010). These data support
earlier molecular evidence indicating that intraclonal
genetic variation occurs in nature and it is a potentially important force for generating variation in
asexual lines (Loxdale & Lushai, 2003; Lushai, Loxdale & Allen, 2003).
The high frequency of karyotype variants in
M. persicae can be explained by considering that the
holokinetic structure of aphid chromosomes facilitates the inheritance of chromosomal fragments
because they can be attached to microtubules and,
furthermore, they can be inherited without the constraint of the homologous pairing typical of meiosis
(Manicardi et al., 2002). At the same time, the
apomictic mode of the aphid parthenogenesis, characterized by the absence of both homologous chromosome pairing and genetic recombination, also makes
it possible for rearranged karyotypes to be passed to
offspring. Lastly, M. persicae possesses a high plasticity in the mode of reproduction such that it is able
to adapt to different climatic conditions in terms of
day length and temperature (Blackman, 1974). In
particular, the coexistence of asexuals and sexual
M. persicae populations has been reported in different countries (e.g. Italy, Greece and Australia: Margaritopoulos et al., 2002; Vorburger, Sunnucks &
Ward, 2003; Rivi et al., 2012) and the presence of
only obligate parthenogenetic populations has been
documented in Scotland (Kasprowicz et al., 2008).
The absence of sexual generations could indeed further facilitate the inheritance of karyotype bearing
rearrangements, thus explaining the numerous
M. persicae populations with an unusual chromosomal complement.
The involvement of autosome 3 and, more rarely,
autosome 1 in M. persicae karyotype rearrangements
is a well known phenomenon (as noted above). By
contrast, the nonreciprocal translocation of a complete autosome A3 onto an A1 and the fragmentation
of the X chromosomes were not reported previously
in Myzus. In particular, fissions of the X chromosomes were considered very rare in aphids not only
in natural populations, where only one case has been
reported in Schoutedenia lutea (van der Goot; Hemiptera, Aphididae, Greenidinae) (Hales, 1989), but
also in X-ray irradiated aphids (Khuda-Bukhsh &
Pal, 1985). Indeed, irradiation of Aphis gossypii (Glover) and Aphis nerii (Boyer de Fonscolombe) specimens produced several breaks occurring mainly on
autosome 1 and never on the X chromosome (KhudaBukhsh & Pal, 1985). Studies on natural populations
of the same species confirmed these results, showing
fragmentations of autosomes 1 and 4 but never of X
chromosomes (Khuda-Bukhsh & Pal, 1985).
The fission of the X chromosome observed in
M. persicae clones frequently involved a subtelomeric
heterochromatic band enriched in satellite DNAs
(Monti et al., 2012a, b) and this change is presumably unlikely to lead to any significant phenotypic
change and therefore could possibly be described as
© 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 116, 519–529
524
G. C. MANICARDI ET AL.
selectively neutral (John, 1983; Blackman, Spence &
Normark, 2000). The presence of chromosome breakpoints occurring within constitutive heterochromatin
is well established in the scientific literature and, for
example, much of the evolution of mammalian karyotypes involved pericentromeric heterochromatin
that is known to be particularly variable, as also
reported in some insects, such as grasshoppers
(John, 1983; Blackman et al., 2000).
Inversions and duplications occurring in intercalary heterochromatic bands of the X chromosomes
have also been observed (Monti et al., 2012a, b; Mandrioli et al., 2014a, b), suggesting that different portions of the X chromosome could be rearranged. This
evidence demonstrates that, currently, the most relevant limit to the study of the aphid karyotype is the
availability of only a few chromosomal markers that
can be used for the identification of chromosomal
rearrangements, such that the presently reported
karyotype variants could represent an underestimation of the changes that have occurred. Accordingly,
interesting results could be obtained, at least for
autosome 1, by extensive use of the histone genes
that have recently mapped in a single cluster located
in an interstitial position of autosome 1 (Mandrioli &
Manicardi, 2013) (Fig. 3).
As a whole, the currently available data on the
M. persicae karyotypes indicate that the main chromosomal rearrangements generally involved not only
the same chromosomes, but also often the same
region of the same chromosome. For example, fissions of the M. persicae X chromosome always
occurred on the telomeric region opposite to the
nucleolar organizer regions (NORs)-bearing one
(Monti et al., 2012a, b). For many years, chromosome
evolution has been generally explained by considering the random-breakage model (Becker & Lenhard,
2007). By contrast, a number of comparative cytogenetic studies provided evidence for a relationship
between chromosomal rearrangements and specific
chromosomal architecture, and suggested a role for
the repetitive DNAs in chromosome rearrangements.
The nature of the repetitive DNA within chromosomal breakpoint regions varies significantly, from
clusters of rRNA and tRNA genes to simple di- and
tri-nucleotide expansions (Coghlan & Wolfe, 2002).
In addition, computational analyses of breakpoints
suggested that recurrent evolutionary breaks are
found in fragile regions or hot spots, such that the
random breakage model required substantial
reassessment in favour of models that place the
architecture of the chromosomes in a pivotal position
for revealing the molecular basis of chromosomal
evolution among species. Very recently, regions of
the genome with a high frequency of mitotic crossovers that may be analogous to common fragile sites
in the human genome have been observed in Drosophila melanogaster (La Fave, Andersen & Stoffregen,
2014).
In view of the recurrent fission of the same chromosomes in the same region, the M. persicae genome therefore appears to have some fragile sites
that could be the basis of the observed changes in
the chromosome number. Hot spots of chromosomal
recombination have already been identified in
aphids within rDNA genes that contain specific
sequences with high similarity to the consensus core
region of human hypervariable minisatellites (Jeffreys, Wilson & Thein, 1985) and with the v
sequence of Escherichia coli (Smith, 1983). As a consequence of such hot spots, the occurrence of X chromosomes paired at NORs has been observed in
several aphid species (Mandrioli et al., 1999)
together with the occurrence of intra- and inter-individual NOR heteromorphism as a result of unequal
crossing over between the two X chromosomes
(Mandrioli et al., 1999).
A last change in the M. persicae karyotype
involves the identification of a small heterochromatic
chromosome resembling a B chromosome (Monti
et al., 2012b). As reviewed by Camacho (2005), B
chromosomes are additional dispensable chromosomes originating from A chromosomes and show a
remarkably higher heterochromatin storage compared to A ones. In view of the small length and
large size of the subtelomeric repeat cluster at the
telomeres, M. persicae autosomes 3 and 4 are the
most probable candidates for the origin of the
observed B chromosome. In aphids, previous studies
have identified supernumerary B chromosomes in
species of the genus Euceraphis, where they putatively derived from nonfunctional X chromosomes
(Blackman, 1988). The results obtained by Monti
et al. (2012a, b) suggest that aphid B chromosomes
could also originate from autosomes. Moreover, C
banding, followed by DAPI (4’,6-diamidino-2phenylindole) staining, showed that B chromosomes
are rapidly heterochromatinized, being highly
enriched in heterochromatin (Monti et al., 2012a, b).
B chromosomes are generally enriched in satellite
DNA and transposable elements, and the accumulation of repeated DNA appears to be a very common
event in B chromosome differentiation. However, the
occurrence of B chromosomes at an intraclonal level
in aphids suggests that the accumulation of repetitive DNA is not the primary cause of B chromosome
differentiation. In particular, the rapid process of
heterochromatinization could primarily result from
epigenetic changes of the B chromosomes that are
subsequently followed by structural modifications,
including transposable element invasion and repetitive DNA amplification.
© 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 116, 519–529
KARYOTYPE INSTABILITY IN THE APHID M. PERSICAE
According to previous data, it has been hypothesized that chromosomal rearrangements could affect
some complex phenotypic traits in aphids, such as
the host choice (Blackman, 1987; Ffrench-Constant,
Devonshire & White, 1988). For example, karyotypic
variants observed in the corn leaf aphid R. maidis
have been associated with changes in the host choice
and, similarly, an association of chromosome number
with host plant has been described within the Sitobion genus, which shows 2n = 12 on ferns and
2n = 18 on grasses (Brown & Blackman, 1988; Hales
et al., 1997). A similar host-related chromosomal
variation was suggested for M. persicae strains feeding on tobacco, for which morphometric analyses of
specific taxonomic markers revealed that they are
distinguishable from those living on other host plant
(Blackman, 1987). However, Rivi et al. (2012) provided evidence showing that, even if most of the
Myzus strains collected on tobacco plants possess
karyotype variations, they do not always show the
same rearrangements, which suggests that the
observed chromosomal rearrangements are not a consequence of a sort of host adaptation but, instead,
they might rely in the clastogenic effect of nicotine.
Indeed, nicotine is a naturally occurring alkaloid
found primarily in members of the solanaceous plant
family, including Nicotiana tabacum. Several studies
showed that nicotine produces genotoxic effects in
Chinese hamsters (Trivedi, Dave & Adhvaryu, 1990,
1993), mice (Sen, Sharma & Talukder, 1991), and
human lymphocytes (Sassen et al., 2005) and spermatozoa (Arabi, 2004). Nicotine, together with ultravilet exposure, has also been considered an
exogenous factor that can contribute to the generation of chromosomal mosaicism (De, 2011), a very
rare phenomenon we observed in M. persicae strains
collected on tobacco plants (Rivi et al., 2012).
Chromosomal fragile sites are defined as regions of
the genome (generally late-replicating) that exhibit
gaps or breaks on metaphase chromosomes under
conditions of partial replication stress (Dillon, Burrow & Wang, 2010). Interestingly, nicotine is able to
induce DNA replication fork stress (Richards, 2001;
Freudenreich, 2005) and the data reported for Myzus
strongly support the presence of fragile sites that
became evident after nicotine damage. Furthermore,
common fragile sites can be observed in all individuals of a species and they are therefore considered to
represent a normal component of chromosome architecture (Glover, 2006). This feature is very intriguing
in aphids because it helps to explain why similar
chromosomal rearrangements can also be observed
in M. persicae clones not feeding on tobacco. Indeed,
as reported in the human genome, various dietary
and environmental factors can significantly increase
fragile site breakage, including pesticides (Richards,
525
2001), and we suggest that the genome architecture,
rather than random breakages, is driving the rearrangements of the M. persicae genome.
TELOMERASE EXPRESSION AND DE NOVO
SYNTHESIS OF TELOMERIC ARRAYS AT
BREAKAGE SITES
The holokinetic structure of aphid chromosomes
ensures the attachment of chromosomal fragments to
the spindle microtubules, although it is insufficient
by itself to stabilize chromosome fragments. Indeed,
chromosomal breakpoints may be highly unstable,
displaying a propensity to fuse with other broken
ends such that breakpoints need to be stabilized
before the transmission of chromosomal fragments to
the daughter cells (Vermeesch & Price, 1994; Pennaneach, Putnam & Kolodner, 2006).
The stabilization of chromosomal fragment, process
known as ‘healing of breakpoints’ or ‘de novo telomere synthesis’, generally involves the addition of
repetitive telomeric sequences at the breakpoints by
telomerase. Indeed, the essential function of telomeres is to protect chromosome ends from nucleolytic
degradation, chromosome fusion and the inappropriate engagement of checkpoint signalling (Lydall,
2003). Hence, the addition of telomere repeats
results in the stabilization of the new chromosome
end and allows the resumption of cell cycling (Vermeesch & Price, 1994; Hug & Lingner, 2006; Pennaneach et al., 2006). In the absence of healing,
irreparable double-strand breaks lead to programmed cell death, as reported in yeast (Sandell &
Zakian, 1993), or to the activation of proto-oncogenes, as described in mammals (Lee & Myung, 2009).
To evaluate the role of telomere and telomerase
and the stabilization of chromosomal fragments, the
presence of a telomerase coding genes has been
assessed in aphids together with the occurrence of
(TTAGG)n repeats located at terminal ends of each
chromosome (Monti et al., 2011, 2012b; Mandrioli,
Monti & Manicardi, 2012). M. persicae telomerase
(TERT) coding gene is highly conserved and it is
expressed in different body parts, such as the gut
and head (Monti et al., 2011), in full agreement with
data reporting telomerase activity in different organs
and tissues of other insects (Sasaki & Fujiwara,
2000). Overall, this evidence suggests that a robust
telomerase expression is present in somatic insect
tissues and not just in germ and pluripotent stem
cells, as observed in human tissues (Krupp, Klapper
& Parwaresch, 2000; Donate & Basco, 2011).
Interestingly, as assessed in different clones,
M. persicae is able to carry out a de novo synthesis
of telomeric sequences at chromosomal breakage
© 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 116, 519–529
526
G. C. MANICARDI ET AL.
sites (Monti et al., 2012b; Mandrioli et al., 2014b).
Furthermore, semi-quantitative reverse transcriptase-polymerase chain reaction analyses indicated
that M. persicae telomerase is expressed at a higher
level in clones possessing intra-individual chromosomal rearrangements than in clones with a stable
karyotype, suggesting an active role of telomerase in
the stabilization of chromosomal fragments (Mandrioli et al., 2012; Monti et al., 2012b).
Telomere sequence addition at chromosomal breakage sites has been observed in many organisms, from
yeast to man (Vermeesch & Price, 1994; Hug &
Lingner, 2006; Pennaneach et al., 2006), although,
until now, in two insect species only (the fly D. melanogaster and the coccid Planococcus lilacinus)
(Biessmann et al., 1990; Mohan et al., 2011). Considering that Drosophila exhibits a noncanonical telomere-telomerase system, aphids and coccids are
therefore the first insect models to exhibit a de novo
telomere synthesis.
FISH experiments showed a clear hybridization
signal on each telomere of all chromosomes without
any interstitial labelling of chromosomes (Monti
et al., 2011). In M. persicae nuclei, telomeres
appeared to be clustered into a few foci, as observed
in the interphase nuclei of most organisms, where
the telomeric regions are arranged in an ordered
fashion showing an association with the nuclear
matrix, and also as a part of the well-studied Rabl
configuration (Pryde, Gorham & Louis, 1997; Ostashevsky, 2002). As generally occurs for all organisms
possessing holocentric chromosomes, aphids cannot
adopt a sensu stricu Rabl configuration because centromere clustering is not possible. However, the
study of chromatin organization at a large scale suggested that a telomere-clustering could constitute a
Rabl-like configuration involved in nucleus organization in the holocentric chromosomes of the nematode
Caenorhabditis elegans (Ostashevsky, 2002). Data
obtained by Mandrioli et al. (2014b) confirmed this
hypothesis, suggesting an anchored positioning to
the nuclear lamina of M. persicae chromosomes by
means of both telomeres for autosomes and a single
telomere for the two X chromosomes (Mandrioli
et al., 2014b).
The different behaviour of the two telomeres of the
X chromosomes is a result of the clear-cut structural
difference between these telomeres as a consequence
of the absence of telomere-associated repeated
sequences, such as satellite DNAs and TRAS retrotransposable elements, at the NOR telomeres (Spence
et al., 1998; Mandrioli et al., 2012). This structural
difference was previously related to the need to favour
rDNA genes pairing, which is involved in the reduction of X chromosome number as the basis of male
determination in aphids (Mandrioli et al., 1999). By
contrast, more recent data suggest that the observed
structural difference could be also related to the spatial organization of the X chromosomes in the interphase nucleus. The clear-cut structural difference
between the two telomeres could indeed permit their
different positioning in the nucleus regulating different transcriptional activities.
CONCLUSIONS
At present, more than 100 M. persicae strains have
been analyzed at a cytogenetic level, showing that
this species consists of several populations with standard karyotypes mixed with others possessing variant karyotypes (involving both chromosomal
rearrangements and changes in the chromosome
number). As a whole, M. persicae appears to be a
complex but intriguing aphid species, with the
tobacco-adapted form, M. persicae ssp. nicotianae,
being more prone to karyotype rearrangement, such
that it has been recently considered as a species ‘in
the making’ (Kati et al., 2014).
Evolutionary changes over short periods can be
observed only in the presence of a strong selection
pressure and the agricultural practices (especially
the chemical control against M. persicae) could
favour rapid changes measurable over a perceptible
time scale (Lauritzen, 1982; Blackman, 1987; Fenton
et al., 1998; Spence & Blackman, 1998; Mandrioli
et al., 1999; Loxdale, 2007; Monti et al., 2012a, b).
The fine-scale patchwork of chromosome rearrangements observed in M. persicae could increase its
potential for local adaptation and differentiation,
such as on different host plants, also explaining the
success of this aphid species as a polyphagous pest
crop species.
In this respect, different studies have suggested
the presence of morphological and physiological plastic responses of M. persicae populations under distinct environmental regimes imposed by the host
plant characteristics (Borja Peppe & Lomonaco,
2003). However, in view of the presence of the large
number of karyotypic and genetic changes observed
in this taxon (Loxdale, 2008a, b; Monti et al., 2011,
2012a, b), it is of interest to revisit these results aiming to confirm the nature of the observed plasticity
in terms of true plasticity (Whitman & Agrawal,
2009) vs. the sum of different strains with specific
traits within the same species.
In view of their impact in agriculture (in particular
for virus transmission), M. persicae needs to be controlled by pesticides and/or using biological control
agents. However, in the absence of a thorough
understating of its genetics, it could be difficult to
properly evaluate the presence of transmissible and
© 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 116, 519–529
KARYOTYPE INSTABILITY IN THE APHID M. PERSICAE
adaptative variations that make biological and chemical controls less effective. The idea that aphid populations are genetically stable in time and space
remains controversial because aphid clones do not
appear to be genetically homogeneous as previously
expected. By contrast, aphid populations could be the
sum of individuals with different karyotypes and,
consequently, could give different responses to the
selective environmental forces. A deeper cytogenetic
approach, hopefully aided by the identification of further chromosomal markers, could therefore provide
precious information helping to evaluate the adaptative potential of M. persicae over short temporal and
spatial scales, resulting in a difference in reproductive rate, host choice, pesticide resistance and speciation events.
ACKNOWLEDGEMENTS
We thank the two anonymous reviewers for their helpful comments. This work was supported by the grant
‘Experimental Approach to the Study of Evolution’ from
the University of Modena and Reggio Emilia to MM.
REFERENCES
Arabi M. 2004. Nicotinic infertility: assessing DNA and
plasma membrane integrity of human spermatozoa. Andrologia 36: 305–310.
Becker TS, Lenhard B. 2007. The random versus fragile
breakage models of chromosome evolution: a matter of resolution. Molecular Genetics and Genomics 278: 487–491.
Biessmann H, Mason JM, Ferry K, d’Hulst M, Valgeirsdottir K, Traverse KL, Pardue ML. 1990. Addition of
telomere-associated HeT DNA sequences ‘heals’ broken
chromosome ends in Drosophila. Cell 61: 663–673.
Blackman RL. 1974. Life cycle variation of Myzus persicae
(Sulz.) (Hom., Aphididae) in different parts of the world, in
relation to genotype and environment. Bulletin of Entomological Research 63: 595–607.
Blackman RL. 1980. Chromosome numbers in the Aphididae and their taxonomic significance. Systematic Entomology 5: 7–25.
Blackman RL. 1985. Aphid cytology and genetics. Evolution
and Biosystematics of Aphids. In: Szelegiewicz H, ed. Proceedings of the International Symposium at Jablonna. Wroclaw: Ossolineum, 170–237.
Blackman RL. 1987. Morphological discrimination of a
tobacco-feeding form of Myzus persicae (Sulzer) (Hemiptera:
Aphididae), and a key to New World Myzus (Nectarosiphon)
species. Bulletin of Entomological Research 77: 713–730.
Blackman RL. 1988. Stability of a multiple X chromosome
system and associated B chromosomes in birch aphids
(Euceraphis spp.; Homoptera: Aphididae). Chromosoma 96:
318–324.
527
Blackman RL, Takada H, Kawakami H. 1978. Chromosomal rearrangement involved in insecticide resistance of
Myzus persicae. Nature 271: 450–452.
Blackman RL, Spence JM, Field LM, Devonshire AL.
1995. Chromosomal location of the amplified esterase genes
conferring resistance to insecticides in Myzus persicae (Homoptera: Aphididae). Heredity 75: 297–302.
Blackman RL, Spence JM, Field LM, Javed N, Devine G,
Devonshire AL. 1996. Inheritance of the amplified esterase
genes responsible for insecticide resistance in Myzus persicae
(Homoptera: Aphididae). Heredity 77: 154–167.
Blackman RL, Spence JM, Field LM, Devonshire AL.
1999. Variation in the chromosomal distribution of amplified esterase (FE4) genes in Greek field populations of
Myzus persicae (Sulzer). Heredity 82: 180–186.
Blackman RL, Spence JM, Normark BB. 2000. High
diversity of structurally heterozygous karyotypes and rDNA
arrays in parthenogenetic aphids of the genus Trama
(Aphididae: Lachninae). Heredity 84: 254–260.
Borja Peppe F, Lomonaco C. 2003. Phenotypic plasticity
of Myzus persicae (Hemiptera: Aphididae) raised on Brassica oleracea L. var. acephala (kale) and Raphanus sativus
L. (radish). Genetics and Molecular Biology 26: 189–194.
Brown G, Blackman RL. 1988. Karyotype variation in the
corn leaf aphid, Rophalosiphon maidis (Fitch), species complex (Hemiptera, Aphididae) in relation to host plant and
morphology. Bullettin of Entomological Research 78: 351–
363.
Camacho JPM. 2005. B chromosomes. In: Gregory TR, ed.
The evolution of the genome. Burlington, MA: Elsevier Academic Press, 223–289.
Coghlan A, Wolfe KH. 2002. Fourfold faster rate of genome
rearrangement in nematodes than in Drosophila. Genome
Research 12: 857–867.
De S. 2011. Somatic mosaicism in healthy human tissues.
Trends in Genetics 27: 217–223.
Devonshire AL. 1989. The role of electrophoresis in the biochemical detection of insecticide resistance. In: Loxdale HD,
den Hollander J, eds. Electrophoretic studies on agricultural pests. Oxford: Clarendon Press, 363–374.
Dillon LW, Burrow AA, Wang Y. 2010. DNA instability at
chromosomal fragile sites in cancer. Current Genomics 11:
326–337.
Donate LE, Basco MA. 2011. Telomeres in cancer and
aging. Philosophical transactions of the Royal Society of
London Series B, Biological Sciences. 366: 76–84.
Fenton B, Woodford JAT, Malloch G. 1998. Analysis of
clonal diversity of the peach–potato aphid, Myzus persicae
(Sulzer), in Scotland, UK and evidence for the existence of
a predominant clone. Molecular Ecology 7: 1475–1487.
Ffrench-Constant RH, Devonshire AL, White RP. 1988.
Spontaneous loss and reselection of resistance in extremely
resistant Myzus persicae (Sulzer). Pesticide Biochemistry
and Physiology 30: 1–10.
Field LM, Blackman RL. 2003. Insecticide resistance in the
aphid Myzus persicae (Sulzer): chromosome location and epigenetic effects on esterase gene expression in clonal lineages.
Biological Journal of the Linnean Society 79: 107–113.
© 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 116, 519–529
528
G. C. MANICARDI ET AL.
Field LM, Crick SE, Devonshire AL. 1996. Polymerase
chain reaction-based identification of insecticide resistance
genes and DNA methylation in the aphid Myzus persicae
(Sulzer). Insect Molecular Biology 5: 197–202.
Field LM, Blackman RL, Tyler Smith C, Devonshire
AL. 1999. Relationship between amount of esterase
and gene copy number in insecticide-resistant Myzus persicae (Sulzer). Biochemical Journal (London) 339: 737–
742.
Foster SP, Devine D, Devonshire AL. 2007. Insecticide
resistance. In: van Emden HF, Harrington R, eds. Aphids
as crop pests. Oxford: CABI, 261–285.
Freudenreich CH. 2005. Molecular mechanisms of chromosome fragility. ChemTracks – Biochemistry and Molecular
Biology 18: 141–152.
Glover TW. 2006. Common fragile sites. Cancer Letters 232:
4–12.
Hales D. 1989. The chromosomes of Scoutedenia lutea (Homopteraa, Aphididae, Greenidinae) with an account of
meiosis in the male. Chromosoma 98: 295–300.
Hales D, Tomiuk J, Wohrmann K, Sunnucks P. 1997.
Evolutionary and genetic aspects of aphid biology: a review.
European Journal of Entomology 94: 1–55.
Hug N, Lingner J. 2006. Telomere length homeostasis.
Chromosoma 115: 413–425.
Hughes-Schrader E, Schrader F. 1961. The kinetochore of
the Hemiptera. Chromosoma 27: 327–350.
Imai HT, Crozier RH, Taylor RW. 1977. Karyotype evolution in Australian ants. Chromosoma 59: 341–393.
Imai HT, Taylor RW, Crozier RH. 1994. Experimental
bases for the minimum interaction theory. I. Chromosome
evolution in ants of the Myrmecia pilosula species complex
(Hymenoptera: Formicidae: Myrmeciinae). Japanese Journal of Genetics 69: 137–182.
Jeffreys AJ, Wilson V, Thein SL. 1985. Hypervariable
minisatellite regions in human DNA. Nature 314: 67–73.
John B. 1983. The role of chromosome change in the evolution of orthopteroid insects. In: Sharma AK, Sharma A, eds.
Chromosomes in evolution of eukaryotic groups, Vol. I. Boca
Raton, FL: CRC Press, 1–110.
Karnik N, Channaveerappa H, Ranganath HA, Gadagkar R. 2010. Karyotype instability in the ponerine ant
genus Diacamma. Journal of Genetics 89: 173–182.
Kasprowicz L, Malloch G, Pickup J, Fenton B. 2008.
Spatial and temporal dynamics of Myzus persicaeclones in
fields and suction traps. Agricultural and Forest Entomology 10: 91–100.
Kati AN, Mandrioli M, Skouras PJ, Malloch GL, Voudouris CCh, Venturelli M, Manicardi GC, Tsitsipis
JA, Fenton B, Margaritopoulos JT. 2014. Recent
changes in the distribution of carboxylesterase genes and
associated chromosomal rearrangements in Greek populations of the tobacco aphid Myzus persicae nicotianae. Biological Journal of the Linnean Society 113: 455–470.
Khuda-Bukhsh AR, Pal NB. 1985. Cytogenetical studies on
aphids (Homoptera: Aphididae) from India: I. karyomorphology of eight species of Aphis. Entomologia 10: 171–177.
Krupp G, Klapper W, Parwaresch R. 2000. Cell proliferation, carcinogenesis and diverse mechanisms of telomerase
regulation. Cellular Molecular Life Sciences 57: 464–486.
La Fave MC, Andersen SL, Stoffregen EP. 2014. Sources
and structures of mitotic crossovers that arise when BLM
helicase is absent in Drosophila. Genetics 196: 107–118.
Lauritzen M. 1982. Q- and G- band identification of two
chromosomal rearrangements in the peach-potato aphids
Myzus persicae (Sulzer), resistant to insecticides. Hereditas
97: 95–102.
Lee SE, Myung K. 2009. Faithful after break-up: suppression of chromosomal translocations. Cellular and Molecular
Life Sciences 66: 3149–3160.
Loxdale HD. 2007. Population genetic issues: the unfolding
story revealed using molecular markers. In: Van Emden
HF, Harrington R, eds. Aphids as crop pests. CABI Millennium volume. Oxford: CABI, 31–67.
Loxdale HD. 2008a. Was Dan Janzen (1977) right about
aphid clones being a ‘super-organism’, i.e. a single ‘evolutionary individual’? New insights from the use of molecular
marker systems. Mitteilungen der DGaaE 16: 437–449.
Loxdale HD. 2008b. The nature and reality of the aphid
clone: genetic variation, adaptation and evolution. Agricultural and Forest Entomology 10: 81–90.
Loxdale HD. 2009. What’s in a clone: the rapid evolution of
aphid asexual lineages in relation to geography, host plant
adaptation and resistance to pesticides. In: Schon I, Martens K, van Dijk P, eds. Lost sex: the evolutionary biology of
parthenogenesis. Heidelberg: Springer, 535–557.
Loxdale HD, Lushai G. 2003. Rapid changes in clonal lines:
the death of a ‘sacred cow’. Biological Journal of the Linnean Society 79: 3–16.
Loxdale HD, Massonnet B, Sch€
ofl G, Weisser WW. 2011.
Evidence for a quiet revolution: seasonal variation in colonies of the specialist tansy aphid, Macrosiphoniella tanacetaria (Kaltenbach) (Hemiptera: Aphididae) studied using
microsatellite markers. Bulletin of Entomological Research
101: 221–239.
Lushai G, Loxdale H, Allen JA. 2003. The dynamic clonal
genome and its adaptive potential. Biological Journal of the
Linnean Society 79: 193–208.
Lydall D. 2003. Hiding at the ends of yeast chromosomes:
telomeres, nucleases and checkpoint pathways. Journal of
Cell Science 116: 4057–4065.
Mandrioli M, Manicardi GC. 2012. Unlocking holocentric
chromosomes: new perspectives from comparative genomics? Current Genomics 13: 343–349.
Mandrioli M, Manicardi GC. 2013. Chromosomal mapping
reveals a dynamic organization of the histone genes in
aphids. Entomologia 1: e2.
Mandrioli M, Manicardi GC. 2014. Mapping the aphid
genome: the cytogenetic dimension of a pest crop insect. In:
Sharakhov IV, ed. Protocols for cytogenetic mapping of
arthropod genome. Boca Raton, FL: Taylor and Francis
Group, 325–348.
Mandrioli M, Bizzaro D, Manicardi GC, Gionghi D,
Bassoli L, Bianchi U. 1999. Cytogenetic and molecular
© 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 116, 519–529
KARYOTYPE INSTABILITY IN THE APHID M. PERSICAE
characterization of a highly repeated DNA sequence in the
peach potato aphid Myzus persicae. Chromosoma 108: 436–442.
Mandrioli M, Monti V, Manicardi GC. 2012. Starting at
the end: telomeres and telomerase in arthropods. Biomolecular Concepts 3: 465–470.
Mandrioli M, Bandinelli S, Manicardi GC. 2014a. Occurrence of a Rabl-like telomere clustering in the holocentric
chromosome of the peach potato aphid Myzus persicae
(Hemiptera; Aphididae). Cytogenetic and Genome Research
144: 68–75.
Mandrioli M, Zanasi F, Manicardi GC. 2014b. Karyotype
rearrangements and telomere analysis in Myzus persicae
(Hemiptera, Aphididae) strains collected on Lavandula sp.
plants. Comparative Cytogenetics 8: 259–274.
Manicardi GC, Mandrioli M, Bizzaro D, Bianchi U.
2002. Cytogenetic and molecular analysis of heterochromatic areas in the holocentric chromosomes of different
aphid species. In Sobti RG, Obe G, Athwal RS, eds. Chromosome structure and function. 47–56. New Delhi: Narosa
Publishing House.
Manicardi GC, Mandrioli M, Blackman RL. 2015. The
cytogenetic architecture of the aphid genome. Biological
Reviews 90: 112–125.
Margaritopoulos JT, Tsitsipis JA, Goudoudaki S,
Blackman RL. 2002. Life cycle variation of Myzus persicae
(Sulzer) (Hemiptera: Aphididae) in Greece. Bulletin of Entomological Research 92: 309–320.
Mohan KN, Rani BS, Kulashreshta PS, Kadandale JS.
2011. Characterization of TTAGG telomeric repeats, their
interstitial occurrence and constitutively active telomerase
in the mealybug Planococcus lilacinus (Homoptera; Coccoidea). Chromosoma 120: 165–175.
Monti V, Giusti M, Bizzaro D, Manicardi GC, Mandrioli
M. 2011. Presence of a functional (TTAGG)n telomeretelomerase system in aphids. Chromosome Research 19:
625–633.
Monti V, Lombardo G, Loxdale H, Manicardi GC, Mandrioli M. 2012a. Continuous occurrence of intra-individual
chromosome rearrangements in the peach potato aphid,
Myzus persicae (Sulzer) (Hemiptera: Aphididae). Genetica
140: 93–103.
Monti V, Mandrioli M, Rivi M, Manicardi GC. 2012b. The
vanishing clone: occurrence of repeated chromosome fragmentations in the aphid Myzus persicae (Homoptera, Aphididae). Biological Journal of the Linnean Society 105: 350–358.
Ostashevsky JA. 2002. Polymer model for large-scale chromatin organization in lower eukaryotes. Molecular Biology
of the Cell 13: 2157–2169.
Pennaneach V, Putnam CD, Kolodner RD. 2006. Chromosome healing by de novo telomere addition in Saccharomyces cerevisiae. Molecular Microbiology 59: 1357–1368.
Pryde FE, Gorham HC, Louis EJ. 1997. Chromosome
ends: all the same under their caps. Current Opinion in
Genetics and Development 7: 822–828.
Richards RI. 2001. Fragile and unstable chromosomes in
cancer: causes and consequences. Trends in Genetics 17:
339–345.
529
Rivi M, Monti V, Mazzoni E, Cassanelli S, Bizzaro D,
Mandrioli M, Manicardi GC. 2012. Karyotype variations
in Italian population of the peach potato aphid Myzus persicae (Hemiptera, Aphididae). Bullettin of Entomological
Research 102: 663671.
Rivi M, Monti V, Mazzoni E, Cassanelli S, Panini M,
Anaclerio M, Ciglini M, Corradetti B, Bizzaro D, Mandrioli M, Manicardi GC. 2013. A1-3 chromosomal
translocations in Italian populations of the peach potato
aphid Myzus persicae (Sulzer) not linked to esterase-based
insecticide resistance. Bullettin of Entomological Research
1: 1–8.
Sandell LL, Zakian VA. 1993. Loss of a yeast telomere:
arrest, recovery and chromosome loss. Cell 75: 729–739.
Sasaki T, Fujiwara H. 2000. Detection and distribution
patterns of telomerase activity in insects. European Journal
of Biochemistry 267: 3025–3031.
Sassen A, Richter E, Semmler M, Harreus U, Gamarra
F, Kleinsasser N. 2005. Genotoxicity of nicotine in miniorgan cultures of human upper aerodigestive tract epithelia. Toxicological Sciences 88: 134–141.
Sen S, Sharma A, Talukder G. 1991. Inhibition of clastogenic effects of nicotine by chlorophyllin in mice bone-marrow cells in vivo. Phytotherapy Research 5: 130–133.
Smith RG. 1983. Chi hotspots of generalized recombination.
Cell 34: 709–710.
Spence JM, Blackman RL. 1998. Chromosomal rearrangements in the Myzus persicae group and their evolutionary
significance. In: Nieto Nafria JM, Dixon AFG, eds. Aphids
in Natural and managed ecosystem. Leon: Universidad de
Leon, Secretario de Publicaciones, 113–118.
Spence JM, Blackman RL, Testa JM, Ready PD. 1998. A
169 bp tandem repeat DNA marker for subtelomeric heterochromatin and chromosomal rearrangements in aphids of
the Myzus persicae group. Chromosome Research 6: 167–
175.
Trivedi AH, Dave BJ, Adhvaryu SG. 1990. Assessment of
genotoxicity of nicotine employing in vitro mammalian test
system. Cancer Letters 54: 89–94.
Trivedi AH, Dave BJ, Adhvaryu SG. 1993. Genotoxic
effects of tobacco extract on Chinese hamster ovary cells.
Cancer Letters 70: 107–112.
Vermeesch JR, Price CM. 1994. Telomeric DNA sequence
and structure following de novo telomere synthesis in Euplotes crassus. Molecular and Cellular Biology 14: 554–566.
Vorburger CP, Sunnucks PJ, Ward SA. 2003. Explaining
the coexistence of asexuals with their sexual progenitors:
no evidence for general-purpose genotypes in obligate
parthenogens of the peach-potato aphid, Myzus persicae.
Ecology Letters 6: 1091–1098.
Whitman DW, Agrawal A. 2009. What is phenotypic plasticity and why is it important? In: Whitman DW, Ananthakrishnan TN, eds. Phenotypic plasticity of insects:
mechanisms and consequences. Enfield, NH: Science Publishers, 1–63.
© 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 116, 519–529