The importance of genetic diversity in successful biological invasions is unclear. In animals, but not necessarily plants, increased genetic diversity is generally associated with successful colonization and establishment of novel habitats. The Oleander aphid, Aphis nerii, though native to the Mediterranean region, is an invasive pest species throughout much of the world. Feeding primarily on Oleander (Nerium oleander) and Milkweed (Asclepias spp.) under natural conditions, these plants are unlikely to support aphid populations year round in the southern US. The objective of this study was to describe the genetic variation within and among US populations of A. nerii, during extinction/recolonization events, to better understand the population ecology of this invasive species.
We used five microsatellite markers to assess genetic diversity over a two year period within and among three aphid populations separated by small (100 km) and large (3,700 km) geographic distances on two host plant species. Here we provide evidence for A. nerii “superclones”. Genotypic variation was absent in all populations (i.e., each population consisted of a single multilocus genotype (MLG) or “clone”) and the genetic composition of only one population completely changed across years. There was no evidence of sexual reproduction or host races on different plant species.
Aphis nerii is a well established invasive species despite having extremely low genetic diversity. As this aphid appears to be obligatorily asexual, it may share more similarities with clonally reproducing invasive plants, than with other animals. Patterns of temporal and geographic genetic variation, viewed in the context of its population dynamics, have important implications for the management of invasive pests and the evolutionary biology of asexual species.
Citation: Harrison JS, Mondor EB (2011) Evidence for an Invasive Aphid “Superclone”: Extremely Low Genetic Diversity in Oleander Aphid (Aphis nerii) Populations in the Southern United States. PLoS ONE 6(3): e17524. https://doi.org/10.1371/journal.pone.0017524
Editor: William Etges, University of Arkanas, United States of America
Received: November 22, 2010; Accepted: February 4, 2011; Published: March 9, 2011
Copyright: © 2011 Harrison, Mondor. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This work was supported by Georgia Southern University and Advisement and Scholarship Promoting Inquiry-based Research Experiences in STEM (ASPIRES) funded through NSF-STEP grant (DUE-0622460). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
While studies are few, genetic diversity is believed to facilitate successful biological invasions . In animals, for example higher levels of genetic diversity increased population persistence and colonization success  and may contribute to increased range expansion , . In invasive plants however, genetic diversity is high in some species, but many successful invaders have little or no genetic diversity (for a review see ).
Aphids are valuable study systems for investigating the roles of genetic variation and phenotypic plasticity on population, ecological, and evolutionary dynamics , , . The Oleander aphid, Aphis nerii Boyer de Fonscolombe, is a pest of several plant families including Apocynaceae (Nerium and Vinca), Asclepiadaceae (Asclepias, Calotropi, and Gomphocarpus), Asteraceae, Convolvulaceae, Euphorbiaceae, and Rutaceae . This aphid, along with its principal host plant (Oleander), is thought to be Mediterranean in origin. Aphis nerii has since become a common invasive species in warm temperate and tropical regions of the world .
In the United States, the Oleander aphid commonly infests two plant families, Apocynaceae and Asclepiadaceae . Oleander, Nerium oleander (Apocynaceae), is a common ornamental plant in southern and coastal states and frequently grows along US highways . Native and ornamental Milkweed (Asclepiadaceae) is the other common US host. Milkweed distribution overlaps with that of Oleander, but extends into northern and central states , . Although these two host plant types are ecologically different, both are patchily distributed, contain cardiac glycosides which A. nerii sequesters for defense, and are unable to support aphid populations throughout the year , .
Aphid fitness tradeoffs among host plant species results in selection for host fidelity which can inhibit gene flow and result in the development of host races; i.e., host-associated population genetic structure , . Increased use of molecular markers to study aphid populations has revealed that host races are common , , . There is currently no evidence of differential fitness for A. nerii inhabiting species of milkweed that differed in the amount cardiac glycosides that they possess . Fitness tradeoffs in A. nerii among different host plant families such as Milkweed and Oleander, to our knowledge, have not been examined.
Aphid species vary in their mode of reproduction from obligate to cyclical parthenogens, but less than 3% of species are strictly clonal . Aphis nerii is believed to be an obligate parthenogen; males have never been found in natural populations . Males, and sexual reproduction, have been induced in laboratory lines under short-day conditions , . These laboratory-induced sexuals, however, had low fecundity and the extent to which sexual reproduction occurs in nature is unclear , . Sexual reproduction and recombination increases variation and sets the stage for selection and adaptation , . Conversely, asexual reproduction may limit genetic variation and adaptive potential, but provides reproductive assurance in stable environments and during colonization events , ,  (see also ).
The genetic structure of aphid populations is shaped spatially and temporally by habitat distribution, dispersal capabilities, and life-cycle. Many aphids, including A. nerii, produce winged forms in response to overcrowding and/or decreasing host plant quality , . Dispersal range is unknown for most species, but reports of less than one to hundreds of kilometers when habitat is continuous, are not uncommon . Aphis nerii, however, is found in patchily distributed habitats throughout the United States, and both Oleander and Milkweed are unable to support aphid populations year round except in the southernmost latitudes of the US . Consequently, most populations are characterized by frequent extinction events followed by re-colonization from unknown source population(s). Spatial and temporal genetic variation is driven by the magnitude of population bottlenecks, the number of founding individuals during re-colonization, and the genetic variation of the source population(s).
The aim of this study was to characterize genetic variation within and among southern US populations of A. nerii with the goal of gaining insight into the population dynamics, life history, and ecology of this well-established invasive species. In so doing, we asked the following questions: 1) Is there any evidence of sexual reproduction in US populations of A. nerii?; 2) Do aphids inhabiting different hosts comprise host races?; 3) Does the patchy distribution of suitable habitat result in population genetic subdivision over small and/or large geographic ranges?; and 4) Is there temporal variation in population genetic structure? To address these questions, we used microsatellite markers to assess genetic diversity over a two year period within and among three populations separated by roughly 100 km and 3,700 km on two host plant species.
Here, we report no evidence of sexual reproduction or the existence of host races in A. nerii. Whilst genetic variation was extremely low within and among populations, the genetic composition of one population was found to change drastically over time. Our findings suggest that A. nerii is an efficient colonizer that demonstrates true metapopulation dynamics.
Aphids were collected from Oleander (N. oleander) at two locations in Georgia and one location in central California, June-August 2008, 2009. This “overwintering” period between samples allowed us to assess temporal genetic variation, and determine if sexual reproduction occurred. Sampling locations were: Statesboro, Georgia (SGO; 32°24′N, 81°46′W); Tybee Island, Georgia (TIGO; 31°59′N, 80°50′W); and Concord, California (CCO; 37°57′N, 121°56′W). Sampling areas consisted of multiple patches of Oleander plants in close proximity but intermittently up to 2.5 km apart. To survey genetic diversity within each population, aphids were collected from different parts of the same plant and from as many different plants as possible. To assess genetic variation among aphids inhabiting different host plant species, aphids were collected from Milkweed (Asclepias amplexicaulis) at a second site in Statesboro, Georgia (SGM; 32°25′N, 81°47′W) in June 2008.
Individual aphids were genotyped at five microsatellite loci (Ago24, Ago66, Ago69, Ago89, and Ago126) using primers originally designed for the Cotton/Melon aphid, Aphis gossypii . These loci have been used previously to study at least three different Aphis species , . No linkage disequilibrium among loci has been detected in any species suggesting that they are unlinked in A. nerii as well , . These five loci were surveyed in 50 individuals from each population for each year except for SGM where 19 individuals were genotyped (n = 319 total aphids).
DNA was extracted by macerating individual aphids in 70 µl cell-lysis/proteinase-K buffer (10 mM of Tris, 50 mM KCl, 0.5 tween, 0.2 mg/ml proteinase-K, pH. 8.0) followed by incubation for 1 h at 65°C and 15 min at 99°C (Lee and Frost 2002). PCR reactions were carried out in 20 µl final volumes with 2.5 mM MgCl2. Reactions consisted of a 4 min denaturation at 94°C, followed by 35 cycles of 30 s at 94°C, 35 s at 58°C, and 45 s at 72°C, and a final 10 min extension step at 72°C.
PCR products were run on small (15 cm×17 cm×0.8 mm), non-denaturing TAE (tris-acetate-EDTA, pH 8.0) buffered polyacrylamide gels of either 9% or 10% concentration, with the lower portion of the gel supplemented with EnhanceIT polymer (Elchrom Scientific, Switzerland). Gels were run for 2 h at 30 mA, stained with ethidium bromide, and visualized on a UV light box. Allele sizes at each locus were estimated using the M3 size standard (Elchrom Scientific, Switzerland).
Observed and expected heterozygosities were calculated for each locus in each population. Deviations from Hardy-Weinberg expectations were tested according to Guo and Thompson  and estimates of the inbreeding coefficient FIS  were estimated using FSTAT v. 2.9.3 . Molecular subdivision among populations and over time were calculated by estimating both FST and RST using the program FSTAT v. 2.9.3 . The significance of F-statistics was tested using the randomization procedure available in FSTAT using 5000 permutations. Estimates of genetic distance between multilocus genotypes (MLGs, see below) (δμ2 and DSW) were calculated according to Shriver et al. . A genotypic diversity index was calculated as the ratio of the number of distinct genotypes out of the total number of samples (G/N ratio). We also compared the ratio of observed multilocus genotypic diversity (GO) to that expected under conditions of sexual reproduction (GE), as described by Stoddart and Taylor . The presence of null alleles in the genotype data was estimated using Microchecker .
All five microsatellite loci used in this study were polymorphic in A. nerii. The number of alleles was low; 3 to 4 alleles per locus. Three of the five loci each had an observed heterozygosity (HO) of 0.143, the two loci an HO of 1. Null alleles were not detected for any loci in any populations sampled.
Extremely low levels of genotypic variation were observed in Aphis nerii. Only 2 multi-locus genotypes (MLGs) or “clones” were detected among the 319 individuals assayed across all populations and both years (Table 1). MLG 1 was dominant both spatially and temporally, comprising 84.3% of the samples. The remaining 15.7% of the samples consisted of MLG 2. MLG 2 was found in one population (TIGO) in 2009 only. The MLG's differed in levels of heterozygosity, with 2 of 5 loci (40%) heterozygous in MLG 1 and 5 of 5 (100%) in MLG 2.
Despite the low level of genotypic diversity observed, the genotypes of the two MLGs were divergent. That is, the two MLGs did not share any alleles, clearly indicating that they were not the product of recombination through sexual reproduction. Estimates of genetic distance, based on the stepwise mutation model, were large (δμ2 = 15.35; DSW = 15.35) suggesting that the MLGs do not share a close genealogical relationship .
No evidence of sexual or mitotic recombination was found. First, no homozygous allelic arrangements were found at any of the heterozygous loci (Table 1). Second, levels of heterozygosity were high, ranging from 0.40 to 1.00 (Table 2), and there were significant deviations from Hardy-Weinberg expectations at polymorphic loci within all populations due to heterozygote excess. Third, the ratio of the number of observed genotypes (G) to the number of individuals sampled (N) ranged from 0.02 to 0.05 (0.024±0.011, mean±SD), and the ratio of the observed multi-locus genotypic diversity to that expected under sexual reproduction (GO/GE) from 0.141 to 0.048 (0.128±0.035, mean ± SD). Fourth, estimates of FIS were −1.0 for all populations.
To test for a non-random distribution of genetic variation among host plant species, we sampled A. nerii from two common host plants (Oleander and Milkweed) from Statesboro, GA (SGO and SGM, respectively). All samples from both hosts consisted of MLG 1, indicating that there is no host associated subdivision at this location (FST = 0, RST = 0) (Table 3).
There was no genotypic variation within any population for either 2008 or 2009, suggesting that each population was composed of a single genotypic “clone” (Table 1). The geographic distribution of MLGs differed between years. Samples collected in 2008 from Tybee Island, Georgia; Statesboro, Georgia; and Concord, California comprised a single MLG, indicating no population subdivision (FST = 0, RST = 0) (Table 3). The Tybee Island, Georgia samples differed significantly from both the Statesboro, Georgia and Concord, California samples in 2009 (FST = 0.650, RST = 0.787) (Table 3). During the 2009 sampling period, the Tybee Island population consisted solely of MLG 2 individuals which were not found in any other population. Between 2008 and 2009 there was, interestingly, a complete change in the genetic composition of the Tybee Island population from MLG 1 to MLG 2.
Small, genetically-uniform populations are subject to ecological and evolutionary forces (i.e., genetic bottlenecks and genetic drift) which threaten population persistence, in both native and introduced habitats . In animals, higher genetic diversity is often associated with an increased ability to establish viable populations in novel environments . This is not necessarily true for plants, however; some species are very successful with little or no genetic diversity, particularly those that are clonally-reproducing, self-pollinating, or apomictic . Here, we analyzed the genetic patterning of the Oleander aphid, A. nerii to better understand the ecology, life history, and population dynamics of this well established invasive species. We found that Oleander aphids are remarkably invasive throughout the southern United States, with extremely low genetic diversity.
Reproduction and Life History
Aphis nerii is believed to be obligately parthenogenetic, based on the complete absence of males under natural conditions . Laboratory lines of the aphid derived from populations in Kyoto, Japan produced males when exposed to short-day conditions , , suggesting that the ability to sexually reproduce is retained in at least some asexual lineages from some populations. From genotypic data, we found no evidence for sexual reproduction in any of the populations we examined, although, of course, further sampling may yet reveal sexual forms. Obligate parthenogenesis is supported by the lack of expected recombinant genotypes observed over the two year sampling period. A high level of heterozygosity between genotypes is consistent with expected genotypic patterns for long term asexual populations; i.e., the “Meselson effect” . That is, in long term asexual populations, heterozygosity is expected to increase because allelic pairs within a genome will continue to diverge over time while meiotic recombination and segregation do not occur to mix and purge alleles , , . This genotypic pattern has been observed in several primarily parthenogenetic taxa , , , , . Our results are consistent with this pattern, suggesting that sexual reproduction is rare or non-existent in natural populations of A. nerii in the southern US.
Geographic Genetic Variation
The most striking pattern observed in this study was the low level of genotypic diversity within and among populations over a large geographic area. We observed only two MLGs and a maximum estimate of genotypic diversity (G/N) within any population of 0.05. Several similar studies of other aphid species using comparable numbers of loci (4 to 7) have revealed higher levels of genetic variation than that observed in A. nerii. For example, long term asexual populations of the pea aphid, Acyrthosiphon pisum (Harris) in Japan were composed of five to seven MLGs, and cyclically parthenogenetic populations harbored much greater diversity than did asexual populations . Genotypic diversity (G/N) estimates in this species range from 0.10 to 0.69 . Several other aphid species show similar or greater levels of genotypic diversity as A. pisum, both within and among populations , , , , .
Aphid populations are sometimes composed of a small number of dominant genotypes (“clones”) and many low frequency (rare) genotypes , , , , . The term “superclone” has been used to describe genotypes that comprise 40–60% of a population in a region . For example, Peccoud et al.  sampled recently introduced asexual populations of A. pisum in Chile. Among the 432 individuals sampled over a 570 km range, 16 MLG's were identified with three MLG's comprising≈90% (47%, 29%, and 14%) of the diversity. Compared to previous studies, the pattern we observe in A. nerii is extreme even for “superclones”. Within any sampling year, each population consisted of a single MLG. In 2008, all populations sampled from both California and Georgia showed only a single MLG (MLG 1). In 2009, populations from Statesboro, Georgia and California comprised the same MLG (MLG 1), whilst a population from Tybee Island, Georgia comprised a single but entirely different MLG (MLG 2).
Variation of morphometric and life history traits in A. nerii has been assessed within and among populations from California, Iowa, and Puerto Rico . Significant variation was observed within populations for traits such as maturation time, fecundity, and wing length while only the proportion of winged offspring differed among populations . Assuming low genetic diversity in this population, these data suggest that phenotypic plasticity and maternal effects may be of utmost importance in shaping the population dynamics of this parthenogenetic species , , .
The genetic uniformity observed in A. nerii likely results from successive genetic bottlenecks or founder events. Subsequent clonal propagation and rapid spread of genotypes throughout the host range would be followed by rapid clonal competition/selection resulting in a few geographically-widespread and dominant clones (but see also ). These processes would occur during the initial introduction of this species into the US and/or during annual extinction and recolonization events. A study of A. nerii on N. oleander in California clearly showed annual colonization and extinction cycles , corresponding with our own observations. Patches of host plants are typically colonized by aphids in late Spring (May or June) followed by rapid increases in population sizes which peak in mid-summer. Aphid numbers decline rapidly in early Fall (September or October), resulting in population extinction . In the southernmost US latitudes, A. nerii has the potential to overwinter as adults, as freezing temperatures are uncommon. Host plant quality, however, substantially decreases in Fall and Winter likely driving the population decline .
Milkweed (Asclepias spp.) is the most common host of A. nerii in the northern US where N. oleander grows infrequently . Asclepias species are perennial plants that sprout in Spring, bloom in early Summer, and then set seed and die-back in the Fall. The life history of this host plant would require A.nerii to have a secondary host (though this has not previously been reported) or to produce sexuals to produce overwintering eggs (and as previously noted, has only been noted under laboratory conditions). On both Oleander and Milkweed, a regular pattern of colonization, rapid increase, and population extinction would be expected.
Host Associated Genetic Variation
Groeters  found no evidence of fitness trade-offs in A. nerii when feeding on different species of milkweed, but differences between Oleander and Milkweeds were not examined. Prior to this study, it was not known if A. nerii populations inhabiting Oleander and Milkweed were genetically different. Several aphid species have “host races” or show non-random distribution of genetic variation among host plants, including the Pea aphid , , the Grain aphid, Sitobion avenae (F.) , , the Cotton /Melon aphid, Aphis gossypii Glover  and others , , . This phenomenon undoubtedly results from habitat choice/host fidelity inhibiting interpopulation gene flow , . If the same process of habitat choice/host fidelity applied to A. nerii populations, we would have expected to see genetic variation between populations inhabiting Oleander and those inhabiting Milkweed, regardless of geographic proximity. Our findings suggest that there is no selection for host specificity and that A. nerii is a polyphagous, i.e. generalist species, although more data are required to confirm this contention. This pattern is consistent with Lynch  who suggested that obligate parthenogenetic species evolve to be ecological generalists (selection favors clones that can survive in all environments).
In summary, populations of A. nerii surveyed in this study show the genetic signature of obligate parthenogenetic reproduction, supporting previous reports that the species is indeed obligately asexual. Within any sampling period, each population was composed of a single MLG. This level of variation is remarkably low compared to the variation observed in other aphid species presumed to be largely or completely asexual , , . Furthermore, only two MLGs were identified among all populations, and populations separated by as far as 3,600 km were genetically homogenous. Temporal variation occurred in one population, one MLG was completely replaced by another between years. There was no correlation between host plant and MLG. In an ecological context, our results suggest that A. nerii is a generalist species with strong dispersal capabilities. As patches of host plants are colonized by few individuals that reproduce rapidly through parthenogenesis, founder individuals likely come from a source population(s) characterized by low genetic diversity.
Despite having extremely low genetic diversity Aphis nerii is a well established invasive species. Understanding the temporal and geographic genetic variation of A. nerii provides great insight for the management of invasive pests and the population dynamics of clonal organisms.
We thank Carlie Smith and Hillary Williams for laboratory and field support and Hugh Loxdale for his valuable comments on an earlier version of this manuscript.
Conceived and designed the experiments: JSH EBM. Performed the experiments: JSH EBM. Analyzed the data: JSH EBM. Contributed reagents/materials/analysis tools: JSH EBM. Wrote the paper: JSH EBM.
- 1. Lockwood JL, Hoopes MF, Marchetti MP (2007) Invasion ecology. Malden, MA: Blackwell Publishing. JL LockwoodMF HoopesMP Marchetti2007Invasion ecologyMalden, MABlackwell Publishing
- 2. Ahlroth P, Alatalo RV, Holopainen A, Kumpulainen T, Suhonen J (2003) Founder population size and number of source populations enhance colonization success in waterstriders. Oecologia 137: 617–620.P. AhlrothRV AlataloA. HolopainenT. KumpulainenJ. Suhonen2003Founder population size and number of source populations enhance colonization success in waterstriders.Oecologia137617620
- 3. Porter SD, Savignano DA (1990) Invasion of polygyne fire ants decimates native ants and disrupts arthropod community. Ecology 71: 2095–2106.SD PorterDA Savignano1990Invasion of polygyne fire ants decimates native ants and disrupts arthropod community.Ecology7120952106
- 4. Krieger MJ, Ross KG (2002) Identification of a major gene regulating complex social behavior. Science 295: 328–332.MJ KriegerKG Ross2002Identification of a major gene regulating complex social behavior.Science295328332
- 5. Ward SM, Gaskin JF, Wilson LM (2008) Ecological genetics of plant invasion: what do we know? Invasive Plant Science and Management 1: 98–109.SM WardJF GaskinLM Wilson2008Ecological genetics of plant invasion: what do we know?Invasive Plant Science and Management198109
- 6. Brisson JA, Stern DL (2006) The pea aphid, Acyrthosiphon pisum: an emerging genomic model system for ecological, developmental and evolutionary studies. Bioessays 28: 747–755.JA BrissonDL Stern2006The pea aphid, Acyrthosiphon pisum: an emerging genomic model system for ecological, developmental and evolutionary studies.Bioessays28747755
- 7. Powell G, Tosh CR, Hardie J (2006) Host plant selection byaphids: Behavioral, evolutionary, and applied perspectives. Annual Review of Entomology 51: 309–330.G. PowellCR ToshJ. Hardie2006Host plant selection byaphids: Behavioral, evolutionary, and applied perspectives.Annual Review of Entomology51309330
- 8. Loxdale HD (2010) Rapid genetic changes in natural insect populations. Ecological Entomology 35: 155–164.HD Loxdale2010Rapid genetic changes in natural insect populations.Ecological Entomology35155164
- 9. Blackman RL, Eastop VF (1984) Aphids on the world's crops: an identification and information guide. New York: Joyn Wiley & Sons. RL BlackmanVF Eastop1984Aphids on the world's crops: an identification and information guideNew YorkJoyn Wiley & Sons
- 10. Stoetzel MB (1990) Some aphids of importance to the southeastern United States (Homoptera, Aphididae). Florida Entomologist 73: 580–586.MB Stoetzel1990Some aphids of importance to the southeastern United States (Homoptera, Aphididae).Florida Entomologist73580586
- 11. Hall RW, Ehler LE (1980) Population ecology of Aphis nerii Homoptera, Aphididae on oleander. Environmental Entomology 9: 338–344.RW HallLE Ehler1980Population ecology of Aphis nerii Homoptera, Aphididae on oleander.Environmental Entomology9338344
- 12. Groeters FR (1989) Geographic and clonal variation in the milkweed-oleander aphid, Aphis nerii (Homoptera, Aphididae), for winged morph production, life-history, and morphology in relation to host plant permanence. Evolutionary Ecology 3: 327–341.FR Groeters1989Geographic and clonal variation in the milkweed-oleander aphid, Aphis nerii (Homoptera, Aphididae), for winged morph production, life-history, and morphology in relation to host plant permanence.Evolutionary Ecology3327341
- 13. Groeters FR (1993) Tests for host-associated fitness trade-offs in the milkweed-oleander aphid. Oecologia 93: 406–411.FR Groeters1993Tests for host-associated fitness trade-offs in the milkweed-oleander aphid.Oecologia93406411
- 14. Via S (1999) Reproductive isolation between sympatric races of pea aphids. I. Gene flow restriction and habitat choice. Evolution 53: 1446–1457.S. Via1999Reproductive isolation between sympatric races of pea aphids. I. Gene flow restriction and habitat choice.Evolution5314461457
- 15. Vanlerberghe-Masutti F, Chavigny P (1998) Host-based genetic differentiation in the aphid Aphis gossypii Glover, evidenced from RAPD fingerprints. Molecular Ecology 7: 905–914.F. Vanlerberghe-MasuttiP. Chavigny1998Host-based genetic differentiation in the aphid Aphis gossypii Glover, evidenced from RAPD fingerprints.Molecular Ecology7905914
- 16. Ruiz-Montoya L, Nunez-Farfan J, Vargas J (2003) Host-associated genetic structure of Mexican populations of the cabbage aphid Brevicoryne brassicae L. (Homoptera : Aphididae). Heredity 91: 415–421.L. Ruiz-MontoyaJ. Nunez-FarfanJ. Vargas2003Host-associated genetic structure of Mexican populations of the cabbage aphid Brevicoryne brassicae L. (Homoptera : Aphididae).Heredity91415421
- 17. Miller NJ, Kift NB, Tatchell GM (2005) Host-associated populations in the lettuce root aphid, Pemphigus bursarius (L.). Heredity 94: 556–564.NJ MillerNB KiftGM Tatchell2005Host-associated populations in the lettuce root aphid, Pemphigus bursarius (L.).Heredity94556564
- 18. Simon JC, Stoeckel S, Tagu D (2010) Evolutionary and functional insights into reproductive strategies of aphids. Comptes Rendus Biologies 333: 488–496.JC SimonS. StoeckelD. Tagu2010Evolutionary and functional insights into reproductive strategies of aphids.Comptes Rendus Biologies333488496
- 19. Takada H, Miyazaki M (1993) Bisexual reproduction of a form of Aphis nerii B. de F. (Homoptera: Aphididae) from Hokkaido. Applied Entomology and Zoology 28: 199–205.H. TakadaM. Miyazaki1993Bisexual reproduction of a form of Aphis nerii B. de F. (Homoptera: Aphididae) from Hokkaido.Applied Entomology and Zoology28199205
- 20. Takada H, Miyazaki M (1992) Occurrence of sexuales of Aphis nerii B. de F. (Homotpera: Aphididae) in Japan. Applied Entomology and Zoology 27: 117–124.H. TakadaM. Miyazaki1992Occurrence of sexuales of Aphis nerii B. de F. (Homotpera: Aphididae) in Japan.Applied Entomology and Zoology27117124
- 21. Simon JC, Rispe C, Sunnucks P (2002) Ecology and evolution of sex in aphids. Trends in Ecology & Evolution 17: 34–39.JC SimonC. RispeP. Sunnucks2002Ecology and evolution of sex in aphids.Trends in Ecology & Evolution173439
- 22. Halkett F, Plantegenest M, Bonhomme J, Simon JC (2008) Gene flow between sexual and facultatively asexual lineages of an aphid species and the maintenance of reproductive mode variation. Molecular Ecology 17: 2998–3007.F. HalkettM. PlantegenestJ. BonhommeJC Simon2008Gene flow between sexual and facultatively asexual lineages of an aphid species and the maintenance of reproductive mode variation.Molecular Ecology1729983007
- 23. Lushai G, Loxdale HD, Allen JA (2003) The dynamic clonal genome and its adaptive potential. Biological Journal of the Linnean Society 79: 193–208.G. LushaiHD LoxdaleJA Allen2003The dynamic clonal genome and its adaptive potential.Biological Journal of the Linnean Society79193208
- 24. Groeters FR, Dingle H (1989) The cost of being able to fly in the milkweed oleander aphid, Aphis nerii (Homoptera, Aphididae). Evolutionary Ecology 3: 313–326.FR GroetersH. Dingle1989The cost of being able to fly in the milkweed oleander aphid, Aphis nerii (Homoptera, Aphididae).Evolutionary Ecology3313326
- 25. Dedryver CA, Le Gallic JF, Haack L, Halkett F, Outreman Y, et al. (2008) Seasonal and annual genotypic variation and the effect of climate on population genetic structure of the cereal aphid Sitobion avenae in northern France. Bulletin of Entomological Research 98: 159–168.CA DedryverJF Le GallicL. HaackF. HalkettY. Outreman2008Seasonal and annual genotypic variation and the effect of climate on population genetic structure of the cereal aphid Sitobion avenae in northern France.Bulletin of Entomological Research98159168
- 26. Loxdale HD, Hardie J, Halbert S, Foottit R, Kidd NAC, et al. (1993) The relative importance of short-range and long-range movement of flying aphids. Biological Reviews of the Cambridge Philosophical Society 68: 291–311.HD LoxdaleJ. HardieS. HalbertR. FoottitNAC Kidd1993The relative importance of short-range and long-range movement of flying aphids.Biological Reviews of the Cambridge Philosophical Society68291311
- 27. Vanlerberghe-Masutti F, Chavigny P, Fuller SJ (1999) Characterization of microsatellite loci in the aphid species Aphis gossypii Glover. Molecular Ecology 8: 693–695.F. Vanlerberghe-MasuttiP. ChavignySJ Fuller1999Characterization of microsatellite loci in the aphid species Aphis gossypii Glover.Molecular Ecology8693695
- 28. Michel AP, Zhang W, Jung JK, Kang ST, Mian MAR (2009) Population genetic structure of Aphis glycines. Environmental Entomolgy 38: 1301–1311.AP MichelW. ZhangJK JungST KangMAR Mian2009Population genetic structure of Aphis glycines.Environmental Entomolgy3813011311
- 29. Guo SW, Thompson EA (1992) Performing the exact test of Hardy-Weinberg proportion for multiple alleles. Biometrics 48: 361–372.SW GuoEA Thompson1992Performing the exact test of Hardy-Weinberg proportion for multiple alleles.Biometrics48361372
- 30. Weir BS, Cockerham CC (1984) Estimating F-statistics for analysis of population structure. Evolution 38: 1358–1370.BS WeirCC Cockerham1984Estimating F-statistics for analysis of population structure.Evolution3813581370
- 31. Goudet J (1995) FSTAT (Version 1.2): A computer program to calculate F-statistics. Journal of Heredity 86: 485–486.J. Goudet1995FSTAT (Version 1.2): A computer program to calculate F-statistics.Journal of Heredity86485486
- 32. Shriver MD, Jin L, Boerwinkle E, Deka R, Ferrell RE, et al. (1995) A novel measure of genetic distance for highly polymorphic tandem repeat loci. Molecular Biology and Evolution 12: 914–920.MD ShriverL. JinE. BoerwinkleR. DekaRE Ferrell1995A novel measure of genetic distance for highly polymorphic tandem repeat loci.Molecular Biology and Evolution12914920
- 33. Stoddart JA, Taylor JF (1988) Genotypic diversity: estimation and prediction in samples. Genetics 118: 705–711.JA StoddartJF Taylor1988Genotypic diversity: estimation and prediction in samples.Genetics118705711
- 34. Van Oosterhout C, Hutchinson WF, Wills DPM, Shipley P (2004) MICRO-CHECKER: software for identifying and correcting genotyping errors in microsatellite data. Molecular Ecology Notes 4: 535–538.C. Van OosterhoutWF HutchinsonDPM WillsP. Shipley2004MICRO-CHECKER: software for identifying and correcting genotyping errors in microsatellite data.Molecular Ecology Notes4535538
- 35. Calabrese PP, Durrett RT, Aquadro CF (2001) Dynamics of microsatellite divergence under stepwise mutation and proportional slippage/point mutation models. Genetics 159: 839–852.PP CalabreseRT DurrettCF Aquadro2001Dynamics of microsatellite divergence under stepwise mutation and proportional slippage/point mutation models.Genetics159839852
- 36. Welch DM, Meselson M (2000) Evidence for the evolution of bdelloid rotifers without sexual reproduction or genetic exchange. Science 288: 1211–1215.DM WelchM. Meselson2000Evidence for the evolution of bdelloid rotifers without sexual reproduction or genetic exchange.Science28812111215
- 37. Balloux F, Lehmann L, de Meeus T (2003) The population genetics of clonal and partially clonal diploids. Genetics 164: 1635–1644.F. BallouxL. LehmannT. de Meeus2003The population genetics of clonal and partially clonal diploids.Genetics16416351644
- 38. De Meeus T, Lehmann L, Balloux F (2006) Molecular epidemiology of clonal diploids: A quick overview and a short DIY (do it yourself) notice. Infection Genetics and Evolution 6: 163–170.T. De MeeusL. LehmannF. Balloux2006Molecular epidemiology of clonal diploids: A quick overview and a short DIY (do it yourself) notice.Infection Genetics and Evolution6163170
- 39. Meselson M, Mark Welch D (2007) Stable heterozygosity? Science 318: 202–203.M. MeselsonD. Mark Welch2007Stable heterozygosity?Science318202203
- 40. Simon JC, Baumann S, Sunnucks P, Hebert PDN, Pierre JS, et al. (1999) Reproductive mode and population genetic structure of the cereal aphid Sitobion avenae studied using phenotypic and microsatellite markers. Molecular Ecology 8: 531–545.JC SimonS. BaumannP. SunnucksPDN HebertJS Pierre1999Reproductive mode and population genetic structure of the cereal aphid Sitobion avenae studied using phenotypic and microsatellite markers.Molecular Ecology8531545
- 41. Butlin RK (2000) Virgin rotifers. Trends in Ecology & Evolution 15: 389–390.RK Butlin2000Virgin rotifers.Trends in Ecology & Evolution15389390
- 42. Corrie AM, Crozier RH, Van Heeswijck R, Hoffmann AA (2002) Clonal reproduction and population genetic structure of grape phylloxera, Daktulosphaira vitifoliae, in Australia. Heredity 88: 203–211.AM CorrieRH CrozierR. Van HeeswijckAA Hoffmann2002Clonal reproduction and population genetic structure of grape phylloxera, Daktulosphaira vitifoliae, in Australia.Heredity88203211
- 43. Delmotte F, Sabater-Munoz B, Prunier-Leterme N, Latorre A, Sunnucks P, et al. (2003) Phylogenetic evidence for hybrid origins of asexual lineages in an aphid species. Evolution 57: 1291–1303.F. DelmotteB. Sabater-MunozN. Prunier-LetermeA. LatorreP. Sunnucks2003Phylogenetic evidence for hybrid origins of asexual lineages in an aphid species.Evolution5712911303
- 44. Muller C, Barker A, Boeve JL, De Jong PW, De Vos H, et al. (2004) Phylogeography of two parthenogenetic sawfly species (Hymenoptera : Tenthredinidae): relationship of population genetic differentiation to host plant distribution. Biological Journal of the Linnean Society 83: 219–227.C. MullerA. BarkerJL BoevePW De JongH. De Vos2004Phylogeography of two parthenogenetic sawfly species (Hymenoptera : Tenthredinidae): relationship of population genetic differentiation to host plant distribution.Biological Journal of the Linnean Society83219227
- 45. Kanbe T, Akimoto SI (2009) Allelic and genotypic diversity in long-term asexual populations of the pea aphid, Acyrthosiphon pisum in comparison with sexual populations. Molecular Ecology 18: 801–816.T. KanbeSI Akimoto2009Allelic and genotypic diversity in long-term asexual populations of the pea aphid, Acyrthosiphon pisum in comparison with sexual populations.Molecular Ecology18801816
- 46. Fuller SJ, Chavigny P, Lapchin L, Vanlerberghe-Masutti F (1999) Variation in clonal diversity in glasshouse infestations of the aphid, Aphis gossypii Glover in southern France. Molecular Ecology 8: 1867–1877.SJ FullerP. ChavignyL. LapchinF. Vanlerberghe-Masutti1999Variation in clonal diversity in glasshouse infestations of the aphid, Aphis gossypii Glover in southern France.Molecular Ecology818671877
- 47. Delmotte F, Leterme N, Gauthier JP, Rispe C, Simon JC (2002) Genetic architecture of sexual and asexual populations of the aphid Rhopalosiphum padi based on allozyme and microsatellite markers. Molecular Ecology 11: 711–723.F. DelmotteN. LetermeJP GauthierC. RispeJC Simon2002Genetic architecture of sexual and asexual populations of the aphid Rhopalosiphum padi based on allozyme and microsatellite markers.Molecular Ecology11711723
- 48. Massonnet B, Simon JC, Weisser WW (2002) Metapopulation structure of the specialized herbivore Macrosiphoniella tanacetaria (Homoptera, Aphididae). Molecular Ecology 11: 2511–2521.B. MassonnetJC SimonWW Weisser2002Metapopulation structure of the specialized herbivore Macrosiphoniella tanacetaria (Homoptera, Aphididae).Molecular Ecology1125112521
- 49. Vorburger C (2006) Temporal dynamics of genotypic diversity reveal strong clonal selection in the aphid Myzus persicae. Journal of Evolutionary Biology 19: 97–107.C. Vorburger2006Temporal dynamics of genotypic diversity reveal strong clonal selection in the aphid Myzus persicae.Journal of Evolutionary Biology1997107
- 50. Sunnucks P, DeBarro PJ, Lushai G, Maclean N, Hales D (1997) Genetic structure of an aphid studied using microsatellites: Cyclic parthenogenesis, differentiated lineages and host specialization. Molecular Ecology 6: 1059–1073.P. SunnucksPJ DeBarroG. LushaiN. MacleanD. Hales1997Genetic structure of an aphid studied using microsatellites: Cyclic parthenogenesis, differentiated lineages and host specialization.Molecular Ecology610591073
- 51. Haack L, Simon JC, Gauthier JP, Plantegenest M, Dedryver CA (2000) Evidence for predominant clones in a cyclically parthenogenetic organism provided by combined demographic and genetic analyses. Molecular Ecology 9: 2055–2066.L. HaackJC SimonJP GauthierM. PlantegenestCA Dedryver2000Evidence for predominant clones in a cyclically parthenogenetic organism provided by combined demographic and genetic analyses.Molecular Ecology920552066
- 52. Peccoud J, Figueroa CC, Silva AX, Ramirez CC, Mieuzet L, et al. (2008) Host range expansion of an introduced insect pest through multiple colonizations of specialized clones. Molecular Ecology 17: 4608–4618.J. PeccoudCC FigueroaAX SilvaCC RamirezL. Mieuzet2008Host range expansion of an introduced insect pest through multiple colonizations of specialized clones.Molecular Ecology1746084618
- 53. Vorburger C, Lancaster M, Sunnucks P (2003) Environmentally related patterns of reproductive modes in the aphid Myzus persicae and the predominance of two ‘superclones’ in Victoria, Australia. Molecular Ecology 12: 3493–3504.C. VorburgerM. LancasterP. Sunnucks2003Environmentally related patterns of reproductive modes in the aphid Myzus persicae and the predominance of two ‘superclones’ in Victoria, Australia.Molecular Ecology1234933504
- 54. Lushai G, Loxdale HD (2002) The biological improbability of a clone. Genetical Research 79: 1–9.G. LushaiHD Loxdale2002The biological improbability of a clone.Genetical Research7919
- 55. Wilson ACC, Sunnucks P, Hales DF (2003) Heritable genetic variation and potential for adaptive evolution in asexual aphids (Aphidoidea). Biological Journal of the Linnean Society 79: 115–135.ACC WilsonP. SunnucksDF Hales2003Heritable genetic variation and potential for adaptive evolution in asexual aphids (Aphidoidea).Biological Journal of the Linnean Society79115135
- 56. Loxdale HD, Massonnet B, Schöfl G, Weisser WW (in press) 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 100: 613–622.HD LoxdaleB. MassonnetG. SchöflWW Weisserin pressEvidence 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 Research100613622
- 57. Vialatte A, Dedryver CA, Simon JC, Galman M, Plantegenest M (2005) Limited genetic exchanges between populations of an insect pest living on uncultivated and related cultivated host plants. Proceedings of the Royal Society B-Biological Sciences 272: 1075–1082.A. VialatteCA DedryverJC SimonM. GalmanM. Plantegenest2005Limited genetic exchanges between populations of an insect pest living on uncultivated and related cultivated host plants.Proceedings of the Royal Society B-Biological Sciences27210751082
- 58. Akimoto S (1990) Local adaptation and host race formation of a gall-forming aphid in relation to environmental heterogeneity. Oecologia 83: 162–170.S. Akimoto1990Local adaptation and host race formation of a gall-forming aphid in relation to environmental heterogeneity.Oecologia83162170
- 59. Via S, Hawthorne DJ (2002) The genetic architecture of ecological specialization: Correlated gene effects on host use and habitat choice in a pea aphids. American Naturalist [print] 159: S76–S88.S. ViaDJ Hawthorne2002The genetic architecture of ecological specialization: Correlated gene effects on host use and habitat choice in a pea aphids.American Naturalist [print]159S76S88
- 60. Lynch M (1984) Destabilizing hybridization, general-purpose genotypes and geographic parthenogenesis. Quarterly Review of Biology 59: 257–290.M. Lynch1984Destabilizing hybridization, general-purpose genotypes and geographic parthenogenesis.Quarterly Review of Biology59257290