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Keywords:

  • Myzus persicae;
  • clones;
  • adaptation;
  • dynamics

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

1 The population of peach-potato aphid Myzus persicae in Scotland is comprised almost entirely of long-term asexual clones.

2 Over a ten year period, M. persicae from Scottish fields and suction traps were analysed with six microsatellite markers.

3 Out of 1497 individuals analysed, 14 clones (denoted A–N) comprised over 98% of the collection.

4 Some clones were particularly abundant but most clones had a widespread distribution on all available plants.

5 Clones E and L had distinct features in their distributions as clone L was geographically totally restricted to the north east of Scotland and clone E showed a marked preference for brassica crops.

6 Clones E and L provide direct evidence of a role for local adaptations in the distribution of M. persicae clones.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

The peach-potato aphid Myzus persicae is one of the most adaptable aphids, alternating between populations on peach, its primary host, and ephemeral populations on many secondary hosts. Sexually reproducing (holocyclic) populations are only present in peach-growing areas, whereas, asexual populations are more widespread, remaining more or less active all year round depending on temperature. Myzus persicae sexual populations are genetically diverse (Wilson et al., 2002; Fenton et al., 2003; Guillemaud et al., 2003; Vorburger et al., 2003a). Studies of long-term asexual populations of this aphid show instead an extreme effect of genetic bottlenecking (Fenton et al., 1998, 2005; Zamoum et al., 2005; Kasprowicz et al., 2007; van Toor et al., 2007). Asexual populations are particularly important in vectoring plant viruses when they move from plant to plant and crop to crop. They usually either produce no male or egg-laying female forms (anholocyclic) or rarely male forms (androcyclic). Long-term study of such stable asexual populations will help to understand what the fundamental drivers are that affect these populations and the species as a whole.

Myzus persicae is an efficient virus vector and it has successfully overcome at least three classes of chemical insecticides (Foster et al., 2000). As a result, it has become a major agricultural pest. The application of high resolution genetic markers to samples in peach-growing areas, taken from peach, from plants growing near peach and from suction traps, have all confirmed that these are diverse populations. Some genotypes have expanded asexually at local scales, ranging from a small number of clonal individuals on neighbouring branches in peach orchards (Fenton et al., 2003) to many clonal individuals spread over a wide area (Guillemaud et al., 2003; Vorburger et al., 2003a).

One of the greatest challenges to an asexual pest aphid is the use of insecticides, compounded by an inability to accumulate resistance genes through sexual recombination. In M. persicae, the spread of insecticide resistance beyond sexual and into asexual populations is rapid and generally associated with expansion of resistant clones but not resistance genes. Resistant clones eventually become mixed with stable asexual populations (Fenton et al., 2005) and these appear in waves, with new clones appearing in successive years (Kasprowicz et al., 2007). Three resistance mechanisms have been identified in M. persicae populations including elevated carboxylesterase, which sequester and degrade organophosphates; modified acetylcholinesterase (MACE), which confers resistance to dimethylcarbamates; and knockdown resistance (kdr) and super-kdr (skdr), which modify the nerve sodium channels gating system, thereby rendering pyrethroids ineffective (Foster et al., 2000). The selective advantage conferred by insecticide resistance is considerable and, under ideal conditions, resistant clones can multiply in sprayed crops to reach damaging levels (Foster et al., 1998; Fenton et al., 2005) and these can be found in large quantities in suction traps (Malloch et al., 2006). However, in the absence of insecticide selection, resistant clones appear to be at a considerable disadvantage both within and between seasons (Foster et al., 2002; Fenton et al., 2005; Kasprowicz, 2007).

There are other natural factors that can potentially affect the clonal structure of aphid asexual populations. Host specialization can affect the natural distribution of aphid clones in a population because any genotypes that grow better on a particular plant will be found in a greater proportion on that plant. This has been found in other aphid species; for example, Acyrthosiphon pisum (Harris) (Via, 1991), Sitobion avenae sensu stricto (F.) (Sunnucks et al., 1997; Lushai & Loxdale, 2002) and Brevicoryne brassicae (L.) (Ruiz-Montoya et al., 2003). Another important factor in aphid ecology is temperature, which influences how an aphid continues its life cycle overwinter and how fast it can reproduce. Genetic variation between different life cycle stages (Clough et al., 1990) and between clones in their temperature tolerance has been shown in M. persicae (Vorburger, 2004).

One of the experimental advantages of studying a pest aphid species is the ready availability of samples. These range from samples taken directly from the field, where plant type and local conditions can be recorded, to samples from the aphid suction trap network (Woiwod & Harrington, 1994), which should provide a random collection of individuals from a much wider area. The present study aimed to investigate the geographical and temporal dynamics of Scottish M. persicae clones and elucidate the effect of crop type, insecticide application and environment on the frequencies of individual clones.

Materials and methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Myzus persicae samples

1995 M. persicae samples In 1995, single parthenogenetic females were collected in Scottish fields for a study using intergenic spacer (IGS) fingerprinting (Fenton et al., 1998). Some of these were grown in culture and their DNA extracted and stored. Individuals of these cultures were also preserved in 95% ethanol. One hundred and eighty-nine of these samples were analysed by the microsatellite methods described below, either by using the stored DNA, or if unavailable, extracting DNA from ethanol preserved individuals.

2003–2005 samples In 2003, specimens were collected from fields of potato or vegetable brassica crops, some of which had been treated with insecticide. In 2004 and 2005, aphid samples were mostly taken from unsprayed brassica or potato crops, which were collected up to three times throughout the season. It was not possible to use the same fields between years, but fields in the same areas, representing north- east Scotland (Elgin/Thurso, E/T), Fife, east Scotland (Angus/Perthshire, A/P) and the Scottish Borders (Lothian/Borders, L/B) were visited. Details of these sampling sites are provided elsewhere (Kasprowicz et al., 2007). Live aphids were taken from single plants separated by at least 3 m. Either one or, if present, several aphids from each plant were put into 1.5-mL Eppendorf tubes and stored at −70 °C until an individual could be analysed by microsatellite analysis. On the rare occasions when the DNA from an individual failed to produce polymerase chain reaction (PCR) products, a second individual from the same tube was used instead. This direct method of analysis avoids any losses or potential selection due to aphid parasitoids in the bodies of aphids. To obtain living material for resistance testing of new clones, wingless individuals (apterae) from colonies in areas where particular clones were expected to be frequent were placed onto potato roots in plastic containers. Once a colony had been established, an individual was genotyped and several live aphids were sent to Rothamsted Research (Harpenden, U.K.) for insecticide resistance testing (for details, see below).

2003–2004 suction trap samples To help monitor and predict aphid movement a network of 12.2-m high suction traps sample airborne aphid populations (Woiwod & Harrington, 1994). In Scotland, these traps are located at Elgin (57°3903″N, 03°25′28″W), Dundee (56°27′05″N, 03°03′08″W), Ayr (55°27′09″N, 04°33′03″W), and at two sites in Edinburgh. The traps in Edinburgh were located at East Craigs (55°56′09″N, 03°18′09″W) and at Gogarbank (55°55′23″N, 03°20′11″W) but, as they are less than 2 km apart, the results from these were combined and they are referred to as the ‘Edinburgh traps’. In 2003, it was only possible to analyse alatae caught in the Elgin suction trap because aphids from the other traps were stored in a solution that does not preserve their DNA. From 2004, all the aphids in Scotland were collected in an ethanol-based preservation solution. DNA was thus extracted and analysed from alatae preserved in this solution.

DNA extraction

DNA from aphids collected in 1995, 2002 and 2003 was extracted using the method of Chia et al. (1985). It was then treated with 0.5 μl of RNase. DNA from the ethanol-preserved aphids collected in 1995, aphids from the suction traps and those collected in 2004 from the field was extracted as described by Malloch et al. (2006).

Genotyping

Six microsatellite loci; M35, M40, M49, M63 and M86 (Sloane et al., 2001) and myz9 (Wilson et al., 2004), were chosen on the basis of their resolution (based on allele numbers of 8, 7, 15, 11, 11 and 11, respectively, giving 3.5 × 1010 possible combinations in the Scottish samples described below). These loci were amplified using fluorochrome primers labelled at the 5′ end of the reverse primer (M35 TET, M40 FAM, M49 HEX, M63 FAM, M86 TET, myz9 HEX; MWG Biotech, Germany) and PCR ready-to-go beads (Amersham Biosciences, U.K.; for the conditions used, see Malloch et al. (2000)). Products were then analysed on an ABI 377 (96) automated sequencer with Genescan v3.4 and Genotyper v2.5 software (Applied Biosystems, Foster City, California), for both visualization and analyses (Fenton et al., 2003).

Insecticide resistance tests

Clones identified for the first time in 2003 and 2004 were sent to Rothamsted Research for insecticide resistance testing. Esterase levels were measured by enzyme-linked immunosorbent assay and the aphids categorized as S (susceptible), R1 (moderately resistant), R2 (highly resistant) or R3 (extremely resistant) (Foster et al., 2000). MACE activity was tested by kinetic enzyme assay in the presence or absence of a diagnostic concentration of insecticides (Moores et al., 1994). Finally, kdr and skdr were determined by high-throughput real-time PCR techniques (Anstead et al., 2004).

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Microsatellite marker analysis

The multilocus microsatellite analysis, using six microsatellite loci, namely myz9, M40, M35, M49, M63, and M86, defined 21 clones amongst the M. persicae samples collected in Scotland in 1995 and 2003–2005. Fourteen clones comprised over 98% of the collections. An additional seven clones were collected in a single year, usually only once, and are the subject of a separate study. In accordance with the convention adopted by Fenton et al. (2005), the 14 clones were designated A–N (Table 1).

Table 1.  The clones found in Scotland from 1995 to 2005
Marker microsatelliteClone
ABCDEFGHIJKLMN
A1A2A1A2A1A2A1A2A1A2A1A2A1A2A1A2A1A2A1A2A1A2A1A2A1A2A1A2
  1. Column 1 denotes the marker used to define each clone and columns 2–15 give the marker profile for each clone. Rows 4–9 identify the microsatellite allele sizes in base pairs for each clone for six loci. As each individual can have two copies of each marker, A1 and A2 denotes alleles 1 and 2. Homozygous individuals will have the same numbers. Rows 11–14 give the results of four insecticide resistance tests for each clone: esterase, knockdown resistance (kdr); superknockdown resistance (skdr) and modified acetyl cholinesterase (MACE).

  2. S, R1, R2 and R3 denoted the various levels of esterase resistance. Genotypes of insecticide resistance: s/s, homozygous sensitive; r/s, heterozygous resistant; r/r, homozygous resistant.

M35196196196200186196186196198202186196178192180196186196186186186198186198198198178178
M40124134128136124124128134124134124128128134134136122134122124124134134134116136116126
M49149156156159153165153153156163149156137179138176153203153165138186138160121169138143
M63174184169204169172174204167172184204165186167169169169169172159184169184176178159172
M861131389913813614012513810310712513810713110310712514011514011913813814010312599103
myz9196224210224196204204224204208210224220228204206202222204222214222196210202206204222
Insecticide resistance
EsteraseR3R1/2R1/2R2/3R2/3R2/3S/R1S/R1S/R1S/R1R3S/R1R2S/R1
kdrr/sr/sr/sr/ss/sr/rs/sr/ss/ss/sr/ss/ss/sr/s
skdrr/ss/ss/sr/ss/ss/ss/ss/ss/ss/ss/ss/ss/ss/s
MACE+++++

Intraclonal variation was identified at loci M49 and M86. In clone L, the M49 locus (ACn) always contained an allele of 138 bp, and another allele of either 156 or 160 bp, with the latter being present in 75% of individuals (results not shown). The L clonal lineages were co-localized and were identical for other markers, which is strong evidence that they share a common asexual ancestor. The 156 bp allele was found in other Scottish clones, but the 160 bp allele was not. The 156 bp allele therefore appears to be shared with other clones colonizing Scotland. This would be consistent with the original asexual lineage of clone L having a 156 bp allele and the 160 bp allele mutating from this to form the now more common clone L lineage.

In clone I, there was consistent variation in one allele at both M49, either 203 or 207 bp with a 153 bp allele always present, and M86 (CAn), 125 or 127 bp with a 140 bp allele always present. Unlike the variation in clone L, the 127 and 207 bp variants of clone I were rare, being found in only three samples. In one case, there were a number of individuals in the sample and these were all characterized and found to carry both mutations. This confirmed that the size variants were present and in the same clonal lineage and that this is a double mutation. These lineages had identical microsatellite patterns for the other loci examined, which is strong evidence that they share a common asexual ancestor. As with lineages in clone L, one of the variants at M63 (125) was shared with other Scottish clones but the other rare one (127) was not. This implies a direction of mutation with a common allele changing to a rare allele in one of the asexual lineages. Both variants at locus M49 in the lineages of I were not shared with any other Scottish clone, although it can still be suggested that a common allele (203) mutated to a rare allele (207) in an asexual lineage.

Spatial and seasonal distributions of clones from field samples

Clones C, I and J were present in each year and area sampled, apart from clone J which was absent from Fife in 2004 (Fig. 1). In total, these three clones comprised 91%, 61%, 71% and 66%, of the respective collections from 1995 to 2003–2005. When data from 2003 to 2005 were combined, clone I was present in the largest proportion (36%) at the northern field sites, Elgin/Thurso (E/T). However, in other areas, clones C and J were more abundant than I. In Angus/Perthshire (A/P), J comprised 35%, and I and C, 17% and 14%, respectively. In Fife, clone C was predominant occurring at a frequency of 21%, whereas J was at 8% and I at 4% and, in Lothian/Borders (L/B), J was present in 45% of samples, with I making up 19% and C comprising 17%.

image

Figure 1. The clonal composition of field samples collected from four areas of Scotland from 1995 to 2005. The numbers of each clone is expressed as a percentage of the total number, which was calculated by dividing the number of individuals by the total number, n. Un, unclassified.

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In 2003, clones D and E were collected, but only two aphids were identified as belonging to clone D (<  1%), whereas 16% of aphids comprised clone E (Fig. 1). Clones D and E were first identified in 2001 (Fenton et al., 2005) where they made up only a small proportion of the sample (7% and 4% respectively). Clone D was not collected again; however, clone E comprised 3% and 14% of the 2004 and 2005 samples, respectively.

Clone K was first collected in 2003 in Fife, and is unusual as it is a red–brown colour. When this clone was tested for insecticide resistance, it was found to be R3 and heterozygous for kdr. It was only collected once in 2003 and 2004 in A/P, but it was collected three times in 2005 at sites in E/T and A/P suggesting that, although rare, it is widespread in Scotland.

In 2003, 80 M. persicae were collected from fields close to Elgin (Fig. 1). Here, a new clone was found, designated as clone L, which made up 20% of the collection that year. In 2004 and 2005, it constituted 40% and 20% of the Elgin samples, respectively, making it the second most dominant clone in that area. Clone L was not collected in any of the other sample areas in any year, including Thurso, which is 120 km north of Elgin.

Clone F made up 5% of the collections in 1995 but it was not found again. DNA from clone F samples was tested for insecticide resistance status and the samples were all found to be R2/3 esterase and homozygous for kdr (kdr/kdr) (Foster et al., 2000; Anstead et al., 2004). Clone G, an insecticide sensitive clone, was found once in 1995 and once in 2003.

Three new MACE carrying clones were identified in 2003 and 2004 (H, M and N) and are the subject of a separate study (Kasprowicz et al., 2007). These supplemented the two MACE clones, A and B, identified by Fenton et al. (2005).

Spatial and seasonal distributions of clones in Scottish suction traps

Composition of trap samples Clones I and J were found in all of the Scottish suction traps in each of the years. Clone C was present in all the traps except Elgin in 2004 and 2005 (Fig. 2). The sample size for Elgin in these two  years was small. When results from all years and traps were combined, the three dominant clones in the field samples, C, I and J, were also dominant in the traps, comprising between 43% and 89% of the catch. In agreement with the E/T field sites, clone I was the most abundant clone in the Elgin suction trap (31%). Clones J and C comprised 20% and 6% of the Elgin catch (Fig. 2). Chi-square analysis allowed the hypothesis that clones J, C and I would be distributed proportionately in the combined collections and in the E/T field sites and suction trap to be rejected (χ2 = 43.94, d.f. = 2, P < 0.001). This was due to clone I occurring in a much larger proportion and J in a smaller proportion than expected in the Elgin area. The Dundee and Edinburgh traps collected more of clone J (43 – 50%) than clones C (12–27%) and I (7–18%), which was a similar trend to field observations in these areas.

image

Figure 2. The percentage clonal composition of three suction trap collections at three sites in Scotland from 2003 to 2005. The percentage was calculated by dividing the number of individuals by the total number, n.

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Clones D, F and G were not identified in the Scottish suction traps in 2003–2005. Clone E was collected in all the traps in all years, except in Elgin in 2004 (Fig. 2). Clone K was collected once in the Dundee trap and once in the Edinburgh suction traps in 2005. Only the Elgin trap caught clone L (Fig. 2).

Chi-square analysis showed that the numbers of each clone in field collections were significantly different from those of the suction trap samples (χ2= 42.26, d.f. = 7, P < 0.001). However, if rare clones and clone L were excluded, the difference was no longer significant (χ2 = 10.73, d.f. = 5, P > 0.05). This is because clones A, G and N were never identified in the traps despite being found in the field whereas clone L was found in much larger proportions in the suction traps than would have been predicted from its known occurrence in the field.

Suction trap temporal dynamics For most years and most traps, only five clones were collected in the first 2 weeks of the trapping season (Fig. 3). Clone I appeared first most often, in five of the possible trap-years. This was closely followed by clone J in four of the trap-years. Despite its abundance, C only appeared twice in the first 2 weeks of a trap-year (Elgin 2003 and Edinburgh 2004). L and K were collected once in the first 2 weeks at Elgin in 2004 and at Dundee in 2005, respectively.

image

Figure 3. The clonal temporal dynamics of the Scottish suction trap collections from 2004 to 2005. The length of the line indicates the period of time during which each clone was caught in that season. Vertical lines indicate that an alata was trapped on that day with the height representing the number. Individual symbols represent clones trapped once. The arrow indicates the peak activity of the season (13/14 July) in Elgin 2003.

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When the length of time that a clone was caught (from first to last capture) in each trap for each year was plotted against the numbers of that clone in the trap, a positive correlation was noted (r = 0.695; P < 0.001). Based on the above relationship, it could be hypothesized that each clone’s length of time being trapped would predict its numbers in the trap. Chi-square analysis on the clones that had sufficient numbers (C, E, H, I, J, L and M) demonstrated a significant difference as some clones were not trapped for the expected length of time (χ2 = 316.70; P < 0.001). When clones I and L were removed from the analysis, the result was no longer significant (χ2 = 3.11; P > 0.05). Clone I was found in smaller numbers in the trap than expected from the length of time it was collected. Conversely, clone L was collected for a shorter period than expected from the numbers in the trap.

Distribution of clones on crop plants

Information gathered from unsprayed crops was used to examine the distribution of clones C, E, H, I, J and L on potato or brassica host plants (Fig. 4). Clones with numbers too small to be analysed individually (A, F, G and K) were combined into a single category. Chi-square analysis was used to compare the observed and expected distributions based on an assumption of equal proportions on both plant types: C, χ2 = 3.17, P > 0.05; E, χ2 = 10.35, P < 0.01; H, χ2 = 0.59, P > 0.20; I, χ2 = 0.30, P > 0.50; J, χ2 = 0.46, P > 0.20; L, χ2 = 1.85, P > 0.10; A + F + G + K, χ2 = 0.22, P > 0.50 (all with 1 d.f.). Therefore, there was no evidence that most clones were significantly associated with either host type. However, the increased proportion of clone E on brassica crops was highly significant, evidence that it demonstrated a preference for this particular host (Fig. 4).

image

Figure 4. The percentage distribution of abundant clones on potato and brassica crops at unsprayed sites. **P < 0.01.

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Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Clones in the Scottish M. persicae population

Seven M. persicae clones were found to be present in Scotland in 2001 (A, B, C, D, E, I and J) using four microsatellite markers (Fenton et al. 2005). The current analysis has applied an additional two microsatellite markers, M40 and myz9 and has identified the same clones along with an additional seven genotypes (F, G, H, K, L, M and N). Although the additional two microsatellite markers increased the analytical power considerably, all the clones (A–N) would have been identified with the previous set of four markers. Two loci exhibited intraclonal variation in allele size of 2–4 bp. This is consistent with unit jumps in microsatellite size and with mutation within an asexual clone lineage (Loxdale & Lushai, 2003). Microsatellite mutation has been noted in asexual populations of Sitobion in New Zealand (Wilson et al., 1999). In the current study for clone I the rare variants were only found at two field sites. This is consistent with the arrival of an individual colonizer, which spread through those fields as a result of asexual propagation during the growing season. In clone L, both variants were common in the population, suggesting that one of the mutations has undergone positive genetic drift. There can be little doubt that these are microsatellite lineages of clone L because of their identity at other markers and their colocalisation.

Retrospective analysis of the 1995 population

Analysis of the 1995 Scottish M. persicae population with polymorphic microsatellite markers reduced by ten-fold the number of clones identified previously by IGS fingerprinting (i.e. eighty to eight). Five of the clones, C, F, G, I and J, made up over 95% of the sample. The other three clones occurred rarely in 1995 and have not been found subsequently. The current analysis confirmed that the population in 1995 was dominated by clone J. Most of the IGS variants then identified belonged to clone C, which shows intraclonal variation in IGS pattern (Fenton et al., 2005). Such variation in ribosomal IGS is consistent with the nature of these multicopy genes and has utility in understanding the ecological and evolutionary fate of clonal lineages of predominant clones (Shufran et al., 2003). Clone F, which was a kdr homozygote, was only detected in this year.

Clone diversity

The Scottish M. persicae population was characterized by low clonal diversity. Only 21 clones have been identified from 1497 field and suction trap individuals over a 10-year period. Clone diversity can be measured simply by dividing the number of genotypes by the number of individuals (G) (Zamoum et al., 2005). For the Scottish material, G = 0.015, which differs from French aerial M. persicae populations, with 100 genotypes among 174 alatae (G = 0.57) (Guillemaud et al., 2003) and an Australian study, which detected 73 genotypes among 167 samples (G = 0.44) (Vorburger et al., 2003a).

Factors affecting clonal structure of the Scottish M. persicae population

Clones A, B, C, E, F, H, K, M and N are known to carry insecticide resistance and are associated with sprayed fields (Kasprowicz et al., 2007). The absence of certain of these clones or the low numbers of others (with the exception of clone C) at unsprayed sites provides further evidence that there is a strong fitness cost linked with high levels of insecticide resistance. Myzus persicae clones with high levels of esterase have maladaptive behaviour affecting their ability to overwinter (Foster et al., 1996, 1997) whereas clones with the kdr gene are known to be less responsive to alarm pheromones, which may affect their ability to respond to unfavourable conditions (e.g. over crowding, poor plant nutrition, predation and parasitism; Foster et al., 1999, 2003, 2005).

By contrast to the current study, in 2001, clone C was not associated with sprayed fields but was significantly associated with unsprayed fields (Fenton et al., 2005). The two findings could account for the widespread success of clone C. As clone C carried kdr, it has an advantage in areas where pyrethroids are being used, but no major disadvantages in unsprayed fields because, apparently, it does not have the same fitness penalty associated with other highly insecticide resistant clones carrying kdr.

Clones G, I, J and L are not resistant to insecticides, but there are differences in their abundance and distribution. Clone I is significantly more abundant in the Elgin area but, in the A/P and L/B areas, clone J dominates. Also, despite its apparent susceptibility to insecticides, clone G is far less numerous in the field than I and J and clone L is only abundant at one location. If the fitness penalties associated with resistance were the only factors involved in the success of a susceptible clone then clones G and L should be equally as successful ecologically and widespread. This paradox suggests that, within insecticide sensitive clones, there are considerable differences in fitness.

Restricted geographical distribution of clone L

Of the many studies performed on aphid populations, none has reported the presence over several seasons of an abundant asexual clone so clearly restricted to a small geographical area. The present study provides strong evidence, gathered from field sites and suction traps over a 3-year period, that clone L is restricted to the north-east of Scotland. It is abundant in the Elgin trap, which means that it is clearly capable of aerial dispersal. The large proportion of L in 2003 suggests that it was present in this area beforehand. Analysis of earlier suction trap samples has not been successful.

Many samples were taken south of Elgin but, to examine the situation, some 120 km further north, a field containing M. persicae was sampled near Thurso at the northern tip of Scotland. None of the thirty individuals sampled was clone L, therefore, it is unlikely that clone L is selected by colder northerly latitudes. This argues for tolerance to a very narrow temperature range. It is difficult to believe, however, that climatic conditions alone would restrict an aphid clone to such a confined area. There are few geographical barriers that could stop this clone from moving, with the notable exception of the Cairngorm Mountains. Other clones have overcome this barrier, for example clones H and M appeared in the 2003 Elgin suction trap having spread there within one season. Clone L is found on both potatoes and brassicas in equal proportions and these plants are equally abundant elsewhere in Scotland. Hence, there is no direct evidence that its host range restricts its distribution. The geographical distribution of L remains the subject of further ongoing studies.

Evidence that clone E shows host-plant specialization

Host specialization has been reported in several aphid species (Via, 1991; Sunnucks et al., 1997; Ruiz-Montoya et al., 2003), but there are no reported cases of strong host specialization in a M. persicae clone with the exception of M. persicae nicotianae on tobacco (Blackman & Paterson, 1986; Blackman, 1987; Nikolakakis et al., 2003). In the laboratory, some M. persicae lineages appear to reproduce slightly better on some hosts (Weber, 1985, 1986; Edwards, 2001). It is only with the advent of high resolution molecular markers that it has been possible to analyse the success of M. persicae clones in an ecological and laboratory context. Vorburger et al. (2003a) examined Australian M. persicae genotypes for plant association over a wide area and found none. However, a later study at a farm scale found that one of the predominant clones in Victoria, Australia (genotype 58) was over-represented on Solanum physalifolium, suggesting that it was well adapted on this host (Vorburger, 2006). Vorburger (2006) also thought some brassica specialization may have been present in genotype 61, which was overrepresented on broccoli, although this was not statistically significant (Vorburger, 2006). It is clear from our studies that the proportions of different clones in a crop can vary stochastically at a local level, and that the effects of the various resistance mechanisms and spray regimes have a pronounced effect on clone distribution and need to be considered.

We have shown that at a landscape scale one of the clones, E, showed a significant preference for brassica crops when only unsprayed fields were compared. It was one of the more successful clones and contributed up to 15% of any of the seasonal samples tested. Such success may well be due to specialization on brassica crops, which thus allows it to reproduce on them with maximum efficiency all year. In Scotland, brassica specialization is potentially advantageous in winter as M. persicae is thought to overwinter on these plentiful hosts (Jacob, 1941; van Emden et al., 1969). By contrast to E, the three dominant clones C, I and J, and clones A, H and L, show no significant differences in their abundance on potato or brassica in unsprayed fields. Having defined the clones in this population, it will now be possible to study them, and clone E in particular, in more detail.

Vorburger et al. (2003b) compared microsatellite defined sexual and asexual lines of M. persicae for their performance on different host plants in the laboratory but found no overall difference between the life-cycle categories. However, as all lines showed individual differences in plant performance, none was considered a ‘general purpose genotype’ (GPG). Despite these differences in the laboratory, only one of these genotypes exhibited host association at a restricted location (Vorburger, 2006). According to the GPG hypothesis, environmental variation over time drives specialized genotypes to extinction and selects for generalized genotypes (Lynch, 1984). Hence, the older the aphid asexual lineage, the higher the probability it is a GPG (Vorburger et al., 2003b). However, the ecological background selecting a GPG has to be considered. A successful GPG lineage in Scotland only needs to exhibit good performance on the different host plants it encounters in the field (potatoes and brassicas) (e.g. survival on tomato would not be an advantage in this area where it is not grown as a major crop).

Analysis of clones of M. persicae in the Scottish suction traps

The 12.2-m high suction traps are designed to collect a sample of airborne aphids that represent the surrounding area (Woiwod & Harrington, 1994). In M. persicae, it has been shown that the spatial patterns of abundance are weakly positively correlated with temperature and negatively with rainfall (Cocu et al., 2005b). However, a given trap is affected by local characteristics (e.g. availability of host plant and insecticide spraying) of which both may affect the genotypic composition of the M. persicae catch. The effects of local differences between traps can be seen in Scotland (e.g. Dundee collects more M. persicae than the other Scottish traps). Peaks in the Scottish suction traps were observed in 2001, which did not fit a model based on climatic conditions (Cocu et al., 2005b) eventually this was attributed to a large increase in a single clone, A, selected by insecticide spraying (Malloch et al., 2006). In contrast the large peak seen in the Elgin trap on 13/14 July 2003 (Fig. 3, arrow) was not due to a single clone, as several clones increased in abundance at this time, suggesting the rise to be a result of a local event that triggered the production of alatae by the whole population.

Estimates for the radius captures by a suction trap represent range from 30 km using 8-m high traps (Halbert et al., 1998), to 80–290 km for 12.2-m high traps (Taylor, 1974; Cocu et al., 2005a). Factors such as trap height, climate, topography and host plant availability are all thought to influence the nature and composition of the catch (Cocu et al., 2005b). The distribution of clone L helps estimate the range of the Dundee and Elgin traps. The traps are approximately 130 km apart and, as Dundee does not collect any clone L, there is seemingly no overlap of the two traps’ catchment areas, giving an estimated catch radius of less than 130 km. This could be reduced to 122 km, as one of the northern field sites containing clone L was 8 km south of Elgin. These figures are in line with the original 80 km estimates of Taylor (1974).

There is no prior evidence that the Scottish M. persicae clones show host specificity (Fenton et al., 2005); hence, it was considered that the main host plants surrounding a suction trap would not affect the clonal composition of the catch. Yet, evidence from the present study appears to indicate that clone E is indeed more numerous on brassica crops, which certainly may drive changes in trap compositions as previously described.

Temporal dynamics of clones of M. persicae in the Scottish suction traps

The length of time the individual clones were caught in the traps was positively correlated with the numbers found in both trap and field. This is probably because the more individuals of a clone there are flying in a season, the greater the chance of catching this clone over an extended period. Clone I was present in the trap catches for much longer than expected from the numbers collected in the traps and fields. Clone I was also the first to be collected in all the traps in 2005 and in the Elgin trap in 2004. This early appearance and longevity of clone I in the trap catches probably reflects physiological differences between it and the other clones, including better adaptation to low temperatures. This may arise because alatae are produced earlier to expand host-range, have higher reproductive rates at colder temperatures or a lower temperature threshold to fly. In 2004, clone J was the first to be collected in Dundee and Edinburgh and may also be adapted to early season activity, but less so than clone I.

Foster et al. (2005) found that clones with high levels of esterase and kdr were less responsive to alarm pheromones. This may also make them less sensitive to the cues that would normally trigger flight. This would mean that sensitive clones are more likely to be abundant in the suction traps. However, the proportions of both insecticide resistant and sensitive type clones in the trap were similar to those found in the field, suggesting that there is no detectable difference in alate production and behaviour between these clones. Only clone L showed a statistically significant difference being more abundant in the traps than the field. Clone L was also caught in the trap for a shorter length of time than expected from its numbers. This could be evidence that, although production by this clone does not seem to be inhibited and may be increased, it responds to the environment in a different way to the other clones.

The resistant clones, C, E, H and K, are more variable in their contributions to trap captures with no clear patterns to the timing and duration of their periods of capture. In general, they were caught for much shorter periods than clones I and J. In the 2001 Dundee suction trap samples, clones J, I and C were collected first, in that order, and they were captured for the longest period (13, 12 and 10 weeks, respectively; Malloch et al., 2006). Clone A dominated the trap in overall numbers in 2001, but was present only for 8 weeks. The differences in the temporal dynamics of clones in the suction traps clearly demonstrate that there are selective forces acting on clones, such as availability of hosts, insecticide spraying, and different aphid responses to temperature and seasonal or behavioural cues, which have yet to be fully understood. The present study has begun to throw light on some possibilities. Investigation of physiological parameters in the laboratory (e.g. reproductive fitness, alate production and host preference) may yield more clues to why some asexual aphid clones have become successful.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

This work was supported by a policy flexible fund project grant from the Scottish Executive Environment and Rural Affairs Department (SEERAD). SCRI was supported by SEERAD. The authors wish to thank colleagues at Rothamsted Research, particularly James Anstead and Diana Cox for insecticide resistance testing of the clones and Steve Foster and Ian Denholm for helpful discussion, as well as the many growers and agronomists who helped provide aphid material. The authors also thank Rocio Alarcon Reverte, University of Valencia, Spain and Sheena Lamond, SCRI for technical assistance, and the entomologists at SASA for the suction trap data and specimens. We also thank the Scottish Society for Crop Research for their support, and Roy Neilson, Mark Phillips and four anonymous referees for their comments on the manuscript.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
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