G. Malloch

Analysis of clonal diversity of the peach–potato aphid, Myzus persicae (Sulzer), in Scotland, UK and evidence for the existence of a predominant clone

Clones of the peach–potato aphid, Myzus persicae (Sulzer), mostly from Scotland, UK were examined using an rDNA fingerprinting technique. Eighty patterns (genotypes) were found amongst the 276 clones. A large number of clones (30%) from all sample areas in Scotland exhibited the same simple pattern, suggesting the presence of a single M. persicae clone. There was no difference in genotype distributions between M. persicae collected from brassica or potato crops, suggesting that host‐adapted genotypes have no advantage in the field. Different fingerprints were randomly distributed in the environment, although clones taken from the same leaf were more often the same fingerprint. Highly distinctive fingerprints, which were more widely distributed, suggest that this technique could be used to follow individual clones. In addition to the common clonal type, multiple fingerprint bands were found over successive years, implying that, in Scotland, local overwintering asexual populations are the most common source of M. persicae in the following year.

Micro‐evolutionary change in relation to insecticide resistance in the peach–potato aphid, Myzus persicae

1. Phenotypic diversity is the fuel that powers evolution.

Tracking the global dispersal of a cosmopolitan insect pest, the peach potato aphid

BackgroundGlobal commerce and human transportation are responsible for the range expansion of various insect pests such as the plant sucking aphids. High resolution DNA markers provide the opportunity to examine the genetic structure of aphid populations, identify aphid genotypes and infer their evolutionary history and routes of expansion which is of value in developing management strategies. One of the most widespread aphid species is the peach-potato aphid Myzus persicae, which is considered as a serious pest on various crops in many parts of the world. The present study examined the genetic variation of this aphid at a world scale and then related this to distribution patterns. In particular, 197 aphid parthenogenetic lineages from around the world were analysed with six microsatellite loci.ResultsBayesian clustering and admixture analysis split the aphid genotypes into three genetic clusters: European M. persicae persicae, New Zealand M. persicae persicae and Global M. persicae nicotianae. This partition was supported by FST and genetic distance analyses. The results showed two further points, a possible connection between genotypes found in the UK and New Zealand and globalization of nicotianae associated with colonisation of regions where tobacco is not cultivated. In addition, we report the presence of geographically widespread clones and for the first time the presence of a nicotianae genotype in the Old and New World. Lastly, heterozygote deficiency was detected in some sexual and asexual populations.ConclusionThe study revealed important genetic variation among the aphid populations we examined and this was partitioned according to region and host-plant. Clonal selection and gene flow between sexual and asexual lineages are important factors shaping the genetic structure of the aphid populations. In addition, the results reflected the globalization of two subspecies of M. persicae with successful clones being spread at various scales throughout the world. A subspecies appears to result from direct selection on tobacco plants. This information highlights the ultimate ability of a polyphagous aphid species to generate and maintain ecologically successful gene combinations through clonal propagation and the role of human transportation and global commerce for expanding their range.

The attack of the clones: tracking the movement of insecticide-resistant peach–potato aphids Myzus persicae (Hemiptera: Aphididae)

Myzus persicae (Sulzer) collected in Scotland were characterized for four microsatellite loci, intergenic spacer fingerprints and the resistance mechanisms modified acetylcholinesterase (MACE), overproduced carboxylesterase and knockdown resistance (kdr). Microsatellite polymorphisms were used to define a limited number of clones that were either fully susceptible to insecticides or possessed characteristic combinations of resistance mechanisms. Within these clones, intergenic spacer fingerprints could either be very consistent or variable, with the latter indicating ongoing evolution within lineages, most likely derived from the same zygote. Two clones (termed A and B) possessed all three resistance mechanisms and predominated at sites treated with insecticides. Their appearance on seed potatoes and oilseed rape in Scotland in 2001 coincided with extensive insecticide use and severe control failures. Clones C, I and J, with no or fewer resistance mechanisms, were found in samples from 1995 and were dominant at untreated sites in 2001. A comparison of Scottish collections with those from other UK and non-UK sites provides insight into the likely origins, distribution and dynamics of M. persicae clones in a region where asexual (anholocyclic) reproduction predominates, but is vulnerable to migration by novel genotypes from areas of Europe where sexual (holocyclic) reproduction occurs.

Analysis of eriophyid mite rDNA internal transcribed spacer sequences reveals variable simple sequence repeats.

Ribosomal DNA internal transcribed spacers of the eriophyid mites Cecidophyopsis ribis , C. selachodon , C. spicata , C. alpina , C. aurea , C. grossulariae and Phylocoptes gracillis were amplified using PCR, cloned and sequenced. Sequences for the ITS1 of Cecidophyopsids were 92–99% homologous. Cecidophyopsis inter‐specific differences were found in seventeen simple sequence repeats (vSSRs), fourteen point mutations and two indels. No intra‐specific variation in vSSRs was detected. A hypothetical structure for ITS1 was obtained and vSSRs were mapped onto this. Changes in vSSRs were compensated for by changes in complementary vSSRs or through multiple point mutations. A comparison with vSSRs of other arthropods suggested that the levels of intra‐specific variation in Cecidophyopsis mites was less than in organisms which do not use arrhentoky for male determination.

Molecular analysis of ribosomal DNA from the aphid Amphorophora idaei and an associated fungal organism

The ribosomal DNA cistron from the large raspberry aphid, Amphorophora idaei (Hemiptera: Aphididae), has been mapped by restriction analysis. The results showed that the map of A. idaeiwas similar to those of the previously characterized aphids Schizaphis grami‐num and Acyrthosiphon pisum. An extra Bglti site was found in some of the ribosomal DNA intergenic spacer repeats in A. idaei. Using in‐situ hybridization to aphid mitotic chromosomes it was demonstrated that probes derived from this region mapped to the pair of X chromosomes and it was therefore aphid in origin. Polymerase chain reaction using conserved rDNA primers also detected significant amounts of a fungal genome in the DNA samples. Microscopic investigation showed that the external surface of A. idaei harboured fungal propagules, hyphae and yeast‐like organisms.

Species identification of Cecidophyopsis mites (Acari: Eriophyidae) from different Ribes species and countries using molecular genetics.

Cecidophyopsis mites were studied by PCR amplification of parts of their ribosomal DNA, followed by restriction enzyme analysis. Mite specimens on Ribes nigrum (black currant) from six countries gave the same digestion pattern, which was distinct from the pattern for mites found on R. rubrum from Poland and Finland and for R. grossularia from the USA. This suggests that each Ribes species is host to a different mite species: C. ribis, C. selachodon and C. grossulariae, respectively. Two other mite samples from R. alpinum and R. aureum were identical but were distinct from each of the other species.

Molecular evidence for multiple infections of a new subgroup of Wolbachia in the European raspberry beetle Byturus tomentosus

Wolbachia, a group of maternally inherited intracellular parasitic bacteria, alter host reproduction, including the induction of thelytokous parthenogenesis, feminization of genetic males, son killing and, most commonly, the induction of cytoplasmic incompatibility (CI), in a diverse array of arthropods. CI can result in infertility and has attracted attention because of its potential in biological control and as an agent in speciation. Although there has been some analysis of overall infection rates in arthropods and within individual insect orders, there has been little exploration of within‐species variation. In this study, primers specific for the ftsZ gene of Wolbachia were used to amplify it from different geographical samples of the European raspberry beetle (Byturus tomentosus), confirming the presence of Wolbachia. More than 99% of UK individuals were found to be infected with Wolbachia and 97% of these B. tomentosus beetles harboured multiple infections. Preliminary analysis of B. tomentosus beetles from continental European populations revealed a lower level of infection (24%) than those from the UK. Phylogenetic analysis using the ftsZ DNA sequences places Wolbachia from B. tomentosus into a new clade (Abt) within the A division, with some revisions to the existing Wolbachia phylogeny.

Super‐infections of Wolbachia in byturid beetles and evidence for genetic transfer between A and B super‐groups of Wolbachia

Wolbachia are maternally inherited bacteria responsible for altering host reproduction. The two main groups found in insects, A and B, are based on molecular characterization using ribosomal, ftsZ, wsp (Wolbachia surface protein) or groE genes. We have used the wsp and ftsZ genes to study Wolbachia in byturid beetles. Byturus affinis contained a single copy of the ftsZ gene which grouped with A ftsZ sequences and a single copy of the wsp gene which grouped with B wsp sequences. This suggests that genetic exchange between A and B groups has occurred in the Wolbachia of this beetle. FtsZ and wsp sequences that were identical or nearly identical to those of B. affinis were found in B. tomentosus, suggesting that it also contains the same recombinant Wolbachia genotype. Most other byturids had more than one wsp sequence with at least one from the A and B groups, suggesting multiple copies of bacterial genes or multiple infections. B. ochraceus and B. unicolor both had four distinct wsp gene sequences. All the byturids had a closely related A wsp sequence and most a closely related B wsp sequence. Therefore, there appears to be an association between specific A and B wsp types.

Gall Mite Molecular Phylogeny and its Relationship to the Evolution of Plant Host Specificity

The phylogenetic relationships of all seven known species of Cecidophyopsis mites (Acari: Eriophyidae) with Ribes hosts have been inferred from ribosomal DNA sequences. This analysis found groups of closely related mites. The five gall-forming species, four of which are monophagous and one which has two hosts, were found in two groups. Another group consisted of the two non gall-forming species, one of which has two hosts, while the other is monophagous. The molecular phylogeny of their known Ribes host plants was calculated using the equivalent ribosomal regions as the mites. The structure of the two trees (mites vs hosts) was clearly different, implying that mite speciation did not closely follow speciation events in the plant hosts. Instead, the three groups of Ribes-infesting Cecidophyopsis mites have derived from a common galling ancestor millions of years ago. Each mite group has recently diversified onto different primary hosts. One group of mites has also lost the galling habit. The results have implications for host range changes and the durability of mite-resistance genes in cultivated Ribes.