ABSTRACT
The peach-potato aphid, Myzus persicae (sulzer), is an important arable pest species throughout the world. Extensive use of insecticides has led to the selection of resistance to most chemical classes including organochlorines, organophosphates, carbamates and pyrethroids. Resistance to pyrethroids is often the result of mutations in the para-type sodium channel protein (knockdown resistance or kdr). In M. persicae, knockdown resistance is associated with two amino-acid substitutions, L1014F (kdr) and M918T (super-kdr). In this study, the temporal and spatial distributions of these mutations, diagnosed using an allelic discriminating polymerase chain reaction assay, were investigated alongside other resistance mechanisms (modified acetylcholinesterase (MACE) and elevated carboxylesterases). Samples were collected from the UK, mainland Europe, Zimbabwe and south-eastern Australia. The kdr mutation and elevated carboxylesterases were widely distributed and recorded from nearly every country. MACE and super-kdr were widespread in Europe but absent from Australian samples. The detection of a strongly significant heterozygote excess for both kdr and super-kdr throughout implies strong selection against individuals homozygous for these resistance mutations. The pattern of distribution found in the UK seemed to indicate strong selection against the super-kdr (but not the kdr) mutation in any genotype, in the absence of insecticide pressure. There was a significant association (linkage disequilibrium) between different resistance mechanisms, which was probably promoted by a lack of recombination due to parthenogenesis.
Subject(s)
Acetylcholinesterase/metabolism , Aphids/genetics , Carboxylesterase/metabolism , Insecticides , Pyrethrins , Alleles , Animals , Aphids/enzymology , Europe , Gene Frequency , Genotype , Geography , Insecticide Resistance , Mutation , Time Factors , VictoriaABSTRACT
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.
Subject(s)
Aphids/drug effects , Aphids/physiology , Insecticide Resistance , Insecticides/pharmacology , Movement/physiology , Animals , Aphids/genetics , DNA, Intergenic/genetics , Demography , Genotype , Microsatellite Repeats/genetics , Plants/parasitology , Time Factors , United KingdomABSTRACT
A 1.0 kb region of the mitochondrial cytochrome oxidase subunit I gene from the greenbug aphid, Schizaphis graminum (Rondani), was sequenced for 24 field collected clones from non-cultivated and cultivated hosts. Maximum likelihood, maximum parsimony and neighbour-joining phylogenies were estimated for these clones, plus 12 previously sequenced clones. All three tests produced trees with identical topologies and confirmed the presence of three clades within S. graminum. Clones showed no relationship between biotype and mtDNA haplotype. At least one biotype was found in all three clades, suggesting exchange among clades of genetic material conditioning for crop virulence, or the sharing of a common ancestor. However, there was a relationship between host and haplotype. Clade 1 was the most homogeneous and contained 12 of 16 clones collected from cultivated hosts and five of the six collected from johnsongrass, Sorghum halepense, a congener of cultivated sorghum, S. bicolor. Four of the six clones collected from Agropyron spp. were found in clade 2. Clade 3 contained two clones from wheat, Triticum aestivum, and four from non-cultivated hosts other than Agropyron spp. A partitioning of populations by mtDNA haplotype and host suggests the occurrence of host adapted races in Schizaphis graminum.
Subject(s)
Aphids/genetics , DNA, Mitochondrial/genetics , Animals , Aphids/classification , DNA, Mitochondrial/analysis , Electron Transport Complex IV/genetics , Haplotypes , Likelihood Functions , PhylogenyABSTRACT
The full complement of known greenbug, Schizaphis graminum (Rondani), biotypes found in the USA were subjected to a molecular phylogenetic analysis based on a 1.2-kb portion of the cytochrome oxidase I mitochondrial gene. In addition to these nine biotypes (B, C, E, F, G, H, I, J and K), a probable isolate of the enigmatic biotype A (NY), a 'new biotype' collected from Elymus canadensis (L.) (CWR), and an isolate from Germany (EUR) were included. Schizaphis rotundiventris (Signoret) was included as an outgroup. Genetic distances among S. graminum biotypes ranged from 0.08% to 6.17% difference in nucleotide substitutions. Neighbour-joining, maximum parsimony and maximum likelihood analyses all produced dendrograms revealing three clades within S. graminum. Clade 1 contained the 'agricultural' biotypes commonly found on sorghum and wheat (C, E, K, I, plus J) and there were few substitutions among these biotypes. Clade 2 contained F, G and NY, and Clade 3 contained B, CWR and EUR, all of which are rarely found on crops. The rarest biotype, H, fell outside the above clades and may represent another Schizaphis species. S. graminum biotypes are a mixture of genotypes belonging to three clades and may have diverged as host-adapted races on wild grasses.