In a previous study, samples of the grain aphid Sitobion avenae (F.) were collected from wheat and adjacent cocksfoot hosts in a population thought to be primarily parthenogenetic, and DNA from individual aphids was analysed with a multilocus technique. Here we have applied single-locus microsatellites and a mitochondrial DNA marker to a subset of the same DNA extracts, and have made several additional inferences about important genetic and population processes in S. avenae. Microsatellite analysis indicated very high levels of genic and genotypic variation. S. avenae fell into three genotypic groups inferred to be almost noninterbreeding, while analysis of linkage and Hardy-Weinberg equilibria suggested high levels of sexual recombination within each genotypic group. Host specialization was evident: one lineage was found only on wheat, and one (bearing many alleles inferred to be introgressed from the blackberry-grass aphid S. fragariae (Walker)) was found only on cocksfoot. The third group of interrelated genotypes was found commonly on both hosts. Although most genotypes were found only once, some were much more numerous in the sample than expected from the frequency of the alleles they contained. This, and rapid temporal changes in genotypic composition of samples, indicates strong selective differences between genotypes and lineages. In the major genotypic group, the commonest genotypes were significantly more homozygous than were rare ones: thus these data may help to explain the frequent observation of homozygous excess in aphid allozymes. The genotype group showing S. avenae-like as well as S. fragariae-like alleles also carried S. fragariae-like mitochondrial DNA in at least 25/31 cases, indicating gender-asymmetrical hybridization.

Most organisms represent specialized forms that arose as a result of natural selection and genetic drift to occupy distinct ecological niches. In animals, this process of specialization includes the behaviour of the organisms concerned, honed by locally-induced adaptations to specific host food plants (in herbivores) or prey items (in predators and parasitoids), and possibly reinforced by kairomones, including sex pheromones. The major thrust of evolution is towards ecological specialization as a result of the direct effects of intra-and interspecific competition. Adaptation to new resources lowers such competition and allows survival in new habitats/niches. Other benefits of food resource/habitat switching include 'enemy free space'. If specialism is the norm for the vast majority of species, what of so-called generalists and generalism, which are widely used terms, but perhaps wrongly so? Does generalism exist or is it a mirage that disappears the closer that it is inspected? We review some of the aspects of specialism and generalism and argue that even apparent generalists are filling distinct ecological niches. Often, generalists are rather specific in terms of food preferences, although they may nevertheless remain opportunistic with an overall broad niche/resource width. When apparent 'good' species are examined using molecular (DNA) markers, they are often found to comprise cryptic species. Many generalists may be of this kind. If so, generalism warrants additional investigation to establish its scope and credentials.

A diverse range of novel molecular (DNA) markers are now available for entomological investigations. Both DNA and protein markers have revolutionized the biological sciences and have enhanced many fields of study, especially ecology. Relative to DNA markers, allozymes are cheap, often much quicker to isolate and develop, even from minute insects (aphids, thrips, parasitic wasps, etc.), and subsequently easy to use. They display single or multi-locus banding patterns of a generally easily interpretable Mendelian nature, and the statistics for their analysis are well established. DNA markers are also suitable for use with small amounts of insect material and can be used with stored, dry or old samples. They have an expanding range of applications, many involving intra- and interspecific discriminations. Like allozymes, they can be single or multilocus, whilst methods for their statistical analysis have recently been published. However, they can be considerably more expensive than allozymes, require more complex preparatory protocols, expensive equipment, may involve lengthy development procedures (e.g. isolating cloned oligonucleotides to develop primers to detect microsatellite regions) and some have complex multi-locus banding patterns which may be of a non-Mendelian nature (e.g. RAPDs, randomly amplified polymorphic DNA), and are in some cases, not easily repeatable. In this review, we hope to inform the general reader about the methodology and scope of the main molecular markers commonly in use, along with brief details of some other techniques which show great promise for entomological studies. Thereafter, we discuss their applications including suitability for particular studies, the methods used to load and run samples, subsequent band detection, band scoring and interpretation, the reliability of particular techniques, the issues of safety involved, cost effectiveness and the statistical analyses utilized.

It is well established that asexually reproducing viruses and prokaryotes mutate rapidly. In contrast, the eukaryotic clone is often still treated as if it is genetically homogeneous within and between populations, i.e. that it is assumed to show genetic fidelity. However, such fidelity has rarely been tested empirically using the range of high‐resolution molecular markers now available, culminating with direct sequencing of the DNA. If such a biological entity as a ‘clone’ really did exist, it would be a fantastic entity, differing from everything else known in biology, i.e. it would possess a population mean but no variance for any particular trait. It would not be amenable to selection and adaptive variation and would thus be unchanging in time and space. In this paper, we argue that the general acceptance of clonal fidelity is a scientific convenience, since the rate of asexual reproduction of eukaryotes is not as fast as that of bacteria and hence it is easier to accept fidelity as a ‘fact’ rather than test for it. We propose that part of the acceptance of fidelity may have a cultural basis and thereby is a kind of ‘pre‐Darwinian relic’. Instead, a clonal genotype is perhaps largely a function of marker resolution, i.e. dependent on the number and type of markers employed. If this is so and were enough of the genome explored, perhaps each individual within a clone would be found to differ genetically at particular regions of the chromosomes. The question of what constitutes a clone is not just a semantic one and impacts directly on recent attempts to understand and produce ‘artificial’ clones, especially of mammals. New research is already confirming that mutations and epigenetic influences play a crucial role in the success of cloning attempts. © 2003 The Linnean Society of London. Biological Journal of the Linnean Society, 2003, 79, 3–16.

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.

The majority of insect species do not show an innate behavioural migration, but rather populations expand into favourable new habitats or contract away from unfavourable ones by random changes of spatial scale. Over the past 50 years, the scienti¢c fascination with dramatic long-distance and directed mass migratory events has overshadowed the more universal mode of population movement, involving much smaller stochastic displacement during the lifetime of the insects concerned. This may be limiting our understanding of insect population dynamics.In the following synthesis, we provide an overview of how herbivorous insect movement is governed by both abiotic and biotic factors, making these animals essentially`slaves of their environment'. No displaced insect or insect population can leave a resource patch, migrate and £ourish, leaving descendants, unless suitable habitat and/or resources are reached during movement. This must have constrained insects over geological time, bringing about species-speci¢c adaptation in behaviour and movements in relation to their environment at a micro-and macrogeographical scale. With insects that undergo longrange spatial displacements, e.g. aphids and locusts, there is presumably a selection against movement unless overruled by factors, such as density-dependent triggering, which cause certain genotypes within the population to migrate. However, for most insect species, spatial changes of scale and range expansion are much slower and may occur over a much longer time-scale, and are not innate (nor directed).Ecologists may say that all animals and plants are ¢guratively speaking`slaves of their environments', in the sense that their distribution is de¢ned by their ecology and genotype. But in the case of insects, a vast number must perish daily, either out at sea or over other hostile habitats, having failed to ¢nd suitable resources and/or a habitat on which to feed and reproduce. Since many are blown by the vagaries of the wind, their chances of success are serendipitous in the extreme, especially over large distances. Hence, the strategies adopted by mass migratory species (innate pre-programmed £ight behaviour, large population sizes and/or fast reproduction), which improve the chances that some of these individuals will succeed. We also emphasize the dearth of knowledge in the various interactions of insect movement and their environment, and describe how molecular markers (protein and DNA) may be used to examine the details of spatial scale over which movement occurs in relation to insect ecology and genotype.

Populations of clonal organisms are often represented as being evolutionary inert with persistent genetic fidelity. The advent of molecular methods and the corresponding increased genetic resolution of clonal populations forces a reconsideration of this viewpoint. We review molecular data from viruses, prokaryotes and eukaryotes to support the argument that clones possess a highly dynamic and adaptive genome. © 2003 The Linnean Society of London, Biological Journal of the Linnean Society, 2003, 79, 193–208.

Elucidation of the genetic variability of a model insect species, the grain aphid, Sitobion avenae (Fabricius), a predominantly asexual herbivore within the temperate agro-ecosystem tested, was initiated using molecular DNA markers (RAPDs). This revealed genetic profiles that appeared related to host adaptation at the specific level amongst the natural populations colonizing different grasses and cereals (Poaceae) within the same geographic location. These profiles were recorded either as 'specialist' genotypes found on specific grasses, or as 'generalist' genotypes colonizing several host types including cultivated cereals or native grasses. These findings are compared with analogous systems found amongst insect species, including at a higher trophic level, i.e. interactions between hymenopterous aphid parasitoids. As the aphids and their respective plant hosts occur in the same geographical region at the same time, this appears to be a rare example of the evolutionary transition leading to sympatric speciation in insects. Hence, this study highlights the importance of understanding not only the demographic parameters to genetic diversity, but also the more intricate correlation of genetic diversity to host types in agricultural environments.