Genetic structuring of populations reflects the interaction of genetic drift, mutation, migration and selection, with influences from life history. Aphids are interesting in this regard as they have the potential for unusually high levels of dispersal and natural selection, which typically counter each other. In the present study, winged grain aphids Sitobion avenae (F.) were collected in four 12.2-m high suction traps along a north-south transect in Britain in order to eliminate sampling bias from plant hosts (cereals and grasses; Poaceae), it being known that these insects show host adaptation demonstrable using molecular markers. Samples were analysed at four polymorphic microsatellite loci over two consecutive years. Population allele frequencies were similar nationally during the two years, although clonal diversity varied greatly between sites and years. In the first sampling year following a harsh winter, diversity was found to display a latitudinal clinal trend: the proportion of unique clones (genotypes) increased with latitude. However, this pattern was less apparent the following year, after a milder winter. Nonetheless, overall FST analysis showed that there was little spatial genetic structuring in either sampling year. These data support the view that the insect is highly migratory and also support a theoretical model and previous data suggesting that the reproductive mode is clinal in S. avenae. This appears to be because natural selection (reduced reproductive success of asexual genotypes under cold conditions) is sufficiently powerful to overcome the homogenizing effects of strong migration. There was no clear evidence for isolation by distance for the genetic data obtained. The data are compared with similar data from other aphid species and other insects. Only by the collection of such data sets can an accurate picture be built up relating genetic variability to flight behaviour, including migratory ambit in this group of insects since, due to their small size and rapid dilution in the air, other marking approaches are impracticable over large geographical distances.

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Ninety-two individuals of Sitobion avenae (F.) collected throughout Britain in 1979 and, were cloned and investigated genetically by electrophoresis of 14 enzymes representing 26 loci. Percentage polymorphism (P) differed considerably between years, 64% (16/25 loci) in 1979 and 19% (5/26) in 1980, whereas average heterozygosity (H) was low (ca. 2%) in both years and confined mainly to one locus, EST-1. The prevalence of homozygous allozyme variation supports ecological findings suggestingS. avenae to be largely anholocyclic in Britain. In 1981 and 1982, large populations, sampled from 11 sites in Britain and Spain, were examined at 13 loci. H ranged from 2 to 7 • 6%, with heterozygosity restricted again mainly to EST-1. Some alleles were unique to certain geographical regions (including Britain or Spain); others showed significant spatial, and in two British populations examined in successive years, temporal frequency differences. Calculating Nei's genetic identity (/) and distance (D) coefficients for each population pair mostly gave 7>0-8, Z?<0-2 with overall means (± s.e.m.) of 0-896 ± 0-006 and 0-111 ± 0-006, respectively, which are comparable with geographical population values for other insects. D was poorly correlated with geographical distance, although values were slightly greater (ca. 0-025) for international population comparisons and did not overlap on a principal coordinates plot. The overall population similarity suggests substantial inter-population gene flow; geographical barriers may have restricted movements but seemingly have not led to allopatric race formation. P and H values resemble those for many other aphids, but H values are lower than typically found in other insects. Possible causes of this heterozygote deficiency are discussed.

One-dimensional slab polyacrylamide gel electrophoretic techniques, staining systems and isoenzyme banding patterns for 14 soluble enzymes separated from crude homogenates of individuals of six species of cereal aphids {Sitobion avenae (F.), 5. fragariae (Wlk.), Metopolophium dirhodum (Wlk.), M. festucae (Theo.), Rhopalosiphum padi (L.) and R. maidis (Fitch)) are described. The value of the techniques and banding patterns to taxonomic and population genetic studies of these and other aphid species are briefly discussed. With the six species, it was possible to separate the different genera as well as individual species within genera. The enzymes found to be most useful for inter-generic and/or -specific separations were adenylate kinase (AK), esterase (EST), glucose-6-phosphate dehydrogenase (G-6-PDH), hexokinase (HK), malate dehydrogenase (MDH), peptidase (PEP), phosphatase (PHOS), phosphoglucomutase (PMG), 6-phosphogluconate dehydrogenase (6-PGD) and sorbitol dehydrogenase (SORDH), whilst glutamate oxaloacetate transaminase (GOT), a-glycerophosphate dehydrogenase (a-GPD), malic enzyme (ME) and peroxidase (POD) were of relatively little taxonomic use. There were no banding pattern differences between the various morphs of 5. avenae (first to fourthinstar nymphs, apterous and alate adults) using the 14 enzymes.

Summary The economically important grain aphid, Sitobion avenae (F.) shows colour polymorphism, with brown and green forms predominating. Colour is determined both genetically and in response to environmental factors, including nutrition. The biological significance of the colour polymorphism is unknown, although seasonal changes occur in the frequency of colour morphs in the field, whilst the brown morph may have adaptive significance in terms of hymenopterous endoparasitism. The ground colour of aphids is produced by haemolymph pigments, aphins (glucosides) and carotenoids. The latter may be under the synthetic control of intracellular endosymbiotic bacteria. In this study, the major carotenoid pigments of a brown and a green clone of S. avenae were examined using thin layer chromatography (TLC) and high‐performance liquid chromatography (HPLC), and their absorbance spectra recorded. Using TLC, the brown clone produced five bands of different Rf, ranging from yellow, to orange‐pink to pink in colour. In contrast, the green clone gave only a single yellow band of higher Rf than any of the bands of brown aphids. Following separation of carotenoids by HPLC, brown aphids gave seven peaks and green aphids five. Comparison of absorbance maxima with known published values for carotenoids provides strong evidence for the identification of four of the carotenoid pigments from brown aphids (RB‐4, 3,4‐didehydrolycopene; RB‐5, torulene; RB‐6; lycopene; RB‐7, γ‐carotene) and one from green aphids (RG‐2, α‐carotene). The other carotenoids remain unidentified. The biosynthesis and possible biological relevance of the various pigments of S. avenae are briefly discussed.