Electrophoretic study of enzymes from cereal aphid populations. V. Spatial and temporal genetic similarity of holocyclic populations of the bird-cherry oat aphid, Rhopalosiphum padi (L.) (Hemiptera: Aphididae), in Britain

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Abstract
Large colonies of holocyclic Rhopalosiphum padi (L.) were sampled in May-June in 1985 and 1986 from the principal host, Prunus padus, in various parts of Britain, mainly in the south-east. They were tested electrophoretically using nine enzymes representing 14 putative genetic loci. Only two loci (14%) were polymorphic (GOT, glutamate-oxaloacetate transaminase, and SORDH, sorbitol dehydrogenase), the remainder being monomorphic. Allele and genotype frequencies at each locus were generally similar both spatially and temporally, and genotype frequencies were close to Hardy-Weinberg expectations. The low level of genetic variability may reflect population bottlenecks which have purged most of the variation and which have been imposed by the distribution of P. padus, especially in the south of Britain where the tree is generally scarce. In addition, it is probable that adverse climatic conditions directly affect winter survival of anholocyclic populations of the aphid and thus also reduce variability. In the long term, such bottlenecks would tend to preclude the maintenance of effectively neutral enzyme variants that arise within populations. The similarity of allele and genotype frequencies may reflect the highly migratory behaviour of R. padi. Thus, in southern Britain, where P. padus is generally much rarer in comparison with the north of the country, R. padi inter-population gene flow is still sufficient to alleviate the influences of genetic drift and possible directional selection on the allelic variation that remains.

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