Temporal Genetic Stability Within and Restricted Migration (Gene Flow) Between Local Populations of the Blackberry-Grain Aphid Sitobion fragariae in South-East England

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Abstract
During the 1980s, blackberry-grain aphids, Sitobion fragariae (Walker), were collected in May-June from the main primary host, Rubus fruticosus and in June-July from the main secondary host, Dactylis glomerata at sites in a 50-km radius around Rothamsted, south-east England. Allozymes from individual aphids were used as biochemical markers to examine local population structure and migration between populations. Three populations were tested electrophoretically using six enzymes representing fourteen putative loci. Percentage biochemical polymorphism was found to be high (50%; 7/14 loci). As with most other aphids examined, average heterozygosity over all loci was low (c. 1.5%). Thereafter, using one of the variable enzymes, glutamate-oxaloacetate transaminase (GOT), populations were monitored spatially and temporally, mainly over four consecutive years. Six GOT alleles were found in total, in four homozygous and seven heterozygous combinations. Population allozyme frequencies were compared (genetic chi-square contingency test). Populations from R. fruticosus were fairly homogeneous, whereas those from D. glomerata were more heterogeneous and allowed broad categorization of samples into ''western'' and ''eastern'' populations. Most population samples had genotype frequencies close to or in accordance with Hardy-Weinberg (H-W) expectations, even samples infesting D. glomerata. However, high non-equilibrium frequencies of a slow (S) allele were sometimes detected in samples from grass, especially at two western sites. It is proposed that such S-bearing genotypes may represent a predominantly anholocyclic (asexual) form of S. fragariae. At sites sampled for temporal changes, GOT allozyme and genotype frequencies were often stable. Alleles appear to be neutral or weakly selected. The concordance of genotype frequencies with H-W expectations, even on the secondary host, the temporal stability of allele and genotype frequencies and the spatial allelic heterogeneity shown especially by asexual aphids on the secondary host, all suggest that gene flow may be restricted between local S. fragariae populations. This view is consistent with the scarcity of S. fragariae in 12.2-m Rothamsted suction trap samples in Britain compared to common cereal aphid pests such as Sitobion avenae (F.) and Rhopalosiphum padi (L).

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