Seasonal and annual genotypic variation and the effect of climate on population genetic structure of the cereal aphidSitobion avenaein northern France

Abstract

AbstractChanges in the genetic structure and genotypic variation of the aphidSitobion avenaecollected from cereal crops in northern France were examined by analysing variation at five microsatellite loci across several years and seasons. Little regional and temporal differentiation was detected, as shown by very lowFSTamong populations. Repeated genotypes, significant heterozygote deficits, positiveFISvalues and frequent linkage disequilibria were found in nearly all samples, suggesting an overall pattern of reproductive mode variation inS. avenaepopulations. In addition, samples from Brittany (Bretagne) showed greater signs of asexual reproduction than those from the north of France, indicating a trend toward increasing sexuality northward. These patterns of reproductive variation inS. avenaeare consistent with theoretical models of selection of aphid reproductive modes by climate. Contrasting with little changes in allelic frequencies, genotypic composition varied substantially in time and, to a lesser extent, in space. An important part of changes in genotypic arrays was due to the variation in frequency distribution of common genotypes, i.e. those that were found at several instances in the samples. Genotypic composition was also shown to vary according to climate, as genotypic diversity in spring was significantly correlated with the severity of the previous winter and autumn. We propose that the genetic homogeneity amongS. avenaepopulations shown here across large temporal and spatial scales is the result of two forces: (i) migration conferred by high dispersal capabilities, and (ii) selection over millions of hectares of cereals (mostly wheat) bred from a narrow genetic base.