Abstract
Quantitative trait differences are often assumed to be correlated with molecular variation, but the relationship is not certain, and empirical evidence is still scarce. To address this issue, we sampled six populations of the cereal aphid Sitobion avenae from areas north and south of the Qinling Mountains, and characterized their molecular variation at seven microsatellite loci and quantitative variation at nine life-history traits. Our results demonstrated that southern populations had slightly longer developmental times of nymphs but much higher lifetime fecundity, compared to northern populations. Of the nine tested quantitative characters, eight differed significantly among populations within regions, as well as between northern and southern regions. Genetic differentiation in neutral markers was likely to have been caused by founder events and drift. Increased subdivision for quantitative characters was found in northern populations, but reduced in southern populations. This phenomenon was not found for molecular characters, suggesting the decoupling between molecular and quantitative variation. The pattern of relationships between FST and QST indicated divergent selection and suggested that local adaptation play a role in the differentiation of life-history traits in tested S. avenae populations, particularly in those traits closely related to reproduction. The main role of natural selection over genetic drift was also supported by strong structural differences in G-matrices among S. avenae populations. However, cluster analyses did not result in two groups corresponding to northern and southern regions. Genetic differentiation between northern and southern populations in neutral markers was low, indicating considerable gene flow between them. The relationship between molecular and quantitative variation, as well as its implications for differentiation and evolution of S. avenae populations, was discussed.
Highlights
Barriers to gene flow presented by mountain ranges can have significant consequences for ecology and evolution of various organisms, and even lead to the isolation of populations
Northern and southern populations showed significantly different quantitative traits and G-matrices, so high quantitative variation was evident between populations from the two regions
The decoupling of molecular and quantitative variation was evident in Principal component analysis (PCA) analyses where clustering patterns from molecular and quantitative trait data were distinct
Summary
Barriers to gene flow presented by mountain ranges can have significant consequences for ecology and evolution of various organisms, and even lead to the isolation of populations. The genetically-determined components of life-history trait variation are a significant factor which drives evolution by supplying raw material for evolutionary changes. Genetic variation can be affected by gene flow, but by natural selection, genetic drift, colonization history, mutation, and their interactions [6,7] Among these causes, natural selection is generally considered as the major force generating changes in the level of genetic variation within and among populations (especially in heterogeneous environments), and driving adaptation and evolutionary changes [7,8,9]. Due to technical and logistic difficulties in measuring such variation, many studies rely solely on molecular markers to evaluate adaptive genetic variation, the degree to which molecular variation reflects quantitative genetic variation between populations is still controversial [10]
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