Estimating levels of gene flow between natural populations of cereal aphids (Homoptera: Aphididae)

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Abstract
Gene flow between field populations of the cereal aphidsSitobion avenae(Fabricius),Sitobion fragariae(Walker) andRhopalosiphum padi(Linnaeus) is examined using statistical analyses of allozyme frequency data (both Slatkin's andFstatistics). Analysis using Slatkin's method confirms the conclusions previously drawn from earlier analyses. Thus there is found to be high gene flow between subpopulations ofS. avenaeandR. padi, and a more restricted level between localS. fragariaesubpopulations, especially when infesting the secondary host (cocksfoot grass,Dactylis glomerata). The value ofNm(number of immigrants per generation) calculated forS. avenaewas approximately 2 and is indicative of high gene flow i.e.Nm>1.0. Also, as previously found by x2genetic contingency testing, theFstatistical analysis confirms thatS. fragariaesubpopulations infestingD. glomerataare genetically more heterogeneous than those infesting the primary host, blackberry,Rubus fruticosusagg. Hence, there may be less gene flow between populations on the secondary host compared with that on the primary. The results obtained are compared with the flight behaviour of these particular aphid species and to their respective host plant abundances. The advantages and disadvantages of both Slatkin's method andFstatistics for analysing insect population genetic structure are noted.