Genetic consequences of an invasion through a patchy environment — the cynipid gallwasp Andricus quercuscalicis (Hymenoptera: Cynipidae)


  • This paper is the result of collaborative research between two institutions. The Institute of Zoology's Conservation Genetics Group (led by Dr R. K. Wayne) uses molecular techniques to understand genetic structures of small populations. The Department of Biology (group led by Dr M. J. Crawley) and NERC Centre for Population Biology (headed by Prof. J. H. Lawton) at Imperial College are engaged in long-term studies of the population biology of invading organisms. This research will be extended to include genetic analyses of other invading cynipids in the genus Andricus with known patterns of colonization and a variety of patterns of host plant utilization.


Over the last 300–400 years, the cynipid gallwasp Andricus quercuscalicis has invaded northern and western Europe following human introduction of an obligate host plant, the Turkey oak (Quercus cerris) from south-eastern Europe. In the introduced range, distances between Turkey oak patches are greater and the numbers of oaks in each patch are far lower than in its native range. These changes in spatial distribution of Turkey oak are predicted to result in high genetic subdivision of A. quercuscalicis in invaded areas relative to its native range. Allozyme electrophoresis was used to examine genetic variation in 823 gall wasps from 39 populations of A. quercuscalicis. No new electrophoretic variants were found in the invaded range and both allelic diversity and mean heterozygosity decreased significantly with distance from the native range. Spatial autocorrelation analysis and values of Wright's Fst indicate that differences in allele frequences between populations were substantially greater in the invaded range than in the native range. Spatial autocorrelation analysis also suggests that changes in allele frequencies across Europe are unlikely to be the results of selection, but rather of strong directional migration followed by limited gene flow between populations. Patterns of genetic differentiation across Europe suggest that populations of A. quercuscalicis have been founded sequentially from the east through a process of random genetic subsampling and not by source-sink colonization directly from the native range.