GENETIC CONSEQUENCES OF AN INVASION THROUGH A PATCHY ENVIRONMENT - THE CYNIPID GALLWASP ANDRICUS-QUERCUSCALICIS (HYMENOPTERA, CYNIPIDAE)

Over the last 300-400 years, the cynipid gallwasp Andricus quercuscali cis has invaded northern and western Europe following human introducti on of an obligate host plant, the Turkey oak (Quercus cerris) from sou th-eastern Europe. In the introduced range, distances between Turkey o ak patches are greater and the numbers of oaks in each patch are far l ower than in its native range. These changes in spatial distribution o f Turkey oak are predicted to result in high genetic subdivision of A. quercuscalicis in invaded areas relative to its native range. Allozym e electrophoresis was used to examine genetic variation in 823 gall wa sps from 39 populations of A. quercuscalicis. No new electrophoretic v ariants were found in the invaded range and both allelic diversity and mean heterozygosity decreased significantly with distance from the na tive range. Spatial autocorrelation analysis and values of Wright's F( st) indicate that differences in allele frequences between populations were substantially greater in the invaded range than in the native ra nge. Spatial autocorrelation analysis also suggests that changes in al lele frequencies across Europe are unlikely to be the results of selec tion, but rather of strong directional migration followed by limited g ene flow between populations. Patterns of genetic differentiation acro ss Europe suggest that populations of A. quercuscalicis have been foun ded sequentially from the east through a process of random genetic sub sampling and not by source-sink colonization directly from the native range.