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We analyzed 725 Ixodes ricinus ticks (the principal vector of
Lyme disease in Europe) collected in Switzerland in 1995 and 1996 (three and eight samples, respectively) and in Tunisia in 1996 (one sample) with five microsatellite markers. We found highly significant genetic differentiation between Swiss and Tunisian samples but detected almost no differentiation within Switzerland, even between those samples separated by the Alps. Interestingly, we found that I. ricinus females were more genetically related to one another than were males at a local scale, which would indicate a higher dispersal rate of immature males. Possible explanations for these findings in terms of sex-specific association of ticks with certain hosts (e.g., birds) and their epidemiological consequences are discussed.