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Evidence for introgression between Anopheles gambiae and An. arabiensis has accumulated for some time. We examined the fate of introgressed DNA directly, using microsatellite markers located throughout the genome. Introgressed X chromosomes were removed within two generations. Furthermore, substantial differences in introgressive capacity between the two autosomes were found. After introgression from An. arabiensis into An. gambiae, most introgressed alleles at third chromosome markers, particularly those on 3R, decreased steadily, indicating selection against them. No such pattern was observed for 2L markers and several 2R markers. The frequency of introgressed alleles on 2L were close to the original frequency even after 19 generations, whereas only two 2R markers showed a modest decrease. Even though limited information was available on the reciprocal cross, the pattern appears to be identical. Although the decrease in frequency of the introgressed X chromosome can be attributed to the presence of sterility and inviability effects, the variation in introgressive capacity of the autosomes does not appear to be explained by the presence of inversion polymorphisms, or regions causing hybrid sterility and inviability. These results can have some important implications for the spread of insecticide resistance and the control of these vector populations via the release of transgenic mosquitoes.
Received October 7, 2004. Accepted for publication February 17, 2005.
Acknowledgments: We thank M. Coluzzi for general support of the work, and G. Petrangeli for instrumental technical support. We also thank two anonymous reviewers, F. Tripet, and D. A. Elnaiem for constructive comments on the manuscript.
Financial support: This study was supported by National Institutes of Health grant R01 46018. Michel A. Slotman was also supported by the Centers for Disease Control and Prevention Fellowship Training Program inVector-Borne Infectious Disease (T01/CCT122306). Alessandra della Torre was supported by the United Nations Development Program for Research and Training in Tropical Diseases (Tropical Disease Research) and the Ministro dell’Istruzione, Dell’ Università e della Ricerca/COFIN.
* Address correspondence to Dr. Michel A. Slotman, Department of Entomology, University of California, 1 Shields Avenue, Davis, CA 95616. E-mail: maslotman{at}ucdavis.edu
Authors’ addresses; Michel A. Slotman, Department of Entomology, University of California, 1 Shields Avenue, Davis CA 95616, Telephone: 530-752-7333, Fax: 530-752-1537, E-mail: maslotman{at}ucdavis.edu. Alessandra della Torre and Maria Calzetta, Istituto di Parassitologia, Fondazione Pasteur-Cenci Bolognetti, Università di Roma La Sapienza, P.le A. Moro 5, 00185 Rome, Italy. Jeffrey R. Powell, Department of Ecology and Evolutionary Biology, Yale University, 165 Prospect Street, New Haven, CT 06511.
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