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VARIATION IN RECOMBINATION RATE ACROSS THE X CHROMOSOME OF ANOPHELES GAMBIAE

MARCO POMBI, ARAM D. STUMP, ALESSANDRA DELLA TORRE, AND NORA J. BESANSKY^*

The M and S molecular forms of Anopheles gambiae are considered to be incipient species, despite residual gene exchange. Of the three small genome regions that are strongly differentiated between the molecular forms ("speciation islands"), two are located near centromeres, on the left arm of chromosome 2 and the X chromosome. To test the prediction of reduced recombination in these islands, we estimated recombination rates between microsatellite loci on the X chromosome using two M-form strains. Across most of the chromosome, recombination occurred at ~1 centimorgan per megabase (cM Mb^–1), a value closely matching the genome-wide average estimated for A. gambiae and for other eukaryotes. Recombination was much higher at the telomeric end, > 7 cM Mb^–1. In the speciation island at the centromeric end, recombination was sharply reduced to ~0.2 cM Mb^–1, consistent with a role for reduced recombination in maintaining differentiation between nascent species despite gene flow.

Received April 7, 2006. Accepted for publication July 18, 2006.

Acknowledgments: The authors thank Bryan Cassone for assistance with genotyping.

Financial support: This work was supported by the National Institutes of Health (AI63508).

^* Address correspondence to Nora J. Besansky, Center for Global Health and Infectious Diseases, Department of Biological Sciences, University of Notre Dame, Notre Dame, Indiana. E-mail: nbesansk{at}nd.edu

These authors contributed equally to this work.

Authors’ addresses: Marco Pombi and Alessandra della Torre, Sezione di Parassitologia, Dipartimento di Scienze di Sanità Pubblica, Università degli Studi di Roma "La Sapienza," P.le Aldo Moro 5, 00185, Roma, Italy, Telephone: +39-06-4991-4932, Fax: +39-06-4991-4653, E-mails: ale.dellatorre{at}uniroma1.it and marco.pombi{at}uniroma1.it. Aram D. Stump and Nora J. Besansky, Center for Global Health and Infectious Diseases, Department of Biological Sciences, Galvin Life Sciences Building, University of Notre Dame, Notre Dame, IN 46556-0369, Telephone: +1-574-631-9321, Fax: +1-574-631-3996, E-mail: nbesansk{at}nd.edu. Current address for Aram D. Stump: Department of Biology, Williams College, Williamstown, MA 01267, E-mail: Aram.D.Stump{at}williams.edu.