Open Access Methodology

PCR-based karyotyping of Anopheles gambiae inversion 2Rj identifies the BAMAKO chromosomal form

Mamadou B Coulibaly12, Marco Pombi3, Beniamino Caputo3, Davis Nwakanma4, Musa Jawara4, Lassana Konate5, Ibrahima Dia6, Abdrahamane Fofana2, Marcia Kern1, Frédéric Simard78, David J Conway4, Vincenzo Petrarca9, Alessandra della Torre3, Sékou Traoré2 and Nora J Besansky1*

Author Affiliations

1 Center for Global Health and Infectious Diseases, Department of Biological Sciences, University of Notre Dame, Notre Dame, IN, USA

2 Malaria Research and Training Center, University of Bamako, Bamako, Mali

3 Istituto Pasteur-Fondazione Cenci Bolognetti and Dipartimento di Scienze di Sanità Pubblica, Università di Roma "La Sapienza", Rome, Italy

4 Medical Research Council Laboratories, Fajara, Banjul, The Gambia

5 Département de Biologie Animale, Faculté des Sciences et Techniques, Université de Dakar, Senegal

6 Medical Entomology Unit, Pasteur Institute, Dakar, Senegal

7 Organisation de Coordination pour la Lutte contre les Endémies en Afrique Centrale, Yaoundé, Cameroon

8 Institut de Recherche pour le Développement, Unité de Recherche 016, Yaoundé, Cameroon

9 Istituto Pasteur-Fondazione Cenci Bolognetti and Dipartimento di Genetica e Biologia Molecolare, Università di Roma "La Sapienza", Rome, Italy

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Malaria Journal 2007, 6:133  doi:10.1186/1475-2875-6-133

Published: 1 October 2007

Abstract

Background

The malaria vector Anopheles gambiae is polymorphic for chromosomal inversions on the right arm of chromosome 2 that segregate nonrandomly between assortatively mating populations in West Africa. One such inversion, 2Rj, is associated with the BAMAKO chromosomal form endemic to southern Mali and northern Guinea Conakry near the Niger River. Although it exploits a unique ecology and both molecular and chromosomal data suggest reduced gene flow between BAMAKO and other A. gambiae populations, no molecular markers exist to identify this form.

Methods

To facilitate study of the BAMAKO form, a PCR assay for molecular karyotyping of 2Rj was developed based on sequences at the breakpoint junctions. The assay was extensively validated using more than 700 field specimens whose karyotypes were determined in parallel by cytogenetic and molecular methods. As inversion 2Rj also occurs in SAVANNA populations outside the geographic range of BAMAKO, samples were tested from Senegal, Cameroon and western Guinea Conakry as well as from Mali.

Results

In southern Mali, where 2Rj polymorphism in SAVANNA populations was very low and most of the 2Rj homozygotes were found in BAMAKO karyotypes, the molecular and cytogenetic methods were almost perfectly congruent. Elsewhere agreement between the methods was much poorer, as the molecular assay frequently misclassified 2Rj heterozygotes as 2R+j standard homozygotes.

Conclusion

Molecular karyotyping of 2Rj is robust and accurate on 2R+j standard and 2Rj inverted homozygotes. Therefore, the proposed approach overcomes the lack of a rapid tool for identifying the BAMAKO form across developmental stages and sexes, and opens new perspectives for the study of BAMAKO ecology and behaviour. On the other hand, the method should not be applied for molecular karyotyping of j-carriers within the SAVANNA chromosomal form.