Studies on hybridization, gene flow and resistance status of M and S molecular forms of Anopheles Gambiae S.S

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JUNE 2015
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Anopheles gambiae s.s is the most efficient malaria vector in Africa, particularly in sub-Sahara African countries. An. gambiae s.s is composed of two sibling species differentiated by the chromosomal form and named M and S molecular forms. New advances in complete mapping of the distribution of the vector have recently encountered the gene flow between them. Though the two forms have just been considered as different species and named An. coluzzii and An. gambiae s.s for M and S form respectively, yet M/S hybrid specimen have been found in some West Africa countries. This study was designed to investigate in the laboratory the hybridization between the two species, the resistance mechanisms of the hybrid mosquitoes and its probable impact on the vector control management. Three different mosquito colonies, including an An. gambiae s.s M form resistant to pyrethroid and DDT, An. gambiae s.s S form resistant to all classes of insecticides and An. gambiae s.s S form susceptible to all insecticides were used to establish a hybrid mosquito colony. A slight mass crossing was done by inversely crossing 10 males and 10 females of each form. Resistance status and mechanisms of the progenies were determined using WHO susceptibility test and PCR methods respectively. An average of 50 males and 50 females were analyzed per crossing and per generation. The results showed that 100% of the females of the first generation of each crossing was M/S hybrid irrespective of the origin of the parent. Also all the males of each crossing inherited the female parent form, but none was found to be hybrid. Furthermore, all the hybrids showed similar DNA sequences whatever the crossing and the generation analyzed. In addition the progeny of the two resistant strains crossed recorded 66% and 80% kdr frequencies according to the form of male or female and higher than the resistant and susceptible hybrid progeny, yielding 50% kdr frequency. The ace-1 genotype was about 50% for all the hybrid progenies with a slight increment for the female M and male S forms progeny (52%). But no significant difference was observed among the ace-1 frequencies of the three M/S hybrids progenies. This study confirmed the hybridization of the M and S molecular form of An. gambiae s.s as reported, and also showed the resistance mechanisms of the hybrids according to the parent resistance status. However, additional investigations need to be undertaken to fully characterize the M/S resistance mechanisms and furthermore, their susceptibility to malaria parasites
A Thesis submitted to the Department of Theoretical and Applied Biology, Kwame Nkrumah University of Science and Technology in partial fulfillment of the requirements for the award of the Degree of Master of Philosophy.