Complexities in the genetic structure of Anopheles gambiae populations in west Africa as revealed by microsatellite DNA analysis

Chromosomal forms of Anopheles gambiae, given the informal designations Bamako, Mopti, and Savannah, have been recognized by the presence or absence of four paracentric inversions on chromosome 2. Studies of karyotype frequencies at sites where the forms occur in sympatry have led to the suggestion that these forms represent species. We conducted a study of the genetic structure of populations of An. gambiae from two villages in Mali, west Africa. Populations at each site were composed of the Bamako and Mopti forms and the sibling species, Anopheles arabiensis. Karyotypes were determined for each individual mosquito and genotypes at 21 microsatellite loci determined. A number of the microsatellites have been physically mapped to polytene chromosomes, making it possible to select loci based on their position relative to the inversions used to define forms. We found that the chromosomal forms differ at all loci on chromosome 2, but there were few differences for loci on other chromosomes. Geographic variation was small. Gene flow appears to vary among different regions within the genome, being lowest on chromosome 2, probably due to hitchhiking with the inversions. We conclude that the majority of observed genetic divergence between chromosomal forms can be explained by forces that need not involve reproductive isolation, although reproductive isolation is not ruled out. We found low levels of gene flow between the sibling species Anopheles gambiae and Anopheles arabiensis, similar to estimates based on observed frequencies of hybrid karyotypes in natural populations.

Variations in frequencies of drug resistance in Plasmodium falciparum

Continual exposure of malarial parasite populations to different drugs may have selected not only for resistance to individual drugs but also for genetic traits that favor initiation of resistance to novel unrelated antimalarials. To test this hypothesis, different Plasmodium falciparum clones having varying numbers of preexisting resistance mechanisms were treated with two new antimalarial agents: 5-fluoroorotate and atovaquone. All parasite populations were equally susceptible in small numbers. However, when large populations of these clones were challenged with either of the two compounds, significant variations in frequencies of resistance became apparent. On one extreme, clone D6 from West Africa, which was sensitive to all traditional antimalarial agents, failed to develop resistance under simple nonmutagenic conditions in vitro. In sharp contrast, the Indochina clone W2, which was known to be resistant to all traditional antimalarial drugs, independently acquired resistance to both new compounds as much as a 1,000 times more frequently than D6. Additional clones that were resistant to some (but not all) traditional antimalarial agents acquired resistance to atovaquone at high frequency, but not to 5-fluoroorotate. These findings were unexpected and surprising based on current views of the evolution of drug resistance in P. falciparum populations. Such new phenotypes, named accelerated resistance to multiple drugs (ARMD), raise important questions about the genetic and biochemical mechanisms related to the initiation of drug resistance in malarial parasites. Some potential mechanisms underlying ARMD phenotypes have public health implications that are ominous.