Genetic recombination in field populations of Plasmodium falciparum

Malaria parasites undergo a mainly haploid life-cycle. The only diploid stage is the zygote, formed by fusion of gametes in the mosquito stomach. The first division of the zygote is a meiotic one, producing, after further mitotic divisions, haploid sporozoites. Genetic recombination occurs at meiosis, following cross-fertilization of gametes of parasites with different genotypes. This has been shown in laboratory studies by feeding mosquitoes on a mixture of Plasmodium falciparum clones and analyzing the resulting progeny for parasites with non-parental combinations of the clone markers. Such recombinants are produced at a higher than expected frequency. There is considerable genotype diversity in field populations of P. falciparum. Evidence that recombination in mosquitoes is the principal cause of this diversity is two-fold. First, parasites isolated from patients in small isolated communities at the same time are genetically very diverse. No two isolates examined for polymorphic markers at some 20 loci have been found to possess identical combinations of the allelic variants of these genes. Second, examination of oocysts in wild-caught mosquitoes by the PCR technique has shown that a high proportion are heterozygotes. There is thus frequent crossing in natural populations of this parasite. In addition to recombination at meiosis, it is also clear that genetic changes can occur during asexual multiplication of P. falciparum blood forms, as shown by deletions of regions of certain chromosomes during in vitro culture. The extent to which this occurs in nature is not known