TRANSACTIONS OF THE ROYAL SOCIETY OF TROFICALMEDICINE AND HYGIENE (1999) 93, SUPPLEMENT 1, Slil l-S1114 s1:11 The epidemiology of multiple Plasmodium falciparum infections 3. Genetic structure and dynamics of Plasmodium falciparum infections in the Kilombero region of Tanzania Hamza A. Babiker*, Lisa C. Ranford-Cartwright and David Walliker Institute of Cell, Animal and Population Biology, University of Edinburgh, West Mains Road, Edinburgh, EH9 3JT Scotland, UK Abstract Plasmodium falciparum parasites exist as genetically distinct haploid clones in infected people. In the Kil- ombero vallev in south-east Tanzania, at least 85% of the inhabitants of Michenga villane harbour more than one clone. Using 2 highly polymorphic unlinked markers, it has been estimated that each infected person harbours between one and 6 l?faZciparum clones at any one time, with a mean of 3.5 clones. When mosquitoes acquire gametocytes of 2 different clones in a blood meal, crossing generates recombinant clones differing from their parental genotypes. The inbreeding coefficient of the parasite population has been estimated as 0.33. Keywords: malaria, Plasmodium falciparum, multiple infection, genetics, inbreeding,Tanzania Introduction A knowledge of the population genetics of a pathogen is important for designing effective control measures. While information on the frequency of genes conferring, for example, resistance to a given drug or a vaccine in a given area has obvious relevance to the implementation of control measures using such agents, an important ad- ditional factor is an understanding of the breeding structure of the parasites. Cross-mating between para- sites allows clones with novel genotypes to be produced, bringing together genes which could additively generate phenotypes such as multiple drug-resistance. Until now, little information has been available on this sub- ject. The complexity of the malaria parasite’s life cycle makes it difficult to study its population structure and dynamics in natural conditions. Differences from one region to another can be expected depending on factors such as the endemicity of the disease, the seasonality of infection, entomological biting rate, etc. A critical first step is to obtain information on the genetic polymor- phisms of the parasites in both human and mosquito hosts in the community to be investigated. The village of Michenga in the Kilombero region of Tanzania is a very suitable community for such a study. It is in an area holoendemic for Plasmodium falciparum in which an in- dividual may receive over 300 infectious mosquito bites each year and where there is a stable parasite rate of more than 60% (SMITH et al., 1993). This means that parasites can easily be obtained from people and from mosquitoes for genetic characterization. In this paper, we review briefly some of the studies carried out in Michenga which address the questions of gene flow, ge- netic diversity and genetic recombination in this com- munity. Genetics of Plasmodium The malaria parasite is haploid in its human host, and an individual may be infected with a multiplicity of ge- netically diverse clones. The only diploid phase in the life cycle is the zygote (ookinete) in the mosquito mid- gut, produced by fertilization between male and female gametes. Meiosis occurs shortly after zygote formation, resulting in haploid sporozoites. Meiosis is critically im- portant for the generation of diversity in eukaryotes. It is at this stage that genetic recombination occurs, by re- assortment of genes on different chromosomes, as well as by crossing-over events between linked genes and, more rarely, within genes. In considering genetic events during mosquito trans- *Author for correspondence: phone +44 131 650 8658, fax +44 667 3210, e-mail <h.babiker@ed.ac.uk> mission, the gametes of a single haploid parasite clone taken up into the mosquito midgut will undergo self- mating to produce homozygous zygotes, containing identical alleles at all loci. Recombination during meio- sis of homozygotes does not have major genetic conse- quences, and the resulting haploid products will have the same genotype as the original clone. However, if gametocytes of 2 or more genetically distinct clones are taken up into the mosquito, crossing as well as selfing events among the gametes are expected. Cross-mating results in heterozygotes, and recombination during mei- osis of these forms will result in parasites with novel gene combinations among the haploid progeny (Fig- ure). Genetic recombination was first demonstrated in the laboratory in rodent malaria parasites by BEALE et al. (1978), and subsequently in I? falciparum by WALLIKER ab ab I HWKUl l+JTiL+ -1 Self-mating Cross-mating Self-mating 1 AABB AaBb . aabb Mosquito oocyit AR Ab aB ab Sporololtr Benolypcs I _I KecomblnanlP Figure. The process of genetic recombination in malaria para- sites when 2 clones undergo cross-mating in mosquitoes. A and a are alleles of one gene, and B and b are alleles of a second gene. One parent clone has genotype AB and the other ab; both produce gametocytes, capable of infecting mosquitoes, be- tween which mating can occur. Recombinants Ab and aB are produced following meiosis of diploid zygotes AaBb. et al. (1987). This work involved feeding mosquitoes on deliberate mixtures of 2 parasite clones to allow crossing to occur, infecting mammal hosts with the resulting sporozoites, and then characterizing clones derived from the resulting blood forms for the parent clone characters. In all such crosses, recombinant forms have invariably been isolated. More recently, the genetic con- stitution of individual oocysts derived from such mixed feeds has been examined, and homo- and heterozygous forms identified (RANFORD-CARTWRIGHT et al., 1993).