Do malaria parasites mate non-randomly in the mosquito midgut?

Polymerase chain reaction (PCR)-based genotyping of oocysts dissected from mosquito midguts has previously been used to investigate overall levels of inbreeding within malaria parasite populations. We present a re-analysis of the population structure of Plasmodium? falciparum malaria using diploid g enotypes at three antigen-encoding loci in 118 oocysts dissected from 34 mo squitoes. We use these data to ask whether mating is occurring at random wi thin the mosquito midgut. as is generally assumed. We observe a highly sign ificant deficit of heterozygous oocysts within mosquitoes at all three loci , suggesting that fusion of gametes occurs non-randomly in the mosquito gut . A variety of biological explanations, such as interrupted feeding of mosq uitoes, positive assortative mating and outcrossing depression, could accou nt for this observation. However. an alternative artefactual explanation - the presence of non-amplifying or null alleles - can account for the observ ed data equally well, without the need to invoke non-random mating. To eval uate this explanation further, we estimate the frequencies of null alleles within the oocyst population using maximum likelihood, by making the assump tion that non-amplifying oocysts at any of the three loci are homozygous fo r null alleles. Observed levels of visible heterozygotes fit closely with t hose expected under random mating when non-amplifying oocysts are accounted for. Other lines of evidence also support the artefactual explanation. Ove rall inbreeding coefficients have been recalculated in the light of this an alysis, and may be considerably lower than those estimated previously. In c onclusion, we suggest that the deficit of heterozygotes observed is unlikel y to indicate non-random mating within the mosquito gut and is better expla ined by misscoring of heterozygotes as homozygotes.