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Gametocyte sex ratios as indirect measures of outcrossing rates in malaria

Published online by Cambridge University Press:  06 April 2009

A. F. Read ,
A. Narara ,
S. Nee ,
A. E. Keymer  and
K. P. Day
A. F. Read
Affiliation:
Zoology Department, University of Oxford, South Parks Road, Oxford OX1 3PS, U.K.
A. Narara
Affiliation:
Papua New Guinea Institute of Medical Research, P.O. Box 378, Madang, Papua New Guinea
S. Nee
Affiliation:
AFRC Unit of Ecology and Behaviour, Zoology Department, University of Oxford, South Parks Road, Oxford OX1 3PS, U.K.
A. E. Keymer
Affiliation:
Zoology Department, University of Oxford, South Parks Road, Oxford OX1 3PS, U.K.
K. P. Day
Affiliation:
Walter and Eliza Hall Institute of Medical Research, Victoria 3050, Australia
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Extract

The frequency of recombination between unlike genotypes is central to understanding the generation of genetic diversity in natural populations of malaria. Here we suggest a way of investigating the problem which could complement conventional biochemical approaches to the population genetics of malaria. Sex allocation theory is one of the most successful areas of evolutionary biology. A well-supported prediction is that progressively less female-biased sex ratios are favoured with more outcrossing; equal numbers of males and females being evolutionarily stable in randomly mating outbred populations. We present a simple game theory model to support the idea that outcrossing rates in malaria will be correlated with the sex ratio of gametocytes in the peripheral blood of vertebrate hosts. Blood films from epidemiological surveys and culture-adapted isolates from Madang Province, Papua New Guinea, were used to estimate average gametocyte sex ratio of Plasmodium falciparum in the area. The geometric mean proportion of males in the population was 0.18 (95% confidence limits: 0.15–0.22). From our model, we estimate that, on average, 36% of zygotes are the result of outcrossing. This estimate assumes that most microgametes released following exflagellation are capable of fertilization. If, on average, fewer than about 70% of microgametes are capable of fertilization (as is the case in at least one other species of Plasmodium), the observed sex ratio would be consistent with between zero and 36% of zygotes being the result of outcrossing. These estimates suggest that there is usually a numerically dominant genotype in the gametocyte population in a blood meal, and that a considerable amount of selfing is occurring in P. falciparum populations in the Madang region, even though it is an area of intense year-round transmission.

Keywords

sex allocation theorylocal mate competitioninbreedingPlasmodium falciparumPapua New Guinea
Type
Research Article
Information
Parasitology , Volume 104 , Issue 3 , June 1992 , pp. 387 - 395
Copyright
Copyright © Cambridge University Press 1992

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