Nature 453, 609-614 (29 May 2008) | doi:10.1038/nature06954; Received 29 February 2008; Accepted 28 March 2008

Sex ratio adjustment and kin discrimination in malaria parasites

Sarah E. Reece1,2, Damien R. Drew2,3 & Andy Gardner1

  1. Institute of Evolutionary Biology,
  2. Institute of Immunology and Infection Research, Ashworth Laboratories, School of Biological Science, University of Edinburgh, West Mains Road, Edinburgh EH9 3JT, UK
  3. The Walter and Eliza Hall Institute of Medical Research Biotechnology Centre, Research Avenue, Bundoora, Victoria 3086, Australia

Correspondence to: Sarah E. Reece1,2 Correspondence and requests for materials should be addressed to S.E.R. (Email:


Malaria parasites and related Apicomplexans are the causative agents of the some of the most serious infectious diseases of humans, companion animals, livestock and wildlife. These parasites must undergo sexual reproduction to transmit from vertebrate hosts to vectors, and their sex ratios are consistently female-biased. Sex allocation theory, a cornerstone of evolutionary biology, is remarkably successful at explaining female-biased sex ratios in multicellular taxa, but has proved controversial when applied to malaria parasites. Here we show that, as predicted by theory, sex ratio is an important fitness-determining trait and Plasmodium chabaudi parasites adjust their sex allocation in response to the presence of unrelated conspecifics. This suggests that P. chabaudi parasites use kin discrimination to evaluate the genetic diversity of their infections, and they adjust their behaviour in response to environmental cues. Malaria parasites provide a novel way to test evolutionary theory, and support the generality and power of a darwinian approach.


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