1. Divisions of Parasitology, National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, UK
2. Divisions of Neurophysiology and Neuropharmacology, National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, UK
3. Department of Infection and Tropical Medicine, Imperial College School of Medicine, Northwick Park Hospital, Lister Unit, Harrow, Middlesex HA1 3UJ, UK
4. Present address: INSERM U442, Universite Paris-Sud, Bat. 443, Orsay 91405, France.

Correspondence to: P. R. Preiser¹ Correspondence and requests for material should be addressed to P.R.P. (e-mail: Email: ppreise@nimr.mrc.ac.uk).

Abstract

The recognition and invasion of host cells are mediated by components of the apical complex of the ookinete, sporozoite and merozoite stages of Plasmodium parasites¹. The paired rhoptries (organelles involved in host-cell recognition) in the apical complex contain many proteins of as-yet unknown function. In the rodent malaria agent P. yoelii yoelii, a multigene family codes for merozoite rhoptry proteins of relative molecular mass 235,000 (p235 proteins)^{2, 3}; these proteins are thought to determine the subset of erythrocytes that the parasites invade^{4, 5}. Further support for this idea came from the identification of a region in p235 with weak but significant homology to reticulocyte-binding protein-2 of P. vivax⁶,⁸ and the demonstration that at least one p235 member binds to the erythrocyte surface membrane⁹. Here, using single, micromanipulated P.y.yoelii parasites, we describe a new mechanism of gene expression by which the merozoites originating from a single schizont each express a distinct member of this multigene family. We propose that this new type of clonal phenotypic variation provides the parasite with a survival strategy in the mammalian host; this strategy contributes to the observed chronicity of malarial infections. This phenomenon is genetically and functionally distinct from classical antigenic variation, which is mediated by the var multigene family of P. falciparum^{10, 11, 12, 13}.

1. Divisions of Parasitology, National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, UK
2. Divisions of Neurophysiology and Neuropharmacology, National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, UK
3. Department of Infection and Tropical Medicine, Imperial College School of Medicine, Northwick Park Hospital, Lister Unit, Harrow, Middlesex HA1 3UJ, UK
4. Present address: INSERM U442, Universite Paris-Sud, Bat. 443, Orsay 91405, France.

Correspondence to: P. R. Preiser¹ Correspondence and requests for material should be addressed to P.R.P. (e-mail: Email: ppreise@nimr.mrc.ac.uk).