Tuesday, August 4, 2009

Source of a Scourge

We now know that humans are the hosts of five different species of malaria parasites. The most virulent of these is Plasmodium falciparum, which is thought to be responsible for close to two million deaths a year. Early molecular phylogenetic studies proposed that humans acquired this parasite from chickens, but subsequent studies showed that that result was an artifact of taxon sampling and paralogous genes and that the closest relative of P. falciparum is a chimpanzee parasite, named Plasmodium reichenowi, which was discovered in the 1920's. For decades, though, only a single strain of P. reichenowi existed in the lab or in any form from which DNA could be extracted and sequenced. Recently, eight new isolates of P. reichenowi were obtained from wild and wild-born chimps and the phylogenetic history of these lineages was studied and published online yesterday in PNAS by Steve Rich of U Mass Amherst and colleagues. These new results now show very nicely that P. falciparum represents a recent host shift from chimps into humans, which may have occurred as recently as 10,000 years ago. It has long been known that these two parasites cannot be reciprocally cross-reared, i.e., chimps cannot be infected with P. falciparum and humans cannot be infected with P. reichenowi (the discoverers of this species tried to give it to themselves!). Rich et al. cite another recent study that demonstrated a different preference for erythrocyte receptors in the two parasites. Perhaps our human ancestors acquired these mutations for receptors and enjoyed a period of being malaria-free, only to have a new - and deadlier - parasite evolve that preferred the human receptor form. Unfortunately, blood smears were not made from these new chimpanzee samples, so it's not yet possible to confirm that these samples share the same distinctive morphology of P. falciparum, but Rich has assured me that they're working on this. It would also be interesting to add the second "ape" parasite, Plasmodium schwetzi, from gorillas, a species that no one has samples of for DNA analysis. What I liked the best about this new study, though, is that these results mean that P. falciparum as a species should be sunk. Think the medical community could handle that?

1 comment:

sergios-orestis kolokotronis said...

And see a commentary by Varki & Gagneux in PNAS, and their 2005 PNAS paper where they suggested on the basis on protein binding properties that the MRCA of falciparum and reichenowi must have resembled to reichenowi.