Saturday, April 23, 2011

Human malarial parasite came not from Chimps but Gorillas

London: The parasite that causes the deadliest form of malaria in humans - Plasmodium falciparum-was not transmitted by chimpanzees, but instead came from western gorillas. The researchers also concluded that the parasite may have made the jump between species just once.

They hope that understanding the parasite’s origins will help to inform medical strategies for tackling the disease. Until now, scientists believed P. falciparum ’s closest relative to be P. reichenowi, a parasite of chimpanzees (Pan troglodytes), but studies were limited to a few apes, many of them from captive populations. Whether wild populations were acting as natural reservoirs for Plasmodium species was not known.

The latest study, led by Beatrice Hahn of the University of Alabama at Birmingham, took in wild populations of chimpanzees, bonobos and gorillas from across sub-Saharan Africa to analyse the genes of ape parasites related to P. falciparum. The team used faecal samples from specimen banks built up to investigate the evolution of HIV, including 1,827 from chimpanzees, 803 from gorillas and 107 from bonobos.

They then sequenced the Plasmodium DNA found in the samples, looking particularly at DNA from mitochondria, the cells’ energy factories. They found high levels of malarial infection among chimpanzees and western gorillas (Gorilla gorilla), populations of which act as natural reservoirs for Plasmodium species, but no infections among eastern gorillas (Gorilla beringei) or bonobos (Pan paniscus).

The team used the mitochondrial DNA sequences to produce phylogenetic trees, which indicate relationships between organisms on the basis of DNA. The researchers’ analyses reveal that the apes were infected with at least nine species of Plasmodium, three of which are new to science. With one exception, the parasitic species were all very closely related, belonging to the subgenus Laverania, and were highly host-specific.

The P. falciparum samples from humans included in the study were most closely related to parasites that infected western gorillas in Cameroon, the Central African Republic and the Republic of the Congo, and is likely to have originated from a single transmission event.

Daniel Jeffares, an evolutionary biologist at University College London, describes the findings as “striking”. “In terms of our understanding of parasites, this paper is a game-changer,” Nature quoted him as saying. The study could help scientists to pinpoint the genetic changes that allowed the parasite to infect humans.

Jeffares says that it would be relatively simple and inexpensive to sample entire genomes of P. falciparum and its close relatives. “You could look throughout the whole genome and find out where rapid evolution has been taking place,” he said. The study has been published in Nature1.Source