Plasmodium Falciparum - Origins and Evolution

Origins and Evolution

The closest relative of Plasmodium falciparum is Plasmodium reichenowi, a parasite of chimpanzees. P. falciparum and P. reichenowi are not closely related to the other Plasmodium species that parasitize humans, or indeed mammals generally. It has been argued that these two species originated from a parasite of birds. More recent analyses do not support this, however, instead suggesting that the ability to parasitize mammals evolved only once within the genus Plasmodium.

New evidence based on analysis of more than 1,100 mitochondrial, apicoplastic, and nuclear DNA sequences has suggested that Plasmodium falciparum may in fact have speciated from a lineage present in gorillas.

According to this theory, P. falciparum and P. reichenowi may both represent host switches from an ancestral line that primarily infected gorillas; P. falciparum went on to primarily infect humans while P. reichenowi specialized in chimpanzees. The ongoing debate over the evolutionary origin of Plasmodium falciparum will likely be the focus of continuing genetic study.

A third species has been discovered that appears to related to these two: Plasmodium gaboni. This putative species is currently (2009) known only from two DNA sequences and awaits a full species description before it can be regarded as valid.

Molecular clock analyses suggest that P. falciparum is as old as the human line; the two species diverged at the same time as humans and chimpanzees. However, low levels of polymorphism within the P. falciparum genome suggest a much more recent origin. It may be that this discrepancy exists because P. falciparum is old, but its population recently underwent a great expansion. Some evidence still indicates that P. reichenowi was the ancestor of P. falciparum. The timing of this event is unclear at present but it has been proposed that it may have occurred about 10,000 years ago.

More recently, P. falciparum has evolved in response to human interventions. Most strains of malaria can be treated with chloroquine, but P. falciparum has developed resistance to this treatment. A combination of quinine and tetracycline has also been used, but there are strains of P. falciparum that have grown resistant to this treatment as well. Different strains of P. falciparum have grown resistant to different treatments. Often the resistance of the strain depends on where it was contracted. Many cases of malaria that come from parts of the Caribbean and west of the Panama Canal as well as the Middle East and Egypt can often be treated with chloroquine, since they have not yet developed resistance. Nearly all cases contracted in Africa, India, and southeast Asia have grown resistant to this medication and there have been cases in Thailand and Cambodia in which the strain has been resistant to nearly all treatments. Often the strain grows resistant to the treatment in areas where the use is not as tightly regulated.

Like most Apicomplexa, malaria parasites harbor a plastid similar to plant chloroplasts, which they probably acquired by engulfing (or being invaded by) a eukaryotic alga, and retaining the algal plastid as a distinctive organelle encased within four membranes (see endosymbiotic theory). The apicomplexan plastid, or apicoplast, is an essential organelle, thought to be involved in the synthesis of lipids and several other compounds, and provides an attractive target for antimalarial drug development, particularly in light of the emergence of parasites resistant to chloroquine and other existing antimalarial agents.