July 2, 2004 The developing AIDS epidemic over the past 20 years has renewed interest for studying the parasitic fungus, Pneumocystis, responsible for an opportunist mycosis which infects humans. This micro-organism is usually non-pathogenic yet is present in the lungs of most Mammals including humans. In immunodepressed individuals it causes a kind of pneumonia which can prove fatal if not treated. IRD researchers working with other scientific institutes (1) used molecular data to assess the evolutionary phylogenetic relationships of different species of pneumocyst sampled from Primates. Comparison of respective pneumocyst phylogenies and their hosts suggested that they evolved alongside each other over geologic time. This contradicted the hypothesis suggesting that the pneumocystosis stemmed from the emergence of a new pathogenic agent. This investigation brought clues to the epidemiology of a serious parasitic disease. It opens up new avenues for research on Mammalian phylogeny.
The progressing AIDS epidemic has prompted a revival of interest in pneumocytosis, a respiratory infection which appears only in immunodepressed humans and can be fatal if treatment is not given. The agent responsible is a microscopic fungus, albeit usually non-pathogenic, which lives in the pulmonary alveoli. Searching for possible sources of human contamination in animal reservoirs proved negative: only the species specific for humans, Pneumocystis jirovecii, can trigger pneumonia in humans. This work has at the same time revealed the existence of numerous other species of pneumocysts in Mammals: primates, domestic animals, rodents, marsupials, and so on. Attempts at cross-infestation between hosts of different species have confirmed the high degree of pneumocyst host-specificity: each animal species harbours a different pneumocyst. Preliminary investigations on hereditary relationships between pneumocysts, using molecular data (from mitochondrial or nuclear DNA sequence comparison), suggested that this specificity would result from a process of strict co-evolution between host and parasite.
Reconstitution of the evolutionary relationship between pneumocysts and consequent verification of this hypothesis was conducted by the IRD researchers. Working with other institutes (1), they compared the sequences of three genes from samples taken from the respiratory passages or lung tissue of about 20 Primate species. The relationships between the micro-organisms were then compared with those of their hosts (2). The phylogenetic trees obtained appeared very similar, the pneumocyst phylogeny following mirror-like that of the Primates. The few differences observed appeared at the branching-off of pneumocysts of three South-American monkeys: the saki (Pithecia pithecia), the squirrel monkey (Saimiri sciureus) and the owl monkey (Aotus nancymai). Previous work on the phylogeny of these monkeys shows a comparable uncertainty concerning the evolutionary relationship of these three species. Thus, the pneumocyst phylogeny is similar to that of its hosts, even when it is uncertain.
The finding of a co-phylogeny situation confirms the hypothesis of a strict co-evolution between the host and its parasite over geologic time. Pneumocystosis is probably not therefore linked to the emergence of a new pathological agent. It is caused by a symbiotic opportunist micro-organism, which only becomes pathological when the host's immune defences decline. Beyond its interest for understanding the epidemiology of a severe parasitic disease linked to the development of AIDS, this investigation suggests that the pneumocysts, which are genetically independent organisms, may provide taxonomists with a series of new markers for elucidating phylogenetic relationships between Mammals. The value of these markers is increased by the fact that they have not been subjected to pressures from the external environment, seeing that the pneumocysts live as symbionts in their hosts' lungs.
New investigations will show if this new model can be adapted to Mammalian phylogenetic research as a whole.
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