With 16 million people affected worldwide, mainly in developing countries, leishmaniosis (also called leishmaniasis) is a major public health problem. It is, however, a neglected disease, from the point of view of both treatment and research effort spent on it. In particular, little is yet known in biological terms about the parasites responsible, called Leishmania. How do they reproduce? How do they evolve and adapt to their environment and hosts, and to drugs, and so on? A recent study by IRD lifts the veil on their complex biology.
A range of environments, hosts and forms of infection
Leishmania parasites live in an extremely diverse range of ecosystems. There are more than twenty species, able to infect a broad range of wild and domesticated mammals. The majority of infections in humans are asymptomatic. When the disease does present, it does so in three different forms, namely cutaneous, mucocutaneous or visceral, being the most serious. Genetically, biologically, ecologically and epidemiologically, Leishmania are consequently complex parasites about which researchers still have much to learn.
A "mixed" reproduction method
The reproduction strategy of Leishmania parasites is the subject of much debate. For a long time, scientists thought they were asexual, in other words, that they reproduced clonally, with occasional genetic recombination. However, as IRD researchers have just revealed, matters are not as simple as that in reality. Statistical analysis of the genetic data available actually shows that the genome of different species of the Leishmania genus bears the hallmarks of sexual reproduction.
DNA samples of parasites from Europe, Asia, Africa and Latin America in fact show genetic mixing by sexual recombination, usually between related individuals, hence the high consanguinity. More precisely, biologists deduce from this that a feature of Leishmania parasites is a "mixed" reproduction method, alternating between clonal and sexual reproduction. It seems, however, that depending on the ecosystem and the Leishmania species involved, these parasites use one or other method to a greater degree.
Widely heterogeneous populations
The discovery of this "mixed" method of reproduction in Leishmania is a major step forward for the genetics of these parasites' populations. The reproduction method is in fact an influence on the distribution of genetic information within populations. This data then makes it possible to deduce the structure and breakdown of natural populations, in particular for organisms that are difficult to observe, such as parasites. During this study, scientists accordingly noted that naturally-occurring Leishmania populations are structured into small sub-populations, where genetic evolution is rapid. Hence, the widely heterogeneous parasite populations in any given environment.
Although some aspects of these organisms and their method of natural evolution remain obscured, this work shed new light on genetic studies of Leishmania populations. Understanding the precise role of clonal or sexual reproduction, somewhat consanguineous, will make it possible to understand how these parasites evolve, adapt to their environment and their hosts, and how they develop resistance to treatment.
Cite This Page: