Discovery clears way to find genes that help Leishmania cause disease

The importance of this scientific research lies in the ability of organisms to cross, in the mechanisms they use to do this and in the diversity of evolution


Dr. Stephen Beverley co-authored the study that found that Leishmania, a unicellular parasite, is capable of reproducing sexually

The unicellular parasite Leishmania can reproduce sexually. This is revealed by the article Whole genome sequencing of experimental hybrids supports meiosis-like sexual recombination in Leishmania. The discovery may pave the way for finding genes that help the parasite cause disease. By studying the hybrid parasites and their recombined progeny, researchers will be able to map the location on chromosomes of genes involved in causing disease and resisting the immune response. This genetic map will help to understand why some strains cause a disease worse than others and how to strengthen the immune response against the parasites. To learn more about the subject, the Brazilian Society of Tropical Medicine (BSTM) communication advisory interviewed Dr. Stephen M. Beverley, co-author of the article published in May at PLOS Genetics.

Find the full interview below:

BSTM: What is the relevance of your finding?

Dr. Stephen M. Beverley: The ability of organisms to mate, the mechanisms they use to do this, and the diversity of all of these in evolution, are a very popular topic of scientific investigation.

For pathogens as well as other organisms, mating provides the opportunity to generate new genotypes that may prove more fit in evolution.

However, mating can also break up gene combinations that were quite successive. This is where microbes have a unique position since they have the option of usually propagating without mating or sex.

BSTM: How did you conduct the test to know whether the hybrid parasites were fertile?

Dr. Stephen M. Beverley: We took a hybrid of parasite-bearing cells, and mixed them with either one of the original parental lines (back cross) or a different WT line (outcross). The lines used in these crosses bore to 3rd independent drug resistance marker.

Since mating has only been described to occur during the parasite stages that occur in the vector sect, the mixture was added to blood and then fed to sand flies through a thin membrane.

The flies were maintained long enough for the parasite developmental cycle within the fly to take place (2 weeks or less), and then the midguts were dissected and the parasites placed in culture.

At that time drug selections were applied to recover potential mating progeny – in the above experiments, it would be one of the markers from the hybrid, and one for the marker in the out or back cross.

If mating occurred, doubly drug resistant parasites would grow out, and be further characterized.

BSTM: Could the reproductive cycle influence virulence?

Dr. Stephen M. Beverley: Yes definitely. However, as noted above mating can both create better adaptive combinations, but also break up ones. this is where microbes have an advantage as successful combinations can be propagated without sex.

BSTM: How will it be possible to know if one strain causes the mild form of the disease and another causes the lethal form?

Dr. Stephen M. Beverley: We can use the information from the patient to predict this; more commonly we use various animal models of human disease and follow what happens to parasites in these.

BSTM: How will the genetic map be built?

Dr. Stephen M. Beverley: One first has a physical map which these days most commonly is the genome sequence. As there are polymorphisms amongst the strains or species we use, we can map these onto the genome to generate markers.   Then, we ask after mating how often the markers are inherited together, or separately. The more often they are cosegregate, the closer they are genetically. We then map the genetic distances onto the physical distances, to create a genetic map.

BSTM: What is expected from the genetic map?

Dr. Stephen M. Beverley: Potentially we can use this to map virulence traits when we acquire more hybrids and phenotypes.

BSTM: Why do hybrid parasites rarely mate?

Dr. Stephen M. Beverley: This is something we are trying to understand. In other organisms mating is controlled by mating type systems which act first to promote diversity by preventing self-mating, and perhaps that is what is happening in Leishmania. The other possibility is that there may be some advantage to preventing mating from occurring too frequently, so as to avoid breaking up good gene combinations.

BSTM: Would you like to add something?

Dr. Stephen M. Beverley: I would like to stress that this work is the fruit of a wonderful, long term collaboration with my colleague David Sacks at the NIH. David has been a pioneer in studying the biology of the parasite and insect vector hosts, and has invested tremendous work over the years to the point that he is really the leader. It has been our pleasure to combine his expertise with ours in parasite genetics very productively.

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