It took a year of work and required North America’s largest colony of lab-reared sand flies, but NIAID researcher David Sacks, Ph.D., and his colleagues finally proved something they long suspected: the parasites that cause leishmaniasis have sex. The finding is important because scientists may now be able to use hybrid parasites to pinpoint genes responsible for a severe form of leishmaniasis or other traits of interest.
Dr. Sacks and his team worked with NIAID grantee Stephen Beverley, Ph.D., and others from Washington University School of Medicine in St. Louis. Dr. Beverley created two strains of Leishmania. One strain carried a gene conferring resistance to the antibiotic hygromycin. The other contained a gene that made it resistant to a different antibiotic, nourseothricin. The drug-resistance genes were located on different chromosomes.
Next, Dr. Sacks and his colleagues tested whether hybrid parasites could be transmitted to mammals. They raised sand flies that were co-infected with both parasite strains and allowed the flies to bite mice. The mice developed skin sores, or lesions, typical of leishmaniasis infection. Within the lesions, the investigators found doubly drug-resistant parasites that were genetically similar to the hybrid parasites extracted directly from sand fly midguts. In contrast, when the scientists injected a mixture of both parasite strains directly into mouse ears, they were never able to find any doubly drug-resistant parasites in the resulting lesions. This strongly suggests that doubly drug-resistant parasites are produced through sexual reproduction inside their insect host but the parasites do not recombine sexually in the mouse.
Leishmania sex appears to be very rare both in nature and in the lab, says Dr. Sacks. However, genetic hybrids do arise often enough to act as windows into the spectrum of disease caused by the parasites. For example, Dr. Sacks and his colleagues observed that some hybrids produced lesions in mice that grew at least as fast as the fast-growing neurseothricin-resistant parent strain, while others produced lesions that grew as slowly as the slow-growing hygromycin-resistant parent strain. Thus, the genes controlling growth rate, which can be used as a surrogate marker of disease virulence, appear to be distributed differently in different hybrids. The genetic hybrids can therefore be used to map the locations of the genes responsible for growth rate or other measurable traits, including drug resistance.
Akopyants NA et al. Demonstration of genetic exchange during cyclical development of Leishmania in the sand fly vector. Science DOI: 10.1126/Science.1169464 (2009).
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Last Updated February 09, 2011