Secondary loss of chloroplasts in trypanosomes

On page 1067 in this issue of PNAS, Hannaert et al. (1) present evidence for the idea that trypanosomes once possessed a chloroplast that they lost some time in their distant evolutionary past. It would probably be going overboard to call trypanosomes algae in disguise, but for a protozoan parasite without any trace of chloroplast remnants, the trypanosome is now shown to possess a remarkable number of enzymes of plant-related origin. This finding advances our understanding of trypanosome biology and could even help efforts to discover drugs to treat trypanosomatid-caused diseases.

Typanosomatids are unicellular eukaryotes belonging to the order of Kinetoplastidae; they are among the most versatile parasites in nature, infecting mammals, fish, and plants, and are usually transmitted by insect vectors. Major human diseases caused by trypanosomatids are leishmaniases (Leishmania species), sleeping sickness (Trypanosoma brucei variants), and Chagas' disease (Trypanosoma cruzi). Drugs for treating these diseases are few and toxic, and resistance is on the rise (2, 3). Because these diseases predominantly affect poor people in (sub)tropical developing countries, development of new drugs is not a high priority of the pharmaceutical industry. Yet academic labs are hunting for new drug targets, so the discovery of plant-like enzymes in T. brucei is good news indeed.

The evidence for secondary loss of plastids in trypanosomes stems from the genome sequencing programs that are now well underway for Leishmania, T. brucei, and T. cruzi.§ Hints for the presence of plant- and plastid-derived genes in trypanosomatids came from analyses of two enzymes involved in the hexose phosphate shunt (4) and of trypanosomatid peroxidases (5). Hannaert et al. (1) now report a substantial number of plant-like trypanosomatid genes, most of which are involved in core carbohydrate metabolism. For many of these genes, the sequence comparisons reveal a specific evolutionary …