Endosymbiosis of Beta-Proteobacteria in Trypanosomatid Protozoa

Concluding Remarks

The endosymbiont-bearing trypanosomatids represent a useful model to study prokaryote-eukaryote interactions for integration relevant to the origin of cell organelles. Protozoa of the Trypanosomatidae family themselves live mostly in association with other animals or plants. The interrelationships of these associations with the origin of the trypanosomatid endosymbionts are unknown. Many digenetic trypanosomatid species are well-known and have been extensively studied as the disease-causing agents. Interestingly, only monogenetic non-pathogenic species, which have one host in their life cycle, are naturally infected with prokaryotic endosymbionts. Their phylogenetic affiliation with beta-Proteobacteria is unique. With few exceptions, other intracellular symbiotic and pathogenic bacteria mostly belong to the alpha-Proteobacteria. The trypanosomatid endosymbionts are each enclosed by two “unit” membranes, but apparently with a degenerative cell wall. Each protozoan usually contains only one symbiont, resulting apparently from their synchronous division. Integration of the symbionts into the physiology of the hosts is thus indicated. However, the symbiont-containing protozoa can be rendered permanently symbiont-free by antibiotic treatment. Biochemical and nutritional comparisons of the symbiont-containing and aposymbiotic strains are thus made possible. It is clear from such studies that the symbiont supplies its host with essential nutrients, i. e. heme, amino acids and vitamins, while the symbiont probably obtains ATP from the host in return. Interestingly, some symbiont-free and symbiont-containing flagellates differ in their externally exposed saccharide residues, surface charges and other cytological features. Characterization of the symbiont genome is crucial for further elucidation of symbiont-host interrelationships in this system by molecular genetic approach.