Trypanosomiasis: Parasite, plastid-related genes

Trypanosomes harbour numerous genes sharing apparent common ancestry with bacteria and/or plants. Many of these horizontally acquired genes seem to function in the glycosome. The glycosome is being studied as a possible therapeutic drug target. How did the trypanosomes acquire these "foreign" genes? Trypanosomes fall within the Phylum Euglenozoa, which includes the beautiful euglenid algae. These algae have chloroplasts, green plastids surrounded by three membranes. This is thought to reflect an engulfed eukaryotic algal endosymbiont similar to the apicoplast in Plasmodium parasites. Chloroplasts themselves are thought to have arisen form an endosymbiotic merger of a cyanobacterial prokaryote with an eukaryote. Although there is no evidence of a plastid in trypanosomes, the presence of such genes suggest that lateral gene transfer from some photosynthetic organism(s) occurred in the distant past. It is possible that the ancestors of the currect trypanosomes had plastids, but that they lost their passengers during evolution. There are known examples of plastid loss, e.g. in oomycetes and possibly in ciliates. Bodonids are free-living bacteriovorous kinetoplastids. Some kinetoplastids contain their own bacterial endosymbionts (e.g. Crithidia oncopelti). It is conceivable that independent endosymbionts could have been an alternative source for some of the "foreign" genes. If further phylogenetic analyses of lateral transferred genes consistently point to a single genetic source, the endosymbiont hypothesis will be supported. If the genes derive from multiple independent lineages, multiple independent gene transfers from ingested food items will be more probable.

Category: Medicine Notes