Compelling visual evidence of sexual reproduction in African trypanosomes, single-celled parasites that cause major human and animal diseases, has been found by researchers from the University of Bristol.
The research could eventually lead to new approaches for controlling sleeping sickness in humans and wasting diseases in livestock which are caused by trypanosomes carried by the bloodsucking tsetse fly.
Biologists believe that sexual reproduction evolved very early and is now ubiquitous in organisms with complex cell structure (the eukaryotes, essentially all living organisms except bacteria). However, real evidence is lacking for a large section of the evolutionary tree.
Trypanosomes represent an early and very distant branch of the eukaryote tree of life and until now it was unclear whether they do indeed reproduce sexually.
Offspring that result from sexual reproduction inherit half their genetic material from each parent. At the core of this process is meiosis, the cellular division that shuffles the parental genes and deals them out in new combinations to the offspring. In organisms which cause diseases, sexual reproduction can spread genes which make them more virulent, or resistant to drugs used for treatment, as well as creating completely new strains with combinations of genes not previously encountered.
Some time ago it was shown that genetic shuffling could occur when two different trypanosome strains were mixed in the tsetse fly, but it was far from clear that this was true sexual reproduction. Direct visualization of the process was difficult because it happened inside the insect.
To get round this problem, Professor Wendy Gibson and colleagues used fluorescently-tagged proteins to make trypanosomes light up like tiny light bulbs [see image]. The tagged proteins only function during meiosis in other well-studied eukaryotes such as yeast.
Professor Gibson said: "It seems that meiosis in trypanosomes has eluded observers because it occurs hidden inside the insect carrying the parasite -- a difficult and technically challenging system to work with. These new results will further our understanding of events at the very beginning of eukaryote evolution, and of the way that new strains of disease-causing microbes emerge."
The study, carried out by researchers from Bristol's Schools of Biological Sciences and Veterinary Sciences in collaboration with the University of Cambridge, is published this week in Proceedings of the National Academy of Sciences (PNAS).
The research was funded by the Wellcome Trust.
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