Not that many people would mourn its loss, but the parasite responsible for a condition known as African sleeping sickness may one day drive itself to extinction as a result of its own mode of reproduction, according to a new study in the journal eLife. The researchers came to this prediction after identifying a particular genetic quirk known as the Meselson effect, which has been proposed for over 20 years but until now had never been fully observed.
The basic premise of the Meselson effect relates to the way in which genes are passed on and distributed throughout a species. These genes are contained within chromosomes, which occur in pairs in most organisms, so that during sexual reproduction the DNA from both parents can become randomly jumbled up across these chromosomes in order to create genetic variation.
This is important as it means unfavorable mutations can be eliminated over time via a process of natural selection. However, asexual reproduction does not involve this process of recombination of genes, so this vital genetic variation fails to materialize. The Meselson effect therefore states that in asexual species, each chromosome should evolve independently of its partner chromosome.
In other words, if a genetic mutation occurs on one of the chromosomes, this mutation will be passed on to all future generations, since there is no jumbling of genes between chromosome pairs during reproduction.
To search for signs of the Meselson effect, a team of researchers conducted genetic analyses of large numbers of the parasite Trypanosoma brucei gambiense, which they obtained from several different countries in West Africa.
The parasite is carried by tsetse flies, which can transmit it to humans by biting them. Initially, the infection causes fevers and headaches, although once the parasite crosses the blood-brain barrier it begins to affect the central nervous system, generating a number of symptoms including irregular sleep patterns – hence its name. If left untreated it can be fatal.
Trypanosoma brucei gambiense is carried by tsetse flies. International Atomic Energy Agency, via Wikimedia Commons.
By analyzing the genetic makeup of their vast number of specimens, the researchers discovered that the entire species “comprises just two independently evolving haplotypes,” or groups of genes. As such, they conclude that “this remarkable observation confirms the Meselson effect at a whole-genome level for the first time.”
Evolutionary theory predicts that this type of genetic dispersal should always lead to extinction, as it means that when deleterious mutations occur they cannot be erased or compensated for by future scrambling of genes. Looking deeper into their analysis of the Trypanosoma brucei gambiense genome, the study authors found evidence that some unfavorable genetic mutations have already occurred at certain loci – or points on a chromosome.
To mitigate the effects of these mutations, the organism has developed a mechanism known as gene conversion, whereby the healthy version of a gene is essentially copied and pasted in order to replace the mutated version.
However, according to the study authors, this tactic is only likely to be effective in the short term, as it only papers over the cracks but cannot fully eliminate a mutation or stop it from being passed on.
1