Millions of people in Latin America have been invade by a parasite – a trypanosome called Trypanosome cruzi. They are passed on through the bite of the blood-sucking assassin bug and they cause Chagas disease, a potentially fatal illness that affects the heart and digestive system. The infections are long-lasting; it can take decades for symptoms to show and a third of infected people eventually die from the disease. But T.cruzi does much more than invade our flesh and blood; it also infiltrates our genomes.
T.cruzi is unusual in that a massive proportion of its DNA, around 15-30%, lies outside of its main genome. These accessory sequences are stored in the form of thousands of interlinked DNA rings. In the parasite, these sequences are found in the mitochrondria – small structures that provide it with energy – but they have found a way to spread much further.
According to new research from Mariana Hecht and a team of Brazilian scientists, T.cruzi has the ability to inveigle its DNA rings into the genomes of those it infects. Once inside, the parasite genes can hop around, hitchhiking from one chromosome to another and leaving genetic chaos in their wake. They can even be passed on from one generation to the next. Hitching a ride aboard sperm and eggs, they can add themselves to the genomes of children, who’ve never been in direct contact with trypanosomes.
Hecht’s discovery suggests that T.cruzi is an unexpected source of genetic diversity in our species. It’s certainly not the only parasite to do this. Viruses have been infiltrating our genes since time immemorial and a massive part of our genome has a viral origin. These events, where viruses joined our family tree, provided raw material for natural selection. Some viral genes wreaked havoc by disrupting important genes, while others were eventually domesticated to act as helpful, even necessary, parts of our genome.
But T.cruzi is a different story. Despite its microscopic, single-celled nature, it’s a vastly complex creature compared to a simple virus. And it continues to breach our DNA today. Now that we’re getting technically better at detecting such “horizontal gene transfers“, we may find that many other parasites are also smuggling their genes into ours. In Hecht’s words, the “human population may be a patchwork of all the organisms to which it has ever been exposed.”