Fluke of nature: Schistosome genome
Schistosomes - flukes or parasitic flatworms - are responsible for a huge global health burden. Over 200 million cases of schistosomiasis occur every year, disabling millions and killing hundreds of thousands. The genome sequence of Schistosoma mansoni, sequenced by Matt Berriman and colleagues at the Wellcome Trust Sanger Institute (ref. 1), is suggesting new ways to break the transmission cycle but is also providing clues to pivotal stages of evolution: the development of organs and of the bilateral body plan.
The S. mansoni genome consists of some 360 million bases, and shows some curious features. Within its 12 000 genes, the gaps in its coding regions (introns) are small near the starts of genes and much larger towards their ends. The genome sequence also revealed families of genes with very small exons (chunks of coding sequence) that seem to be mixed and matched in multiple combinations - possibly a mechanism to increase protein variability and help the parasite to evade the host immune system.
A detailed analysis of the genome has identified many possible avenues for drug development, such as proteins not seen in vertebrates and new members of protein families typically targeted by drugs. Indeed, some S. mansoni proteins resemble those for which potential drugs already exist.
Schistosomes are an important evolutionary stepping-stone. Comparisons with more simple organisms such as sea anemones have shed light on the new genetic features that led to anatomical innovations maintained throughout the evolution of higher animals - such as the three-layered body plan and the formation of organs. Similarly, comparisons up the family tree are providing insight into the steps needed to create the complex anatomical structures seen in higher animals.
Image: Scanning electron micrograph of a pair of Schistosoma mansoni. Credit: Uniformed Services University of the Health Sciences.
Reference
1 Berriman M et al. Nature 2009;460(7253):352-8.