15 July 2014
By Adam Reid
Sequencing the genome of the chicken parasite Eimeria has uncovered a fascinating quirk and could help us to develop more cost-effective vaccines that will target all seven species of the parasite. Coccidiosis, the disease caused by Eimeria parasites, poses a major threat to food security as chickens are one of the most important sources of animal protein worldwide.
When we looked at the DNA of this parasite, we noticed that each chromosome had an ordered, barcode-like pattern of repetitive sections of DNA code. These repeats often occur within genes and it turns out that this parasite has the most repeat-rich genes ever described.
While these repeat-rich regions disrupt the majority of protein-coding sequences in the genome, we have every reason to believe they are beneficial to the parasite, as they have been present in the genome for millions of years. It could be that this unusual pattern of repeated DNA has helped Eimeria increase the rate at which it evolves. That theory would certainly go some way to explaining why this parasite has been so difficult to defeat.
The Eimeria parasite causes an intestinal disease in chickens called coccidiosis. It infects billions of chickens each year, costing farmers £2.5 billion annually and jeopardising stocks of the 55 billion chickens produced. Given the dire consequences for global food security, it’s crucial that we work to develop a vaccine for this parasite that it won’t be able to avoid.
By sequencing all seven species of Eimeria that infect domestic chickens, we have provided a basis for a better understanding of how Eimeria interacts with its host to cause disease. We have also selected key developmental stages in the parasites’ lifecycle and sequenced the relevant transcriptome at base-pair resolution. Our data is publically available to help further research community-wide and to inform the development of new drugs and vaccines.
As well as helping to tackle chicken coccidiosis, these genomes have completed the genetics parts list for all the most important species of Apicomplexan parasite. This family of parasites include species that cause malaria and a number of other severe diseases in humans and animals. By comparing the genomes of the parasites in this phylum, we have begun to patch together a family history and learn more about why some cause destructive disease and others are relatively harmless.
The Eimeria genome is full of surprises that have posed important questions about parasite biology and that may hold the key to future drug targets for coccidiosis and a host of diseases within the Apicomplexan phylum.
Adam Reid is a Staff Scientist in the Parasite Genomics group led by Matt Berriman at the Wellcome Trust Sanger Institute. He is interested in using comparative genomics and transcriptomics to understand host-parasite interactions.
- Reid, AJ et al (2014). Genomic analysis of the causative agents of coccidiosis in domestic chickens. Genome Research. DOI:10.1101/gr.168955.113