Malaria parasites have around 5,000 genes. Only a small proportion of these genes have homologs in well-studied model systems, and as a result the function of many is unclear. We performed the first genome-scale genetic screen in a malaria parasite.
We used linear DNA vectors which integrate into the Plasmodium genome. We were able to create vectors to target around half of all Plasmodium berghei genes. Each vector integrates into the parasite genome in the place of its target gene. It carries a drug resistance marker that enables us to select only parasites in which a gene has been deleted. The vectors also carry a "genetic barcode" into the parasite genome. We can amplify all of the barcodes from a mixture of thousands of parasites and read them out using DNA sequencing. This allowed us to measure the growth of many different mutants in a mixed population of parasites. By doing so we could work out whether deleting a gene had a profound effect on parasite growth ("essential" gene), a more moderate effect ("slow" gene) or no evidence of a change in growth rate ("dipsensable gene").
We unexpectedly found that most genes are important for parasite growth, even in a single stage of parasite growth - in the blood of an infected host. This is a higher proportion of essential genes than has been seen in any other organism screens and suggests that there may be more multi-stage drug targets encoded by the parasite than was previously supposed.
All phenotypes are available on the PlasmoGEM site: http://plasmogem.sanger.ac.uk/phenotypes