The importance of reverse genetics in determining gene function in apicomplexan parasites.

The phylum Apixomplexa includes obligate intracellular parasites that are of enormous medical and veterinary significance, as they are responsible for a wide variety of diseases including malaria, toxoplasmosis, coccidiosis, cryptosporidiosis, theileriosis and babesiosis. The EST sequencing projects in Toxoplasma gondii and the Plasmodium falciparum genome sequencing project have greatly accelerated gene discovery, revealing for example novel coding sequences restricted to the Apicomplexa. However, easy acquisition of sequence is almost useless if the function of any given gene cannot be tested. The establishment of transfection systems in Toxoplasma gondii, Neospora and in several Plasmodium species has provided us with the reverse genetics methods appropriate to the functional analysis of genes. Over the past few years, the discovery of novel genes coupled to the ability to introduce or modify genes has already contributed to a better understanding of cell biology and pathogenesis of these obligate intracellular parasites. Some insights into the complex processes of parasite invasion, differentiation, regulation of gene expression and protein trafficking are emerging although identification of the exact functional roles for many molecules is still awaiting more investigation. This review summarizes progress in this area. It also emphasises the tight link and synergy between Toxoplasma and malaria research. The use of reverse genetics does not guarantee the answer to gene function, so we can learn from both failed and successful experiments about how better and more efficiently to use 'genomics' to accelerate discoveries relevant to the understanding of parasitism by Apicomplexa.