The biology of kinetoplastid parasites: insights and challenges from genomics and post-genomics

Abstract

Kinetoplastid parasites exhibit a rich and diverse biology which mirrors many of the most interesting topics of current interest and study in the broader biological sciences. These evolutionarily ancient organisms possess intriguing mechanisms for control of gene expression, and exhibit complex patterns of cell morphogenesis orchestrated by an internal cytoskeleton. Their cell shapes change during a set of complex cell type differentiations in their life cycles. These differentiations are intimately linked to interactions with mammalian hosts or insect vectors, and often, these differentiations appear central to the successful transfer of the parasite between vector and host, and host and vector. The basics of this rich and complex cell and life cycle biology were described (with often rather forgotten clarity and prescience) in the early period of the last century. The last 30 years have seen major developments in our understanding of this biology. Ultrastructural differences in the various cells of the life cycle stages of Trypanosoma brucei, Trypanosoma cruzi and the various Leishmania species have been documented, and such studies have proven highly informative in defining important aspects of parasite adaptation. They have also proven to be a rich source of information for defining unusual aspects of parasite cell biology, novel organelles and cell architecture. This ultrastructural cell biology has been mirrored in a set of biochemical explanations defining unusual aspects of metabolism, surface molecules, and organelles. Finally, the application of molecular biology to these parasites revealed fascinating layers of complexity in the control of gene expression. These molecular studies have given us particular insights into polycistronic transcription, trans-splicing, RNA editing and gene rearrangements during antigenic variation. In contrast to other microbial systems, these cell biological, biochemical and molecular studies have not been greatly aided by insights gained from genetics--the diploid nature of the genome has discouraged the application of selectional genetics, mutant isolation and analysis. This is an important fact, since in general, it means that we have only recently started to analyse the phenotypes of mutants produced in the context of reverse genetics. In the following, I will argue that this lack of investment in the analysis of mutant phenotype is just one of the challenges that will need to be met if we are to gain the expected added value from the parasite genome projects. In this presentation, I will use some of the current areas of interest in the biology of T. brucei, T. cruzi and the Leishmania species to rehearse some of the insights and challenges that are likely to stem from the application of genomics and post-genomic studies to the kinetoplastid parasites. In some cases, I will exemplify points by illustrations from my laboratory's work, interests and hypotheses. The presentation slants therefore towards T. brucei biology, however, in each case the reader will, no doubt, see the generalities of application to other kinetoplastid parasites.