The Giardia lamblia genome

Giardia lamblia is a protozoan parasite of humans and other mammals that is thought to be one of the most primitive extant eukaryotic organisms. Altho ugh distinctly eukaryotic, it is notable for its lack of mitochondria, nucl eoli: and perixosomes. It has been suggested that Giardia spp, are pre-mito chondriate organisms, but the identification of genes in G. lamblia thought to be of mitochondrial origin has generated controversy regarding that des ignation. Giardi lamblia trophozoites have two nuclei that are identical in all ways that have been studied. They are polyploid with at least four, an d perhaps eight or more, copies of each of five chromosomes per organism an d have an estimated genome complexity of 1.2x10(7) bp of DNA, and GC conten t of 46%. There is evidence for recombination at the telomeres of some of t he chromosomes, and multiple size variants of single chromosomes have been identified within cloned isolates. However, the internal regions of the chr omosomes demonstrate no evidence of recombination. For example. there is no evidence for control of vsp gene expression by DNA recombination, and no e vidence for rapid mutation in the vsp, genes. Single pass sequences of appr oximately 9% of the G. lamblia genome have already been obtained. An ongoin g genome project plans to obtain approximately 95% of the genome by a rando m approach, as well as a complete physical map using a bacterial artificial chromosome library. The results will facilitate a better understanding of the biology of Giardia spp. as well as their phylogenetic relationship to o ther primitive organisms. (C) 2000 Australian Society for Parasitology Inc. Published by Elsevier Science Ltd. All rights reserved.