| Summary: | <p>Abstract</p> <p>Background</p> <p><it>Giardia </it>are a group of widespread intestinal protozoan parasites in a number of vertebrates. Much evidence from <it>G. lamblia </it>indicated they might be the most primitive extant eukaryotes. When and how such a group of the earliest branching unicellular eukaryotes developed the ability to successfully parasitize the latest branching higher eukaryotes (vertebrates) is an intriguing question. Gene duplication has long been thought to be the most common mechanism in the production of primary resources for the origin of evolutionary novelties. In order to parse the evolutionary trajectory of <it>Giardia </it>parasitic lifestyle, here we carried out a genome-wide analysis about gene duplication patterns in <it>G. lamblia</it>.</p> <p>Results</p> <p>Although genomic comparison showed that in <it>G. lamblia </it>the contents of many fundamental biologic pathways are simplified and the whole genome is very compact, in our study 40% of its genes were identified as duplicated genes. Evolutionary distance analyses of these duplicated genes indicated two rounds of large scale duplication events had occurred in <it>G. lamblia </it>genome. Functional annotation of them further showed that the majority of recent duplicated genes are VSPs (Variant-specific Surface Proteins), which are essential for the successful parasitic life of <it>Giardia </it>in hosts. Based on evolutionary comparison with their hosts, it was found that the rapid expansion of VSPs in <it>G. lamblia </it>is consistent with the evolutionary radiation of placental mammals.</p> <p>Conclusions</p> <p>Based on the genome-wide analysis of duplicated genes in <it>G. lamblia</it>, we found that gene duplication was essential for the origin and evolution of <it>Giardia </it>parasitic lifestyle. The recent expansion of VSPs uniquely occurring in <it>G. lamblia </it>is consistent with the increment of its hosts. Therefore we proposed a hypothesis that the increment of <it>Giradia </it>hosts might be the driving force for the rapid expansion of VSPs.</p>
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