Giardia lamblia (also Giardia intestinalis and Giardia duodenalis) is medically important as a cause of diarrhea and malabsorption throughout the world，and is thought to be one of the earliest-branching eukaryotes on a phylogenetic tree. G. lamblia has two apparently genetically identical, functionally equivalent nuclei, which are at least diploidy. Both nuclei are transcriptionally active and contain two complete copies of the genome. The two nuclei in the trophozoite remain physically and genetically distinct during mitosis, with two autonomous spindles segregating the parental nuclei into the daughter cells. Neither mating nor meiosis has been reported in Giardia. However, if Giardia is asexual, it should accumulate substantial allelic sequence heterozygosity (ASH) within and between the two nuclei, and this has not been observed. What has caused this intriguing result? Two work were done to explore the factors. Firstly, Two genes ( fen1，pdi ) were selected, amplified by single-cell PCR, and sequenced multitudinously to continuously track their dynamic variation in single-cell-cloned line established our own for eighteen months. Except for rare mutations, however, no variation was found in both genes, indicating that ASH is exceedingly low within and between the two nuclei. Statistical analysis of mutation sites in both genes found that some sites changed intermittently in pdi, indicating the mutation was repaired, which telling us that a biological mechanism repairing allelic mutations does exist in Giardia. We speculated that it is gene conversion. Secondly, we investigated systematically the crucial steps of gene conversion and those corresponding genes according to the current theories and models concerning gene conversion, laying a foundation for the further research. We searched the Giardia genome and did find homologues of those genes involved in gene conversion. Furthermore, we matched homologous genes found in Giardia with those crucial steps of gene conversion, and found that they can be definitely identified with each other, indicating that gene conversion may happen in Giardia. Then, fluorescence in situ hybridization (FISH) and RT-PCR assay were designed to futher demonstrate the specific cases of homologous genes found in Giardia. FISH results showed that each gene exists in both nuclei, RT-PCR results showed that all these genes are transcriptionally active, which indicating that gene conversion maybe occurred as an efficient mechanism to reduce heterozygosity within two nuclei.
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