Giardia lamblia is a unicellular parasitic protist that inhabits the small intestine and commonly causes diarrheal disease called giardiasis. Giardia, aside from its medical importance, has been one of the earliest-branching eukaryotes on a phylogenetic tree. The most intriguing feature of the G. lamblia trophozoites is that they have two nuclei that are morphologically identical. Although many studies have been done on the two nuclei, it is still unknown the actual number of chromosomes, and whether the genes in the two nuclei are the same. In the present work, firstly we demonstrated the equal gene contents in the two nuclei of G. lamblia. Based on the background of our studying gene conversion in Giardia, we searched the Giardia genome and did find 13 homologues of those genes involved in gene conversion: spo11、rad50、mre11、hop1、rad52、dmc1(dmc1a、dmc1b)、pms1、mlh1、 msh2、 msh6、mnd1、hop2、mlh2. Fluorescence in situ hybridization (FISH) and RT-PCR assay were designed to study whether these genes exist in both nuclei and whether they all express. FISH results showed that these genes are found in both nuclei, confirming they are both present in each nucleus and indicating the two nuclei of Giardia contain the same gene; RT-PCR results showed that all these genes are transcriptionally active, indicating that gene conversion maybe occurred in both nuclei. Secondly, we analyzed the karyotypes of Giardia nuclei. To enrich the cultured population with mitotic cells, we synchronized trophozoite cell cycle (WB strain), which was followed by hypotonic treatment and fixation. Through lots of experiments, we applied these conventional cytogenetic techniques to Giardia successfully and got good karyotyoes. The results showed that the chromosome number is in a range of 8-12, and that the two nuclei in a single cell G. lamblia (C2 strain), indicating karyotype diversity of different Giardia strains. These results maybe have great significance to study the strain differentiation of Giardia, even the species differentiation, Giardia’s primitiveness and the adaptability of parasitism.exhibited different chromosome numbers. This indicated that the chromosome number of Giardia (WB strain) isn’t stable, which was different from the karyotype of another strain of Key Words: Giardia lamblia FISH gene conversion karyotype
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