Giardia lamblia is a flagellated unicellular parasite and one of the earliest diverging eukaryotic cells. Of all the cellular features within the trophozoite, the most puzzling and intriguing is the presence of two nuclei in mirror symmetry, which are identical or similar in several respects. They replicate at nearly the same time, they are equivalent with respect to the amount of DNA harbored in each nucleus, the presence of ribosomal DNA sequences, and the transcriptional activity，and genes from each of the five chromosomes are found in both nuclei, confirming that each nucleus has at least one complete copy of the genome. However, whether each nucleus of Giardia contains the same DNA sequences, and whether the allelic genes in the same nucleus are uniform is not known. If the sequences are uniform, why Giardia needs two equal nuclei? How the Giardia to keep the sequences in the two nuclei equal even though the spontaneous gene mutation? Also, various data indicate the trophozoite of Giardia is tetraploid, that is each nucleus is haploid. Then, How the Giardia to keep the allelic genes in the same nucleus equal? The research into these questions is intriguing, and it can also reveal the unusual genetic mechanism of Giardia and provide useful clue to the ploidy evolution of eukaryote. In the present work, we first determined the two nuclei of Giardia contain the same gene. We randomly selected three genes: topII、krr1 and fes, which functioned differently. We used fluorescence in situ hybridization (FISH) to demonstrate that these genes are found in both nuclei, confirming they are both present in each nucleus. We can also infer the two nuclei maybe functioned uniformly in these respects. Secondly，we determined the sequences in the same nucleus is uniform. Based on this, we attempted to elucidate the mechanism of Giardia keeping the allelic genes of the same nucleus and of the two nuclei equal. In order to see whether the gene sequences were still identical after many generations, we selected genes fen1 and pdi which had been reported having polymorphic sites and tracked their dynamic variation in our established single-cell-cloned line. In the gene fen1, we found no variation after 18 months long. However, we found some sites that changed intermittently in pdi1, which indicated gene conversion maybe occurred. So we searched the Giardia genome and did find the homologous genes of gene conversion. We concluded the gene conversion maybe occurred as an efficient mechanism to eliminate the allelic sequence heterozygousity of Giardia. The another question we have addressed in the present work is how Giardia eliminate sequence difference between nuclei. We stained the nuclei of the Giardia expecting to reveal a clue. Surprisingly, trophoziotes with single nucleus and two nuclei in one lateral were found. The potential reason for this could be intermittent loss of one nucleus, and another nucleus replicated and the two nuclei repositioned to the normal pattern. So the homozygosity of the two nuclei was maintained.
修改评论