The single-celled parasite Giardia intestinalis (Diplomonadida) has two equally sized nuclei in one cell. The nuclei have been considered identical. We have previously shown that they contain different chromosomal sets and proceed through the cell cycle with some asynchrony. Here, we demonstrate by fluorescence in situ hybridization that several genes from chromosome 5 are lost in one of the two nuclei of the WBc6 Giardia line. The missing segment stretches over at least 50 kb near the 5' chromosome end. In both WB and WBc6 Giardia cell lines, chromosome 5 is trisomic in one nucleus and monosomic in the other nucleus. The described chromosomal deletion has always been observed at the monosomic chromosome in WBc6; however, the deletion was not detected in the parent line WB. The chromosomal segment was thus initially lost after biological cloning of WB, which gave rise to clone WBc6. We show that Giardia is capable of carrying out gene expression from only one nucleus. The two nuclei display a certain level of diversity, making each of them irreplaceable. The doubled karyomastigonts of diplomonads likely have separate functions both in the mastigont/flagellar organization and in chromosomal and gene content. To our knowledge, our results offer the first methodical approach to differentiating the two, so far indistinguishable nuclei.

Institute of Immunology and Microbiology 1st Faculty of Medicine and General University Hospital Charles University Prague Czech Republic

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