In situ analyses reveal that the two nuclei of Giardia lamblia are equivalent.

The protozoan parasite Giardia lamblia has the unusual morphology of bearing two equal-sized nuclei. This organism probably represents the earliest diverging lineage of eukaryotes, suggesting that its biological tactics may be transitional. To begin to understand the role played by the two equal-sized nuclei in this organism, and perhaps the role this organism has played along the path to higher eukaryotes, we have analyzed the structure and function of these two nuclei. We show that the two nuclei are equivalent with respect to the amount of DNA harbored in each nucleus, the presence of ribosomal DNA sequences, and the transcriptional activity. We begin also to address the question of how these bilaterally symmetrical ancestors divide, by illustrating the mitotic plane of division.