Evidence for Karyogamy and Exchange of Genetic Material in the Binucleate Intestinal Parasite Giardia intestinalis

Science 319, 1530 (2008)

Speaker: Wei-Ting Chang (張瑋庭)                                             Date: 2009.3.18

Commentator: Chih-Li Hsu, Ph.D. (胥直利 老師)             Place: Room 601

Abstract:

        Giardia intestinalis (syn. lamblia, duodenalis) is a common intestinal protozoan parasite and a major cause of water-borne diarrheal disease worldwide. The parasite has two developmental stages during life cycle: a binucleate, flagellated trophozoite attaches to the upper small intestine, and the quadrinucleate infectious cyst. As an asexual diplomonad, it has two apparently genetically identical, functionally equivalent diploid nuclei; both nuclei are transcriptionally active and contain two complete copies of the genome. However, Giardia has low levels of allelic heterozygosity that indicate the two nuclear genomes may exchange genetic material. The results of fluorescence in situ hybridization (FISH) performed with probes to an episomal plasmid suggests that plasmids are transferred between nuclei in the cyst, and transmission electron micrographs (TEM) demonstrate fusion between cyst nuclei. In addition, the green fluorescent fusion protein of giardial homologs of meiosis-specific genes (HMGs) were expressed and localized in the nuclei of cysts, but not in the trophozoite. All of the evidences in this report suggest that reproduction of the Giardia is parasexual, which including nuclei fusion (karyogamy) and somatic homologous recombination facilitated by the meiosis gene homologs, occur in the giardial cyst.

Reference:

1.        M. A. Ramesh, S. B. Malik, J. M. Logsdon. A phylogenomic inventory of meiotic genes; evidence for sex in Giardia and an early eukaryotic origin of meiosis. Curr. Biol. 15, 185 (2005).

2.        M. S. Sagolla, S. C. Dawson, J. J. Mancuso, W. Z. Cande. J. Three-dimensional analysis of mitosis and cytokinesis in the binucleate parasite Giardia intestinalis. Cell Sci. 119, 4889 (2006).

3.        H. G. Morrison et al. Genomic Minimalism in the Early Diverging Intestinal Parasite Giardia lamblia. Science 317, 1921 (2007).