Giardia: Nuclear and Chromosomal Structure and Replication
Rodney D. Adam and Staffan G. Svärd
from: Anaerobic Parasitic Protozoa: Genomics and Molecular Biology (Edited by: C. Graham Clark, Patricia J. Johnson and Rodney D. Adam). Caister Academic Press, U.K. (2010)
Giardia and the other diplomonads are unique in their possession of two nuclei that are similar in appearance, DNA content, transcription, and time of replication. Trophozoites have a ploidy of four and the ploidy of cysts is eight, which in turn raises the question of how Giardia maintains homogeneity between the chromosomes of the same and opposite nuclei. Giardia has been assumed to be primitively asexual and with no means of transferring DNA between nuclei. These assumptions make it very difficult to explain the remarkably low level of allelic heterozygosity (< 0.01%) in the genome isolate, WB. However, all these assumptions are now in doubt with the identification of meiotic genes in Giardi, evidence for recombination among isolates, and the evidence for exchange of genetic material between nuclei during the process of encystation. These findings raise a new set of questions that will be answered only by further investigation read more ...