Date Published: August 20, 2008
Publisher: Public Library of Science
Author(s): Chandrama Mukherjee, C. Graham Clark, Anuradha Lohia, Daniel Eichinger
Abstract: Under axenic growth conditions, trophozoites of Entamoeba histolytica contain heterogenous amounts of DNA due to the presence of both multiple nuclei and different amounts of DNA in individual nuclei. In order to establish if the DNA content and the observed heterogeneity is maintained during different growth conditions, we have compared E. histolytica cells growing in xenic and axenic cultures. Our results show that the nuclear DNA content of E. histolytica trophozoites growing in axenic cultures is at least 10 fold higher than in xenic cultures. Re-association of axenic cultures with their bacterial flora led to a reduction of DNA content to the original xenic values. Thus switching between xenic and axenic growth conditions was accompanied by significant changes in the nuclear DNA content of this parasite. Changes in DNA content during encystation-excystation were studied in the related reptilian parasite E. invadens. During excystation of E. invadens cysts, it was observed that the nuclear DNA content increased approximately 40 fold following emergence of trophozoites in axenic cultures. Based on the observed large changes in nuclear size and DNA content, and the minor differences in relative abundance of representative protein coding sequences, rDNA and tRNA sequences, it appears that gain or loss of whole genome copies may be occurring during changes in the growth conditions. Our studies demonstrate the inherent plasticity and dynamic nature of the Entamoeba genome in at least two species.
Partial Text: In most eukaryotes, genomic stability is dependant on maintenance of the same genome content among individuals of the same species. This is achieved in part through the regulation of genome duplication, repair of DNA damage, and equal segregation of the duplicated genome copies. In contrast, a large number of organisms, including some plants, Drosophila, ciliates and other protists, and a few bacterial species, display dynamic changes in their genome or DNA content during their life cycle. Variations in DNA content between individuals of the same species may consist of duplication/deletion of large regions of chromosomes, and variable copy number of individual chromosomes or of whole genomes . Endo-replication can lead to an accumulation of multiple genome copies before segregation instead of the more normal simple duplication .
This study highlights several important properties of the protozoan parasites E. histolytica and E. invadens. Firstly, during the change of growth conditions from xenic to axenic culture, E. histolytica undergoes increases in nuclear size and DNA content. This phenomenon is reversible when axenic cultures are re-associated with a bacterial flora. In the social amoeba Dictyostelium discoideum, it was demonstrated that amoebae grown in axenic culture tended to be multi-nucleate and had a greater DNA content per nucleus than amoebae grown on a bacterial substrate, which were uni-nucleate . Wild type cells grew xenically while axenic growth was favoured in phagocytosis and motility deficient mutants of Dictyostelium. These mutants have also been associated with defects in cytokinesis and increased multi-nuclearity during axenic growth . Although loss of microbial substrates appears to be linked with increased DNA content during axenic growth in both D. discoideum and E. histolytica it may not be the only factor involved since an increase in nuclear DNA content of 40–50 fold is also observed when E. invadens trophozoites emerge from cysts into axenic cultures. This increase is also accompanied by nuclear size expansion (data not shown).