Genome ploidy in different stages of the Giardia lamblia life cycle

Abstract

The early diverging eukaryotic parasite Giardia lamblia is unusual in that it contains two apparently identical nuclei in the vegetative trophozoite stage. We have determined the nuclear and cellular genome ploidy of G. lamblia cells during all stages of the life cycle. During vegetative growth, the nuclei cycle between a diploid (2N) and tetraploid (4N) genome content and the cell, consequently, cycles between 4N and 8N. Stationary phase trophozoites arrest in the G2 phase with a ploidy of 8N (two nuclei, each with a 4N ploidy). On its way to cyst formation, a G1 trophozoite goes through two successive rounds of chromosome replication without an intervening cell division event. Fully differentiated cysts contain four nuclei, each with a ploidy of 4N, resulting in a cyst ploidy of 16N. The newly excysted cell, for which we suggest the term ‘excyzoite’, contains four nuclei (cellular ploidy 16N). In a reversal of the events occurring during encystation, the excyzoite divides twice to form four trophozoites containing two diploid nuclei each. The formation of multiple cells from a single cyst is likely to be one of the main reasons for the low infectious doses of G. lamblia.

The Giardia cell cycle progresses independently of the Anaphase Promoting Complex

“…By mining the Giardia genome database for cyclin homologs (Reiner et al, 2008), several genes have been identified as candidates for a mitotic cyclin on the basis of sequence similarity. Each cell contains two diploid nuclei that are replicated concurrently (Bernander et al, 2001;Sagolla et al, 2006), and then simultaneously segregated to opposite poles of the cell by two separate spindles, prior to cytokinesis (Nohýnkova et al, 2000;Sagolla et al, 2006). Thus, the cell cycle in Giardia has G1, S, G2 and M phases similar to other eukaryotes.…”

Characterisation of the subtelomeric regions of Giardia lamblia genome isolate WBC6

“…Each nucleus has at least one copy of each chromosome (Yu et al, 2002) and the two nuclei have approximately equal quantities of DNA. Therefore, since trophozoites have an estimated ploidy of four (Bernander et al, 2001), it is likely that each nucleus has a ploidy of approximately two. However, the exact number of copies of each chromosome in each nucleus may vary somewhat, as suggested by a cytogenetic evaluation in which the number of chromosomes per nucleus varied between nine and 11 (Tumova et al, 2006), leaving open the possibility of a small degree of aneuploidy.…”

The genome of an Encephalitozoon cuniculi type III strain reveals insights into the genetic diversity and mode of reproduction of a ubiquitous vertebrate pathogen

“…Perhaps, these must first fuse and form tetraploid diplokaryons (Bernander et al, 2001;Lee et al, 2014) to trigger meiosis and create genetic diversity? This hypothesis is supported by evidence of diploidy in mononucleate species (Katinka et al, 2001;Cuomo et al, 2012;Selman et al, 2013;Desjardins et al, 2015;Watson et al, 2015) and tetraploidy in Nosema spp.…”