|Families, subfamilies, and genera|
The diplomonads (Greek for "two units") are a group of flagellates, most of which are parasitic. They include Giardia lamblia, which causes giardiasis in humans. They are placed among the metamonads, and appear to be particularly close relatives of the retortamonads.
Most diplomonads are double cells: they have two nuclei, each with four associated flagella, arranged symmetrically about the body's main axis. Like the retortamonads, they lack both mitochondria and Golgi apparatuses. However, they are now known to possess modified mitochondria, in the case of G. lamblia, called mitosomes. These are not used in ATP synthesis the way mitochondria are, but are involved in the maturation of iron-sulfur proteins.
Possible sexual reproduction in Giardia
The common intestinal parasite Giardia intestinalis (synonyms Giardia lamblia, G. duodenalis) was once considered to be a descendant of a protist lineage that predated the emergence of meiosis and sex. However, G. intestinalis has now been found to have a core set of genes that function in meiosis and that are widely present among sexual eukaryotes. These results suggested that G. intestinalis is capable of meiosis and thus sexual reproduction. Furthermore, Cooper et al. found direct evidence in G. intestinalis for infrequent meiotic recombination, indicative of sexual reproduction between individuals. Lasek-Nesselquist et al. also detected molecular signatures consistent with meiotic sex. The possibility of sexual reproduction is still debated.
G. intestinalis contains two functionally equivalent nuclei that are inherited independently during mitosis. In the giardial cyst these nuclei fuse (karyogamy) and undergo homologous recombination facilitated by meiosis gene homologs. The recombination associated with karyogamy may primarily function to repair DNA damage.
G. intestinalis is divided into eight assemblages based on host specificities and genetic divergence of marker genes. Although recombination can occur infrequently within assemblages, Xu et al. found that recombination between individuals from different assemblages is very rare. They suggested that the assemblages are genetically isolated lineages, and thus could be viewed as separated Giardia species.
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- Tovar J, León-Avila G, Sánchez LB, Sutak R, Tachezy J, van der Giezen M, et al. (November 2003). "Mitochondrial remnant organelles of Giardia function in iron-sulphur protein maturation". Nature. 426 (6963): 172–6. Bibcode:2003Natur.426..172T. doi:10.1038/nature01945. PMID 14614504. S2CID 4402808.
- Ramesh MA, Malik SB, Logsdon JM (January 2005). "A phylogenomic inventory of meiotic genes; evidence for sex in Giardia and an early eukaryotic origin of meiosis". Current Biology. 15 (2): 185–91. doi:10.1016/j.cub.2005.01.003. PMID 15668177. S2CID 17013247.
- Cooper MA, Adam RD, Worobey M, Sterling CR (November 2007). "Population genetics provides evidence for recombination in Giardia". Current Biology. 17 (22): 1984–8. doi:10.1016/j.cub.2007.10.020. PMID 17980591. S2CID 15991722.
- Lasek-Nesselquist E, Welch DM, Thompson RC, Steuart RF, Sogin ML (2009). "Genetic exchange within and between assemblages of Giardia duodenalis". The Journal of Eukaryotic Microbiology. 56 (6): 504–18. doi:10.1111/j.1550-7408.2009.00443.x. PMID 19883439.
- Maciver SK, Koutsogiannis Z, de Obeso Fernández Del Valle A (March 2019). "'Meiotic genes' are constitutively expressed in an asexual amoeba and are not necessarily involved in sexual reproduction". Biology Letters. 15 (3): 20180871. doi:10.1098/rsbl.2018.0871. PMC 6451372. PMID 30836881.
- Poxleitner MK, Carpenter ML, Mancuso JJ, Wang CJ, Dawson SC, Cande WZ (March 2008). "Evidence for karyogamy and exchange of genetic material in the binucleate intestinal parasite Giardia intestinalis". Science. 319 (5869): 1530–3. Bibcode:2008Sci...319.1530P. doi:10.1126/science.1153752. PMID 18339940. S2CID 206510785.
- Xu F, Jerlström-Hultqvist J, Andersson JO (October 2012). "Genome-wide analyses of recombination suggest that Giardia intestinalis assemblages represent different species". Molecular Biology and Evolution. 29 (10): 2895–8. doi:10.1093/molbev/mss107. PMID 22474166.