Diplomonad Explained

The diplomonads (Greek for "two units")^[1] are a group of flagellates, most of which are parasitic. They include Giardia duodenalis, which causes giardiasis in humans.^[2] 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. duodenalis, called mitosomes. These are not used in ATP synthesis the way mitochondria are, but are involved in the maturation of iron-sulfur proteins.^[3]

Possible sexual reproduction in Giardia

The common intestinal parasite Giardia duodenalis (synonyms Giardia lamblia, G. intestinalis) was once considered to be a descendant of a protist lineage that predated the emergence of meiosis and sex. However, researchers found G. duodenalis to have a core set of genes that function in meiosis and that are widely present among sexual eukaryotes.^[4] These results suggested that Giardia duodenalis is capable of meiosis and thus sexual reproduction. Furthermore, Cooper et al.^[5] found direct evidence in Giardia duodenalis for infrequent meiotic recombination, indicative of sexual reproduction between individuals. Lasek-Nesselquist et al.^[6] also detected molecular signatures consistent with meiotic sex. The possibility of sexual reproduction is still debated.^[7]

Giardia duodenalis 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.^[8] The recombination associated with karyogamy may primarily function to repair DNA damage.

Giardia duodenalis 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.^[9] 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.

Notes and References

1. Web site: monad. https://web.archive.org/web/20160926143456/https://en.oxforddictionaries.com/definition/monad. dead. September 26, 2016. Oxford Dictionary. 16 October 2017.
2. Web site: CDC - DPDx - Giardiasis . www.cdc.gov . en-us . 22 April 2021.
3. Tovar J, León-Avila G, Sánchez LB, Sutak R, Tachezy J, van der Giezen M, Hernández M, Müller M, Lucocq JM . 4402808 . 6 . Mitochondrial remnant organelles of Giardia function in iron-sulphur protein maturation . Nature . 426 . 6963 . 172–6 . November 2003 . 14614504 . 10.1038/nature01945 . 2003Natur.426..172T .
4. Ramesh MA, Malik SB, Logsdon JM . 17013247 . A phylogenomic inventory of meiotic genes; evidence for sex in Giardia and an early eukaryotic origin of meiosis . Current Biology . 15 . 2 . 185–91 . January 2005 . 15668177 . 10.1016/j.cub.2005.01.003 . free .
5. Cooper MA, Adam RD, Worobey M, Sterling CR . 15991722 . Population genetics provides evidence for recombination in Giardia . Current Biology . 17 . 22 . 1984–8 . November 2007 . 17980591 . 10.1016/j.cub.2007.10.020 . free .
6. Lasek-Nesselquist E, Welch DM, Thompson RC, Steuart RF, Sogin ML . Genetic exchange within and between assemblages of Giardia duodenalis . The Journal of Eukaryotic Microbiology . 56 . 6 . 504–18 . 2009 . 19883439 . 10.1111/j.1550-7408.2009.00443.x . 24848706 . free .
7. Maciver SK, Koutsogiannis Z, de Obeso Fernández Del Valle A . 'Meiotic genes' are constitutively expressed in an asexual amoeba and are not necessarily involved in sexual reproduction . Biology Letters . 15 . 3 . 20180871 . March 2019 . 30836881 . 6451372 . 10.1098/rsbl.2018.0871 .
8. Poxleitner MK, Carpenter ML, Mancuso JJ, Wang CJ, Dawson SC, Cande WZ . 206510785 . Evidence for karyogamy and exchange of genetic material in the binucleate intestinal parasite Giardia intestinalis . Science . 319 . 5869 . 1530–3 . March 2008 . 18339940 . 10.1126/science.1153752 . 2008Sci...319.1530P .
9. Xu F, Jerlström-Hultqvist J, Andersson JO . Genome-wide analyses of recombination suggest that Giardia intestinalis assemblages represent different species . Molecular Biology and Evolution . 29 . 10 . 2895–8 . October 2012 . 22474166 . 10.1093/molbev/mss107 . free .