The diplomonads (Greek for "two units") are a group of flagellate
s, most of which are parasitic. They include ''Giardia lamblia
'', which causes giardiasis
in humans. They are placed among the metamonad
s, and appear to be particularly close relatives of the retortamonad
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 apparatus
es. However, they are now known to possess modified mitochondria, in the case of ''G. lamblia
'', called mitosome
s. These are not used in ATP
synthesis the way mitochondria are, but are involved in the maturation of iron-sulfur protein
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
. 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 eukaryote
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