Entamoeba
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The entamoebae are a group of parasitic and commensal amoebae which lack mitochondria. They include several species that are pathenogenic in humans, most notably Entamoeba histolytica, which causes amoebic dysentery. Cells are uninucleate and small, typically 10-100 μm across, and usually have a single lobose pseudopod taking the form of a clear anterior bulge. Cysts are common.
The absence of mitochondria is considered a secondary loss, possibly associated with their parasitic life-cycle. Studies show the entamoebae are close relatives of the pelobionts, another group of amitochondriate amoebae, but unlike them retain dictyosomes. Both groups may belong among the Amoebozoa.
David Biron of The Weizmann Institute of Science and coworkers found that about one third of Entamoeba invadens cells are unable to separate unaided and recruit a neighboring amoeba (dubbed the "midwife") to complete the fission. He writes :
When an amoeba divides, the two daughter cells stay attached by a tubular tether which remains intact unless mechanically severed. If called upon, the neighbouring amoeba midwife travels up to 200 μm towards the dividing amoeba, usually advancing in a straight trajectory with an average velocity of about 0.5 μm/s. The midwife then proceeds to rupture the connection, after which all three amoebae move on.
They also reported a similar behavior in Dictyostelium.