The parasite

Entamoeba histolytica lacks many organelles seen by electron microscopy in eukaryotes, such as mitochondria, Golgi apparatus, rough endoplasmic reticulum, and microtubules. This was earlier interpreted as evidence for E. histolytica being an ancestral eukaryote. However, ribosomal RNA-based molecular phylogeny shows that E. histolytica diverged from other eukaryotes much later than lineages containing all the missing organelles. Direct evidence for secondary loss of organelles has recently been shown by the identification, in the E. histolytica nuclear genome, of genes that in other eukaryotes reside in the mitochondrion. Interestingly a putative mitochondrial remnant has been identified recently in the cytoplasm of E. histolytica (Mai et al. 1999; Tovar et al. 1999).

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