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Giardia lamblia: cell biology and microscopy of one of the most primitive eukaryotes (Soltys lab)

This web site from Bohdan Soltys who worked in Gupta’s lab where he worked on Giardia shows how the finding of mitochondrial proteins in Giardia led to the conclusion that Giardia lost their mtiochondria. Also claims that hydrogenosomes were reduced mitochondria. All this is based on the endosymbiotic hypothesis, but is also in line with the thoery posed on this website that mitochondria were derived from within the eukaryotic cell and that hydrogenosomes and mitosomes were precursor mitochondria. The fact that mitochondrial proteins were found in amitochondriate eukaryotes would actually not be expected in the endosymbiotic theory.

   A large number of protists, including Giardia, lack mitochondria. In the past this has been taken as evidence that these organisms existed before the endosymbiosis event which led to mitochondria, and hence were more primitive than other protists. In endosymbiosis, a theory made popular in its modern version by Dr. Lynn Margulis 30 years ago, oxygen respiring bacteria invaded a host cell and formed a permanent relationship living within it, evolving into mitochondria. This endosymbiotic event is thought to have occured more than 1000 million years ago [our planetary system formed 4600 million years ago; the first bacterial cell appeared 3900 million years ago; the first protists appeared 2000 milllion years ago; man’s ancestors appeared 4 million years ago]. Mitochondria in cells actually still look like bacteria and grow and divide at their own pace. They even have their own DNA, although most genes over time have been transferred to the nucleus.

Despite the fact that Giardia lacks mitochondria, I and Rad Gupta published work showing the presence of a protein related to mitochondrial hsp60 in Giardia (9). The evidence included biochemical immunoblot detection of a protein of the correct molecular weight and both immunoflourescence and electron microscopic localization of reactivity at discrete sites in the cytoplasm.

Electron microscopic localization of hsp60 showed that hsp60 labeling was in the cytoplasm and was not associated with any type of membranous structure (not shown). To explain the findings we suggested that Giardia originally had mitochondria but lost them in evolution. More recent studies in the higher protist Trichomonas vaginalis, which contain hydrogenosomes but no mitochondria, showed molecular evidence for the presence of mitochondrial heat shock proteins within hydogenosomes (the hydrogenosome is a double membraned redox organelle found in certain anaerobic protists). Palmer et al (10) have  reviewed this work. The results led to the suggestion that hydogenosomes evolved (or de-evolved, depending on how you look at it) from mitochondria by a process of reductive, as opposed to acquisitive, evolution. Since Giardia has been regarded as the most primitive eukaryote in existence, Palmer et al (10) also cite our work as evidence from diplomonads to support the idea that the earliest eukaryotic cell contained mitochondria which were subsequently lost. Thus, the timing for the endosymbiotic event that gave rise to mitochondria is currently being pushed backwards. We are faced with the possibility that no representative of the premitochondrial stage of eukaryotic evolution may be alive today. The endosymbiotic event that gave rise to mitochondria in fact may have occurred as far back as the very origin of the first eukaryotic cell. The key to resolving this issue would be to obtain further molecular data in Giardia. The cloning of a variety of mitochondrial proteins will be necessary. It may be very difficult, however, to exclude lateral gene transfer of proteins from another species, particularly bacterial. The proteins would have to contain mitochondrial targeting sequences to definitively distinquish them from prokaryotic homologs.

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