This is a background story about the dogma change regarding primitive eukaryotes that are claimed to be not so primitive after all (leading to the strange situation where there are no living precursors of the eukaryotic cell, a blatant contradiction of what is expected in gradualism). See also how strong Bill Martin’s words are regarding this dogma change and the lack of scientific evidence for claiming victory. Don’t get me started of how I dislike the telological phrasing. I also think dr Tovar’s and Dr’ Martin’s attitude is quite arrogant, especially since they must know that the data is still controversial. Anyway, a nice example of how the PR machine of contemporary science works.
RESEARCHERS ARE CLAIMING a “landmark in the fields of cell biology, microbiology and evolutionary biology,” having proved the true nature of one of the world’s most successful parasites.
The discovery concerns Giardia intestinalis, a common diarrhoea-inducing bug that is all too familiar to international travellers. Until recently it was generally thought that Giardia embodied the link between primitive cells and modern cells, but new findings suggest a different background that could change the way biologists think about evolutionary history.
Modern cells such as our own store their genes in the nucleus, and contain power-generating organelles mitochondria that help them respire. Primitive cells, on the other hand, lack both nucleus and mitochondria. The bridge between these two states is of great interest to scientists, who have for many years focused their attention on cells such as Giardia.
This is because in oxygen-poor environments like the gut, where there is no need for aerobic respiration, there are organisms that have nuclei but appear to lack mitochondria. These ‘amitochondrial’ organisms, of which Giardia. is a prime example, have conventionally been posited as the evolutionary link between primitive and modern cells, as they appeared to have characteristics of both.
Now an international research team led by Dr Jorge Tovar of Royal Holloway, University of London, has demonstrated that Giardia is not as straightforward as was previously thought. They proved that the cells in fact contain very small organelles, surrounded by double membranes that resemble downsized mitochondria.
Using advanced microscopy techniques and highly specific antibodies that target two mitochondrial proteins, they have shown that Giardia’s tiny organelles actually contain typical, ‘modern’ mitochondrial proteins.
“This bug is an impostor, posing as a primitive cell when in reality it is a highly evolved organism with mitochondrion-bearing ancestors,” explains Dr Tovar. “Giardia has done a sterling job of concealing all evidence of its mitochondrial ancestry. Little wonder that over the past 100 years or so most biologists were fooled into thinking of it as a primitive microbe.”
Although the shrunken organelles are unusable for aerobic respiration by Giardia, the researchers have shown that they nevertheless fulfil an essential function. Employing biochemical techniques, they found that vital iron-sulphur complexes could only be assembled in the presence of the organelles, which is probably the reason that Giardia retains them in their current form. “Giardia does not need oxygen to respire and so it could afford to dispense with all functions related to aerobic respiration, but it cannot do away with iron-sulphur proteins that are required for anaerobic proliferation,” says Dr Tovar.
While other eukaryote cells now classified as ‘amitochondrial’ are yet to be investigated in this way, Dr Tovar now believes that the odds are stacked against anyone proving they do not have a mitochondrial past, and says that the textbooks will have to be rewritten.
Professor Bill Martin of the University of Dusseldorf says that the UK is producing much of the cutting edge science in this area, and sees the latest discovery as the culmination of important work by several successful teams:
“The old view that Giardia lacked mitochondria is the keystone of a 15-year-old dogma that has held back progress in the field like a yoke. It is a real relief that we can now look forward. David Lloyd, Martin Embley and Jorge Tovar all from the UK have contributed the decisive findings that brought the old dogma down. But the nail in the coffin of the old theory were the electron micrographs delivered by Tovar et al.
Professor Martin added: “Of course, there will be folks who will even doubt the evidence presented by Tovar and his colleagues, and there will be folks who dream up hair-brained scenarios to suggest that the Giardia organelles descend from something else other than mitochondria. Let them dream on.”