Evolution’s Double Take

The Eukarya domain is broken down into four Kingdoms: animals, plants, fungi, and protists. All eukaryotes are characterized by having their DNA enclosed within a cell nucleus. In most eukaryotes, mitochondria act as the powerhouses of the cell. Mitochondria convert food into energy through the respiration of oxygen.

cyanobacteria
Cyanobacteria (above) became the first microbes to produce oxygen by photosynthesis.
Credit: UC Berkeley

But not all eukaryotes rely on mitochondria for their energy. For instance, the cells of plants and some protists also contain plastids, where photosynthesis takes place and provides the organism with food. Organisms that live in environments without oxygen, such as anaerobic fungi and protozoa (a type of protist), don’t use mitochondria at all — instead their power is generated by hydrogenosomes, organelles that, while similar in many ways to mitochondria, follow a different metabolic pathway. While mitochondria need oxygen to produce energy, hydrogenosomes produce energy under anoxic conditions.

Researchers for the Netherlands Organization for Scientific Research (NWO) believe that hydrogenosomes have repeatedly evolved from mitochondria in the course of evolution. They hypothesize that protozoans and fungi, which had once lived in an oxygen-rich environment, found themselves in a completely anoxic environment. To survive this loss of oxygen, the researchers say the mitochondria evolved into hydrogenosomes.

The origin of hydrogenosomes has been debated for some time. Like the NWO researchers, some scientists believe that hydrogenosomes evolved from mitochondria. Others believe that hydrogenosomes and mitochondria evolved from a common ancestor. Because hydrogenosomes work in anoxic environments, whereas mitochondria thrive under an oxygen atmosphere, understanding how the two cellular power systems developed may tell us something about the evolution of early life on Earth. The answer to these questions also could indicate the pathways life is most likely to follow on other worlds.

carol_cleland
Mitochondria are the cells’ power sources. They are distinct organelles with two membranes. Usually they are rod-shaped, however they can be round. The outer membrane limits the organelle. The inner membrane is thrown into folds or shelves that project inward. Credit: UIUC

"Any clear insights into microbial evolution have important implications for astrobiology," says Mitch Sogin, microbiologist with the Marine Biological Laboratory and NAI-member. "We need to understand how diversity has evolved in all microbial lineages."

To convert into hydrogenosomes, mitochondria acquire genetic material from anaerobic bacteria. This passing of genetic material from one microorganism to another is called lateral gene transfer.’

"Lateral gene transfer involves the movement of genes between organisms," says Sogin. "This allows the sudden introduction of one or more new biochemical capabilities into an organism."

According to Sogin, lateral gene transfer differs from what we normally think of as evolution, in which genetic mutation changes the gene pool over time. As mutations accumulate in the genome, new capabilities develop or other functions are lost. Scientists can track these gradual changes caused by mutations, but lateral gene transfer is harder to track because it involves the abrupt introduction of genes that do not correspond to the natural development of the gene pool.

In the Earth’s early history, when free oxygen was limited, the planet was dominated by single-celled organisms that had little or no intercellular organization. It is thought that blue-green algae eventually produced so much oxygen that the oceans could no longer absorb it, and the excess oxygen accumulated in the Earth’s atmosphere. At that time, oxygen was a poisonous gas to the vast majority of life on Earth. For life to survive, new forms of metabolism based on oxygen respiration had to develop.

Molecular and fossil data indicate that eukaryotes emerged between 2 to 3 billion years ago, when the Earth’s atmosphere was still mostly devoid of oxygen. The earliest eukaryotes did not have mitochondria. Many scientists now believe that mitochondria were originally aerobic bacteria that were consumed by a larger, anaerobic organism (the early eukaryotic cell). Rather than be broken down as food, the bacteria survived to become a part of the cell’s inner machinery. The introduction of the aerobic bacteria allowed the anaerobic organism to survive in the oxygenated atmosphere, while the anaerobic organism provided nutrients for the aerobic bacteria.

krauss_books
Tree of life, divided between major cell types, those with a nucleus (eukaryotes) and without a nucleus (prokaryotes: the bacteria and archaea).

Because mitochondria and hydrogenosomes share many proteins in common, and because the metabolic pathway followed by hydrogenosomes is thought to be similar to that of the earliest eukaryotes before the development of mitochondria, some scientists believe that hydrogenosomes provide a view of how eukaryotes functioned before the rise of atmospheric oxygen. Sogin, however, does not think that hydrogenosomes tell us anything about the origin of eukaryotes.

"The hydrogen hypothesis is a very clever model that is intended to explain the origins of eukaryotes," says Sogin. "In my opinion, this area of research is on the one hand very exciting but on the other hand lacks meaningful molecular sequence data capable of proving anything about the validity of the hypothesis. I think the transformation from aerobic mitochondria to metabolisms based upon anaerobic hydrogenosomes has little or nothing to do with the origins of eukaryotes."

The NWO research team compared the hydrogenosomes of various protozoans and anaerobic fungi and discovered that the contents and form of these cell organelles differ from species to species. The hydrogenosomes of Nyctotherus ovalis, a protozoan from the gut of the cockroach, still very much resemble mitochondria. Those of Neocallimastix and Piromyces, fungi from the gut of the llama and the Indian elephant, look entirely different.

Besides the differences, there are also many points of similarity between the hydrogenosomes of different species. The researchers concluded that the hydrogenosomes of the different species constantly evolved anew from mitochondria in the course of evolution.

In other words, says Sogin, hydrogenosomes are evolving from mitochondria even today whenever organisms move from an oxygenated environment to one lacking in oxygen. This switch in environmental conditions can be as simple as becoming buried by mud, or being ingested into the gut of an animal.

"What is clear to me is that hydrogenosomes have arisen multiple times in the evolutionary history of protists, and in fact might even be an inevitable consequence of a eukaryote moving into an anaerobic niche," says Sogin.