Lateral Contributions to Evolution
(An editorial review of an article in
The Scientific American, February 2000, p90)

There is increasing evidence that the evolutionary tree of life is something other than a neatly branching tree. Ever increasing evidence from molecular taxonomy indicates that it is more like a "shrub of life" in which there is much self-grafting between the branches and twigs.

Lynn Margulis was the first to propose and give important backup proof that chloroplasts and mitochondria are actually symbiotic prokaryotes within the eukaryotic cells. Not only are there many morphologically different "species" of both organelles - spherical, rod-shaped and helical, but there is important evidence that, while the cytoplasm has larger ribosomes with larger subunits containing 5, 18 and 28-S rRNA, these two types of organelles have ribosome component sizes that are identical to those in prokaryotic cells with smaller subunits containing 5, 16 and 23 S rRNA. Furthermore, while the base-ratios ([A+T]/[G+C]) of the nuclear DNA range narrowly across the whole of the animal and plant kingdoms, the DNA base-ratios of the various sorts of mitochondria and chloroplasts are very diverse - just as is found within the bacterial realm.

This strongly indicates that bacteria, and perhaps archebacteria also, colonized the cytoplasm of early cells. Furthermore, this was NOT a one-time event in both cases: it must have happened many times! How else could the fact that so many different kinds of mitochondria and chloroplasts are seen in nature?

Thus, somewhere in the early stages of eukaryotic life, highly aerobic bacteria took up residence in the larger nucleated cells and, in this safe haven, lost many of their genes as no longer being needed, thus becoming today's mitrochondria. Later, various cyanobacteria made the same lateral maneuver and took up residence on some of the cells already containing mitochondria, and have become what we today call chloroplasts.

For a little side-note: cyanobacteria are not the only photosynthetic bacteria. So why didn't the other photosynthetic bacteria take up residence in the eukaryotic cells? Well, the reason probably rests on the fact that oxygen is detrimental to these other photosynthesizers. So you would expect that, since these host cells with mitochondria must live in oxygen to use it as the electron acceptor, they would not offer friendly environments to the bacteria that use non-water electron donors.

Because this is an editorial, allow me to say that I found that the article had a paucity of data to add to its "convincingness". Along with the 5S rRNA data, the author spoke only superficially of these two examples of lateral contributions to evolution. Are there any other evidences of the fusion of other branches on this "shrub of life?" The intrinsic nature of evolution is that it is an on-going process. The mitochondria and chloroplast examples form a historical set of long accomplished biological activity. Are there any more recent examples to fill in the needs of the continuing nature of evolutionary theory? Are there even examples of probings going on today that might lead to new symbiotic relationships? I believe there are!

Normally, when one thinks of photosynthesis, one considers what is called "C3" photosynthesis. This is characterized by layers of cells in leaves. I will not go further than that because only this gross anatomical aspect will show up in the fossil record. And that layered record is literally as old as the hills! BUT we also know of another type called "C4" photosynthesis, which is possessed by most of those plants that grow amazingly fast - corn, bamboo (both monocots), and in some dicots such as pigweed. Actually there are several thousand possessors of C4 photosynthesis. Again we are interested in the gross anatomy of those plants, especially those characteristics that would show up in the fossil record. C4 plants are characterized by concentric tubular design in their leaves. Fossils are very sparse for this design - none so far are very old - and certainly not as old as the hills. It is as if we are in the midst of the epidemic of C4 as it spreads throughout the plant kingdom and allows its host to grow faster and be more fit than C3 plants.

Of course, there is the case of euglena, which can lack any chloroplasts and act like an animal, or it can possess chloroplasts and act more like a Venus flytrap - both photosynthesizing and eating its neighbors. But, interestingly, euglena can serve as home for several different types of chloroplasts, and, in fact, for several different types of cyanobacteria! In many instances the bacteria can enter the cells on their own. Are euglena in the very earliest stages of a new line of plants?

What about people? We are home to mitochondria for sure. But some of us are home to other organisms - usually in not too friendly a way, but there are cases of tolerance, and some of those seem asymptomatic. Malaria, typhoid fever and, importantly, asymptomatic "Typhoid Mary's". Many infectious diseases are caused by organisms that invade cells. But there are cases when the disease symptoms do not arise. Now only to have the host cells make good use of the invader! Voila! Symbiont! And possibly a HUGE leap in evolution!

Also missed in that Scientific American article were the probable roles that viruses, plasmids and transposons (jumping genes) play in lateral contributions to evolution. Transduction of genes from one species to another by viruses is a very likely mechanism for moving useful characteristics laterally within the shrub of life. This might especially be the case with regard to those viruses that target mitochondria and chloroplasts, which might be more closely related to each other than would be the host cells themselves. Interspecific plasmid transfer is not widely recognized, but is the most likely mechanism for the "guard dog" theory of immunology mentioned elsewhere in this website. These "dairy bacteria" seem to be able to inject their killer plasmids into many different types of bacteria and fungi. These few types of bacteria coat the linings to all the entrances to our bodies, and apparently coat the tender, moist cells in the air spaces of leaves. Thus we eukaryotes have leashed these "guard dogs" to fight for our lives by moving plasmids quite freely into passing cells (and fortunately not into the cells of us host organisms!).

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