Nature’s self-plagiarism rewards life forms by sidestepping evolution

Red algae, fruitfly, and humans may have more in common than expected.
 

Red algae are among the oldest manifestations of life. They have been around for well over a billion years and arguably will endure for as long as this planet will remain habitable for gene-based life. Galdieria sulphuraria, a stunningly successful variant of this extremophilic life form, has been found in the boiling-hot springs of Yellowstone National Park, or in the acidic waste water drains of mine shafts where they are exposed to extreme levels of heavy metals, as they have emerged in some of the highest saline concentrations on Earth. Its metabolism is extremely adaptable as well: at times, it “forages” by photosynthesis while at other times it devours a wide array of bacteria in its immediate vicinity, growing either photoautotrophically or heterotrophically on over fifty carbon sources.

How is this possible? Simple: Galdieria, a microbial eukaryote, has concluded that it is better to decline the arduous path of evolution just to reinvent the wheel. It plagiarizes.

Since nature does not run a patent office nor a copyright register, Galdieria figured out a simple way to extend to the successfully adapted elements of its environments the sincerest form of flattery: it copies them by way of horizontal gene transfer. By absorbing genes outside of sexual transmission and across species barriers, this form of algae has adopted at least five percent of its protein-coding genes from organisms in its environment. Analyses of the Galdieriagenome show that it has assumed achaea’s heat resistance while it took its resistance to heavy metals such as mercury and arsenic from bacteria that had developed transport proteins and enzymes. 

While Galdieria sulphuraria presents a fascinating example of unexpected effects of crossing the species barrier, other examples are even more relevant to us: a fragment of human DNA was found in Neisseria gonorrhoeae, the bacterium that causes one of the oldest human scourges of sexually transmitted diseases. Such a horizontal transfer of genes from the highly developed human life form to a bacterium constitutes a huge jump. Studies concluded that absorption of human DNA by Neisseria gonorrhoeae must have happened quite recently in evolution’s timeline. It also explains the bacterium’s high adaptability and its persistence throughout human history.

Genetic transfer can also work from lower to higher species. For example, genetic material of a bacterial parasite called wolbachia is found in 70 percent of the world's invertebrates. But there exists at least one species, the fruitfly Drosophila ananassae, that contains the entire genetic material of wolbachia and continues to replicate it as its own. Here, the genetic transfer clearly benefits the parasite, not its genetic host. 

In the human genome, as many as 223 of some 23,000 genes appear to have been acquired directly from bacteria through horizontal transfer through incidents such as bacterial infections. Those genes are present only in prokaryotes and in humans, having skipped entirely all intermediate life forms such as invertebrates. 

Recent studies questioned the idea of gene transfer between human and lower species. Analysis of a larger spectrum of non-human genomes suggested a reduction in the number of human genes serving as potential proofs of gene transfer to less than fifty, implying that even that remaining set can be disqualified with further research. However, considering the staggering scale of contamination of non-human species’ genomes in databases with DNA of scientists handling the samples, any such identification needs to be carefully screened for false positives. 

In view of the fact that a healthy human body contains trillions of microorganisms, ten times as many microorganisms as it contains human cells, and the number of present microbial genes at an overwhelming 3.3 million dwarfs the human genome's 23,000, it is conceivable that some horizontal gene transfer would indeed occur.  Current research mapping the human microbiome is considered of importance equal to the human genome project, since the 10,000 microbial species present in the human body, mostly in its gastro-intestinal tract, play a critical role in the very survival of the human species. Microbes are responsible for digesting our food, for producing vitamins and anti-inflammatories needed for our immune response, but they also need to be taken in consideration when devising treatments for human diseases, not only for those of a bacterial or viral nature. 

The next time we venture into the great outdoors and feel like we need to protect ourselves from the “dirt and contamination” ubiquitous outside of our aseptic homes, maybe we should consider that we are, in fact, an integral part of nature, and that its microbes are part of us in far more ways than one.