Horizontal Gene Transfer

Eastern Emerald Elysia - a sea slug. Photo by Patrick Krug of the
Encyclopedia or Life

Everyone knows the usual story of evolution. An organism wishes to pass on its genes to the next generation must do so by surviving and reproducing in a dynamic environment. It must evade predation, avoid the pitfalls associated with weather, find enough food to make it to the next day and convince a partner (if the organism reproduces sexually) to mate in order to pass on those genes. If the genes are well-suited to the context in which it evolved, then the next generation will be given the necessary tools to continue the same struggle. If the genes are not well-suited for some reason, or the environment or competition is too harsh, then the survival of the organism becomes questionable. This story works because of a mechanism we will call Vertical Gene Transfer (VGT). This is where genes are passed from parent to offspring through reproduction.

So, in light of this scientific truth, it may be surprising to know that VGT isn't the only game in town. There is a thing in the world called Horizontal Gene Transfer (HGT). Certain organisms are capable, it seems, of exchanging genes in ways that DO NOT involve the standard sexual or asexual reproduction model. There are several different ways in which HGT can happen:

• Transformation - a bacteria cell will intake foreign DNA, which will eventually be expressed inside of the cell, by use of the bacteria's cellular machinery. 
• Transduction - this is where a virus transplants genetic material from one host to another, which ends up being expressed in the new cell. 
• Bacterial Conjunction - a situation where bacteria simply exchange genetic material in cell-to-cell contact. This method appears to be quite common. 

If you allow this idea of HGT to sink in, you might come to the conclusion that a mechanism like this can muddy up the tree of life a bit. In fact, if such a mechanism were widespread, the field of biology can be distinctly certain that the Tree of Life might be more like a spiderweb.

Biologists first became aware of HGT in 1951. Since then, a few studies have tackled its implications. In the intervening 50 years since, some interesting HGT candidate organisms have come to be known. For example, certain viruses are capable of HGT. It is quite common among prokaryotic bacteria. HGT may have occurred within the eukaryotic cell, between the nuclear DNA central to the cell and the mitochondria DNA that is also present (see the endosymbiosis theory). Yeast can do it. The Adzuki Bean Beetle, the Eastern Emerald Elysia Sea Slug, too...a small but growing list of HGT-capable organisms is being assembled. 

The most recent addition to this list? Rafflesia arnoldii - the world's largest flower. Harvard University's Charles David and his team have been studying this unique parasitic flower and found that even though the flower and its target host, a vine, are separated by 100 million years of evolution, they share several dozen "actively transcribed genes", genes in the parasitic flower that likely "originated from the host" vine (see this article for more information). A flower that takes genes from its host using HGT?! Clearly, this IS interesting research. Who would have guessed that the world's largest flower, already unusual for its size and odor (reportedly smells like a corpse), is also a GENE THIEF?!

Now, imagine the implications this has for astrobiology. Our best bet so far, in looking for life outside of the Earth, is to assume that a Darwinian evolutionary mechanism supported by water is the basis of life. However, imagining HGT in an alien context changes the game quite a bit. What happens when HGT is more dominant than VGT in an evolutionary system? How does HGT get started? Why is VGT preferred over HGT in our current system? What does this mean for the prediction of where we can find life in the universe? Horizontal Gene Transfer is a brilliant example as to why studying rare, unusual examples of life here on Earth - cases represented by parasitic or predatory plants, extremophiles and chemosynthetic ecosystems on the bottom of the ocean - can help prepare us for the eventualities waiting for us in other corners of our galaxy. 

Read about the HGT capabilities of the world's largest flower at the Harvard Gazette

Click here to get a better grasp of the process of HGT

Some discussion about HGT in Scientific American

The Esalen Center for Theory and Research has an excellent write up about HGT

Rafflesia arnoldii gallery