Category Archives: horizontal gene transfer

Lateral transfer climbs up the tree

Though researchers now generally agree that horizontal gene transfer is relatively common among simple organisms like bacteria, they have continued to assume that it remained relatively rare among complex organisms like plants and animals.

“The thinking has been that there is very little horizontal gene transfer among plants and animals except for a few big, ancient events and maybe the occasional transfer of a single gene here or there,” Slot said. “Our discovery suggests that the horizontal transfer of gene clusters may have been a big player not only in the evolution of bacteria but also in more complex organisms.”

Discovery of Jumping Gene Cluster Tangles Tree of Life

Another Unicellular Genome

Another genome from a single-celled organism has been sequenced. This time it is the green algae, Chlorella. Chlorella are tiny algae that can reproduce quite rapidly. Yet despite the stream-lined nature of the organism, it retains most of the phytohormone biosynthesis pathways necessary to the development and growth of land plants.

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Another interesting feature of the NC64A genome was the presence of homologs of receptors and biosynthetic enzymes of land plant hormones, such as abscisic acid, auxin, and cytokinin. The presence of these homologs does not necessarily imply the existence of plant hormones and their related functions in Chlorella but supports the hypothesis that genes involved in phytohormone biosynthesis and perception were established in ancestral organisms prior to the appearance of land plants.

Not only does this genome add more evidence to the growing plausibility of front-loading, but it also seems to offer a clue that the horizontal transfer of genetic information played a key role in the evolution of one of its key features – it’s unique chitin cell wall.

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Back in 2001, I proposed that the original cells, used to seed this planet, contained the ability to form “viretes.”  The basic idea is that the virete would function something like a gamete, but instead of transmitting genetic information across time to future generations, it would transmit genetic information across space to facilitate the survival of the founding group (see my discussion on cross-talk ) by connecting them.  Here is how I put it back in 2001:

Actually, I have been toying with the idea that viruses were designed  (keeping in mind that I view viruses as non-living, life-dependent phenomena and not organisms).  I would speculate that viruses were originally designed to allow the designed cells to cross-talk extensively. More specifically, I envision  cells designed with the program to disperse part of their genetic constitution laterally through a life-cycle-like stage that involved replicating and packaging genetic material for dispersal. In short, I speculate that what we now know as ‘viruses’ were originally a designed sex-like mechanism for unicellular organisms, important for establishing a foothold on a sterile planet (I call them viretes).  Possible expressions of this mechanism might include:

a. A cell suicide program coupled to the packaging of genetic material for dispersal.

b. An endospore-like program, where instead of forming a spore around the replicated DNA, the DNA is packaged in virus heads which in turn are packaged into a “release” cell.

c. Controlled exocytotic release.

I would further speculate that such sex-like mechanisms may have been important in the early stages of the designed founder effect allowing the heterogeneous cells to adjust, as a consortium, to an unfriendly environment. During this adjustment phase (analogous to the latent phase in a bacteria growth curve), the cells shuffled their material and hit upon global-adaptive state whereby the importance of such transfer was decreased. We still see “rusty remnants” of this state carried on by the vestiges of transposons, natural transformation, and yes, viruses.

Well, almost 10 years later, it’s looking like I was on to something:

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Genetic Cross-Talk

Bacteria can do tricks that your dog cannot do. A bacterium can acquire DNA from another completely unrelated species of bacteria just by living in the same area. For example, if you take cyanobacteria, a type of bacteria that normally live in the oceans and carry out photosynthesis, and mix them in a test tube with a population of E. coli, the bacteria that normally live in your large intestines, something odd can happen. If you let them sit together overnight, some of the E coli cells will link up with the cyanobacteria using a microscopic hose and then transfer some of their DNA into the cyanobacteria [1]. Once the E. coli DNA is inside the cyanobacterium, the cellular machinery will then splice the E. coli DNA into the chromosome. This would be like your dog somehow acquiring cat DNA and the ability to purr simply by sleeping on the same bed with the cat!This process of acquiring foreign DNA is known as lateral gene transfer (LGT) or horizontal gene transfer(HGT). A large number of scientists consider LGT to be a powerful force in bacterial evolution.

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