Lateral Gene Transfer
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Contents |
[edit] Introduction
Lateral gene transfer, also known as horizontal gene transfer, is the transmission of genetic material from one organism to another, as distinct from the more normal vertical gene transfer, where an organism inherits its genes from its parent or parents.
[edit] Examples of lateral gene transfer
In this section we shall review the mechanisms of lateral gene transfer and give some examples.
[edit] Transformation
Transformation occurs when a bacterium takes up naked DNA from its environment and incorporates it into its genes.
This was first observed in 1928 by Frederick Griffith, a British doctor, who observed that when he mixed dead pneumonia bacteria that were virulent with live, non-virulent pneumonia bacteria, the live bacteria could acquire the virulent trait and pass it on to their descendants. It was by following up this observation that scientists obtained the first evidence that DNA was the genetic material of cells.
Acquiring genes from dead cells might seem like a poor idea, since insofar as a dead cell is dead, it has lost out in the game of life. However, computer models show[1] that under a wide range of conditions, transformation does provide an evolutionary advantage.
[edit] Transduction
In transduction, phages (viruses that prey on bacteria) shuttle DNA from cell to cell.
A brief, rough sketch of the life cycle of a phage may be in order. A phage is a bit of DNA wrapped in a protein coat. The phage fastens onto the outside of a bacterium, and injects the DNA into it. When the DNA is there, it subverts the cell so that the cell machinery replicates the phage DNA, and also transcribes and translates it to produce the proteins that make up the protein coat of the phage. These proteins assemble themselves around the phage DNA to produce new phages, which then leave the bacterium, and the cycle begins again. There are subtleties to this picture that we've skimmed over, but that's the basic idea.
Now, if during the stage where the phages assemble themselves inside the host, the protein coat accidentally assembles around a fragment of the DNA of the host bacterium, instead of phage DNA, then the phage will inject this DNA into its next host: this is transduction.
[edit] Conjugation
The F pilus will latch on to a bacterium without an F plasmid, and reel it in until the cells are close enough for a cytoplasmic bridge to form between them, at which point a copy of the F plasmid will pass from the donor to the recipient. The recipient, having acquired an F plasmid, can now make an F pilus of its own and pass on the F plasmid to other cells.
In this respect, the F plasmid acts as a set of selfish genes: its "purpose" seems to be simply to spread itself about.
However, sometimes the genes of the F plasmid become incorporated into the main chromosome of the bacterium. Once this has happened, when conjugation occurs a copy of part or all of the genes of the donor bacterium can be dragged into the recipient cell along with the F genes. This means that copies of useful genes can pass from bacterium to bacterium.
The F plasmid, particularly in E. coli, has been the standard model for the study of conjugation, but there are other plasmids with similar effects, most importantly the R plasmids, which not only cause conjugation, but also carry genes for antibiotic resistance.
Mostly scientists have studied conjugation between bacteria of the same species: such studies were used to map the genome of E. coli. However, conjugation has certainly been observed between bacteria of different species: for example, with E. coli as the donor and Serratia marcescens as the recipient. E. coli has even been observed to transfer genes to that other favorite experimental organism, yeast[2][3][4], which is not a bacterium at all, but a eukaryote, demonstrating the possibility of lateral gene transfer not merely between different species, but between different domains of life.
[edit] Agrobacterium tumefaciens
This has no particular evolutionary importance, because it does not affect the sex cells of the plant, and so the genetic change is not passed down from generation to generation.
[edit] Wolbachia
Wolbachia pipientis is a bacterium that lives in many species of arthropod, and infects their eggs, so that it is passed down from generation to generation. If the DNA leaks out of a Wolbachia into the egg, it has a chance to get mixed up with the genes of the host species, and this seems to have happened with Drosophila ananassae, a species of fruit fly which is a host to Wolbachia and which has a nearly complete set of Wolbachia genes inserted in its genome.[6]
[edit] Chimerism in bacterial evolution
As with sexual recombination in more complex organisms, lateral gene transfer, by definition, does not produce any new genes, it merely puts existing genes into novel combinations. However, its effects are much more dramatic than those of ordinary sexual recombination, because (by definition of species) sexual reproduction occurs between members of the same species: we cannot mate an eagle with a lion and get a griffin.
Lateral gene transfer, by contrast, can and does produce such weird chimeras between bacterial species. Besides what can be directly observed, there is genetic evidence suggesting that lateral gene transfer is implicated in the evolution and transmission of such characteristics as photosynthesis, aerobic respiration, nitrogen fixation, sulfate reduction, methylotrophy, isoprenoid biosynthesis, quorum sensing, flotation, thermophily, and halophily.[7]
Consider the consequences of this for phylogeny. Amongst more sophisticated organisms, vertebrates for example, once two populations have separated into two different species by reproductive isolation, their genes will never get shuffled together again, and so we can draw a family tree of vertebrates that branches whenever reproductive isolation occurs. In bacteria, by contrast, with their promiscuous habit of lateral gene transfer, two long-separated and differentiated species can have their genes mix back together again to produce a third species. So although we should be able to draw up a family tree for individual bacterial genes, a phylogeny of the bacteria themselves should not be a tree, but rather a sort of net or web.
[edit] Chimerism between bacteria and higher organisms
We have already given the example of Wolbachia transferring genes to fruit flies, of Agrobacterium transferring genes to plants, and of E. coli transferring genes to yeast as examples of bacteria transferring genes to eukaryotes.
Recently, with the sequencing of the human genome, evidence has come to light that about 0.5% of the human genome is made up of scraps of bacterial DNA[8]. Before the recent advances in gene sequencing, scientists used to suspect that evidence of bacterial DNA in eukaryotic genomes was the result of contamination of their samples. Now that it is relatively easy to sequence genes, biologists are beginning to realize the true extent of lateral gene transfer, and with further application of the new techniques we can expect to see a lot more discoveries of a similar nature.
[edit] Chimerism between higher organisms
As we have seen, lateral gene transfer was and still is an important factor in bacteria. Why, then, can't we see lateral gene transfer between, let us say, elephants and humans, causing some lucky (or unlucky) person to be born with a prehensile trunk?
We dwell on this point because it is often (correctly) asserted that the discovery of a morphological chimera such as a dragon or a griffin would falsify the Theory of Evolution. But if lateral gene transfer is possible, then on what grounds does the theory rule out the occurrence of such a chimeric form?
There are a number of reasons:
- First, there is no mechanism for such a transfer to take place. Bacteria get inside us: elephants do not. Remember, even if you were to eat a fresh raw haunch of elephant, so that intact elephant DNA was in your body, it would still be a long way from getting inside your germ cells --- your sperm or eggs --- and so could have no evolutionary consequences. The only halfway plausible way to get elephant DNA into a human germ cell would be for a bacterium in an elephant to take up elephant DNA, for that strain to survive, for it to infect humans, and for a further gene transfer to occur to a germ cell. We do not maintain that such a thing is absolutely impossible, but it is surely a very long shot.
- You will often hear it said that DNA is "not a blueprint, but a recipe". To be precise, DNA is a blueprint for polypeptides, which is why lateral gene transfer can be useful to bacteria. However, it is not the blueprint for a body plan: the body plan of complex organisms is the result of a complicated interaction between the proteins. A zygote with half an elephant's genes will not grow up to be the front half of an elephant; and in the same way, it is problematic to speak of "genes for having a trunk". The nearest we might come to this is to identify genes in an elephant which, if damaged, would cause incorrect development of the trunk, but that is not really the same thing, for even when functioning correctly, such genes produce proteins that produce a trunk only in interaction with all the other proteins of the elephant.
- In bacteria, we often find genes that relate to some common purpose, such as manufacturing histidine, are arranged together on the same little stretch of DNA: such clusters of genes are known as operons. In humans, elephants, and other "higher" organisms there is no such organization: so that even if elephants had "trunk genes", they would not all lie on the same snippet of DNA, but distributed all over the elephant's chromosomes.
Hence such the production of such a chimera as a result of lateral gene transfer can safely be ruled out.
[edit] Misconceptions
We know of only one misconception concerning lateral gene transfer: this, of course, originates with creationists, who will sometimes claim that lateral gene transfer is an ad hoc hypothesis to explain why the methods of molecular phylogeny don't sort bacteria into a phylogenetic tree as with higher organisms. But, as we have seen, lateral gene transfer between bacteria is a phenomenon that may readily and frequently be observed directly: there is nothing ad hoc or hypothetical about it.
It would be downright weird if lateral gene transfer between bacteria was a strictly modern phenomenon that never happened prior to 1928 when it was first observed. The only sensible conclusion is that it did affect the history of life before that date, and that therefore we should expect the phylogeny of bacteria to be a web rather than a tree.
