Horizontal gene transfer (HGT), also called lateral gene transfer (LGT), is the process by which genetic information is transferred directly to unrelated cells, in contrast to vertical gene transfer, which is the classic parent-to-offspring heritability. It is undoubtably important in unicellular organisms, be they eukaryotic (Keeling & Palmer, 2008) or bacterial (Ochman et al., 2000), and their evolution.
While my personal interest in HGT is from the point of view of phylogenetics (how to detect it), this post is an answer to a question several readers have converged upon: can all genes be swapped by HGT?
The answer is no.
The key to HGT are mobile genetic elements (MGEs): these are the plasmids, transposons and bacteriophages. Basically, these are collections of genes that are swapped between neighbouring cells, conferring onto the recipient the ability to use those inherited genes for its own purpose. The most famous of these swappings is the endosymbiosis of chloroplasts: when the free-swimming chloroplast was eaten by that eukaryotic cell on that fateful day billions of years ago, massive gene transfer from chloroplast to host had to occur to allow it to be incorporated into the cellular machinery, and we see various stages of this transfer in the various plant and algal groups today. Another famous case of HGT is antiobiotic resistance: while a large part of that is natural selection, HGT also plays a role by allowing the resistance genes to be passed on to the population – a prime example of this is MRSA superbugs (Bloemendaal et al., 2010).
MGE genes are never “core” genes. Chloroplasts have retained the genes responsible for their main function, photosynthesis, as well as those responsible for translation and replication. Those can never be given away, or else the chloroplast will not be able to function anymore. Everything else has been given to the host cell’s nucleus. With the antiobiotic resistance scenario, the MGEs carry with them genes that are advantageous in some situations (resistance to an antibiotic). The transfer is not “purposeful”, but those genes just happened to be on the transferred MGE.
So a longer answer would be that while it might be theoretically possible for “core” genes to transferred as well, it has never, as far as I know, been observed. A case for and against it can be made: chloroplast genomes are prone to natural selection, since they are non-recombinant and thus open to deleterious mutations. The quicker they get their genes transferred to the nucleus, the better. But then the function and biogenesis of the chloroplast might be lost in the process, if the incorporation of those core genes goes wrong.
Bloemendaal ALA, Brouwer EC & Fluit AC. 2010. Methicillin Resistance Transfer from Staphylocccus epidermidis to Methicillin-Susceptible Staphylococcus aureus in a Patient during Antibiotic Therapy. PLoS One 5, e11841.
Keeling PJ & Palmer JD. 2008. Horizontal gene transfer in eukaryotic evolution. Nature Reviews Genetics 9, 605-618.
Ochman H, Lawrence JG & Groisman EA. 2000. Lateral gene transfer and the nature of bacterial innovation. Nature 405, 299-304.