An investigation of an antibacterial gene across domains of life.
Generally when we think of the transfer of genes, we think back to our biology class where cells go through meiosis and become gametes. Then, through the process of fertilization, two independent gametes join and form a new organism that expresses the dominant traits from both gametes. Therefore, if your mom has a widow’s peak and your dad has dimples, you will end up with those same traits.
How you end up with those traits is through the process of transcription and translation. Your DNA in your nucleus transcribes, or writes, its code into RNA. RNA then can travel outside of the nucleus to ribosomes. Ribosomes are essentially tiny protein-producing factories; they translate the genetic code into a protein, which will go do some sort of job in the cell. If there is an error, called a mutation, in the genetic code, the wrong protein will be made.
Horizontal Gene Transfer, which will be referred to as HGT, is when genetic material moves laterally, whether that is between organisms (in a manner other than sexual fertilization) or even between species. This process is just as tricky as you’re imagining it to be. First of all, a part of a cell that has DNA in it (the nucleus, mitochondrion, or a chloroplast) would most likely need to be transferred, or in prokaryotic cells (cells without a nucleus or other organelles), the DNA itself would have to move between organisms without becoming degraded.
Many times, HGT results in dysfunctional genes and has lost its ability to code for proteins. In order for HGT to be a beneficial process in an organism, the gene that is being transferred must convey a positive phenotype to the new organism. It also must be able to get over all the physiological, cellular structure, and ecological niche barriers. For example, if the gene could not get through the cell wall, then HGT wouldn’t happen. Or, if the two organisms did not occupy the same ecological niche (a scientific way of saying they do not live in the same place), then transmission of genes between species also could not happen. In fact, research has shown that HGT has a decreasing frequency gradient from transfer within a domain to transfers between domains and finally to transfers between all domains. These reasons make HGT a very delicate process, and one that we still do not quite understand very well.
In a recent study, scientists set out to examine a gene that codes for a lysozyme, an enzyme that breaks down the outer casing of a bacterial cell. The scientists claimed that this gene gave the cell antibacterial properties. The data was collected through genetic analyses of several different organisms - an archaebacteria that lives at the bottom of the ocean near deep-sea vents, an aphid (a type of insect), clubmosses, and a fungus that aids in the production of soy sauce. All of these organisms have the same lysozyme.
By analyzing their DNA, the scientists found that the gene was homologous to a gene from a eubacterial cell (what you normally think of as a bacterial cell - E. coli and Salmonella are two). Through the analysis, the scientists found that 9 of 12 species of the archaebacteria had the gene, 5 of 6 of the clubmosses had the gene, and 8 of 9 of the aphids had the gene. The gene was also found in the fungus as well as in a type of virus. By looking at the structure of the actual enzyme, the scientists found that the structures were relatively similar across all the different organisms. Since the enzyme confers some antibacterial properties to the host organism, the cell would increase the production of this gene which could increase population numbers of that organism.
The scientists also investigated how the genes could have transferred from a eubacteria to all these different forms of life. What they discovered is that there was reported transfer between several eubacteria phyla to the different domains. Upon investigation of these phyla it was found that the eubacteria that transferred the genes to the fungus and plant species can inhabit soil, which would put it in the same ecological niche as the plants and fungi. The eubacteria that reportedly passed the gene to the aphid is pathogenic, meaning it could infect that species. Last, the eubacteria that passed the gene to the archaebacteria also lives in extreme environments.
In summary, HGT can happen between the same species of organisms or through several different organisms. Not all genes have to travel vertically (meaning travel parent to offspring). The HGT process is very rare, since not only do the organisms have to be in the same place at the same time, but the genetic material must be able to transcend a lot of barriers to become established in a new organism. So in the case of the antibacterial gene discussed above, the event is extraordinary; the very rare event of genetic material being shared laterally through all the domains of life not only occurred, but that gene became an established part of the genome!