XI.5 New genes can originate from interspecific (horizontal) gene transmission
A whole range of molecular mechanisms are known for single-celled organisms, through which the cell is capable of incorporating a nucleic acid derived from its surroundings into its nucleus (Amabile-Cuevas & Chicurel 1992).The best-known example is the transformation of bacteria.Here, it has long been known that, under certain conditions, the bacterial cell can very readily absorb quite long DNA sections, some of which it can incorporate into its chromosome.It has been estimated for the bacteria Salmonella enterica that up to 20% of its genes are strain-specific and the individual strains of the species most probably acquired them relatively recently through horizontal transfer from other species (Lan & Reeves 1996).Of eukaryotic organisms, asexual rotifers have incorporated a large number of foreign genes into their genomes .
Some groups of viruses are also capable of mediating in the transfer of a nucleic acid between two organisms and many of them even have specific mechanisms for incorporation of the relevant DNA section into the chromosome of the host cell.Endosymbiosis andintracellular parasitism can be considered to be further mechanisms for the transfer of genes between various groups of organisms.When one organism becomes domesticated in the cells of another organism, favorable conditions are created for the transfer of genes from one genome to another genome, usually from the genome of the parasite or symbiont (in the sense of a mutualist) to the genome of the host.It has been very well documented that a number of the genes in the cells of modern eukaryotes are derived from the genomes of prokaryotic symbionts or parasites, probably those that caused the formation of mitochondria and chloroplasts.Simultaneously, the mitochondria and chloroplasts still contain part of their original genome, but have long ago transferred most of their genes to the nucleus of their eukaryotic host.
Individual species of organisms live under very different conditions, so that their genes are exposed to different selection pressures.As a consequence, in various species, the same genes gradually develop in different ways and gain different and, from an evolutionary standpoint, frequently new properties.A gene that, in one organism, encoded an enzyme with a certain substrate specificity can, in a different organism, under the influence of a mutation, change into a gene encoding an enzyme with quite different specificity.
If an organism is capable of incorporating foreign genes into its genome and if it is selective advantageous for it to obtain a gene derived from some other species of organism, it is mostly only a question of time before a member of the particular species actually encounters the relevant gene in its surroundings and incorporates it into its genome.Once again, it should be emphasized that it is not at all clear how intensely evolution of new genes or new cellular functions actually occurs in the contemporary phase of biological evolution.It is, in fact, possible that evolution of new genes (as defined in Section XI.2) need not occur at all in multicellular species and that all evolutionary changes occur here through mutation in the genes participating in control of the ontogenetic process.In this case, interspecieshorizontal transfer of genes (HTG) could not be very important for evolution.On the other hand, for example, in bacteria or protozoa, most evolutionary changes could occur just at the level of evolution of new genes.Here, we could reasonably expect that the transfer of genes between different, even very unrelated species would be evolutionarily important.
The possibility of horizontal transfer of genes should be constantly borne in mind in interpretation of the results of molecular phylogenetics of single-celled species.It is, at the very least, very audacious to draw conclusions about the mutual relatedness of two species of protozoa on the basis of the fact that they contain a phylogenetically related enzyme in their genome, which is lacking in other protozoa.If both protozoa require an enzyme with the given specificity, then it is highly probable that, sooner or later, one will gain it from the other or from some third source (Jain, Rivera, & Lake 1999; de Koning et al. 2000).When considering the phylogenesis of organisms, it will probably be safer to base our considerations on the results of comparison, where possible, of neutral mutations in universally occurring molecules of the ribosomal RNA type or protein components of the replication or proteosynthetic apparatus.
At the present time, the evolutionary importance of interspecies transfer of genes is still a subject of discussion.It has been suggested that this phenomenon is basically of no practical importance, while other opinions assume that the evolution of new genes is totally dependent on the existence of gene transfer.