Читать книгу Algorithms in Bioinformatics - Paul A. Gagniuc - Страница 46

The symbiosis between organisms is not possible without the HGT. The HGT is the weak “glue” that unites all species and it has important evolutionary implications [129]. In the future, these implications may very well undo the classification discussed above for the tree of life and how we understand the origin of life on Earth. The HGT refers to the transmission of DNA between different genomes, whereas the vertical gene transfer (VGT) is made between generations by sexual or asexual reproduction. The way in which the classification for the tree of life works is largely based on the VGT concept; thus, one can imagine the issue. HGT was first observed as a phenomenon in Streptococcus pneumoniae species by Frederick Griffith in 1928 [130]. The main observation made by Frederick Griffith was that virulence (pathogenicity) in this species of bacterium is transmitted by contact or proximity. This was an important revelation for the later field of genetics. Since then, increasing evidence shows that DNA fragments of different sizes may be exchanged between the kingdoms of life, to a greater or lesser extent [129]. Not long ago, the transfer of genetic information from the members of the Agrobacterium genus to eukaryote cells was seen as an extraordinary and rare process [131, 132]. Today, evidence indicates clearly that transfer of genetic information between species and inside different cell compartments is a common process, which takes place over the evolutionary time. For instance, bacteria have acquired genetic material from eukaryotic hosts and vice versa [133]. Viruses contain genes derived from their eukaryotic hosts and vice versa [134]. In plants, for instance, the HGT between genomes takes place through intracellular transfer of DNA among the nuclear, mitochondrial, and plastid genomes. The transfer of mitochondrial genes to the nucleus is known to be an ongoing evolutionary process. However, evidence also shows a HGT of mitochondrial DNA to the plastid genome [135]. Moreover, expression of a transferred nuclear gene in a mitochondrial genome was also observed [136]. For example, the orf164 gene in the mitochondrial genome of Arabidopsis is derived from the nuclear ARF17 gene that codes for an auxin-responsive protein [136]. Thus, the transfer of DNA segments from any location to any other location seems to be a rule across all life. However, HGT is most frequent between closely related species with similar genome features and less frequent otherwise [137]. In other words, HGT is a process that occurs at different frequencies between prokaryotes, between eukaryotes, between prokaryotes and eukaryotes and vice versa [138]. Perhaps, the importance of HGT goes as far as the emergence of new species (speciation) [139, 140].