Derek Meyer
Well-Known Member
So, from all of this I gather that all modern organisms are a result of genetic variation coupled with natural selection?I dont brush over that the HGT events happened early and in fact I have stated this. Cross breeding happened earlier as well because creatures were able to cross breed between species more then as they were not as separated as they are now. So these process may have allowed a great variety of life to be created in the early history of life. If all life had access to not just the genetic material available in their own pools but to a vast array in all pools then this would give them a great amount of genetic material to use in adding new features and creating different types of animals.
All I know is paper after paper keeps mentioning that HGT was more prominent in complex life then thought and is still happening today. Doolittle is one of many who have mentioned HGT and these are models to explain the existence of foreign genes in different creatures. So there are going to be different perspectives for how this happened. Doolittle focused more on prokaryotes so therefore he was going to view the pattern for eukaryotes as still being tree like. But there have been new discoveries since the work of Doolittle which show that HGT is far more prevalent in eukaryotes. As stated earlier as time goes by more and more cases of HGT are being discovered.
Available data indicate that no insurmountable barrier to HGT exists, even in complex multicellular eukaryotes. In addition, the discovery of both recent and ancient HGT events in all major eukaryotic groups suggests that HGT has been a regular occurrence throughout the history of eukaryotic evolution.
About a decade ago, Doolittle et al. raised a question about the number of bacterial genes in protists, speculating that many bacterial genes should have accumulated in genomes of protists through feeding activities 1,2. Back then, horizontal gene transfer (HGT) had been documented widely as a mechanism to gain foreign genetic materials in prokaryotes, but remained largely an exotic concept in eukaryotes, with little substantial evidence. It is now clear that HGT has occurred in all major eukaryotic lineages.
There are many straightforward cases of HGT in eukaryotes that involve recently acquired genes 10,18,36,75,76.
Unicellularity is the most common form of eukaryotic life, and it is known that unicellular eukaryotes are prone to HGT 3,4.
In fact, acquired genes can be found in numerous unicellular eukaryotes including many obligate intracellular parasites 76,87–89, which often have streamlined genomes and retain fewer foreign genes. The fact that all multicellular eukaryotes descend from unicellular ancestors points to potentially more frequent ancient HGT 20,90. Indeed, foreign genes were introduced regularly at major historical stages during the evolution of primary photosynthetic eukaryotes 17,20,91–96.
Therefore, HGT during early eukaryotic evolution might occur as frequently as in modern bacteria and archaea, allowing foreign genes to trickle into early eukaryotes continually.
External fertilization occurs in animals inhabiting aquatic environments, meaning gametes and zygotes are, likewise, freely exposed to foreign sources of DNA. Structurally internalized gametes in seed plants and animals in terrestrial environments may be protected from mechanical damages, but not necessarily foreign DNA from symbiotic bacteria, pathogens, or other microbes 8,10,19,38. Propagation of foreign genes also is possible through gene transfer among neighboring cells, as demonstrated in natural plant grafts 102,103. In these respects, the entry points in early developmental stages represent the weak link in recipient organisms for initiating foreign gene transfer; as such, they ultimately control the transmission of foreign genes to offspring.
This model also makes the following specific predictions regarding the occurrence or overall frequency of HGT in eukaryotes of different lifestyles:
Given the difficulties and complications discussed above, it is important that putative cases of HGT in eukaryotes be investigated carefully. To do so, independent lines of evidence and alternative scenarios should be considered. Many cases of patchy distribution probably reflect combined effects of duplication, gene loss, HGT and other processes 80,112,113. Nevertheless, as long as vertical inheritance remains the null hypothesis, HGT in eukaryotes will likely be underestimated. Therefore, it is useful to bear in mind that HGT, although difficult to “prove” in every individual case, offers a valid explanation for many of the atypical gene distributions in eukaryotes.
- Frequent HGT in unicellular eukaryotes. Since all developmental stages of unicellular eukaryotes represent weak-link entry points, there are ample opportunities for foreign genes to be integrated and, therefore, transmitted to offspring.
- Occurrence of foreign genes in multicellular eukaryotes with fully exposed unicellular or early developmental stages (e.g. spores, zygotes, or embryos) in their lifecycles (see above).
- Frequent HGT in asexual multicellular eukaryotes. The absence of specific germ cells means that any cell carrying foreign genes may propagate them into offspring. The frequency of HGT should be even higher if bacterial endosymbionts exist in asexual structures, such as spores and hyphae in fungi 67.
- Existence of many anciently acquired genes in multicellular eukaryotes. Because multicellular eukaryotes are ultimately derived from unicellular ancestors, it is expected that many foreign genes acquired by their unicellular ancestors remain in the genomes of their multicellular descendants.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4033532/
Upvote
0