Because no one has time to scroll down when they're busy learning:
Learning Objectives:
- Explain why horizontal gene transfer can complicate evolutionary hypothesis
- Explain why the "Tree of Life" is being replaced by the "Web of Life"
Section Summary
Section 26.5 describes the role horizontal gene transfer (HGT), or lateral gene transfer, has on evolution. So far in our evolution unit, we've focused on the concept of vertical evolution, which are changes in groups of species due to descent from a common ancestor. It became the "traditional" way biologists have viewed evolution since Darwin's time and had the foremost role in the phylogeny of living things. Recent research has proposed, however, that horizontal gene transfer, which is a process in which an organism incorporates genetic material from one organism to another without being the organism's offspring, has an important role in phylogeny also.
Horizontal gene transfer has been discussed before through bacterial conjugation, transduction, and transformation; we've learned that genes can be transferred from one bacteria to another through a F pilus or virus, and that those genes can encode for desired hormones in labs (e.g insulin), or proteins beneficial for a bacteria's survival. HGT is most common in bacteria and archaea, but it does occur rarely with eukaryotes because many eukaryotic organisms are multicellular despite vertical gene transfer being the more common method of gene transfer. From that, the frequency and importance of HGT in multicellular eurkyotic evolution is difficult to determine as well. Interestingly, HGT may complicate the concept of a universal ancestor in evolution; genomic research suggested that HGT was prevalent when all organisms were unicellular during the early stages of life on earth, and the universal ancestor may have been a community of cells rather than a single prokaryote. In addition, this prevalence of HGT suggests that the tree of life depicting the early evolution of life on Earth should instead be illustrated as a "web" of life. Gene transfers between species, or even domains, may foster evolutionary change in the recipient, and these transfers are indicated by horizontal connections between two branches on the phylogenetic tree of early life, Changes in groups of species can be caused by horizontal gene transfer in addition to descent from a common anscestor. As a result, these connections akin the "Tree of Life" more to a "Web of Life".
Vocabulary:
- Horizontal gene transfer: A process in which an organism incorporates genetic material from one organism to another organism without being an offspring of the organism.
- Vertical evolution: Changes in groups of species due to descent from a common ancestor.
- Vertical gene transfer: The transfer of genes from parent to offspring. Essentially reproduction.
Useful Materials
http://highered.mcgraw-hill.com/sites/dl/free/0072835125/126997/animation46.html
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I highly recommend (meaning, you really should) viewing this short animation before class. This animation describes the difference between vertical and horizontal gene transfer in an easy-to-understand manner. The most important part is that it goes on to show how horizontal gene transfer can occur in a eukaryotic cell.
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| http://www.scienceagogo.com/news/20070029220033data_trunc_sys.shtml |
This is a short article by Kate Melville from scienceagogo.com explaining that HGT may have accelerated rates of evolution according to Rice University researchers. She began by briefly explaining the times when new life emerged according to the fossil record. Then, she went on to say that HGT may have increased rates of evolution according to Rice University research.
Visiting professor Jeong Man-Park and Rice's Michael Deem developed a new mathematical model of evolution in an attempt to understand how HGT changes evolutionary dynamics. They've found that HGT accelerated evolution by "propagating favorable mutations across populations", compared to older models which account only for point mutations and sexual recombination.
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| http://www.astrobio.net/exclusive/5416/digging-down-below-the-tree-of-life |
This is a nice post from Astrobiology magazine about horizontal gene transfer. Here, Schirber discusses the role HGT has on the tree of life, and how it was important during some of life's earliest stages. The article begins with a short introduction to vertical and horizontal gene transfers, It then goes on to describe that early life relied on HGT, and that organisms that partook in HGT had an advantage over those only transferred genes vertically.
This article is much more detailed in comparison to the other article. It's nice because it includes more information about HGT, evolution, and phylogenetic (family) trees. This is certainly a great resource to read to apply your understanding of HGT.
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| http://rspb.royalsocietypublishing.org/content/277/1683/819.full |
Kind of a long paper about HGT, but if you're into that stuff, I think it'll be worth your while. This paper covers a lot of topics, such as the increasing importance of HGT in evolution, and determining how important HGT truly is in evolution. It then goes on to describe in detail the relationship between HGT and microbial evolution, then HGT and current evolutionary thinking.
This paper is really legit. If you still don't quite understand HGT, this resource is really helpful in understanding it if you have the time. If you don't quite understand HGT's role in evolution, it has a lot of big words but explains its role very nicely.
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Primary Literature
http://eqsys.org/2011-Graduate-PS/Week9-Multigene%20analyses%20HGT/nrg2386.pdf
Download: nrg2386.pdf
This is a really good excerpt from a textbook or encyclopedia regarding horizontal gene transfer. It goes into much more detail than the textbook does, so if you need a little more information to understand the topic - or if you're into that kind of stuff - then I highly recommend reading a bit of this. It's structured similarly to our textbook and even provides color diagrams to enhance the material.
This paper covers a lot of topics relating to HGT, including detecting and evaluating HGT, the relationship between endosymbiosis and HGT, and so on. It begins by introducing the reader briefly to the definition of HGT (like many papers), but then goes on to describe more interesting concepts related to HGT. The first section describes the difficulty of detecting and evaluating cases of HGT in organisms, and that the "gold standard" in identifying cases is phylogenetic incongruence. Then, the relationship between endosymbiosis and HGT, and a great example of it would be the transfer of genes for chloroplasts and mitochondria were acquired by eukaryotes from bacteria. Prokaryote and eukaryote gene transfers are explained next, along with even eukaryote-eukaryote gene transfers. Lastly, HGT and eukaryotic phylogeny is explored because HGT has been identified as an important force driving eukaryotic evolution.
Virtual Lecture
PowerPoint Slides
My slides:
Book slides: (Horizontal gene transfer specific slides start at slide 38)
Download: Chapter 26.ppt
Exam Questions
Quiz
Grading Sheet (Monica)
Comments (2)
Monica Macaro said
at 7:55 am on May 2, 2013
UPDATE: The primary literature link has an error. I've uploaded each page of the primary literature instead, with PL1 representing page 1 of the paper, etc. I'm sorry for the inconvenience.
Monica Macaro said
at 8:40 am on May 2, 2013
UPDATE UPDATE: Decided to use another resource rather than the original.
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