Jump to content
Science Forums

'Punctuated' evolution in the human genome


Tormod
 Share

Recommended Posts

Researchers report today that regions of the human genome have been hotspots for acquiring duplicated DNA sequences – but only at specific time-points during evolution.

 

lefthttp://hypography.com/gallery/files/5/2ac8e95064b7bb85e6bfbdaaac938b95-large_thumb.jpg[/img]It appears that long periods of genomic stasis, at least with regard to the accretion of duplicated DNA fragments, are "punctuated" by relatively brief episodes of duplicative activity. This is the first time that such temporal bias has been documented for DNA duplications, and it challenges the evolutionary paradigm that continuous alterations occur during the course of genome evolution.

 

The scientists, who are affiliated with the University of Washington (Seattle, WA), Case Western Reserve University (Cleveland, OH), the University of Bari (Bari, Italy), Washington University (St. Louis, MO), Washington State University (Pullman, WA), and Duke University (Durham, NC), report their findings online today in the journal Genome Research.

 

Dr. Evan E. Eichler, Associate Professor of Genome Sciences at the University of Washington, heads the team. "Primate genomic sequence comparisons are becoming useful for elucidating the evolutionary history and organization of our own genome," he explains. "Such studies are particularly informative within human pericentromeric regions – areas of rapid change in genomic structure."

 

Pericentromeric regions are sequences of DNA that lie in close proximity to the centromere, which plays a critical role in chromosomal separation during cell division. Pericentromeric regions contain an abundance of segmental duplications, which are large DNA sequences that exhibit strong similarity to the euchromatic ancestral loci from which they were copied. According to Eichler, the limited number of comparisons of pericentromeric regions among closely related primates suggests extraordinary dynamism, where duplication, deletion, and rearrangement of large segments of DNA occur at an unprecedented scale.

 

Eichler's group performed a comprehensive structural and evolutionary analysis of a 700-kilobase (Kb) pericentromeric region on the short arm of human chromosome 2. This chromosome has intrigued evolutionary and primate biologists for years because it appears to have formed from the fusion of two mid-sized ape chromosomes, and it is the primary cytogenetic distinction separating humans and their evolutionary progenitors.

 

Within this 700 Kb region of human chromosome 2, the researchers identified segments of DNA that originated from 14 ancestral loci. These DNA segments, or "duplicons," ranged from 4-77 Kb in length and exhibited 94-99% sequence identity to their euchromatic predecessors.

 

The scientists then performed a comparative analysis of these duplicons in other primate species, including chimpanzee, gorilla, orangutan, baboon, and macaque. This analysis revealed that the duplicative transposition events leading to the establishment of these duplicons within the pericentromeric region occurred during a relatively narrow window of evolutionary time between 10-20 million years ago. This corresponds to the time period following the divergence of humans and Old World monkeys, but before the divergence of humans and great apes. For the past 10 million years, however, no such "duplicative seeding" events appear to have occurred in this region of the genome.

 

"It is unclear why pericentromeric seeding events have occurred so frequently during this period of human/great-ape evolutionary history," says Eichler. "It is also unclear as to why they suddenly cease, at least in the case of this pericentromeric region of chromosome 2."

 

Clearly, factors other than DNA sequence are necessary for such "punctuated" duplicative transposition events to occur during genome evolution. During the divergence of the human/great-ape lineage from the Old World monkey lineage, the genome may have been particularly permissive to segmental duplication events. The scientists speculate that the molecular driving forces behind this "punctuated" duplicative activity may have been changes in transcriptional status or chromatin conformation.

 

"Other regions may show different temporal biases," explains Eichler. "The important implication here is that episodic bursts of activity challenge the concept of gradual clock-like changes during the course of genome evolution. Since duplications are important in the birth of new genes and large-scale chromosomal rearrangements, it may follow that these processes may have gone through similar episodes of activity followed by quiescence."

 

Source: Cold Spring Harbor Laboratory

Link to comment
Share on other sites

...I wonder what the trigger for that was....
Boy, me too. This is interesting in that there was not (to my knowledge) any contemporaneous cataclysmic stress to stimulate this complicated biochemical event. But the biochemical behavior certainly is interesting.

 

Although I am probably droning on this. I suspect that most will report this biochemical activity as a mutative mechanism. I would like to underline that there was nothing in this research to suggest that the biochemical behavior was a mutation. To me it looks more like a systematic change.

Link to comment
Share on other sites

Boy, me too. This is interesting in that there was not (to my knowledge) any contemporaneous cataclysmic stress to stimulate this complicated biochemical event. But the biochemical behavior certainly is interesting.

 

Although I am probably droning on this. I suspect that most will report this biochemical activity as a mutative mechanism. I would like to underline that there was nothing in this research to suggest that the biochemical behavior was a mutation. To me it looks more like a systematic change.

 

 

Systematic or Mutative or both really isn't the thrust here at this point. That's better left to side research to determine which. But as you mentioned, contemporaneous cataclysmic stress or whatever factors in the past that influenced this are not at the present acting agents in current evolution. That really does open up the door for determining what exactly does influence shuch a burst.

 

The Miocene was a time of warmer global climates than those in the preceeding Oligocene, or the following Pliocene. It is notable in that two major ecosystems first appeared at this time: kelp forests and grasslands. It was a period of warming followed by a period of cooling. With the mass extinction that ended the time of the dinos all this period could well be termed a rehab period for the earth. So I would suggest looking at factors involved in ecological rehab periods.

Link to comment
Share on other sites

Proceedings of the National Academy of Sciences, a bit back, mentioned that marine benthic diversity in Laurentia,which eventually became North America, recovered to pre-extinction levels within 5 million years, which is nearly 15 million years sooner than suggested by global compilations. There has been a suggestion that the region was operating differently than the globe as a whole which suggests factors involving isolation in the sence of very local environmental changes of a sudden type which also may play into this epoch also. One possible factor put forward was that the quicker recovery was caused by immigration of organisms from other areas of the globe. However, while this works in regional areas that well apply to the rise of man it does not solve global situations.

 

In 1972, natural nuclear reactor was found in a Western Africa in the Republic of Gabon, at Oklo. While the reactor was critical, approximately 1.7 billion years ago, it released 15,000 megawatt-years of energy by consuming six tons of uranium. This in itself might raise the issue of possible radiation based sources for such a rapid mutational change since radiation has long been known to cause different types of mutations. (see THE NATURAL NUCLEAR REACTOR AT OKLO: A COMPARISON WITH MODERN NUCLEAR REACTORS (WWW paper by Andrew Karam - 1998, updated 2005))

 

However, what one would need to look for is a simular event around 10 million years ago. But note, 2.8 million, 1.7 million and 1 million years ago coincide with major steps in human evolution as documented by the fossil record. Another factor in these periods was climatic changes. All of these themselves are over short periods of time. At 2.8 million years ago, the human family tree split into at least two major branches, Paranthropus and Homo. At 1.7 million years ago, humans' most immediate ancestor, Homo erectus, first appeared. At 1 million years ago, Paranthropus had died out and great numbers of Homo erectus began to migrate out of Africa into a variety of regions and habitats in Europe and Asia. Again the key time period was around 1.7 million years ago which does fit well with that natural reactor.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
 Share

×
×
  • Create New...