[Centraal] Evolutie in het nieuws

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Horizontal gene transfer of the algal nuclear gene psbO to the photosynthetic sea slug Elysia chlorotica

The sea slug Elysia chlorotica acquires plastids by ingestion of its algal food source Vaucheria litorea. Organelles are sequestered in the mollusc's digestive epithelium, where they photosynthesize for months in the absence of algal nucleocytoplasm. This is perplexing because plastid metabolism depends on the nuclear genome for >90% of the needed proteins. Two possible explanations for the persistence of photosynthesis in the sea slug are the ability of V. litorea plastids to retain genetic autonomy and/or (ii) more likely, the mollusc provides the essential plastid proteins. Under the latter scenario, genes supporting photosynthesis have been acquired by the animal via horizontal gene transfer and the encoded proteins are retargeted to the plastid. We sequenced the plastid genome and confirmed that it lacks the full complement of genes required for photosynthesis. In support of the second scenario, we demonstrated that a nuclear gene of oxygenic photosynthesis, psbO, is expressed in the sea slug and has integrated into the germline. The source of psbO in the sea slug is V. litorea because this sequence is identical from the predator and prey genomes. Evidence that the transferred gene has integrated into sea slug nuclear DNA comes from the finding of a highly diverged psbO 3′ flanking sequence in the algal and mollusc nuclear homologues and gene absence from the mitochondrial genome of E. chlorotica. We demonstrate that foreign organelle retention generates metabolic novelty (“green animals”) and is explained by anastomosis of distinct branches of the tree of life driven by predation and horizontal gene transfer.

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How the turtle's shell evolved

A newly discovered fossil from China has shed light on how the turtle's shell evolved.

The 220 million-year-old find, described in Nature journal, shows that the turtle's breast plate developed earlier than the rest of its shell.

The breast plate of this fossil was an extension of its ribs, but only hardened skin covered its back.

Researchers say the breast plate may have protected it while swimming.

The turtle fossil, found near Guangling in south-west China, is thought to be the ancestor of all modern turtles, although it differs markedly; it has teeth rather than a bony plate, the shell only covers its underside and it has a long tail.

The fossil find helps to answer key questions about the evolution of turtles, Dr Xiao-Chun Wu from the Canadian Museum of Nature was one of the first to examine the fossil.

Aquatic life

"Since the 1800s, there have been many hypotheses about the origin of the turtle shell," explained Dr. Wu. "Now we have these fossils of the earliest known turtle. They support the theory that the shell would have formed from below as extensions of the backbone and ribs, rather than as bony plates from the skin as others have theorised," Dr Wu explained.

The researchers say this idea is supported by evidence from the way modern turtle embryos develop. The breast plate grows before the shell covering their backs.

The fossilised turtle ancestor, which has been named Odontochelys semitestacea, meaning half-shelled turtle with teeth, probably inhabited the river deltas or coastal shallows of China's Nanpanjiang trough basin - the area where the fossil was unearthed.

Researchers say the development of the shell to first protect the underside points to a mainly aquatic lifestyle.

Dr Olivier Rieppel from Chicago's Field Museum also examined the fossil.

"This strongly suggests Odontochelys was a water dweller whose swimming exposed its underside to predators. Reptiles living on the land have their bellies close to the ground with little exposure to danger," he said.

The researchers say further evidence to support the idea that this species lived mainly in water comes from the structure and proportions of the fossil's forelimbs, which closely resemble those of modern turtles that live in similar conditions.

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All for One and One for All
Few scientists win a Pulitzer Prize. In 1991, Bert Hölldobler and Edward O. Wilson did for their magisterial The Ants (1). Now these distinguished collaborators offer The Superorganism: The Beauty, Elegance, and Strangeness of Insect Societies. In this ambitious book, they seemingly have three goals. Hölldobler and Wilson reprise and update themes from The Ants while broadening their survey to include other social insects (most notably honeybees). In a thread woven throughout the narrative, they argue that colonies of social insects should be seen and studied as superorganisms, a naturally selected level of organization a step above individual organisms. And they present the work in a format apparently intended to attract the broadest possible readership of both specialists and nonspecialists. This suggests the authors' probable primary motivation--they are advocating a conceptual framework for understanding insect sociality that has been out of favor for four decades.

Hölldobler and Wilson begin by updating the superorganism concept. They hold that "[t]he principal target of natural selection in the social evolution of insects is the colony, while the unit of selection is the gene." They then treat this claim in detail through a historical and theoretical dissection of gene-centered, altruistic worker-centered, and colony-centered perspectives along with the controversies, sometimes heated, that have accompanied them. They seek synthesis under the umbrella of multilevel selection, which Wilson and David Sloan Wilson have recently advocated in general and specialist articles (2, 3). Ensuing chapters address the "sociogenesis" of colonies and two main organizing principles for social insect colonies, division of labor and communication. Several chapters on ants, the forte of both authors, round out the book. Each chapter after the introduction is a stand-alone review of its topic, rich in content and sometimes daunting in detail. The chapter on ponerine ants, in particular, exemplifies the quantity, quality, and diversity of research on ants during the past two decades.

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Amish gene 'limits heart disease'

A gene mutation which protects the heart against a high-fat diet has been found in the Amish population.

Researchers found 5% of the US Amish population in Lancaster, Pennsylvania have a mutation in a protein which breaks down fatty particles.

Those with the mutation had higher levels of "good" HDL-cholesterol and lower levels of "bad" LDL-cholesterol, the journal Science reported.

It is hoped the finding will lead to new therapies to reduce cholesterol.

The researchers used blood samples from 800 volunteers in the Old Order Amish community to look for DNA markers that might be associated with levels of fat particles called triglycerides in the blood stream.

High blood levels of triglycerides, one of the most common types of fat in food, have been linked to heart disease.

They found a mutation in the APOC3 gene, which encodes a protein - apoC-III - that inhibits the breakdown of triglycerides.

As part of the study, participants drank a high-fat milkshake and were monitored for the next six hours.

Individuals with the mutation produced half the normal amount of apoC-III and had the lowest blood triglyceride levels - seemingly because they could break down more fat.

They also had relatively low levels of artery-hardening - a sign of cardiovascular disease.

Protection

Study leader Dr Toni Pollin, assistant professor of medicine at the University of Maryland School of Medicine, said: "Our findings suggest that having a lifelong deficiency of apoC-III helps to protect people from developing cardiovascular disease.

"The discovery of this mutation may eventually help us to develop new therapies to lower triglycerides and prevent cardiovascular disease," she added.

The researchers believe the mutation was first introduced into the Amish community in Lancaster County by a person who was born in the mid-1700s.

It appears to be rare or absent in the general population.

Cathy Ross, cardiac nurse at the British Heart Foundation said the benefits of high HDL cholesterol and low LDL cholesterol are already being achieved by drugs such as statins.

"If new drugs can be developed that mimic the effect of this mutation, it may afford more ways in which individuals could be protected from developing cardiovascular disease.

"There are also lots of other simple ways people can reduce your risk of cardiovascular disease such as eating a diet low in saturated fat, having five portions of fruit and vegetables a day and taking regular physical activity."

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Large allele frequency differences between human continental groups are more likely to have occurred by drift during range expansions than by selection.

T. Hofer, N. Ray, D. Wegmann, L. Excoffier (2009).
Annals of Human Genetics, 73 (1), 95-108

Several studies have found strikingly different allele frequencies between continents. This has been mainly interpreted as being due to local adaptation. However, demographic factors can generate similar patterns. Namely, allelic surfing during a population range expansion may increase the frequency of alleles in newly colonised areas. In this study, we examined 772 STRs, 210 diallelic indels, and 2834 SNPs typed in 53 human populations worldwide under the HGDP-CEPH Diversity Panel to determine to which extent allele frequency differs among four regions (Africa, Eurasia, East Asia, and America). We find that large allele frequency differences between continents are surprisingly common, and that Africa and America show the largest number of loci with extreme frequency differences. Moreover, more STR alleles have increased rather than decreased in frequency outside Africa, as expected under allelic surfing. Finally, there is no relationship between the extent of allele frequency differences and proximity to genes, as would be expected under selection. We therefore conclude that most of the observed large allele frequency differences between continents result from demography rather than from positive selection.

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Life makes more of itself.

And now so can a set of custom-designed chemicals. Chemists have shown that a group of synthetic enzymes replicated, competed and evolved much like a natural ecosystem, but without life or cells.

"So long as you provide the building blocks and the starter seed, it goes forever," said Gerald Joyce, a chemist at the Scripps Research Institute and co-author of the paper published Thursday in Science. "It is immortalized molecular information."

Joyce's chemicals are technically hacked RNA enzymes, much like the ones we have in our bodies, but they don't behave anything like those in living creatures. But, these synthetic RNA replicators do provide a model for evolution — and shed light on one step in the development of early living systems from on a lifeless globe.

Scientists believe that early life on Earth was much more primitive than what we see around us today. It probably didn't use DNA like our cells do. This theory of the origin of life is called the RNA World hypothesis, and it posits that life began using RNA both to store information, like DNA does now, and as a catalyst allowing the molecules to reproduce. To try to understand what this life might have looked like, researchers are trying to build models for early life forms and in the process, they are discovering entirely new lifelike behavior that nonetheless isn't life, at least as we know it.

As Joyce put it, "This is more of a Life 2.0 thing."

The researchers began with pairs of enzymes they've been tweaking and designing for the past eight years. Each member of the pairs can only reproduce with the help of the other member.

"We have two enzymes, a plus and a minus," Joyce explains. "The plus assembles the pieces to make the minus enzyme, and the minus enzyme assembles the pieces to draw the plus. It's kind of like biology, where there is a DNA strand with plus and minus strands."

From there, Joyce and his graduate student Tracey Lincoln, added the enzymes into a soup of building blocks, strings of nucleic bases that can be assembled into RNA, DNA or larger strings, and tweaked them to find pairs of enzymes that would reproduce. One day, some of the enzymes "went critical" and produced more RNA enzymes than the researchers had put in.

It was an important day, but Joyce and Lincoln wanted more. They wanted to create an entire population of enzymes that could replicate, compete and evolve, which is exactly what they did.

"To put it in info speak, we have a channel of 30 bit capacity for transferring information," Joyce said. "We can configure those bits in different ways and make a variety of different replicators. And then have them compete with each other."

But it wasn't just a bunch of scientist-designed enzymes competing, like a miniature molecular BattleBots sequence. As soon as the replicators got into the broth, they began to change.

"Most of the time they breed true, but sometimes there is a bit flip — a mutation — and it's a different replicator," explained Joyce.

Most of these mutations went away quickly, but — sound familiar? — some of the changes ended up being advantageous to the chemicals in replicating better. After 77 doublings of the chemicals, astounding changes had occurred in the molecular broth.

"All the original replicators went extinct and it was the new recombinants that took over," said Joyce. "There wasn't one winner. There was a whole cloud of winners, but there were three mutants that arose that pretty much dominated the population."

It turned out that while the scientist-designed enzymes were great at reproducing without competition, when you put them in the big soup mix, a new set of mutants emerged that were better at replicating within the system. It almost worked like an ecosystem, but with just straight chemistry.

"This is indeed interesting work," said Jeffrey Bada, a chemist at the Scripps Institution of Oceanography, who was not involved with the work. It shows that RNA molecules "could have carried out their replication in the total absence" of the more sophisticated biological machinery that life now possesses.

"This is a nice example of the robustness of the RNA world hypothesis," he said. However, "it still leaves the problem of how RNA first came about. Some type of self-replicating molecule likely proceeded RNA and what this was is the big unknown at this point."

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Op zondag 1 februari 2009 20:40 schreef pfaf het volgende:

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Uiteraard. En dat zie ik in deze tijd nog niet zo snel gebeuren. Of heel vrouwelijk moet op 6 vingers gaan vallen.

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Op maandag 2 februari 2009 04:53 schreef vaarsuvius het volgende:
Prachtige utizending van David Attenborough gisteravond weer op de BBC. in een uur Darwin uitgelegd zonder dat het te moeilijk werd, maar toch met alle belangrijke zaken erin. BBC kwaliteit. Waarom hebben we dat niet in Nederland?

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A research team at the Broad Institute of Harvard and MIT and Beth Israel Deaconess Medical Center has uncovered a vast new class of previously unrecognized mammalian genes that do not encode proteins, but instead function as long RNA molecules.

Their findings, published in the February 1st advance online issue of the journal Nature, demonstrate that this novel class of "large intervening non-coding RNAs" or "lincRNAs" plays critical roles in both health and disease, including cancer, immune signaling and stem cell biology.

Standard "textbook" genes encode RNAs that are translated into proteins, and mammalian genomes harbor about 20,000 such protein-coding genes. Some genes, however, encode functional RNAs that are never translated into proteins. These include a handful of classical examples known for decades and some recently discovered classes of tiny RNAs, such as microRNAs.

[..]

By contrast, the newly discovered lincRNAs are thousands of bases long. Because only about ten examples of functional lincRNAs were known previously, they seemed more like genomic oddities than critical components. The new Nature study shows that there are actually thousands of such genes and that they have been conserved across mammalian evolution.

[..]

To uncover this large collection of new genes, the Broad scientific team looked not at the RNA molecules themselves but at telltale signs in the DNA called chromatin modifications or epigenomic marks. They searched for genomic regions that have the same chromatin patterns as protein-coding genes, but do not encode proteins. By surveying the genomes of four different types of mouse cells (including embryonic stem cells and cells from various tissue types), they found an astounding 1,586 such loci that had not been previously described. The researchers also found that the vast majority of these genomic regions are transcribed into lincRNAs, and that these are conserved across mammals.

"The epigenomic marks revealed where these genes were hiding," said Mitch Guttman, a MIT graduate student working at the Broad Institute. "Analysis of their sequence then revealed that the genes are highly conserved in mammalian genomes, which strongly suggested that these genes play critical biological functions."

By correlating the expression patterns of lincRNAs in various cell types with the expression patterns of known critical protein-coding genes in those same cells, the scientists observed that lincRNAs likely play critical roles in helping to regulate a variety of different cellular processes, including cell proliferation, immune surveillance, maintenance of embryonic stem cell pluripotency, neuronal and muscle development, and gametogenesis. Further experimental evidence from several of the identified lincRNAs verified these observations.

Because of the stringent experimental conditions imposed by the researchers in identifying the 1,600 lincRNAs in the Nature study, it is likely that there are many more lincRNA genes hiding in plain sight in the genome, as well as other RNA-encoding genes that are as important to genome function as their better-recognized protein-coding counterparts.

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Guttman, M. et al. (2009) Chromatin signature reveals over a thousand highly conserved large non-coding RNAs in mammals. Nature, advance online

There is growing recognition that mammalian cells produce many thousands of large intergenic transcripts. However, the functional significance of these transcripts has been particularly controversial. Although there are some well-characterized examples, most (>95%) show little evidence of evolutionary conservation and have been suggested to represent transcriptional noise5, 6. Here we report a new approach to identifying large non-coding RNAs using chromatin-state maps to discover discrete transcriptional units intervening known protein-coding loci. Our approach identified approx1,600 large multi-exonic RNAs across four mouse cell types. In sharp contrast to previous collections, these large intervening non-coding RNAs (lincRNAs) show strong purifying selection in their genomic loci, exonic sequences and promoter regions, with greater than 95% showing clear evolutionary conservation. We also developed a functional genomics approach that assigns putative functions to each lincRNA, demonstrating a diverse range of roles for lincRNAs in processes from embryonic stem cell pluripotency to cell proliferation. We obtained independent functional validation for the predictions for over 100 lincRNAs, using cell-based assays. In particular, we demonstrate that specific lincRNAs are transcriptionally regulated by key transcription factors in these processes such as p53, NFkappaB, Sox2, Oct4 (also known as Pou5f1) and Nanog. Together, these results define a unique collection of functional lincRNAs that are highly conserved and implicated in diverse biological processes.

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Rapidly Evolving Gene Contributes To Origin Of Species

ScienceDaily (Feb. 7, 2009) — A gene that helped one species split into two species shows evidence of adapting much faster than other genes in the genome, raising questions about what is driving its rapid evolution.

The paper in the Feb 5 issue of Science shows that the gene has connections to another previously identified "speciation gene." Both genes code for key proteins that control molecular traffic into and out of a cell's nucleus. The researchers believe an arms race of sorts inside the cell drives these genes to evolve rapidly—and as a consequence makes closely related species genetically incompatible with one another.

"When we cross two species of fruit fly, which had split from one another 3 million years ago, some of the hybrid offspring die," says Daven Presgraves, professor of biology at the University of Rochester and Grass Fellow at the Radcliffe Institute for Advanced Study at Harvard University. "This tells us that genes from one species are no longer compatible with genes from the other species. We've now found that a functionally related group of genes is responsible, with different versions of the genes having evolved in the two species. And just as Darwin predicted 150 years ago, they evolved by natural selection."

Presgraves has some ideas why two of the genes in particular, called Nup160 and Nup96, have evolved so quickly: they act as gatekeepers of a cell's nucleus, a favorite target for viruses and even malicious genes within the fly's own genome. Presgraves says that these genes probably experience constant assault and thus must constantly adapt. That these genes also prevent genetic mixing between closely related species is incidental—the origin of new species is just a by-product of evolutionary arms races, he says.

When two populations become separated by a geographic barrier—a mountain range or an ocean—they evolve independently. Presgraves and his graduate student Shanwu Tang studied a fruit fly species from Madagascar that long ago become separated from its sister species in Africa. Separated by an expanse of the Indian Ocean, the two independently evolving species accumulated genetic differences. Tang and Presgraves's unexpected finding, however, was that in both species, the Nup160 and Nup96 genes became so different so quickly that they are no longer compatible.

"When the same genes in two different species evolve quickly, they become so different that they can be incompatible," says Tang. "The genes from one species can't talk to the genes in the other species any more."

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Biologists Find Gene Network That Gave Rise To First Tooth

ScienceDaily (Feb. 10, 2009) — A new paper in PLoS Biology reports that a common gene regulatory circuit controls the development of all dentitions, from the first teeth in the throats of jawless fishes that lived half a billion years ago, to the incisors and molars of modern vertebrates, including you and me.

"It's likely that every tooth made throughout the evolution of vertebrates has used this core set of genes," said Gareth Fraser, postdoctoral fellow at Georgia Tech's School of Biology.

The first vertebrates to have teeth were a group of eel-like jawless fish known as the conodonts that had teeth not in their mouth, but lining the throat. This particular group is long since extinct, but some modern fish retain teeth in the throat (pharynx). Dr. Fraser and colleagues studied tooth formation in a group of fish known for their rapid rate of evolution, the cichlids of Africa's Lake Malawi. The cichlids have teeth both in their oral jaws, like humans, and deep in their throats on a pharyngeal jaw. A co-author of the paper, Darrin Hulsey, first identified a surprising positive correlation between the number of teeth in the oral jaw and in the throat in these fish.

"Originally, I thought there wouldn't be a correlation due to the developmental differences and the evolutionary distinction between the two jaw regions, but it turns out there is," explained Fraser. "So fish that have fewer oral teeth also have fewer pharyngeal teeth. This shows that on some level there's a genetic control that governs the number of teeth in both regions."

The team investigated what this control might be by using a technique localizing gene expression in the cells during tooth development, known as in situ hybridization, and found that a common genetic network governs teeth in the two locations.

"So seemingly, regardless of where you grow a tooth, whether it's in the jaw or the pharynx, you use the same core set of genes to do it," said co-author J. Todd Streelman. "We also think it's probable that this network is not just acting in teeth, but also in other similarly patterned structures like hair and feathers."

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Deducing Diet Of Prehistoric Hominid With Mathematical Models

ScienceDaily (Feb. 11, 2009) — In an unusual intersection of materials science and anthropology, researchers from the National Institute of Standards and Technology (NIST) and The George Washington University (GWU) have applied materials-science-based mathematical models to help shed light on the dietary habits of some of mankind’s prehistoric relatives. Their work forms part of a newly published, multidisciplinary analysis of the early hominid Australopithecus africanus by anthropologists at the State University of New York at Albany and elsewhere.

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A. africanus presented a puzzle. Classical analysis of the skull—large molars and premolars with thick enamel, thick heavy jawbones, strong chewing muscles as evidenced by their anchor points on the bone—pointed to a diet of small, hard seeds. The finite-element analysis threw a spanner in the works. It suggested that A. africanus’s facial and jaw anatomy was optimized to handle stress on the premolars, teeth located farther forward in the mouth and most useful for chewing larger hard objects. But recent studies had shown that the teeth lacked the microscopic wear patterns characteristic of chewing hard objects, a contradiction.

Here, work by NIST researcher Brian Lawn and a group at GWU headed by Peter Lucas came in handy. Driven by an interest in tooth restoration materials, they had been studying teeth using fracture mechanics, a field that considers how materials fail under excessive loads. “Our analytical approach produces equations that predict how each mode of damage will occur under different conditions and this enables us to determine trends for different tooth sizes, different food sizes, different food hardness and so on,” explained Lawn. “What they show is that, under some conditions, teeth will actually fracture before they wear.” This explained the absence of microwear patterns in the teeth, which would normally not be used for chewing small hard seeds. “A lot of people have thought the most important part of the survival of the tooth is wear, but it’s now becoming evident that the fracture properties are also very important because there’s a limit to the force that you can apply. Wear is important, but when you start to bite on harder, larger objects, fracture becomes more important,” Lawn said.

“This is a neat example of how really basic materials science—fracture mechanics—has important implications for biological sciences and anthropology,” Strait observed. In the bigger picture, said Strait, the new understanding about A. africanus’s diet may help to explain its successful adaptation to changing climates. A large hard nut that had to be cracked with the premolars may not have been a preferred meal, but it could be something to fall back on when other foods were scarce.

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Did Burst Of Gene Duplication Set Stage For Human Evolution?

ScienceDaily (Feb. 12, 2009) — Roughly 10 million years ago, a major genetic change occurred in a common ancestor of gorillas, chimpanzees, and humans. Segments of DNA in its genome began to form duplicate copies at a greater rate than in the past, creating an instability that persists in the genome of modern humans and contributes to diseases like autism and schizophrenia. But that gene duplication also may be responsible for a genetic flexibility that has resulted in some uniquely human characteristics.

"Because of the architecture of the human genome, genetic material is constantly being added and deleted in certain regions," says Howard Hughes Medical Institute investigator and University of Washington geneticist Evan Eichler, who led the project that uncovered the new findings. "These are really like volcanoes in the genome, blowing out pieces of DNA."

Eichler and his colleagues focused on the genomes of four different species: macaques, orangutans, chimpanzees, and humans. All are descended from a single ancestral species that lived about 25 million years ago. The line leading to macaques broke off first, so that macaques are the most distantly related to humans in evolutionary terms. Orangutans, chimpanzees, and humans share a common ancestor that lived 12-16 million years ago. Chimps and humans are descended from a common ancestral species that lived about 6 million years ago.

By comparing the DNA sequences of the four species, Eichler and his colleagues identified gene duplications in the lineages leading to these species since they shared a common ancestor. They also were able to estimate when a duplication occurred from the number of species sharing that duplication. For example, a duplication observed in orangutan, chimpanzees, and humans but not in macaques must have occurred sometime after 25 million years ago but before the orangutan lineage branched off.

Eichler's research team found an especially high rate of duplications in the ancestral species leading to chimps and humans, even though other mutational processes, such as changes in single DNA letters, were slowing down during this period. "There's a big burst of activity that happens where genomes are suddenly rearranged and changed," he says. Surprisingly, the rate of duplications slowed down again after the lineages leading to humans and to chimpanzees diverged. "You might like to think that humans are special because we have more duplications than did earlier species," he says, "but that's not the case."

These duplications have created regions of our genomes that are especially prone to large-scale reorganizations. "That architecture predisposes to recurrent deletions and duplications that are associated with autism and schizophrenia and with a whole host of other diseases," says Eichler.

Yet these regions also exhibit signs of being under positive selection, meaning that some of the rearrangements must have conferred advantages on the individuals who inherited them. Eichler thinks that uncharacterized genes or regulatory signals in the duplicated regions must have created some sort of reproductive edge. "I believe that the negative selection of these duplications is being outweighed by the selective advantage of having these newly minted genes, but that's still unproven," he said.

An important task for future studies is to identify the genes in these regions and analyze their functions, according to Eichler. "Geneticists have to figure out the genes in these regions and how variation leads to different aspects of the human condition such as disease. Then, they can pass that information on to neuroscientists and physiologist and biochemists who can work out what these proteins are and what they do," he says. "There is the possibility that these genes might be important for language or for aspects of cognition, though much more work has to be done before we'll be able to say that for sure."

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Survival Of The Weakest? Cyclical Competition Of 3 Species Favors Weakest As Victor

ScienceDaily (Feb. 13, 2009) — The extinction of species is a consequence of their inability to adapt to new environmental conditions, and also of their competition with other species. Besides selection and the appearance of new species, the possibility of adaptation is also one of the driving forces behind evolution. According to the interpretation that has been familiar since Darwin, these processes increase the “fitness” of the species overall, since, of two competing species, only the fittest would survive.

LMU researchers have now simulated the progression of a cyclic competition of three species. It means that each participant is superior to one other species, but will be beaten by a third interaction partner. “In this kind of cyclical concurrence, the weakest species proves the winner almost without exception,” reports Professor Erwin Frey, who headed the study. “The two stronger species, on the other hand, die out, as experiments with bacteria have already shown. Our results are not only a big surprise, they are important to our understanding of evolution of ecosystems and the development of new strategies for the protection of species.”

Ecosystems are composed of a large number of different species, which interact and compete with one another for scarce resources. This competition between species in turn affects the probability with which the individual can reproduce and survive – a matter of life and death, as it were. All of these processes are also largely probabilistic and lead to fluctuations that ultimately lead to the extinction of species. We know that up to 50 species become extinct every day on Earth, which at this high rate can be attributed to the influence of man.

Yet, the phenomenon of extinction of species itself cannot be avoided altogether – and is still only barely understood. Theoretical ecologists and biophysicists are therefore intensively researching conditions and mechanisms that affect the biodiversity of Earth. Cyclic dominance is a particularly interesting constellation of species competing with each other. It means that each participant is superior to one other interaction partner, but will be beaten by a third. In ecosystems, this would be three subpopulations – in the simplified model – which dominate in turn. In fact, communities of subpopulations following such rules have been identified in numerous ecosystems, ranging from coral reef invertebrates to lizards in the inner Coast Range of California.

Such cyclical interaction is also familiarly termed “rock-paper-scissors” interaction. This is where the rock blunts the scissors, which cut the paper, which in turn wraps around the rock. Together, these non-hierarchical relationships form a cyclical motion. “The game can help describe the diversity of species,” explains Frey. “The background is a branch of mathematics called game theory, and in this case evolutionary game theory. It helps analyze systems that involve multiple actors whose interactions are similar to those in parlor games.”

Using game theory, one can also study the collective development of populations. In their study, the scientists working with Frey developed elaborate computer simulations in order to calculate the probabilities with which species in cyclical competition will survive. The games started off with three species coexisting in the systems, and ran until two species became extinct – with the third being the only remaining survivors. “What we saw was that in large populations, the weakest species would – with very high probability – come out as the victor,” says Frey.

This “law of the weakest” even held true when the difference between the competing species was slight. “This result was just as unexpected for us,” reports Frey. “But it shows once more that chance plays a big part in the dynamics of an ecosystem. Incidentally, in experiments that were conducted a couple of years ago on bacterial colonies, in order to study cyclical competition, there was one clear result: The weakest of the three species emerged victorious from the competition.”

The project was supported by the cluster of excellence “Nanoinitiative Munich (NIM)”, of which Professor Erwin Frey is a principal investigator.