[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:

[..]

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?