Published online: 22 March 2007; | doi:10.1038/news070319-12

Mice made to see a rainbow of colours

All you need to see more is more pigments in the eye.

Lucy Odling Smee

Mice can usually only see a dull mix of yellow, blues and greys. Getty

Simply by inserting a piece of DNA that codes for a human eye pigment into the genome of a mouse, scientists have introduced a rainbow array of colour to the dull mix of yellows, blues and greys that normally make up a mouse's visual world.

This suggests that the mammalian brain is very flexible and can interpret signals not normally encountered. It also hints that just a single genetic mutation could have added reds and greens to the visual palette of our ancestors tens of millions of years ago.

Gerald Jacobs from the University of California in Santa Barbara and his colleagues have genetically engineered mice with a human pigment in their eye as well as the normal mouse pigments and shown that this does appear to give the mice the ability to see colours they could not see before.

"The implications are astounding," says David Williams, an expert in vision at the University of Rochester in New York state. "It's stunning to think the rest of the nervous system in the mouse has developed to be able to process the new information."

Most mammals have just two kinds of photopigment in their retinas: one is encoded in the X chromosome and the other in an autosomal (non-sex) chromosome. But many primates, including humans, have a third photopigment, encoded by a second gene on the X chromosome. This allows for a much broader appreciation of colour.

In looking at the evolution of full colour, or trichromatic, vision in humans, most scientists turn to New World monkeys, which have an arrangement mid-way between the two- and three-photopigment systems. They have only one photopigment gene on their X chromosome, but there are different versions of the gene, producing different pigments. As a result, female monkeys (which carry two X chromosomes, and so can potentially have two different pigment genes) can end up with three different photopigments in their eyes.

It's plausible that millions of years ago a single mutation resulted in two different versions of the photopigment gene becoming located on the same X chromosome. That could have paved the way for trichromatic vision in both males and females in descendant primates, says Jeremy Nathans, a co-author of the mouse study.

But was an extra photopigment all that was needed to evolve trichromatic vision? Or does seeing the world in all its colourful splendour require extra brain power too?

http://www.nature.com/news/2007/070319/full/070319-5.html Published online: 21 March 2007; | doi:10.1038/news070319-5

Burrowing dinosaur unearthed

Fossilized family broadens picture of extinct reptiles.

John Whitfield

An artist's impression shows the "digging runner of the lair" had a broad snout for burrowing. Lee Hall

The discovery of a dinosaur family fossilized in its burrow could make us rethink where the animals lived, how they behaved, and even what wiped them out, say researchers.

David Varricchio of Montana State University in Bozeman and his colleagues found the jumbled remains of two juveniles and an adult together in what looks to be the remains of a custom-built hole in southern Montana.

The discovery provides the first evidence that dinosaurs could burrow, and the best evidence yet for long-term parental care in dinosaurs, says team member Anthony Martin, an expert in animal traces at Emory University in Atlanta, Georgia. "I imagine that two juveniles curled up in a small space with an adult," he says.

The team has named the beast Oryctodromeus cubicularis, meaning 'digging runner of the lair'. It belongs to a group of small herbivorous dinosaurs, and lived 95 million years ago during the mid-Cretaceous period.

The skeletons are incomplete, but they show that when fully grown, the animal was about 2.1 metres long, of which more than half was tail. The dinosaur had a broad snout and powerful shoulders well adapted for digging, and sturdy hips that would help it to brace itself as it dug. It could also run on its back legs.

Secret entrance

The burrow's presence was betrayed by a patch of rock that differed from its surroundings, in an area known to contain dinosaur fossils. The burrow seems to have been dug on the edge of a river flood plain and filled with mud during a flood, burying its occupants, the researchers report in Proceedings of the Royal Society¹.

Dig this: an adult Oryctodromeus cubicularis (far left) was probably 2.1 metres long. Lee Hall

The den was just over two metres long, with a pronounced s-bend - making it harder for predators to enter - opening out into a terminal chamber. The close fit between the sizes of burrow and beast convinced the team that Oryctodromeus had dug its own den, rather than simply displacing a previous occupant. "It's not just a random attempt," says Martin. "It's very well constructed."

"It was generally assumed that dinosaurs wouldn't dig - they tend to be either runners or very large," says palaeontologist Paul Barrett of the Natural History Museum, London. "This is quite a departure."

Varricchio and his colleagues had previously found what seemed to be a family of dinosaurs in what could have been a collapsed burrow in China, but no one had seen an actual tunnel space until now.

Safe underground

Burrowing may have helped dinosaurs to survive in harsh climates, increasing the range of habitats available to them. No one knows exactly what the environment of this part of the world was like when these dinosaurs lived there, although it was probably semi-arid.

Related dinosaurs are known to have lived in southern Australia, which was close to the South Pole at the time, and South Africa, which was hot and dry. These species and locations would be good places to look for further evidence of burrowing, says Barrett.

The lack of an ability to burrow has also been suggested as a factor in the demise of dinosaurs 65 million years ago, at the end of the Cretaceous. Many of the mammals, reptiles and amphibians that survived a mass extinction at this time could burrow, perhaps sheltering them from whatever catastrophe caused the massive cull.

"The absence of burrowing has been proposed as one reason why dinosaurs didn't make it," says Martin. "You can't use that as a reason now."

Using one specimen to speculate about the dinosaurs' extinction is "quite a big inference", says Barrett. "It might be taking the data too far," he cautions.

---

http://www.futura-sciences.com/news-dinosaure-son-terrier-sa-famille_10561.php

Life is faster in the temperate zone

Evolution of species is more leisurely in the tropics.

Michael Hopkin

http://www.nature.com/news/2007/070312/full/070312-8.html

This male 'Masked Tityra' is closer to its sister species than birds in the tropics are to theirs. J. Weir

Most people tend to think of the tropics as the hottest scene on the planet when it comes to spawning new life. But Canadian zoologists have found that it is actually the world's temperate zones where new species evolve and become extinct the fastest.

The discovery by Jason Weir and Dolph Schluter of the University of British Columbia in Vancouver threatens to overturn the theory that because tropical regions contain the greatest overall species diversity, that they must also have the fastest rates of 'speciation' - the emergence of new species.

"Our findings contradict the conventional view by suggesting that temperate zones, and not the tropics, are the hotbeds of speciation," says Weir.

Sisters

The researchers surveyed 309 pairs of 'sister' species - those that are closely related to one another, much like humans and chimpanzees - from throughout the Americas. They compared their DNA sequences to work out how much the sister species had diverged from one another, and therefore how long ago their split had occurred.

Those in temperate zones tended to have diverged more recently, implying that new species are being thrown up faster in these regions. Near the Equator, sister species were separated by an average of 3.4 million years, whereas at the most extreme latitudes studied, stretching as far as the northern wilds of Canada, the figure was less than 1 million years, the researchers report in Science¹.

The apparently prodigious rate at which new species appear and disappear in temperate regions might be due to the cycle of ice ages and warm periods, which affect extreme latitudes more than the tropics, Weir suggests. "Intense climatic instability at high latitudes has resulted in increased opportunities for extinction, and increased ecological opportunity during the benign periods," he says.

By contrast, the relatively unchanged climate of the tropical region over millennia has meant that once species gain a foothold they are less likely to become extinct.

The current warming of the planet from greenhouse gases is also changing climate conditions more at extreme latitudes. But the results of this study apply only to more dramatic changes that happened over a longer time scale; they do not cast any light on how today's species might be affected by future climate change.

http://www.nature.com/news/2007/070305/full/070305-6.html

Published online: 7 March 2007; | doi:10.1038/news070305-6

Did a 'light' genome help birds take flight?

A smaller genome evolved in dinosaurs, long before birds learned to fly.

Lucy Odling-Smee

Tyrannosaurus rex may have had the genetic 'lightness' to permit flight, long before their descendents took to the skies. NHPA

A study of dinosaur genomes hints that the early evolution of a smaller genome might have been necessary for later vertebrates to take to the skies.

Birds have long been known to have much smaller genomes than mammals and reptiles living on the ground. And a small genome has been linked to both small cell size and high metabolic rate: the lower volume-to-surface ratio of small cells, which don't have much DNA to pack inside, can allow for faster transport of nutrients and signals across the membrane. Thus, some suggest that the energetic demands of flight require birds to have a 'light' genome.

But which came first: flying birds or the smaller genome?

 

http://www.nature.com/news/2007/070226/full/070226-4.html Published online: 26 February 2007; | doi:10.1038/news070226-4
Ancient DNA solves milk mystery
Analysis of fossilized bones suggests milk-drinking mutations emerged after dairy herding.

Erika Check

When did ancient populations learn that drinking milk 'does a body good'? A team of scientists in Germany has tried to answer this question by studying ancient DNA extracted from skeletons thousands of years old.

Many adult humans can drink cow's milk - a rare feat among mammals, which usually lose the ability to digest the sugar in milk after they are weaned. Scientists have found the genetic mutations that allow many Europeans and some Africans to digest milk (see 'Human evolution: How Africa learned to love the cow'). Geneticists have estimated that these mutations first spread 3,000 to 7,000 years ago in eastern Africa, and slightly earlier than that in Europe.

But some researchers have posed a 'chicken-and-egg' question about milk drinking: was dairy herding adopted only by those populations who could already drink milk? Or did the invention of dairy herding favour those people who had the mutation, so that the mutation quickly spread throughout the population?

Joachim Burger of the University of Mainz, Germany, and colleagues worked with Mark Thomas of University College London, UK, to address this riddle by studying DNA from skeletons scattered throughout Europe. The team examined ten skeletons ranging in age from 3,800 to nearly 6,000 years old. The skeletons were discovered at archaeological sites in Germany, Hungary, Poland and Lithuania.