杏吧原创

Editorial: 鈥�No need to fear rewinding evolution鈥�

Evolution has been rewound to create a 鈥渟nouted鈥� chicken. That means we might also be able to fast-forward it to create the animals of the future

ARHAT ABZHANOV cuts a square hole in the shell of a chicken egg, drops in a small gelatinous bead and watches the embryo develop. By day 14, the chick has formed not a beak but something more snoutish 鈥� a feature, he says, 鈥渕odern birds have not seen since the Cretaceous鈥�. Abzhanov has rewound evolution.

Chickens share a common ancestor with alligators and are descended from dinosaurs, raising the question of how they and other birds switched from snouts to beaks. Because chick and gator embryos start out looking strikingly similar, , an evolutionary biologist at Harvard University, suspected the key might be found in developing embryos. In his open-egg experiment he tweaked a few of the embryo鈥檚 genes to make them behave more like identical genes do in an alligator embryo.

If rewinding evolution has a certain Frankenstein-esque quality, the opposite is even more intriguing. Fast-forwarding evolution to create the chickens of the future may also lie within grasp. And that, in theory, could lead to the creation of species better equipped to handle a changing climate.

Mounting evidence shows that small modifications in when and where genes are switched on are all that鈥檚 necessary to trigger dramatic shifts in anatomy. These changes can lead to the appearance of beaks, turtle shells and jaws (see 鈥淨ucks and duails鈥�).

鈥淪mall changes in when and where genes are switched on may trigger dramatic shifts in anatomy鈥�

Generally, the genes that control these major anatomical changes produce signalling molecules. In a developing embryo, these switch on genes controlling the formation of structures such as limbs, organs and facial features. Other genes dictate where the molecules are produced and therefore where they take effect, ensuring that embryos don鈥檛 grow digits in the wrong places, misshapen bones or an extra pair of eyes.

Abzhanov鈥檚 鈥渟nouted鈥� chicken provides a striking demonstration of just how easy it can be to provoke major evolutionary changes, says Craig Albertson, a developmental biologist at the University of Massachusetts in Amherst. Before such experiments were possible, explanations for how creatures evolved 鈥渞elied on the fossil record, which is incomplete, and mathematical modelling, which is boring鈥�.

So how did he do it? Abzhanov started by trying to pinpoint the gene changes that led to the myriad beak shapes of Galapagos finches. In 2004, he showed that all the finches share a handful of genes crucial to beak development, but instructions for the signalling molecules they control vary from bird to bird (). Abzhanov realised that a similar process might underlie the much bigger evolutionary shift from snouts to beaks.

The tip of an alligator snout is made of a separate set of paired bones called the premaxillary, but in birds, these have fused with the main of the upper jaw to form a single, sharp bone.

Abzhanov scanned signalling molecules in alligator and chick embryos and found that two of them 鈥� known as sonic hedgehog and fibroblast growth factor 8 鈥� show up before the snout and beak form. In gators, however, the molecules were only present along the sides of the face. Chicks express them both at the sides and centre of the developing face. What would happen, he wondered, if he turned that central expression off?

He developed a gel bead full of proteins that stick to the signalling molecules and deactivate them. As the molecules arrived at the centre of the embryonic chick face 鈥� around day 5 鈥� Abzhanov added his bead to the mix. Sure enough, the chicks developed paired bones. 鈥淚t looks exactly like a snout looks in an alligator [at this stage],鈥� says Abzhanov, who at . Ethics regulations mean no such eggs can be hatched.

Long term Abzhanov, dreams of turning chickens back into Maniraptora, small dinosaurs thought to have given rise to the 10,000 species of birds around today. Others have similar musings. described the basic principles in a book he co-wrote with James Gorman, How to Build a Dinosaur (Dutton Books, 2009), and regularly speaks of a future . 鈥淲e are interested in finding a way to extend the tail and create a hand in the chicken,鈥� Horner told New 杏吧原创, but would not elaborate.

鈥淎bzhanov dreams of turning chickens back into the dinosaurs they evolved from鈥�

The realisation that all it takes to create novel traits is a little genetic fine-tuning raises the possibility of engineering those shifts ourselves. Could we build the creatures of the future?

To a degree, we are already doing that, says Albertson. He and others are crossing closely related species 鈥� those that could conceivably pair on their own 鈥� and studying the resulting genetic changes. Sometimes those crosses result in novel creatures. For instance, Albertson crossed blue cichlid fish from neighbouring but separate populations and was surprised to find some of the offspring were red. He is trying to identify the genes and molecules involved, and says there is a possible advantage to the change. Some lakes that are home to cichlids are becoming increasingly murky, making it difficult for males to attract females with their colourful scales. Could it be that the bright red fish might have the edge, allowing the species to survive a more polluted world?

Amplifying the changes in the lab to create say, a fluorescent fish, may still be some way off, says , of the University of California at San Francisco. So far, there are no ways to turn signalling pathways on; we can only rewind, not fast-forward evolution.

Understanding these subtleties could have a huge impact on medicine. Many developmental abnormalities 鈥� cleft palate for instance 鈥� arise from changes in gene signalling. Could we tweak them in a developing embryo? 鈥淚 can envision a day when we eliminate such defects in the womb,鈥� says , a stem cell biologist at Stanford University in California.