sábado, 16 de maio de 2009

Alguns animais nasceram selvagens e outros não

Scientists have Identified Genetic Regions Linked to Tameness
by Denise Chen
June 24, 2009 [posted here]

There are lots of differences between wild and domestic animals. Consider a wolf versus a dog, or a lion versus a cat. Aside from changes to make them look different, domestic animals are bred to be comfortable around people. And to even like human company!

Since these traits are bred into these animals, it makes sense that genes are involved. Now, in a new study, scientists have managed to narrow down genes involved in tameness to five big DNA regions. And they found that two of these genomic regions are particularly important.

These results show that genetics can explain tameness. The next step is to figure out what genes are important in these regions. And to understand which versions of these genes become more common in animal generations that go from being wild to domesticated.

Animal Tameness, a Genetic Thing?

Different versions of many genes likely explain the differences between wild and domesticated animals.
Over 15,000 years ago, dogs were the first animals to become domesticated. Now, there are many more examples all around us. We find these examples in our food (chickens), clothing (sheep), transportation (horses), friendships (pets), and research subjects (rats, mice).

All of these animals came from wild ancestors. Researchers have wondered for a long time how they became tame.

In 1972, scientists in Russia began an experiment to figure this out. They caught wild rats from around the city and sorted them into aggressive and tame groups.

Rats’ tameness was determined with the glove test. A human hand (in a protective glove) is put into a rat’s cage. The aggressive rats will scream, attack, or run away from the hand. The tame rats will allow touching and handling, and some will even approach and investigate the hand.

The scientists have bred the most aggressive rats to each other and the tamest rats to each other twice a year since then. Now there are two very distinct groups, the very aggressive and the very tame.

Remember, these rats are the same species. And they’re raised in very similar conditions. The most likely reason for their different behavior is that they each inherited different versions of the tameness genes from their parents.

Populations tend to have lots of different versions of the same genes. That’s why some people have pale skin and others have dark. Or why some people tend to be happier than other people. And why some animals are tamer than other animals.

The first step in finding out why an animal is tame is to find the DNA regions that have the genes involved in tameness. The next is to find the actual genes in those regions. And the final step is to see what differences there are in these genes between tame and aggressive animals.

The researchers in this study have found five DNA regions important in tameness. They haven’t found the genes or the gene differences that explain the behavior yet. But they have found that the genes there work together to make an animal tame.

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Finding Tameness DNA

Genes can explain why some rodents are aggressive and others are friendly.

To find the tameness regions, the researchers took rats from both extremes and bred them together. Then they bred all the rats from this generation with each other to completely mix up all their genes. The researchers ended up with grandpups ranging from very aggressive to very tame. Some grandpups were even more extreme than their grandparents!

The researchers bred the mice with each other to keep from going down the wrong track. What if, for example, the tame rats’ ancestors happened to have a version of a gene that had to do with digesting food and not with tameness?

All of the rats’ ancestors would share that same gene even though it has nothing to do with tameness. The researchers would find this gene and try to use it in their studies. Mixing the rats beforehand helps get rid of a lot of these sorts of false leads.

Also, the aggressive and tame rats may have had different versions of the tameness genes. In other words, there may not be a single tameness version or a single aggressive version of any given gene. The researchers wanted to figure out how these alleles all worked with each other. And the best way to do that was to mix them up and see what happens.

The researchers tested the grandpups for many different things and looked over their DNA. Because rats have so much DNA, the researchers needed to zoom in on useful regions to look at. They used a process of elimination.

First, they eliminated the genes that were different among the tame rats. Since these genes were different, they could not have contributed to every tame rat’s tameness.

Next, they eliminated the genes that were the same between the aggressive and tame rats. Tame rats shouldn’t share the same versions of tameness genes with the aggressive rats, so any shared gene versions probably aren’t involved in tameness.

When they were done, they found five big DNA regions linked to tameness in these rats. They called these regions Tame-1 all the way to Tame-5. The researchers found that Tame-1 and Tame-2 have a particularly strong influence on rat tameness.

Tameness Genes Work Together

Genes work together like groups of lights in a movie theater that help you see.
When they looked more closely at these genomic regions, the researchers found that they are part of a ‘tameness’ network. This means that the genes in these five regions communicate with each other to determine how a rat should behave. This communication is called epistasis. Epistasis means that genes cooperate with each other to get a job done.

Imagine that someone dropped a tube of chapstick in the movie theatre. She might be able to find it using the walkway lights. But when the movie is over and the lights get turned on, the walkway lights only kind of help.

If tameness is measured by the amount of light in the movie theatre, Tame-1 is the light switch that controls the main theatre lights and Tame-2 controls the walkway lights. When Tame-1 is off, the theatre is completely dark and turning on Tame-2 produces a good amount of light to help her to see. However, when Tame-1 is on, it lights up the entire room and turning Tame-2 on or off has a less of an effect.

Tame-1 is the main region that has the biggest effect on rat tameness, Tame-2 has less of an effect, and Tame-3 through Tame-5 have even smaller effects.

The next steps will be to actually find the genes that make these DNA regions important. And then to find the differences that make an animal tame. Finally, it will be interesting to see if the same genes are involved in tameness for dogs, cats, and all of the other domesticated animals out there.

Of course, tameness is just one trait of domestic animals. Other traits are related to personality and eating habits, physical size, anatomy, color patterns, as well as hormone and chemical levels in the body. This is an early study and so the researchers haven’t yet associated these with the ‘tameness’ genes. Future studies may possibly link some of these traits. There are many changes that take place for an animal to become domestic, but we are on our way to figuring out how we can all have lions as a household pet!

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