How did kaengurus develop their pouch
biology : First jump of the kangaroo cleared up
Charles Darwin was already amazed. An unbeliever might think two different creators were at work in the world, he wrote of the confusing wildlife of Australia. After all, there seem to be numerous mammals here again, just with pouches. Appearances are deceptive, however, because from a biological point of view the koala bear is not a bear, the pouch wolf is not a wolf and the shrew pouch rat is neither a mouse nor a rat. They are similar, but they are not related. The relationships among the marsupials, which only live in America and Australia, were also unclear for a long time. Biologists from the University of Münster have now been able to explain this - and the picture is amazingly simple.
Most researchers today agree that marsupials originated in Southeast Asia and then spread through North America to South America. Numerous different species emerged there, while their ancestors died out in Asia. What happened afterwards, however, is controversial. "Most of the studies started out from complicated hikes," says Jürgen Schmitz, who heads the working group at the University of Münster.
Because some animals in America appeared to be more closely related to Australian marsupials than to the other American ones, researchers assumed that marsupials immigrated to Australia several times, but some also came back to South America. This would be possible because the Antarctic, South America and Australia formed a contiguous land mass until 80 million years ago. Schmitz now considers the repeated hikes to be superfluous. This scenario now seems out of date. Their results indicated a single immigration from South America to Australia, the researchers write in the journal “Plos Biology”. "We can clearly show that all Australian species can be traced back to a common ancestor," he says.
The scientists used so-called retrotransposons for their work. These are small sections in the genome that copy themselves and then paste them somewhere else in the genome. Like a spelling mistake that creeps into a text and is then retained in all copies, the retrotransposons are also retained in the offspring. In this way, the relationship can be reconstructed on the basis of the “jumping genes”. Because in the course of evolution, new "errors" are added, but old ones do not disappear.
The starting point was the completely deciphered genomes of the domestic shrew rat from South America and the Australian derby wallaby, one of the smallest species of kangaroo. More than half of the genome of these animals consists of retrotransposons. The researchers examined more than 200,000 of them to find those who were most active during the interesting period. So they finally found 53 meaningful markers. They examined these in 20 different species of marsupials, from the mountain kangaroo to the wombat to the pouch mullet.
Ten of the markers were found in all marsupials, whether American or Australian. In other mammals, the placentates, there is not a single one of these retrotransposons. This confirms that the marsupials form a cohesive group. The remaining 43 markers were more interesting for the scientists because they allow statements about the relationship within the marsupials. The result: The family tree is much simpler than previously thought. All Australian marsupials apparently have the same origin, an ancient marsupial, father of all kangaroos, koalas and Tasmanian devils. “There were no signs of that before,” says Schmitz.
It is conceivable that when Australia and South America broke up, only this ancient marsupial lived in Australia. But there could also have been several species, of which only the descendants of one species are still alive today. “It is now up to the paleontologists to find out,” says Schmitz.
In any case, the marsupials had the best conditions in Australia. Because there were hardly any other mammals on the new continent and thus hardly any competition. “This is why the diversity we see in Australia today was able to develop,” says Schmitz. And that's why the immigrants from South America seem to be originally Australian today.
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