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The descending testicles were probably present in the first mammals, then disappeared in elephants, manatees and their parents, according to a new study.
Reader, here is an incomplete list of things you should not try with elephants: a memory contest, a jump rope and a castration.
See, in addition to having a weird memory and a firm relationship with gravity, elephants have their testicles tucked deep into their bodies, close to their loins. This is unusual: in most other mammals, the testes are formed during embryonic development near the kidneys and then down, either down the abdomen or to the outer scrotum, at the time of the birth of the male.
Biologists have questioned this discrepancy for decades. Did the first mammals keep their testicles, like elephants, or did they drop their family jewels? A new study, published Thursday in PLOS Biology, says that it was the last.
Studying the DNA of 71 mammals, a German team concludes that testicular progeny is an ancestral trait that was later lost in the so-called afrotherians, a motley group that includes elephants, manatees and several insects eating insects. Africa
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In four subgroups Afrotherians – manatees and dugongs, elephant shrews, golden moles and tenrecs (small insectivores resembling hedgehogs) – the authors found nonfunctional remains of two genes specifically involved in testicular offspring.
Scientists often rely on geological fossils to reconstruct evolutionary history, but this study shows that there is also a "fossil in the genome," said Mark Springer, a biologist . Professor at the University of California, Riverside, who was not involved in the research.
These "molecular fossils" abound through the tree of the life. "For almost all species, you will usually find about one hundred broken genes that existed in the past and have been lost," said Michael Hiller, a research group leader at the # Max Planck Institute for Molecular Cell Biology and Genetics, Dresden, Germany, and lead author of the new article.
He and the lead author of the study, Virag Sharma, did not start targeting the testicles.
Over the years, their group has developed a method of computation for to screen different genomes of broken genes with great precision. They observed remnants of genes rendered unusable by evolution: enamel genes in edentulous whales, fat digestion genes in sugar – dependent bats, and repair genes in the toothless bats. DNA in armadillos with protection against harmful UV rays.
In addition, they noticed that two genes called RXFP2 and INSL3 were inactive in several Afrotherian species.
From a literature search, the researchers learned that if you tore these genes out of male mice, the rodent testicles would not come down. They also learned that evolutionary biologists have long debated whether this absence of testicular offspring – called testicondy – is a primitive trait, or one that afrotherians has evolved uniquely.
"It became clear that we would be able to help resolve this debate," said Dr. Hiller.
Based on the fact that genes begin to accumulate mutations once they lose their function, researchers worked backwards and felt that testicondy arose independently at least four times, ranging from About 25 million years ago about 80 million years ago in cape elephant shrews.
This also means that testicondy evolved after afrotherians about 100 million years ago, other placental mammals that suggest that the common ancestor of all mammals actually lowered their testicles, said Dr. Hiller.
But the mysteries remain. Not all afrotherians have testicles – the aardvarks, for example, have descending testicles. And although elephants and rock damans (which look like guinea pigs) do not have descending testicles, RXFP2 and INSL3 are still intact in both.
Researchers may only "look at part of the picture". Ross MacPhee, a curator in mammalogy at the American Museum of Natural History, who did not participate in the new study.
There is also the question of why testicles fall in the first place. Since they have valuable and living contents, why wear them in vulnerable bags? Scientists know that optimal sperm production requires lower temperatures than the rest of the body, but they do not understand why.
The answer may lie in a more in-depth study of the afrotherians, especially why and how they came to hold their testicles so close.
Previous reports on the mysteries of mammals
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