The first mammals have kept their freshness with the testicles descended | Science

Most male mammals carry valuable cargoes in a terribly precarious package. The external testicles – which sway gently outside the abdominal cavity in a thin-skinned bag exposed – are sensitive, fussy, and an obvious target for all enemies of men (balloons, lost knees and furniture of cut). It is therefore not surprising that the evolution of the descended testicles has baffled and confused biologists for decades.

A study published today in PLOS Biology offers an answer to one of the greatest debates of the mystery: our earliest ancestors? to wear them, or to slaughter them? The researchers conclude that the first mammals already had this disconcerting trait, with millennia of descendants eventually inheriting family jewelry in full exposure. Yet, strangely, it seems that since then, the internal testes have evolved at least four times separately

During embryonic development in male males, reproductive structures turn into proto-testicles deep in the abdomen ( in females, these same structures form ovaries). Before birth, the testicles descend through the abdomen and into the scrotum in a two-step process. This downward migration is set in motion by the action of two key genes, INSL3 and RXFP2 . The removal of one of these "scrotal genes" in mice completely derails the southern trajectory of the testes.

Male males born with undescended testicles (between 2 and 4%) may have problems: if the condition persists in adulthood, it may contribute to hernias, infertility, and testicular cancer. But for a whole group of mammals, having no scrotum is the norm. The mammalian species of the Afrotheria clade – which includes elephants, manatees, golden cape moles, and rocky hyrax – retain their testes inside the abdomen in a state called "testify".

These divergent traits have been problematic for evolutionary biologists. The common ancestor of all living mammals, such as Afrotherians and female mammals, has kept its reproductive organs valid inside its abdomen? Or, like humans and most other mammal lines, did she bring them out of her body so everyone could see them? Because soft tissues such as testicles are poorly preserved in the fossil record, no physical evidence of the ancestral testicles remains, and the location of the mammalian ancestral testes has remained elusive.

Principal author Virag Sharma and senior author Michael Hiller, genomic scientists at the Max Planck Institute of Molecular Cell Biology and Genetics in Dresden, Germany, took a genetic approach to the debate. Knowing how important the scrotal genes were for testicular progeny, they felt that comparing these genes in a variety of mammalian lineages would provide the most direct route for locating the ancestral state. (This method bypasses the boundaries of the fossil record, which can sometimes produce contradictory or vague information about the relationship between species.)

"Being able to use molecular data to answer a question like this is something we We could not do it 10 years ago, "said Natalia Prado-Oviedo, a genomic scientist at the Smithsonian National Zoo, who was not affiliated with the study, Sharma and Hiller's method. works [with any interpretation of the fossil record]. "

When Sharma compared scrotal genes in 71 mammals, he found four species of Afroths devoid of descending testicles: the manatees, the golden moles of cape the shrews and tenrecs -choping mammals that look like hedgehogs-all wore deceased copies of the scrotal genes . Sharma then used this genetic information to estimate when one of the genes had lost its functionality in each species. When genes become non-functional, there is no more pressure to maintain coherence, and they begin to disintegrate and accumulate mutations by negligence. The more errors there are in a gene sequence, the more likely it has been lost for a long time.

On the way back, Sharma also tracked the loss of testicular offspring in all four species 23-83 million years ago. as the estimated divergence of the Afrotherian lineage 100 million years. Unlike other mammals, the Afrotherians separating from the main peloton, their testicles did not do the same thing.

Sharma also found that the types of genetic errors found in these four species differed one from the other and appeared separately. at the hour. If there had been identical mutations, Sharma would have deduced that only one ascrotal ancestor had passed the same broken genes to all four species at once. But the variation showed that scrota has disappeared four times during the history of evolution. In other words, the evolution has "independently invented" four undescended testicles.

Scientists have known for years that one of the most important benefits of scrota is ventilation: spermatozoa mature better at temperatures lower than 2.5 to 3 degrees Celsius. rest of the body, and to drop these organs keeps them cool. But we are much less sure if that is the reason that scrota evolved. This is a classic cock and egg dilemma: the testicles may have leaked to the abdomen because the temperatures were too hot or the sperm may be suitable for love the cold because they had already been hunted for another reason. ] (Other theories abound, including the idea that the testicles are ornaments that boast of male virility.) Or perhaps, as pediatric urologist John Hutson believes, the testicles have been expelled as a by-product or an anatomical rearrangement error. is the main factor, there is still a puzzle that researchers must answer. Elephants and elephant cape shrews – which keep their testicles locked in the abdomen – have internal body temperatures similar to those of humans. How are they coping?

Among the Afrotherians, the costs (exposure, vulnerability) could simply outweigh the benefits (slightly colder temperature), says Hiller. Or maybe these mammals use a method still unknown to keep them cold. To link these phenomena to the retention of testicles, geneticists will probably have to combine with physiologists.

"We can not rely solely on genetics or just on fossils," says computer biologist Melissa Wilson Sayres of Arizona State University. "Genomics is powerful, but we need to understand it in conjunction with natural history and anatomy."

For the moment, we remain suspended.

Do you like this article?
REGISTER NOW for our newsletter