The scrotum is a mystery. Why do most male mammals have their reproductive glands so vulnerable in a sack of skin and muscles outside the body? According to new research, the answer could be found in those unusual mammals that have testes inside the abdomen. These include elephants, aardvarks and others from a group that originated in Africa, known as the Afrotheria.
The new study, published in PLOS Biology, examined 71 placental mammals for two key genes – RXFP2 and INSL3 – needed for the development of ligaments involved in testicular outflow. They found that these genes mutated in many African mammals without external testes, so that they no longer functioned.
These data showed that the common ancestor of all African mammals that lived 70 to 90 million years ago as well as the much older ancestor of all placental mammals were testicles. Research also showed that this property was reversed at least four times during the evolution of some African animals. This suggests that closer scrutiny of Afrotheria and her ancestors could show exactly what evolutionary advantages and disadvantages testicles have outside the body
Scientists often try to find out how extinct animals are and how they evolved, especially when There are no fossils available when looking at their live offspring. If all the modern offspring of a particular species have a particular physical or behavioral trait, then it is probably the ancestral species as well.
The reconstruction of details of such species is known as an existing phylogenetic staple. However, this approach is well founded on how groups of living species are related, and in the case of placental mammalian groups, attempts to draw a precise pedigree are still controversial.
Studying the DNA of Afrotheria circumvents this problem because we do not need to know how they are broadly related to other placental mammalian groups. Instead, the researchers looked for clues to how the animals' genes had changed during their development. They found that only modern mammalian mammals had mutated versions of genes involved in the testicular origin. All other groups of mammals had these genes intact.
The fact that dysfunctional residues of the RXFP2 and INSL3 genes are found in mammals without external testes strongly suggests that their ancestors had working copies of these genes. In the course of evolution, when for some reason it was no longer beneficial to have external testicles, mutations in these genes occurred without diminishing the reproductive chances of the animal. These mutant genes were then passed on to the next generation.
The researchers were also able to roughly date when these gene mutations occurred, with the so-called molecular clock, the idea that gene mutations build at specific rates. If you know what an originally functioning gene looks like, you can find out how many mutations in mutated genes have occurred in other animals. With the help of the molecular clock you can then find out how long these mutations would have needed.
What we do not yet know is why these mutations were successfully passed on and why Afrotheria developed so that their testes are no longer descendants (Testicondy). The researchers suggest that either the testicondylia is beneficial in an unknown way, or that it results from the constraints of another physical development.
Obviously, much remains unknown and these ideas need to be tested. But the latest study also shows how scientists can use this form of molecular paleobiology to find out what extinct organisms are, even if we have no hope of finding their complete physical remains as fossils.
originally published on The Conversation. Read the original article.
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